ROAD DISASTER PREVENTION MANAGEMENT MANUAL

GUIDE V
DISASTER PREVENTION WORKS

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 DISASTER PREVENTION WORKS GUIDE V
CONTENTS

GUIDE V DISASTER PREVENTION WORKS

1 BASIC CONCEPT
2 RECOMMENDABLE PREVENTION WORKS
3 DRAINAGE WORKS
4 RIVER REVETMENT WORKS
5 PREVENTION WORKS AGAINST DEBRIS FLOW
6 PREVENTION WORKS AGAINST ROCK FALL AND SURFACE COLLAPSE

APPENDIX V-1 COUNTERMEASURES

V1-1 SELECTION OF COUNTERMEASURES
V1-2 COUNTERMEASURE WORKSAGAINST SLOPECOLLAPSE
V1-3 COUNTERMEASURE WORKS AGAINST ROCK FALL
V1-4 COUNTERMEASURE WORKS AGAINST ROCK MASS FAILURE
V1-5 COUNTERMEASURE WORKS AGAINST MASS MOVEMENT
V1-6 COUNTERMEASURE WORKS AGAINST ROAD COLLAPSE

APPENDIX V-2 INVESTIGATION

V2-1 BASIC CONCEPT OF INVESTIGATION
V2-2 SITE RECONNAISSANCE
V2-3 INVESTIGATION FOR MASS MOVEMENT
V2-4 INVESTIGATION FOR DEBRIS FLOW
V2-5 OTHER INVESTIGATION

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CONTENTS

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 DISASTER PREVENTION WORKS GUIDE V
OUTLINE

This is a guide for design of prevention measures and

investigation for prevention measures.
Principal methods for preventive measures are described. There
have been many road disasters related to water in Bolivia,
therefore drainage methods are recommended as simple measure
works, even some heavy prevention measures are introduced in
this guide. The standard gradient of cut slopes and simple
prevention measures are included in this guide. These simple
prevention measures are recommendable for the high risk control
sections in preference to the other sections. With limited budget,
the places of prevention measures would be very limited in the
year. The place of the prevention measures shall be selected
deliberately by ABC Head Office taking total condition of the
highways into conditions.
The investigation methodology is ordered for a detailed design.
Investigation instruments, monitoring systems and protection of
equipment are included.

The contents of Sections 1.2, 1.3 and Chapter 2 in this Guide are
excerpted from the following publications.
Figure 0.1 Contents of Guide V
1) Public Works Research Institute Japan, 2004.
Manual for Highway Earthwarks in Japan
2) JICA, 2002.
Guide III: Guide to Early Warning and Site
Investigation.The Study on Slope Disaster Management
for Federal Roads in Malaysia

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 DISASTER PREVENTION WORKS GUIDE V
1 BASIC CONCEPT
inappropriate management of the rain water flowing over the
The basic concept of prevention measure shall be to remove the ground surface (surface water) triggers slope failures. The
primary factor and the contributory factor. contributory factors could be removed and the road disasters
The primary factor (constitution) is the ground’s general could be reduced, if the surface water would be managed
condition of health, especially the ground’s ability to remain properly. Preparation of road drainage such as ditches, cross
health. It is the makings of the ground, such as geological or drainage, proper surface water management on cut slopes, proper
geomorphic characteristics. size of cross sections for cross streams, revetment against side
The contributory factor is direct cause of landslides. It is natural rivers.
phenomena such as heavy rain and earthquake, or factitious And small protection measures could keep the safety traffics in
factors such as construction works. Bolivia.
The primary factor These simple prevention measures can not prevent all of road
(a) Material Characteristics disasters in Bolivia, however, they could be effective and could
(b) Mass Characteristics mitigate the road disasters in Bolivia.
(c) Reduced Shear Strength
The contributory factor
(a) Removal of Support
(b) Imposition of Surcharges
(c) Transitory Stresses

Design concept of prevention measure for road disaster is;-

First : Remove the primary factor and the contributory

factor
Second : Remove th primary factor or the contributory factor.
(mostly contributory factor)
Third : If it is difficult to remove neither the primary factor
nor the contributory factor, control by force.
Fourth : If it is difficult to remove both the factors and using
force, leave from danger (diversion).

In Bolivia, the road on slopes in mountainous areas, daylight

structures of geological stratums or poor vegetation in dry areas
are the primary factors, and rainfall and earthquake are the
contributory factors. There are many cases in Bolivia that

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 DISASTER PREVENTION WORKS GUIDE V
2 RECOMMENDABLE PREVENTION WORKS
Main cause : eroded by surface water which flow out from road surface
Prevention Measures : road side drains, transverse drains, and
2.1 DRAINAGE appropriate drainage
The following four types of disasters are common in Bolivia.
Most of these disasters are caused inappropriate drainage on the
road, slopes or cross channels. Poor Surface
(a) road collapse Drainage
(b) slope collapse (surface collapse)
(c) large road collapse This erosion
(d) debris flow might occur
because of
poor drainage
(a) Road collapse of the road
Drainage is a critical issue for roads. The infiltration of rainwater (Route 3)
and/or groundwater to the subgrade or base course can be a
major factor for road surface damage while slope erosion due to
running rainwater or slope failure due to seepage water
destabilizes the filled up ground of an embankment.

Figure 2.1 Road Shoulder Erosion (above) and Prevention Measures

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(b) Slope Collapse (Surface Collapse) Poor Surface

Drainage
Surface drainage facilities on slopes are designed to reduce the
amount of water running down the slopes to prevent slope
If the proper drainage
erosion by surface water and to prevent the infiltration of surface exists on this slope,
water into the slope. The relevant facilities include a drainage surface erosion could
ditch at the top of the slope, a vertical drainage ditch and a be prevented.
drainage ditch at the berm.

Poor Surface
Drainage

If the proper drainage

exists on this slope,
surface erosion could
be prevented.

Poor Surface
Drainage

This slope has been

cut proper angle and
Figure 2.2 Water Erodes Cut Slope (above) and Prevention Measures has berms on it.
(below) However proper
Main cause : gully or erosion which caused by surface water
(rainfall) developed and cause collapse.
drainages do not exist
Prevention Measures : Appropriate gradient of cut slope, berm and on the bermes.
drain (on berm and cascade)

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(c) Road dollapse (Large Scale) Slopes Eroded by a

River
The most prominent damage to the national roads running along
the river occurs when the bottom of the slopes under the road are
River side of the slope
eroded by rivers. When the slope bottoms are damaged, slope were eroded by a river,
collapses can progress until the roads. even though the
temporary protection
work.

Base of River Wall

Eroded

Base of the river wall

were eroded

Good Example

Figure 2.3 Prevention Measures for Road collapse Base of the wall was
Main cause : eroded by river , Prevention constructed propery
Measures : river revetment works

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(d) Debris Flow Small Cross

Drainage
In Bolivia, when a road crosses an originating or passing area of
debris flow along a mountain stream susceptible to debris flow, Because of limited
the crossing roads do not provided by cross drainages with a capacity of the
sufficient cross-sectional area or a bridge with sufficient cross drainage,
clearance. In the case of a mountain stream where high speed debris flew over
mud flow type debris flow is expected to occur at a mountain the road.
stream with a high debris flow frequency, a road crossing in the
originating or passing area of debris flow should preferably be
provided by a bridge with sufficient debris flow clearance, a
debris flow shed or a tunnel. Small Cross
Drainage

Because of limited
capacity of the
cross drainage,
debris flew over
the road.

Narrow Dranage

Because of limited
capacity of
drainage on hill
side of the road,
debris flew over
the road.
Figure 2.4 Prevention Measures for Debris Flow
Main cause : the cross sections of facilities across the road are not enough
Prevention Measures : enough cross section of facilities across the road

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2.2 PROTECTION MEASURES AGAINST ROCK FALL Steep Slope

AND SURFACE COLLAPSE
Cutting with
The following measures are not common in Bolivia, but they are proper angle and
effective in terms of protection of vehicles on the roads. berms with
(a) appropriate gradient with berms on cut slopes drainage are
(b) shotcrete on natural slopes required on this
(c) rock catch wall slope.

(a) Appropriate Gradient with Bermas on Cut Slopes

The rock mass and soil layer forming the existing ground can be
classified from the viewpoints of the excavation difficulty and
slope stability. Empirically established standard slope gradients
are then applied to the classified ground, assuming non-treatment,
sodding or simple protective work such as netting, to determine
the slope gradient and shape corresponding to the soil and rock
properties, and cutting height.

Figurer 2.5 Example of Cut Slope Gradient

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(b) Shotcrete on Natural Slopes Steep Slope with

Shotcrete works include mortar spraying and concrete spraying. Rock Face
They are commonly used on steep slopes of highly weathered or
Shotcrete works
heavily jointed rocks on which vegetation is not possible. is useful for the
Shotcrete works are intended to chiefly prevent surface slopes consist of
weathering and erosion, and in some cases, to control small-scale hard rocks like
rock falls. this slope to
prevent small
rock fall and
weathering.

Steep Slope with

Rock Face

Shotcrete works
is useful for the
slopes consist of
hard rocks like
this slope to
prevent small
rock fall and
weathering.

Figure 2.6 Surface Collapse on Rock Slope (upper) and Shotcrete

(Mortar Spray) (lower)

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wall without pocket
(c) Rock Catch Wall
A rock fall protection retaining wall is used as a protection works There is no pocket
to prevent rocks from falling onto a road and is often used at behind the wall, but
sites where the slope gradient at the back is gentle or sites where width of the wall is
large enough
there is ample room at the roadside.
comparing the height
of the slope.

example of wall with

pocket

most of falling rocks

has been stopped at the
wall

gabion wall also is effective

Figure 2.7 Prevention Measure for Rock Fall and Collapse

too small pocket

behind the wall
concrete walls with
debris and many rocks pocket
flew over the wall
volume of the pockets
of the wall are not
enough. A lot of debres
flew over the wall.

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 DISASTER PREVENTION WORKS GUIDE V
3 DRAINAGE WORKS
many types of damage which are directly or indirectly caused by
3.1 Surface Drainage Facilities water, including cut slope failure and damage to retaining walls
Damage of slope due to water can be roughly divided into the and other structures due to erosion by rainwater.
surface erosion due to slope surface water and failures due to the Even if water does not cause any structural damage to roads,
increase in pore water pressure or the decrease in shearing poor drainage from the road surface can cause traffic congestion
strength of earth forming the slope by scouring and water or slip accidents by standing water and can also inconvenience
seepage. Figure 3.1 shows the drainage facilities for roadside pedestrians and people living along roads.
slopes. There are many road drainage systems as shown in Figure 3.2.
Depending on the type of target water, these are basically
classified as surface drainage, underground drainage, slope
drainage and drainage from the backfill part.

Figure 3.2 Type of Drainage

According to example of failures after completion of

embankment in Japan, caution is required shown as following
Figure 3.1 Slope of Road and Flow of Water cases.
i) Places where rainwater gathers
3.2 Road Drainage In the case of curved section of road where the cross grade of the
Drainage is a critical issue for roads. The infiltration of rainwater road becomes a super-elevation, and where the surface run-off
and/or groundwater to the subgrade or base course can be a on the road concentrates on such places as A and B as shown in
major factor for pavement damage while slope erosion due to Figure 3.3 and runs away outside of road when water volume
running rainwater or slope failure due to seepage water
destabilises the filled up ground of an embankment. There are

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exceeds the drainage capacity of inlets at A and B thereby
sometimes resulting in the erosion of slope surface. 3.3 Subsurface Drainage
The drainage facilities used for subsurface drainage include sub-
surface drainage ditches, transverse sub-surface drainage ditches
and an impermeable layer. When groundwater inflows from only
one side of a road at sloping land, a sub-surface drainage ditch is
Surface Run-off
introduced at the road side near the mountain as shown in Figure
3.4-(b). When the groundwater surface is virtually flat, a sub-
Superelevation Surface Run-off
Down grade
surface drainage ditch is introduced on both sides of the road
(Figure 3.4-(a)). An additional ditch is required below the
Figure 3.3 Concentration of Water on the Surface of Curved median when the road width is very large (Figure 3.4-(c)). It
Section appears that subsurface drainage is designed based on previous
work which took place at sites with similar conditions instead of
ii) Half-bank and half-cut section conducting new calculation in many cases. However,
Rainwater falling on to the cut slope side may not be drained to examination of the seepage flow based on survey data is
the side ditch at the bottom of the slope and may cross the road necessary for important drainage facilities.
surface to run down the embankment slope, causing scouring
there.
iii) Places with transverse drainage facilities across the road
A major slope collapse or complete washing away of the
embankment may occur due to overflow, in turn caused by an
excessive volume of running water beyond the capacity of the
drainage facilities across the road (culvert) and/or clogging of the
entrance by driftwood and/or sediment.
iv) Embankment at the site of poor subsurface drainage in a
mountain area
When the gradient of the ground is relatively steep and the
seepage water in the ground is poorly drained, deep failure could
occur.

Figure 3.4 Sub-Surface Drain Ditch

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In areas with particularly abundant groundwater, sub-surface permeability compared to the subgrade soil. It is, therefore,
drainage alone is insufficient. A horizontal impermeable layer is, desirable for filtering materials to satisfy the following
therefore, introduced at the boundary between the subgrade and conditions.
base course or inside the subgrade or filled up ground to guide D15 ( filtering material)
the seepage flow into a sub-surface drainage ditch. <5
D85 (subgrade soil)
In the case where it is necessary to reduce the inflow of
groundwater, which is bolstered by much seepage water from a D15 ( filtering material)
>5
cut slope, to the embankment at half-bank and half-cut section of D15 ( subgrade soil)
a road or the longitudinal boundary between the cutting section D85 ( filtering material)
and banking section of a road, the sub-surface drainage ditches >2
D ( pore diameter)
shown in Figure 1.8 should be introduced.
Here, D15 and D85 are the particle sizes corresponding to 15%
In many cases, a depth of some 1.0 – 2.0 m is required for and 85% of the pass percentage by weight respectively in the grain
roadside sub-surface drainage ditches. However, the actual size distribution curve.
requirement varies depending on the topographical and
geological conditions and the groundwater level. In principle, a
drainage pipe should be installed at the bottom of a sub-surface
drainage ditch (Figure 3.5). Although a porous concrete pipe is
often used as the drainage pipe, other types, such as a permeable
pipe, may be used to suit the specific site conditions.
The protection of a porous pipe with a good quality filtering
material is desirable to prevent the inflow of soil into the pipe. It
is essential for the backfilling material for the drainage ditch to
be a highly permeable filtering material which is capable of
preventing the inflow of minute soil grains from both sides of the
ditch.
The requirements for filtering materials are high stability of the
grains to resist weathering or dissolution into water and an
appropriate particle size distribution curve. The particle size
distribution curve required must indicate that the selected
filtering material prevents the clogging of a porous pipe by the Figure 3.5 Example of Sub-Surface Drainage Ditch
inflow of the subgrade or base course soil and provide sufficient

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3.4 Cut Slope Drainage Facilities

The drainage facilities introduced to stabilise slopes target either Table 3.1 Types of Slope Drainage
Purpose Type of Drainage Function
surface water or seepage water as well as groundwater and the Works
main types are listed in Table 3.1.
Surface drainage Drainage ditch at Prevention of descending
Surface drainage facilities on a slope are designed to reduce the (drainage from the top of a slope surface water to the slope
amount of water running down the slope to prevent slope erosion road surface, Vertical drainage surface
by surface water and to prevent the infiltration of surface water adjacent area and ditch Guidance of rainwater on the
into the slope. The relevant facilities include a drainage ditch at slope surface) Drainage ditch at slope surface to a vertical
the top of the slope (Figure 3.6), a vertical drainage ditch and a a berm drainage ditch
drainage ditch at the berm. In addition, extra consideration, such Guidance of water in a
drainage ditch at the top of a
as the introduction of a downward gradient at the berm, is slope and in a drainage ditch at
necessary to prevent the concentration of surface water on the a berm to the base of a slope
berm to create another downward flow of surface water on the Subsurface Subsurface Drainage of groundwater and
slope surface below the berm. drainage drainage ditch seepage water from the slope
The destruction of slope drainage facilities is mainly caused by (drainage of Gabion works surface
scouring outside or below the ditch by water which has failed to seepage water and Lateral drainage Reinforcement of the base of a
groundwater from hole slope along with a subsurface
slope surface) Vertical drainage drainage ditch
run through the ditch. Those drainage facilities of which the hole Drainage of spring water from
purpose is to collect surface water must have a sufficient depth Horizontal the slope surface
into the original ground to easily receive running water. At drainage layer Drainage of seepage water
places where a rapid flow is anticipated, the introduction of from the slope surface through
certain measures is necessary. These include the use of a lid to a drainage well
prevent water from splashing out and drainage ditch protection Drainage of water from an
using turf or stone pitching to prevent scouring due to splashing embankment or seepage water
from the ground to an
water. embankment
In the case where minor spring water is observed at a slope, one
good idea is the digging of holes as shown in Figure 3.7 and the
insertion of porous piping in these holes. The length of these
holes should generally be at least 2 m.

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Figure 3.6 Unsupported Gutter

Figure 3.7 Horizontal Weep Holes

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4 RIVER REVETMENT WORKS

4.1 DESIGN CONDITION FOR REVETMENT WORKS Table 4.1 Design Conditions for Revetment
Design of External forces of running water and soil
Revetment works are constructed for the main purpose of
Stability pressure, etc; riverbed fluctuation at the
protecting dykes from erosion at the time of flooding and consist time of flooding
of several components as shown in Figure 4.1. Abrasion, damage and degradation due to
The conditions to be considered for the design of revetment the impacts, etc. of flowing sand and gravel
works include such external forces as the fluid force and earth Suction due to the seepage of running
pressure, etc., changes of the topography near the river course water or rainwater
due to changes of the riverbed at the time of flooding, abrasion Design of Prevention/mitigation of erosion
and damage due to the impacts of flowing sand and gravel, Functions Preservation/improvement of river
seepage of flowing water and rainwater, natural environment, environment
use of river, workability and construction cost (Table 4.1). Design of Construction cost and workability
Rationality

Prior to the design of revetment works, it is essential to study

past examples of damage in order to establish a full
understanding of the main cause of damage to each component
and the characteristics of damage suffered by different types of
structures of revetment works as listed below.
i) Damage by scouring of the riverbed
ii) Damage starting from the apron
iii) Damage to slope protection works
iv) Loss of crown works and crown protection works
v) Suction of backfilling soil

Figure 4.1 Configuration of Revetment Works

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4.2 SLOPE PROTECTION WORKS ordinary water level and sites where the likely occurrence of
As slope protection works are the dominant structural part of scouring must be taken into consideration.
revetment works, the design must ensure a stable structure Foot protection works are used to reduce the velocity of the
against the impacts of flowing water and driftwood, earth riverwater flow and also to mitigate rapid scouring by means of
pressure and other forces, taking ecology and landscape into directly covering the riverbed. As foot protection works are
consideration. Although weep holes are not generally introduced introduced at sites with a high riverwater velocity, sufficient
for bank protection works, they may be required to combat high weight to withstand the fluid force, sufficient volume to prevent
residual water pressure in some cases. scouring of the front part of the foundations of revetment works,
Suction prevention materials are used to prevent the suction of a high durability level and a flexible structure to follow changes
the backfilling soil through voids of the slope protection works of the riverbed are required. Typical types of foot protection
when residual water behind the bank protection works escapes or works are shown in Figure 4.2.
when high speed flowing water acts on the bank protection
works.

4.3 FOUNDATION AND FOOT PROTECTION WORKS

The most prominent damage to revetment works occurs when the
foundation and slope protection works are damaged by lifting of
the foundations as a result of scouring of the riverbed by a flood.
When the foundations are damaged, suction of the backfilling
soil could occur, resulting in much wider damage. Because of
such a prospect, the decision on the elevation of the foundations
(embedment depth) is the most important aspect of foundation
design.
The elevation of the foundations must be decided by evaluating
the deepest riverbed elevation based on past observation results
as well as the relevant survey/research results so that lifting of
the foundations of the revetment works does not occur despite Figure 4.2 Typical Examples of Foot Protection
scouring by a flood. When the required embedment depth is Works
quite deep, the elevation of the foundations can be raised by
introducing foot protection works.
The type of foundations should be spread foundations in the case
of good ground. Piles or sheet piles are often used for soft
ground. Sheet piles are sometimes used at sites with a high

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5 PREVENTION WORKS AGAINST DEBRIS FLOW
For road sections which include an originating and/or passing
5.1 SELECTION OF COUNTERMEASURES FOR area of debris flow, passage control should be introduced
DEBRIS FLOW
For the selection of a prevention measure for debris flow, the according to need if the level of rainfall which is high enough to
expected type (mud flow type debris flow or gravel type debris cause debris flow is predicted.
flow) and frequency of debris flow are firstly considered. In
general, when a road crosses an originating or passing area of
debris flow along a mountain stream susceptible to debris flow,
crossing should be provided by a culvert with a sufficient cross-
sectional area or a bridge with sufficient clearance. In the case of
a mountain stream where high speed mud flow type debris flow
is expected to occur at the foot of a volcano or a mountain stream
with a high debris flow frequency, a road crossing in the
originating or passing area of debris flow should preferably be
provided by a bridge with sufficient debris flow clearance.
At a site where the gradient of the upstream section of the debris
flow sedimentation area is 3 – 10 , the occurrence of debris flow
causes considerable fluctuation of the streambed. It is, therefore,
desirable to shifted to either the upstream or downstream and a
crossing provided by a bridge with sufficient clearance (Figure
5.1).

When the road surface is not much higher than the streambed,
the introduction of a ford (low level crossing) should be Figure 5.1 Minor Shift in Debris Flow Sedimentation Area
considered. When the road surface is lower than the streambed,
the introduction of a debris flow shed should be considered.
If a change of the route or prevention measures is not deemed to
be sufficient, it may be necessary to consider the construction of
a dam to control debris flow. In the case of this option, it is
necessary to fully coordinate with any sabo (erosion control)
projects in the area.

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5.2 CULVERT 5.3 BRIDGE
The planned cross-section of a culvert should allow passing of The planned cross-section of a bridge should allow passing of
the peak debris flow discharge and both the height and horizontal the peak debris flow discharge. The height of the bridge girders
width must exceed double the maximum particle size of the is determined by adding a clearance to the debris flow wave
gravel contained in debris flow. The axis of a culvert, including height. It is preferable not to place the bridge piers on the
the water channel upstream, should be as straight as possible to streambed. Special attention must be paid to avoiding narrowing
coincide with the flow direction of debris flow. Careful attention
should be paid to avoiding a smaller cross-sectional area and of the channel width at the bridge site. Even if it is necessary to
gradient of the water channel downstream than those at the place piers on the streambed, their positions should avoid the
culvert. central section of the channel so that the piers are not destroyed
Further attention must also be paid to the possible clogging of by debris flow.
the culvert by driftwood. When the outflow of a large quantity of
driftwood is anticipated, it is desirable to set up a boom(s) in the 5.4 FORD
upstream (Figure 5.2). When there is little head between the streambed and the road
surface in an originating or passing area of debris flow, the road
should be given a structure (ford) which cannot be destroyed by
the passing of debris flow over the road surface (Figure 5.3).

Figure 5.2 Culvert and Boom Ford Ford

Figure 5.3 Ford

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5.5 DEBRIS FLOW SHED 5.7 ESTIMATION OF SCALE OF DEBRIS FLOW
The structure of a debris flow shed should be similar to that of a For the design of debris flow countermeasures, the volume of
rock shed. The longitudinal gradient along the flow direction in sediment, peak discharge, velocity of flow, water level (wave
the channel should, in principle, be similar to the streambed height), unit volume weight and maximum particle size of the
gradient in the upstream so that there is no soil deposition above anticipated debris flow should be estimated, if necessary, based
the shed. The width of the flow area should be identical to the on survey results for each mountain stream.
width of the streambed in the upstream.
The side wall should have a height equivalent to the debris flow 1) Peak Discharge
wave height plus a margin height and should be designed to When a debris flow occurs due to destabilization of sediment
guide debris flow from a mountain stream to the shed. deposited on the streambed, the peak debris flow discharge can
be estimated using the following formula.
5.6 CAPTURE OF DISCHARGED SOIL BY DAM AND
FENCE Q sp C /C Cd Q p …………………. (1.1)
One or several dams or fences can be used to catch an entire where.
debris flow or large gravel and driftwood so that only soil and Q sp : peak discharge of debris flow (m3/sec)
water, which can be drained by road drainage facilities in the Qp : peak discharge of water alone (m3/sec)
downstream, are allowed to flow further downwards. The design
C : volume sediment density of deposited sediment
sedimentation gradient should be half of the current streambed
gradient. Concrete or steel permeable dams and fences are used at streambed ( 1 n ; n : void ratio)
to reduce the sedimentation volume during ordinary floods. The Cd : density of debris flow on the move
size of the opening should be less than 1.5 times of the maximum
diameter of large gravel to catch debris flow (Figure 5.4). Meanwhile, the equilibrium sediment density of debris flow is
given by the following formula.
w tan
Cd …………………. (1.2)
( s w )(tan tan )
Where,
3 3
s : unit volume weight of sediment (kN/m (tf/m ))
3 3
w : unit volume weight of water (kN/m (tf/m )
: shear resistance angle of deposited sediment at
Figure 5.4 Capture of Discharged Soil by Permeable Dam or streambed ( )
Fence : streambed gradient ( )

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The peak discharge of water only Q p is calculated by the
following formula for the critical rainfall causing a debris flow.
Q p 1 f re A ………………………… (1.3)
3
Where,
f : peak flow index
re : mean rainfall intensity in flood concentration
time (mm/hr)
A : catchment area (km2)

2) Velocity and Highest Water Level (Wave Height)

A Manning type uniform flow formula is used for the velocity of
flow based on debris flow observation results in Japan.
v 1 h 2 / 3 sin
1/ 2
……………………… (1.4)
n
Where,
v : mean velocity of debris flow (m/sec)
h : wave height of debris flow (m)
The coefficient of roughness n to be used is
approximately 0.03 for a fixed bed water channel
and 0.1 for a movable bed.

The highest high water level can be calculated using the peak
discharge, a velocity-wave height formula and the width of the
streambed in the passing area of debris flow.

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As natural ground is generally complicated and its properties are

not uniform, the rock mass and soil layer forming the existing Table 6.1 Standard Gradients of Cut Slope
ground must be classified from the viewpoints of the excavation Classification of Rock and Soil Slope Gradient
difficulty and slope stability. Empirically established standard Height
slope gradients are then applied to the classified ground, Hard rock 1:0.3 - 1:0.8
assuming non-treatment, sodding or simple protective work such Soft rock 1:0.5 - 1:1.2
Sand Not dense, and 1:1.5 -
as netting, to determine the slope gradient and shape
poorly graded
corresponding to the soil and rock properties, and cutting height. Dense < 5m 1:0.8 - 1:1.0
Sandy soil 5 - 10m 1:1.0 - 1:1.2
6.1 STANDARD SLOPE GRADIENTS Not dense < 5m 1:1.0 - 1:1.2
5 - 10m 1:1.2 - 1:1.5
Natural ground is extremely complicated and not uniform in its
properties, and cut slope tend to gradually become unstable An Sandy soil mixed Dense, or well < 10m 1:0.8 - 1:1.0
overall judgement should be made by fully taking account of the with gravel or graded 10 - 15m 1:1.0 - 1:1.2
rock masses Not dense, or < 10m 1:1.0 - 1:1.2
requirements for stability described later by referring to the
poorly grade 10 - 15m 1:1.2 - 1:1.5
standard slope gradients listed in Table 6.1. The table indicates
Clayey soil 0 - 15m 1:0.8 - 1:1.2
the standard values of the gradient of slopes and have been
Clayey soil mixed < 5m 1:1.0 - 1:1.2
empirically established based upon protection works such as
with rock masses 5 - 10m 1:1.2 - 1:1.5
sodding, netting or non-protection. The gradients referred to here or cobble-stone
are those for the individual slopes without a berm. Notes
The difference between soft and hard rock referred to herein is ha : cut slope height for slope surface a
judged on the basis of the degree of difficulty of excavation, and hb : cut slope height for slope surface b
is mainly governed by the shearing strength of rock and the
amount of rock cracks. The range of standard values shown in
Table 1.3 is wider than that of standard values for embankments
described later, so that determination of gradient of rock slope on
the basis of these standard values alone seems to be difficult
because there are so many factors involved.

-The gradient does not include a berm

-The cut slope height vis-à-vis the gradient means the total cut slope
height covering the entire cut slopes above the cut slope in question.

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1) The cut slope height and gradient when a single gradient is
not opted for because of the soil composition and other
reasons are based on the ideas shown below.
2) Silt is to be classified into the Clayey soil.
3) The table is not applicable to soils not included in the above
table.
4) In planning of planting for slope, it also takes into
consideration the slope gradient suitable for planting.

The standard slope gradients shown in Table 6.1 may not be

applicable in certain cases as follows, and more gentler gradient
may be applicable with other prevention measures.
1) Cuts in Colluvial Deposits or Heavily Weathered Slopes
2) Cuts in Easily Erodible Ground such as Sandy Soil
3) Cuts in Quickly Weatherable Rocks such as Mudstone, Tuff
and Serpentinite
4) Cuts in Rocks with Many Fissures Figure 6.1 Ground Conditions and Shapes of Slopes
5) Cuts in Dip Slope Structures with Fissures
6) Cuts where Much Groundwater is Present BERMS
7) Large-Scale Slopes A berm about l to 2m wide will be generally installed in the
middle of a cut slope with a large height.
SLOPE SHAPE
As shown in Figure 6.1, the gradient of slope varies depending 1) Purpose of berm
upon the soils and the rocks, and berms are provided in many At the lower portion of a continuous, large slope, the discharge
cases at transition points where the gradient changes. and current speed of the surface water increase, causing the
A single slope gradient is generally used where the geology and scouring. In this case, the current speed can be reduced by
soils are almost the same in the depth direction and in the providing a berm in the middle of the slope, or the concentration
longitudinal and transverse directions. Where the geology and of the surface water at the lower portion of the slope may be
soils vary considerably and complicatedly, a single gradient of prevented by making a ditch in the berm for draining water
slope suited to the soil of the gentlest gradient may be used even outside of the slope. The berm also is used as inspection step or
though this is somewhat uneconomical. as scaffo1d for repairing.

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6.2 SHOTCRETE
Among slopes with a rockfall hazard, those made of gravelly soil
or weathered soft rock is liable to small-scale rockfall. For these
slopes, the installation of a rockfall prevention net to hold down
loose stones and/or rockfall prevention fences to prevent rocks
from falling on to the road surface along with planting is
necessary. In the case of highly cracked soft rock without spring
water, mortar concrete spraying is appropriate. It is preferable to
use rockfall prevention works for slopes containing an
exfoliation-type rockfall on highly cracked slopes of hard rock
and the additional use of rockfall protection works is even more
Figure 6.2 Cross- Sectional Gradient of Beam
desirable if the gradient of these slopes is very steep.
2) Gradient of berm Shotcrete works include mortar spraying and concrete spraying.
Where the drainage facilities are not provided, about 5 to 10% of They are commonly used on steep slopes of highly weathered or
cross-grade is normally provided for the berm so as to drain heavily jointed rocks on which vegetation is not possible.
water toward the bottom of slope (toe of slope). However, where
the slope is considered to be easily flaked off or eroded, the 1) Purposes
gradient of the berm should be made in the reverse direction so Shotcrete works are intended (a) to chiefly prevent surface
as to drain water toward the ditch of the berm (Figure 6.2 (c)). weathering and erosion, and in some cases, (b) to control small-
scale rock falls.
3) Location of berms
On the cut slopes, berms about 1 to 2 m wide are normally
provided every 5 to 10 m of height depending upon the soil, rock 2) Design Considerations
and scale of slope. A wider berm is recommended where the Shotcrete works do not have extra support against the mass of
slope is long and large or where the rock fall protection fences the unstable slope. For permanent applications, shotcrete should
are to be installed. be reinforced to reduce the risk of cracking. Two common
berms should be designed by taking account of difficulty of the methods of reinforcement are welded-wire mesh and steel fibre.
inspection and repair, gradient of slope, height of cut, soils of The mesh must be closely attached to the rock face and fully
slope, construction cost and other various conditions. encased in shotcrete, with care being taken to eliminate voids
within the shotcrete. The standard thickness of shotcrete
generally is 8 to 10 centimetres for mortar spraying and 10 to 25

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6 PREVENTION WORKS AGAINST ROCK FALL AND SURFACE COLLAPSE

centimetres for concrete spraying. 6.3 ROCK CATCH WALL

In principle, drain holes should be provided through the shotcrete Rock catch wall is used as a protection works to prevent rocks
to prevent the creation of water pressure behind the face, with from falling onto a road and is often used at sites where the slope
the drain holes usually located on 1 to 2-m centres to a depth of gradient at the back is gentle or sites where there is ample room
about 20 centimetres. The sprayed portion at the top of the slope at the roadside.
should be completely embedded into the ground. This method is commonly used and is cost-effective when scale
of rock fall is likely to be large and difficult to control.
Rock catch wall should be designed to safely absorb the energy
of falling rocks with deformation of the wall itself as well as the
bearing stratum after calculating the value of such energy.
In addition, it is desirable to establish a pocket at the back of this
retaining wall so that fallen rocks as well as fallen soil can be
deposited there to a certain extent. Figure 6.4 shows the
conceptual arrangement of a catch fill and ditch.

Figure 6.3 Typical Example of Mortar or Concrete Shotcrete

Figure 6.4 Diagrammatic layout of catch fill and ditch

Apart from embankment stability analysis, design considerations

are concerned with the shape and dimensions of the catch

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embankment and ditch in terms of their capacity for arresting The energy of falling rocks ( Ei ) used for design purposes is
and accommodating falling stones. Table 6.1 lists the calculated by the following formula.
recommended shapes and dimensions of this structure in relation
to slope gradient. Ei 1 1 tan W H ………………… (1.5)
Where,
Table 6.1 Recommended Shapes and Dimensions of Catch Ditch Ei : energy of falling rocks (kN･m)
Gradient of slope Height of Width of Depth of : coefficient of rotating energy (dimensionless)
(Vertical to slope ditch ditch
Horizontal) (m) (m) (m) : equivalent friction coefficient of slope
5 to 10 4 1.0 (dimensionless)
Nearly vertical 10 to 20 5 1.5 (value ranging from 0.05 to 0.35 is used depending on
the characteristics of the falling rocks and slope)
20 < 6 1.5
: slope gradient ( )
5 to 10 4 1.0
W : weight of falling rocks (kN)
10 to 20 5 1.5
1:0.25 to 1:0.3 H : height of fall (m)
20to 30 6 2.0
30 < 8 2.0
5 to 10 4 1.5
10 to 20 5 2.0
1:0.3 to 1:0.5
20 to 30 6 2.0
30 < 8 2.5
0 to 10 4 1.0
1:0.5 to 1:0.75 10 to 20 5 1.5
20 < 5 2.0
0 to 10 4 1.0
1:0.75 to 1:1.0 10 to 20 4 1.5
20 < 5 2.0
Note) The width of ditch is the horizontal distance from the
toe of slope to the top of embankment.

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APPENDIX V-1 COUNTERMESURE WORKS

APPENDIX V-1

COUNTERMEASURE WORKS

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APPENDIX V-1 COUNTERMESURE WORKS
In slope protection engineering and stabilization, experience
A1-1 SELECTION OF COUNTERMEASURES is a highly important factor, and the standard gradient of fill
(1) General embankments and cut slopes is determined on an empirical
In this guide, the countermeasure means the measure works basis. Design of slopes must be made not by simply applying
against disasters which have occurred and occur repetitiously. the standard gradient, but must also be based on the judgment
of experienced engineers who are well acquainted with the
However, the countermeasures introduced here would be useful
geological, topographic, meteorological and other natural
as the prevention measures. The some countermeasures
introduced here are expensive and may not be suitable for conditions of the area.
Bolivia in terms of the cost. 4) Roads are often constructed initially as simple structures at
low cost and then gradually upgraded and enhanced with
Suitable prevention measures against slope disaster should be varied functions through maintenance. The situation,
based on a sound understanding of the characteristics of road however, changes in due course and the roads have to be
slope disaster. In undertaking investigations of, or planning to constructed as complete a structure as possible to cope with
prevent, road slope disasters, extreme care should be paid to the possible disasters and reduce the cost of maintenance.
following points. 5) Water is an essential factor in controlling slope stability.
1) Field investigations should start with a comprehensive Drainage is the most important factor for the safety of both
evaluation of general conditions (topography, geology, natural and artificial slopes. Drainage water and spring water
vegetation, etc). Investigators should not be unduly absorbed and drainage of groundwater to achieve the largest possible
in detail from the beginning, because initial impressions of drawdown of its level are important methods for stabilizing
such detail may often mislead them from understanding the slopes.
true condition of the site. 6) Unexpected accidents, such as local slope disaster or rock
2) Where an existing road is threatened by large-scale collapse, fall, may sometimes occur in the course of slope cutting work.
rock fall, mass movement or debris flow that would be too In such cases, the design and the work plan have to be
costly or difficult to prevent, safe traffic movement must be reviewed by well-experienced expert engineers. The original
maintained by relocating the road or applying traffic control. design should not be persisted with.
If a new road needs to be constructed, its alignment should 7) The safety of a road from natural disasters should be
be determined with the least risk of slope instability at the maintained and enhanced for constant smooth traffic flow.
planning stage. Periodical field inspection along the route is highly important
3) Large-scale fills or slope cutting in landslide-prone areas 8) If any slope or slope-protection work shows signs of
sometimes causes unforeseen disasters. Field reconnaissance deformation such as swelling or sinking, stabilization works
and other necessary investigations, therefore, are essential for should be started immediately to prevent a large disaster
planning of safe roads. The cost of prevention measures after from developing.
a slope disaster is often several times the cost of taking 9) To preserve knowledge and experience, it is recommended to
proper preventative measures before any failures can occur. compile, and keep for an appropriate time, the data used for

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 DISASTER PREVENTION WORKS GUIDE V
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design, records of field inspection, histories of damage and Debris Flow (FE) ─ the term DEBRIS FLOW is a fast dense
repair works, records of additional stabilization work, and so flow of rock fragments, and earth and mud mixed with water,
on. These records can be very useful for future design, usually within a defined stream or river channel.
maintenance and general development of methods of
preventing and treating road disasters. Road Collapse (FP) ─ the term ROAD COLLAPSE is used to
mean small-scale failure occurring in embankment slopes.
Slope disasters occurring along national roads in Bolivia, are
classified into the following six types based on their occurrence Countermeasures for slope disaster involving roads are classified
mechanism and the types of prevention measures that need to be into seven groups, in consideration of size, purpose, application,
applied. and design method, as follows.
- Earth Work
Slope Collapse (DR) ─ the term SLOPE COLLAPSE is used to - Surface Cover
mean small-scale shallow failures marked by sudden and rapid - Water Drainage
movement without prior indication - Slope Work
- Wall and Resisting Structures
Rock Fall (CR) ─ the term ROCK FALL is reserved for abrupt - Protection Work
free-fall movement of materials away from steep slopes, ranging - Others
in size from individual rock blocks to small-size failures with
A suitable combination of these methods should be implemented
volumes of less than 2 m3. Rock falls of greater size, exceeding
after consideration of the mechanism and dimension of slope
1,000 m3, are referred to as rock mass failure (see below). disasters, the importance of the objects to be protected, and the
cost-effectiveness. Generally, countermeasures involve some or
Rock Mass Failure (FR) ─ the term ROCK MASS FAILURE all of the following objectives:
includes toppling failures and rock slides, as distinct from rock
1) Preventing erosion and weathering of the slope surface by the
falls, is characterized by failure masses of larger than 2 m3. Its
use of vegetation, shotcrete and surface drainage;
occurrence is closely related to geologic structure.
2) Reducing pore-water pressures in the slope by surface and
subsurface drainage;
Mass movement (DL) ─ the term MASS MOVEMENT is used
3) Reducing shear (or destabilising) force by removing the
to describe slow, long-term deformations of slopes underlain by unstable materials from the upper part of the unstable slope;
soils or strongly weathered rocks and are usually characterized 4) Increasing shear strength (or stabilising force) by adding
by recognizable sliding surfaces. weight to the toe of an unstable slope or by increasing shear
strength along the failure surface;

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5) Supporting the unstable area of slope by the construction of

retaining walls and similar structures;
6) Reducing or preventing the damages from slope disasters by
catch works, etc;
7) Avoiding the unstable area by relocating a route or by the
construction of bridge and similar structures.

Figure A1-1.1 Disaster Type

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Catch concrete wall A A L - A -
Table A1-1.1 Applicability of Countermeasures against Slope Precast reinforced L - - L - L
concrete pile
disasters Pilling Steel pile L - - A - L
Applicability Cast in place reinforced
Classification Measure Works concrete pile L - - A - A
DR CR FR DL FE FP
Protection Rock fall catch net L A L - - -
Removal A A A A A - Work Rock fall catch fence L A L - - -
Rock cutting - A A - - -
EARTH Earth Work Rock pre-splitting - A A - - -
PROTECTIO Rock Shed Concrete (or Steel) shed L A L - L -
WORK N WORK Concrete (or Stone) dam - - - - A -
Soil cutting A - - A - - Sabo
(Check) Dam Steel crib dam - - - - A -
Embankment A - - A - A
Slit dam - - - - A -
SURFACE Hydroseeding A L - L - A
COVER Vegetation Avoiding Diversion (Shifting) L L A A A L
Vegetation A L - L - A OTHERS Problem Route relocation L L A A A L
Surface Subsoil drainage L - - A - A Work Tunnel, Bridge L L L A A A
Drainage Berm or roadside drain A A A A - A
A : Applicable L : Limited case - : Not applicable
WATER Culverts - - - L A A DR : Collapse CR : Rock Fall FR : Rock Mass Failure
DRAINAGE Subsurface Horizontal drain hole A - L A - A DS :Mass movement FE : Debris Flow FP : Embankment
Drainage Drainage well - - L A - -
Drainage tunnel - - L A - -
Stone pitching A L L - - A
Pitching Block pitching A L L - - A
Concrete pitching A L L - - A
Mortar spraying A A A - - -
SLOPE Shotcrete
Concrete spraying A A A - - -
WORK Concrete block crib
(Precast) L L - L - A
Crib work Cast-in-place concrete crib A A L L - -
Shotcrete crib A A L L - -
WALL AND Soil nail A L - L - A
RESISTING Anchoring Rock bolt - A A - - -
STRUCTUR
E Ground anchor A - A A - A
Gabion wall A - - L L A
Stone masonry wall A - - L - A
Retaining Gravity type retaining wall A - L L - A
Wall
Concrete block wall A - L L - A
Supported type retaining A - L L - A
wall
Catch Work Catch fill L A L - - -
Catch gabion A A L - A -

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A1-2 COUNTERMEASURE WORKKS AGAINST measures.

SLOPECOLLAPSE (DR) 5) Even though they are costly, anchoring or piling works should
(1) General be planned if other methods are not expected to control collapses.
Heavy rainfall and earthquakes frequently cause collapses in
Figure A1-2.1 shows a flow chart for the selection of
natural or cut slopes. Many slopes are stable during normal
countermeasures to prevent collapse. The success of such
conditions but become unstable during or after heavy rainfall. To
prevent collapse, either the sliding force must be decreased or collapse countermeasures is influenced greatly by topographical,
sufficient resistance to overcome the sliding force must be added geological and meteorological conditions. In principle, cutting
work, drainage work and vegetation are the preferable choices.
by structures. Any prevention plan should be suitable for the
Structural methods such as crib work and anchor work are
field conditions.
adopted only when soil and gradient conditions are unsuitable for
An adequate and effective measure for preventing collapse vegetation and slope stability cannot be secured by cutting and/or
should be selected in consideration of the anticipated causes, drainage works alone.
shape, mechanism, and scale of collapse, as well as appearance
and through discussion. Generally, the following criteria must be
used for selection.
1) Wherever possible, cutting work should be selected,
especially in the cases of overhanging slopes and highly jointed
or weathered rock slopes. In planning cutting work, slope
stability and harmony with the surrounding environment should
be considered.
2) In principle, surface drainage work should be planned
positively. Subsurface drainage works should be adopted if
spring water exists during normal time and/or rainfall, or a
depression exists near the top of the slope.
3) In most cases, vegetation is low-cost, if it is an available
option (gradient and soil). Vegetation should be applied to
prevent erosion due to rainfall by growing plants on the face of
the slope. Where slopes are unsuited to vegetation, such as
jointed or weathered rock slopes, pitching work, shotcrete work,
and crib work should be considered.
4) Retaining wall works should be selected if the foot of a slope
must be stabilized or if it is to be used as the foundation of other

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Table A1-2.1 Classification of Countermeasures against

Collapse START

CLASSIFICATION TYPE OF WORK Can No Can No No

Is
Cutting a stable cutting slope
be secured?
slope stability be assured
by structures?
catch works alone sufficient?
EARTH WORK Earth Work
Embankment
Yes
Hydroseeding
VEGETATION Vegetation ・Cutting work
Yes Yes
Re-Vegetation
Subsoil Drainage Hole
Surface Drainage
Drain Ditch and Cascade
DRAINAGE
Culverts Is Is
Subsurface Drainage subsurface drainage subsurface drainage
Horizontal Drain Hole effective? effective?

Yes No Yes
Pitching Work Stone Pitching No

・Subsurface drainage ・Subsurface drainage

Shotcrete (mortar)
SLOPE WORK Shotcrete Work
Shotcrete (concrete)
Crib Work Cribwork (Precast) Soil and
rock
Scale,
importance of
characters protect objects

Soil Nail
ANCHORING Anchoring Rock Bolt
Ground Anchor
Soil, earth Weathered or Small scale, Large scale,
Gabion Wall or sand jointed rocks unimportant important

Stone Pitching Wall

WALL AND Retaining Wall
・Rockshed

RESISTING Concrete Block Wall ・Catch wall

・Sabo dam
・Route relocation
・Pitching work ・Anchor work ・Subsurface or 　 ・Bridge
STRUCTURES Retaining Wall ・Vegetation ・Crib work ・Retaining wall ・Piling work 　surface drainag ・Culvert
・Shotcrete ・Retaining wall
(Supported Type)
Catch Work Catch Concrete Wall
Steel Pipe Pile Figure A1-2.1 Flow chart for selection of countermeasures for
PILING WORK Piling Work
H Steel Pile collapse

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(2) Earth Works (3) Vegetation

1) Purpose Achieving a dense vegetation cover can prevent the formation of
Cutting work is applied to remove unstable soil and rock and to unstable debris on bare hillsides such as failure surfaces and bare
reduce the load, and hence shear force, at the head of an unstable slopes. The method is one of the most important
or potentially unstable slope. countermeasures, and is normally not expensive.
2) Design Considerations 1) Purposes
The gradient and vertical height of the cutting slope should be The main objectives of vegetation are (a) to reduce surface
determined on the basis of the geological conditions, etc. The erosion caused by running water and rainfall; (b) to reduce
gradient should be between 1V to 0.3H and 1V to1.5H infiltration from rainfall; and (c) to bind subsurface soil with root
depending on subsurface conditions and characteristics. Berms 1 systems.
to 4 m wide should be created at intervals of 5 to 10 m in the 2) Design Considerations
vertical direction. Careful investigation of the stability of the Generally, unstable and bare slopes are unsuitable for vegetation,
back slope should be conducted prior to cutting. This suggestion and surface failures are frequent. There is little possibility for
is shown the only normal gradient, therefore applied gradient successful planting on such a surface without supporting
should be approved by the engineer. measures. Therefore, vegetation of the slope should in principle
be carried out when the slope is stabilized by installation of other
countermeasures.
In selecting the type of vegetation to establish, careful attention
should be paid to the rainfall, plant growth conditions, and the
soil properties of the slope, as well as the timing of construction
and the area of protection works. Table A1-2.2 gives general
selection criteria for the various vegetation establishment
methods in Japan. The table may be applicable in Bolivia,
however it is recommended to revise this table based on the
Bolivian condition of vegetation.
Additionally, brushwood and net are usually set on relatively
steep slopes to stabilize the surface soil. The slope gradient for
vegetation is usually less than 60 degrees.

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of slope with gum. Therefore, this method is suited to relatively
Table A1-2.2 General selection criteria for vegetation steep slopes and high places. Asphalt emulsion is sprayed to
Soil and rock properties Methods perform the film curing.
Hydroseeding, seed mud spraying,
Vegetation mat: Fibrous mat containing seed and fertilizer is
Sand used to cover the face of slope. This method offers the protection
vegetation mat
effects of the mats until the establishment of vegetation.
Sandy soil, Sodding, hydroseeding, seed mud
Loose spraying, vegetation mat, vegetation Vegetation bag: Vegetation bags, made of polyethylene net or
gravel soil,
sandy soil net. cotton net filled with seeds and vermiculite, are usually placed
containing rocks on horizontal ditches on the slope.
Hydroseeding, seed mud spraying,
and cobble Dense vegetation bag, vegetation hole, Vegetation hole: Mixture of seeds, fertilizer and soil is placed in
stones vegetation block, vegetation packet. holes that are made in advance in the face of the slope.
Clay, clayey soil, Sodding, hydroseeding, seed mud Vegetation block: Turf, seeds and mud, are usually placed
Soft
clay or clayey spraying, vegetation mat. linearly along contour lines.
soil containing Hydroseeding, seed mud spraying,,
rocks and cobble Stiff vegetation bag, vegetation hole,
stones vegetation block, vegetation packet
Hydroseeding, seed mud spraying,
Soft rock
vegetation bag, vegetation hole.

Closed turfing: This is the conventional method in which sod is

directly laid on the face of the slope and is suited to erodable
soils. In laying sod, it should tightly contact the face of the slope,
which may require it to be hit, and be laid flat without joins to
prevent scouring.
Hydroseeding: Mixture of seed, fertilizer, fibres and water, is
sprayed over the face of the slope with a pump. This method is Figure A1-2.2 Typical vegetation works
suited to relatively gentler slopes or low land.
Seed mud spraying: Similar to hydroseeding, a mud-like
mixture of seed, fertilizer, soil and water is sprayed over the face

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(4) Pitching Table A1-2.3 Selection of pitching work

Pitching Gradient
Pitching works include concrete pitching, stone pitching and Type of Height Geological
thickness
block pitching. They are commonly used on slopes gentler than pitching (V : H) (m) condition
(cm)
1:1.0. When slope gradient is greater than 1:1.0, these methods
25 to 35 1:1.0 to 1:1.5 ≤5.0 Sediments,
are respectively concrete retaining wall, stone masonry retaining Stone
wall and block masonry retaining wall. ≤25 1:1.5 to 1:1.8 − talus, cone
mudstone and
1) Purposes 35 1:1.0 to 1:1.5 ≤3.0 collapsible
Block
Pitching works are applied chiefly to prevent surface weathering, ≤12 1:1.5 to 1:1.8 − clayey soils.
scouring, stripping and erosion and, in some cases, to prevent
less than Bedrock with
small-scale collapses. Concrete ≥20
1:0.5
−
numerous joints
2) Design Considerations and with a
Stone pitching and block pitching are used for non-cohesive Reinforced possibility of
≥20 Over 1:0.5 − weathering and
sediments, mudstone and collapsible clayey soils with a slope Concrete
gradient less than 1:1.0. On the other hand, concrete pitching is stripping.
employed for jointed rock slope with a possibility of weathering Note : This table is only a preliminary suggestion. Further detailed
and stripping. For large and/or steep rock slopes, it is desirable to analysis should be carried out by an engineer.
reinforce the concrete with reinforcing bars or wire mesh.
Similar to shotcrete works, drain holes of about 5 centimetres in
diameter should be provided every 2 to 4 m2, regardless of the
presence of spring water or seepage water.
Adequate methods should be selected by referring to Table A1-
2.3.

Figure A1-2.3 Examples of pitching works

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(5) Crib Works Table A1-2.4 Application of Crib Works

Crib works include concrete block crib, shotcrete crib and cast- Gradient Vertical height
Type of crib works Condition of slope
(V:H) (m)
in-place concrete crib works. They are commonly used on steep
slopes of highly weathered or heavily jointed rocks accompanied Flat slope with spring
with abundant springs, especially where spalls cannot be fixed Concrete block < 1:0.8 <5m water and large slope of
with shotcrete works. gradient below 1:0.8
1) Purposes Concrete > 1:0.8 < 10 m Slope of gradient above
1:0.8 and weathered or
Similar to shotcrete works, crib works are applied (a) to chiefly jointed rocks with spring
Cast-in-place
prevent surface weathering, scouring and erosion and, in some > 1:0.8 < 10 m water lacking in long-
concrete
cases, (b) to control both rock fall and small-scale slope disaster. term stability
2) Design Considerations Note1: This table is only preliminary suggestion. Further detailed
Concrete block crib work offers little or no resisting force against analysis and analysis should be carried out by an engineer.
the driving force of the unstable slopes, while shotcrete crib and
cast-in-place concrete crib works have some resistance,
depending on the size and space of the cribs. Vegetation, or Concrete
Spraying
Concrete block crib work is used for slopes with gradients less
than 1:1.0 (V:H) and when vegetation is suited to the slopes. 400
500
Shotcrete crib and cast-in-place concrete crib works are used
when the long-term stability of the slope is questionable, or when
concrete block crib work is likely to collapse on a large slope or
on a slope of weathered and jointed rocks with spring water.
The crib (or frame) usually ranges in size from 200 × 200 L B
Anchor Bar
millimetres to 800 × 800 millimetres at an interval of 2 to 5
metres. The spaces inside the cribs are filled and protected by Note: B=300 to 600 mm, L=B×(5
stone pitching, mortar spraying, or vegetation, depending on the to 10)
slope conditions (gradient, spring water, etc). Each intersection Figure A1-2.4 Details of Cast-in-place Concrete Crib Work
of the crib should be anchored with stakes or pre-stressed steel,
depending on the conditions of the slope. Table A1-2.4 shows the
applications of crib works. Figure A1-2.4 presents details of cast-
in-place concrete crib work.

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(6) Soil Nails

Soil nails are slender steel rods inserted into the soil layer to
provide composite action. There are two different types of soil
nails, referred to as flexible nails and stiff nails. Flexible nails are Sliding surface
generally drilled and grouted and oriented to mobilize tension. Concrete
Stiff nailing involves directly insertion without the addition of
grout, and nails are oriented to produce both shear and bending
in the nail as well as a degree of tension.
1) Purposes Soil nail
Soil nails are applied to (a) Stabilize small-scale unstable soil ROAD
slopes and in some cases, (b) to reinforce embankments.
2) Design considerations
The existing design procedures fall into two categories: those
Figure A1-2.5 An example of soil nail
that consider shear bending and tension and those that take into
account only tension as a restraining action. The latter is
recommended as a safer design. The stability analysis of a soil (7) Retaining wall
nailed slope is similar to that of an anchored slope (by referring Frequently, retaining walls are used to support cuttings or
to anchor design). embankment and provide restraint against instability. Retaining
In planning soil nailing, attention should be paid to soil walls can be generally classified into 5 types in terms of their
properties. Soil nail is effective in firm dense low-plasticity soils design criteria, applications, etc. These types are gabion wall,
and is not practical in loose sandy soils and soft clay. Commonly, stone masonry wall, crib retaining wall, gravity type retaining
one nail is used for each 1 to 6 m2 of soil surface area combined wall and supported type retaining wall.
with shotcrete facing (mortar or concrete). Standard nails vary in 1) Purposes
diameter between 12 mm and 32 mm, and are less than 5 m in Retaining walls are used (a) to prevent small-scale shallow
length. Figure A1-2.5 shows a diagrammatic example of soil nail. collapse and toe collapse of large-scale slope disasters, and (b) as
a foundation of other slope protection works such as crib works.
Typically where the toe of slope has collapsed or the collapse is
likely to enlarge upward along the slope, retaining walls are
strongly recommended.

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2) Design Considerations
In principle, retaining wall design includes analyses of (a) sliding,
(b) overturning, typically about the toe of walls, (c) the bearing
capacity of the foundation ground, and (d) overall stability. For
(d), stability analyses must not only consider the stability of the
wall itself, but also the overall slope of which the wall may be a
part. Moreover, loads acting on the retaining wall are normally
considered to be (e) deadweight, (f) surcharge and (g) earth
pressure, for design purposes.
Gabion walls are fabricated from gabion baskets that are
typically 1 meter × 1 meter in cross-section and 2 meters to 5
meters in length. The rock fill for gabion is generally graded
from a maximum of 250 millimetres diameter down to 100
millimetres size. The gabion structures are flexible and the nature
Note: Wall thickness a=35 cm
of the gabion filling provides for good drainage conditions in the
Backfilling concrete thickness b=0 to 20 cm
vicinity of the wall. Therefore, filtration protection between the Top edge thickness of backfilling c=30 to 40 cm
gabion and the wall backfill should be considered. Wall height H=1.5 to 7.0 m
Stone (or concrete block) masonry retaining walls must be wet Foundation height H1=25 to 40 cm
masonry. Wall stability, especially the critical height, should be Gradient N1=1V: 0.3H to 1V: 0.5H
examined (refer to the depth from the wall top edge to the critical
point that protrudes 1/3 outside of the force line centre). In Figure A1-2.6 Detail of stone or concrete block retaining wall
general, the foundation must be embedded at least 30 centimetres.
One drain hole (generally φ75 mm) must be installed every 2 to
3 m2, usually in a zigzag pattern, because of the poor drainage of
the walls (Figure A1-2.6).

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Table A1-2.5 Standard Dimension of Stone or Concrete Block

Retaining Wall ROAD Collapse surface
Height Gradie Wall Backfilling Backfilling Compacted fill
(m) nt thickness thickness concrete
(cm) (cm) thickness (cm)
H N1 a c b
0 to 1.5 1:0.3 35 30 to 40 0 to 10
1.5 to 3.0 1:0.3 35 30 to 40 10 H
3.0 to 5.0 1:0.4 35 30 to 40 15 N
Grained sand fill
5.0 to 7.0 1:0.5 35 30 to 40 20
Note1: This table is only preliminary suggestion. Further detailed
Bedrock
analysis and analysis should be carried out by an engineer. Bedrock
Drain pipe
Dowels
Crib retaining walls, usually being fabricated from precast Note: Wall H=1.5 to 6.0 m, Gradient 1V: 0.3H to 1V: 0.5H.
reinforced concrete elements, are flexible due to the segmental Figure A1-2.7 Detail of crib retaining wall
nature of the elements, and are thus somewhat resistant to
differential settlement and deformation. The stability is
calculated for the whole structure as well as for several
horizontal sections. Slope stability calculations should include Table A1-2.6 Recommended Design Constants
the potential failure surface above the toe of the wall. Earth Materials Unit weight (kN/m 3)
pressure calculations of the walls are similar to that of gravity
type retaining wall (Figure A1-2.7). Reinforced concrete 25
For gravity type and supported type retaining walls, design Concrete 23.5
considerations mainly involve the above-mentioned analyses of Gravel, gravely soil, sand 20
four states, sliding, overturning, bearing capacity and overall
stability. In determining the dimension of the wall, it is desirable Sandy soil 19
that the width, B, of the bottom slab is about 0.5 to 0.7 times the Silt, cohesive soil 18
height of the retaining wall and that the thickness of the member
at the top is greater than 35 cm.
Table A1-2.6 gives some recommended design constants.

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A1-3 COUNTERMEASURE WORKS AGAINST ROCK

(2) Criteria for selection of countermeasures
FALL (CR)
(1) Classification of countermeasures To select adequate and effective measures for preventing damage
from rock fall, one should consider topographical and geological
Countermeasures against rock fall can be classified into works to
conditions, vegetation, rock fall history, and the effects of the
prevent rock fall and works to provide protection from rock fall. countermeasure by predicting the size and height of any potential
Rock fall prevention concentrates on the rock fall source, such as rock fall. Figure A1-3.1 shows a procedure for selecting
removal and crib work. Rock fall protection does not attempt to
countermeasures against rock fall. The following criteria must be
prevent rock fall, but aims at preventing the relevant objects
used for selection.
from being damaged by rock fall. There are a variety of methods
within the two major approaches shown in Table A1-3.1. 1) If there is a possibility of rock fall, the first priority should be
to remove the source of the rock fall. When this is difficult to
implement, other methods should be adopted.
Table A1-3.1 Classification of Countermeasures against Rock 2) In selecting a countermeasure, it is essential to consider not
Fall only the conditions of the slope and rock fall, but also the road
CLASSIFICATION TYPE OF WORK structure, traffic conditions and ground conditions
Removal
Earth Work Earth Work 3) It may be necessary to perform a combination of works
Rock Fall Prevention Work

Cutting
Hydroseeding because the functions of any single type of countermeasures may
Vegetatioin Vegetation
Re-Vegetation be inadequate.
Water Surface Subsoil Drainage Hole
Drainage Drainage Drain Ditch and Cascade
Pitching Work Stone Pitching
Shotcrete Shotcrete (mortar)
Slope Work
Work Shotcrete (concrete)
Crib Work Cribwork
Anchoring Anchoring Rock Bolt
Wall and Catch Fill
Resisting Catch Work Catch Gabion
Structures Catch Concrete Wall
Protection
Rock Fall

Protection Rock Fall Catch Net

Protection
Work Rock Fall Catch Fence
Work

Work
Rock Shed Rock Shed

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(3) Removal
ＳＴＡＲＴ
This method involves the removal of small-scale unstable rock
Small Large overhangs by trimming and removal of loose individual rock
Scale of
the anticiplated rock fall debris by hand scaling. In planning removal, it is important to
Protection work
take account of the rock character. For example, when rocks are
Prevention work
highly degradable and strongly susceptible to weathering and
Possible to
jointing, such as shale, the removal of loose rock from the
control falling rock
individually
Small Large
surface will start a new cycle of weathering and instability.
Energy
of rock fall
No Yes

・Removal (4) Rock fall catch net

・Rock bolt
・Anchor
Rock fall catch nets consist of net and wire rope and include two
major types, covering type and pocket type.
Possible to
cut unstable parts
1) Purposes
No Yes
Rock fall catch nets are used to cover slopes that show a
・Cutting work
potential for rock fall in order to protect road traffic from rock
fall damage.
2) Design considerations
Soil and
rock
characters The design of rock fall catch nets is generally based on the
following steps.
Residual soil, Weathered or
sandy soils jointed rocks

a. Determine the size (diameter) of vertical ropes required to

・Shotcrete crib
・Pitching work
・Ground anchor
・Pitching work ・Shotcrere
・Rock bolt

・Vegetation
・Crib work
・Shotcrete
・Rock bolt
・Ground anchor
・Retaining wall
・Rock fall fence
・Catch fill
support the combined catch net deadweight and weight of
・Soil nail ・Rock bolt
・Ground anchor
・Retaining wall
・Rock fall catch net
・Rock fall catck fence
・Catch gabion
・Catch concrete wall
・Rock shed
falling rocks corresponding to each span of vertical rope.
b. Determine the diameter of horizontal ropes needed to support
its own deadweight and the weight of falling rocks assuming
that rocks are uniformly distributed in spans in the direction
of the slope.
Figure A1-3.1 Flow chart for selection of countermeasures for c. To calculate the strength and stability of anchors needed on
rock fall the assumption that all of the load on the ropes will be
transferred to the anchors.

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b. Find the force R acting on the post from T y of the wire ropes.
The use of two wire ropes is assumed to be capable of
resisting the force of falling rocks.
c. Find the force Fy required for forming a plastic hinge at the
bottom of intermediate post.
d. Compare forces R and Fy and calculate the energy that can be
absorbed by the fence.

Figure A1-3.2 Example of rock fall catch net

(5) Rock fall catch fence

A rock fall prevention fence consists of a fence made of net and
wire rope attached to steel pipe or H-section posts. This type of
fence has the capacity to absorb the energy of falling rocks.
1) Purposes
Rock fall catch fences are also designed to protect road traffic
from rock fall damage, but differ from rock fall catch nets in that
they are installed near the road.
2) Design considerations
The design of a rock fall catch fence mainly involves the energy
of falling rock and the absorbable energy by the fence and
generally involves the following steps.
a. Determine the yield tension Ty corresponding to the diameter
of the wire ropes.

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A1-4 COUNTERMEASURE WORKS AGAINST ROCK (2) Criteria for selection of countermeasures
MASS FAILURE (FR) Rock mass failures generally result from geological structures
(1) Selection of Countermeasures and mostly occur at a relatively large scale. In selecting and
Because of the larger scale of this kind of failure, it may be more designing the countermeasures, the following points should be
considered. Figure A1-4.1 shows a flowchart for selection of
economical to relocate a road route than to prevent rock mass
countermeasures for rock mass failure.
failures. For this reason, route relocation or bridge diversion is
the most preferable method of protecting rock mass failure. 1) A comprehensive investigation should be conducted on the
causes and scale of rock mass failure as well as the regional
Table A1-4.1 gives the general countermeasures for rock mass geological features. Where rock mass failure is closely
failures and their classification. In the case of preventing small related to large geological structures such as faults and folds,
rock mass failures, countermeasures such as cutting, shotcrete, route relocation is the most cost-effective method because of
cast-in-place concrete crib, rock bolt and ground anchor may be the large scale involved. Where a rock mass failure threat is a
the most cost-effective. result of geological composition such as limestone and
metamorphic rock prone to weathering and jointing,
Table A1-4.1 Classification of Countermeasures against Rock framework and shotcrete work may, in some cases, be cost-
Mass Failure effective.
CLASSIFICATION TYPE OF WORK 2) Suitable countermeasures should be selected after
Removal examination of the economics, effectiveness, maintenance
Earth Work Earth Work
Rock Cutting requirements, environment and appearance. In most cases,
Water maintenance is costly, so conservative countermeasures that
Drainage Subsurface Drainage Horizontal Drain Hole
will require little on-going maintenance should be selected.
Shotcrete Work Shotcrete (mortar)
Slope Work Shotcrete (concrete) For example, engineering practice shows that to stabilise
Crib Work Crib Work strongly jointed rock slopes, mortar shotcrete costs less than
Anchoring Anchoring Rock Bolt cast-in-place concrete crib, but is costly to maintain.
Ground Anchor 3) If removal work cannot be done by some conditions,
Wall and Catch Fill
Resisting Catch Work protection works should be done to prevent occurrence of
Catch Concrete Wall rock mass failure. Figure A1-4.5 shows the examples of
Structures
Protection Protection Work Rock Fall Catch Net countermeasures in these cases.
Work Rock Fall Catch Fence
Rock Shed Rock Shed
Others Avoiding Problem Tunnel, Bridge
Work Route Relocation

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(3) Cutting work

ＳＴＡＲＴ The purposes and design considerations for stabilising work are
similar to those for collapse. Cutting work is particularly
Scale effective to safeguard against rock mass failure and an example
of rock mass failure
is shown in Figure A1-4.2.
Small Medium
Large

Yes No No
Cutting of unstable materials
Possible Is there space
to cut ? enough to accommodate
the collapsed mass ?

Small
Removing of rock overhang
Importance of Yes
objects to be protected

Large

・Removal
・Removal ・Catch fill and ditch ・ground anchor
・Removal ・Route relocation
・Removal
・Shotcrete
・Concrete pitching
・Shotcrete
・Frame work
・Rock bolt
・Catch concrete wall ・Catch wire rope
・Catch wire rope net
・Tunnel
・Bridge
Weathered rock
・Ground anchor ・Concrete rock shed
・Steel rock shed

ROAD

Figure A1-4.1 Flow chart for selection of countermeasures for rock

mass failure Figure A1-4.2 An example of cutting work to treat rock mass
failure

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(4) Ground anchor and rock bolt view; portal (gate) type, retaining wall type, arch type and pocket
Where the objects to be protected are very important, and other type (Figure A1-4.4).
works cannot provide enough safety, ground anchors and rock
bolts should be considered. Deposited mass
Rock bolting is a shallow-fitting method, while ground
anchoring is inserted deep into the slope. Therefore, rock bolts Absorption layer
Jointed Rock
are applied to stabilize the slope face by exerting a force that
compresses the joints and prevents loosening of the rock mass.
Ground anchoring is applied to prevent a rockslide by tension
force, generally in association with frame works. Figure A1-4.3 Jointed Rock
shows diagrammatically how unstable rock above a road can be
stabilised by ground anchor and rock bolt.
ROAD ROAD
Loosening
Rock Slide Portal Type Retaining Wall Type
Toppling
Figure A1-4.4 Types of rock sheds
Anchored shotcrete crib
1) Purposes
This method is applied to reduce road disasters due to rock fall
Rock bolt or rock mass failure by absorbing the impact force of a falling
rock mass or shifting the movement direction of rock mass
ROAD failure and rock fall.
2) Design considerations
The most important design consideration should be given to the
calculation of the impact force of the falling mass. Generally,
rock sheds are designed after converting the impact force into a
static force according to the allowable stresses design method.
Figure A1-4.3 Stabilization of unstable rock slope above a road
For the purposes of simplifying calculation, the area on which
the impact load is calculated is assumed to be rectangular rather
(5) Rock shed than circular.
Rock sheds are reinforced concrete or steel structures covering a
road and can be subdivided into four types from a structural

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Figure A1-4.5 Examples of countermeasures against rock mass

failure

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A1-5 COUNTERMEASURE WORKS AGAINST MASS Table A1-5.1 Classification of Countermeasures against Mass
MOVEMENT (DL) movements
(1) General CLASSIFICATION TYPE OF WORK
Mass movements frequently occur because of particular Cutting
Earth Work Earth Work

CONTROL WORK
Embankment
conditions relating to topography, geology, meteorology, and
Hydroseeding
land utilization. Mass movement disasters can be either direct or Vegetation Vegetation
Re-Vegetation
indirect disasters. Direct disasters are the damage caused by Surface Drain Ditch and Cascade
mass movements to public facilities, houses and cultivated lands, Drainage Subsoil Drainage Hole
whilst indirect disasters are damage such as the blocking of Water Drainage Horizontal Drain Hole
Subsurface
rivers and secondary collapses as a result of a mass movement. Drainage
Drainage Well
Therefore, the main purpose of a mass movement Drainage Tunnel
countermeasure plan is to prevent or reduce disasters due to mass Slope Work Crib Work Crib Work
Rock Bolt

RESTRAINT
movements. Anchoring Anchoring
Ground Anchor

WORK
Wall and Resisting Retaining Gabion Wall
(2) Classification of Countermeasures Structures Wall Retaining Wall
Countermeasures for mass movements belong to one of two Steel Pipe Pile
Piling Work Piling Work
broad categories, (A) control works; and (B) restraint works. Shaft Work
Control works involve modifications to natural conditions such Avoiding Diversion (shifting)
as, topography, geology, ground water, or other conditions that Others Problem Route Relocation
indirectly control portions of the entire mass movement Work Bridge, Tunnel
movement. Restraint methods rely directly on the construction of
structural elements. When the potential mass movement is large-
scale, it may be more cost-effective to relocate the route or
bridge.

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ＳＴＡＲＴ

(3) Criteria for selection of countermeasures

Figure A1-5.1 shows a flowchart for selection of
countermeasures against mass movements. Adequate works Is No
the landslide active ?
should be selected in consideration of the following points.
The works selected should address the mechanism(s) of the mass Yes

movement, the relationship between precipitation, groundwater

and mass movement movement, geological, topographical and ・Temporary Cutting
・Horizontal drain hole
soil properties, the scale and type of mass movement and its ・Embankment　or
・Gabion wall
・Surface drainage

likely movement velocity.

Control works should be regarded as the main method of mass
movement control, while restraint works should be adopted for Does landslide
the stabilization of small mass movements to directly protect No movement cease?

public facilities, houses, etc. Yes

Where mass movement movement is closely related to rainfall,

surface drainage work should be immediately performed to Importance
minimise the infiltration of rainwater. Small
of objectsYes
to be protected

When a mass movement continues to move, control works Large ・Drainage well

should be performed first; restraint works can then be done after ・Dranage tunnel
・Earth works

reduction or arresting mass movement movement by the control Scale

　　if necessary

works. of landslide
Large

An adequate combination of various works is cost-effective and Medium and Small

should be selected.
Slope gradient

Steep
Gentle

・Crib work ・Steel pipe pile ・Route relocation

・Drainage works ・Ground anchor ・Diversion ・Shaft work
・Erath works ・Diversion ・Embankment ・Embankment
if necessary ・Cutting ・Cutting ・Cutting
・Embankment

Figure A1-5.1 Flow chart for selection of countermeasures for

mass movement

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(4) Surface Drainage (5) Horizontal Drain Hole
Surface drainage can be classified into catch drain, berm drain Groundwater can generally be divided into two types, shallow
and toe drain. In most cases, surface water should be prevented and deep. Shallow groundwater, 0 to 5 meters below the ground
from infiltrating the mass movement area to avoid any hydraulic surface, is due mainly to rainfall received in the short-term.
thrusts. Especially where mass movements are closely related to Shallow groundwater frequently causes a shallow failure or the
short-term rainfall, the work should be immediately performed toe failure of a large-scale unstable slope. In such cases, culverts
regardless of the results of stability analyses. U-shaped gutter, and horizontal drain holes are effective. Deep groundwater is
centrifugal reinforced concrete or corrugated pipe may be used to related to rainfall received over the longer term and should be
construct the drainage ditch. drained by installation of drainage wells or tunnels with
1) Purposes horizontal drain holes. The following is a brief presentation of
Surface drainage control includes works for drainage collection considerations for horizontal drain holes and drainage wells as
and drainage channels. these are the most effective methods of stabilizing mass
movements.
2) Design Considerations
Drainage collection works are designed to collect surface flow
by installing corrugated half pipes or lined U-ditches along the 1) Purposes
slopes, which are then connected to a drainage channel. The Horizontal drain holes are used to drain both shallow and deep
drainage channel works are designed to remove the collected groundwater to stabilize the mass movement by decreasing the
water out of the mass movement zone as quickly as possible, and pore water pressure that is responsible for activating the slip
are constructed from the same materials as the drainage surface. It is useful as a temporary countermeasure to decrease
collection works. The surface drainage control works are often the moving speed of a mass movement.
combined with subsurface control works. If necessary, the designed reduction in the groundwater level
The drainage ditch beds should, in principle, be covered. may be determined through stability calculations, aiming at
Collecting boxes should be installed at the confluence with achieving the following values in the case of the standard-scale
tributaries, curves and points of change in gradient.Where failure with a failure depth of 20 metres.
conducted in the active area of a mass movement, drainage Horizontal drain 1 to 3 meters
ditches should have the required strength and be easy to repair.
Drainage well 3 to 5 meters
Bed consolidation should be planned every 20 to 30 m to prevent
the drain ditch from sliding.The shoulders and cut slope faces of Drainage tunnel 5 to 8 meters
the ditches must be protected with vegetation, boulder covers,
and so on. 2) Design Considerations
Horizontal drain holes are constructed for the drainage of
shallow groundwater and deep groundwater. If topography
prevents the groundwater from being drained on a gentle

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gradient, then drainage wells or tunnels with horizontal drain well’s safety. In principle, wells should be located in stable
holes should achieve drainage. ground within an area from which it is possible to effectively
Horizontal drain holes, usually 20 to 50 meters in length, should collect groundwater. Wells are usually between 2 meters to 4
be excavated at a gradient of 5 to 10 degrees with a diameter of meters in diameter and 10 to 30 meters in depth. Liner plates,
50 to 100 millimetres and should be designed to traverse aquifers. reinforced concrete segments, and other materials generally
A typical location of horizontal drain holes is shown in Figure support the sidewalls of wells.
A1-5.2. Collecting drains are similar to horizontal drain holes in terms of
design considerations. The safety of drainage wells should be
evaluated by checking the earth pressure of the surrounding area.
Horizontal drain hole The diameter of drainage holes should be based on catchment
quality of groundwater. Figure A1-5.3 shows the details of a
drainage well.
Retaining Wall
Retaining Wall

Roa
Slip Surface Road Trap
Drain Ditch 2 to
4m
1m

Figure A1-5.2 Typical location of horizontal drain holes

Groundwater level

(6) Drainage Wells

Drainage wells consist of wells with horizontally bored
collecting drains and relief drains. This method is used when Collecting boring hole
Drainage boring hole
horizontal drains or culverts cannot achieve efficient drainage
because of the large scale of the mass movement.
1) Purposes
Similar to horizontal drain holes, drainage wells are used to drain Concrete bottom
deep groundwater for stabilization of the mass movement.
2) Design considerations Figure A1-5.3 Details of Drainage Well
The location of drainage wells should be determined on the basis
of the distribution of groundwater and in consideration of the

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(7) Ground Anchor Figure A1-5.4 shows a typical example of a stabilised mass
Compared with other countermeasures, ground anchor works are movement with ground anchors and Figure A1-5.5 shows details
costly, but reliable. Recently this method has been applied of an anchor structure.
increasingly to artificial mass movements to cut off the toe of the
mass movement. Compared with rock bolts and soil nailing,
ground anchor work has a relatively large resistance to sliding
force and is therefore used to stabilize relatively large-scale slope
disasters. Concrete Crib

1) Purposes
Ground anchor works are intended to prevent mass movements ROAD Slip Surface
through the tensile strength of high tensile strength steel wire or
bars installed across the slip surface. Ground Anchor
2) Design Considerations
Important considerations for ground anchors are the bearing Figure A1-5.4 Mass movement Stabilized with Ground Anchors
capacity of the groundmass under the bearing plate and the bond
strength between the anchor grout and rock at the attachment
point. Further, in planning ground anchors, the bond strength test
Anchor materials (steel wire)
at the attachment should be carried out. Ground anchors are in Sheath
principle installed at a spacing of at least 2 meters in 2 rows or Bearing Plate Borehole
more.
Fixation length should be 3 meters to 10 meters in length, and L1
the free length should be more than 4 meters. The settlement 1
angle should not be applied from + 10˚ to -10˚.
The direction of anchoring should be parallel to the direction of L1: Free length L2
movement of the mass movement. L2: Bonded length
Cribs, plates or cross-shaped blocks are used as pressure
resisting bearing plates set on the surface of the ground. The
most appropriate pressure resisting plate should be selected in Figure A1-5.5 Outline of Anchor Structure
consideration of specifications, operation efficiency, cost-
effectiveness, maintenance, landscape, etc.

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The following items are decided or calculated for the designing α : Angle of slope of sliding surface
of anchors
i) Stability analysis φ : Internal frictional angle of sliding surface
ii) Calculate the required restraining power (degree)
iii) Selection of anchor works B: Interval between anchors in horizontal direction
iv) Calculate the required anchor power (location,
(m)
interval, angle)
v) Determine the type of anchor and steel material N: Number of anchor setting in vertical direction
vi) Calculate the fixation length and diameter of
boring
vii) Design of pressure resisting plate Sliding srface
Anchor

The design process can be summarised as follows. α

P・sin(α-θ)
1) The design anchor power (Td) is calculated using the
following formulas: θ
P

Fsp・Σ W・sin α – Σ c・l – Σ ( W－u・b ) cos α ・ tan φ B

Td = × P・cos(α-θ)
Fsp ・ cos ( α +φ ) + sin ( α +φ ) ・tan φ N
Figure A1-5.6 Functional description of an anchor
……………… （1.6）
Po: the required resisting power (kN/m2) 2) Determination of the type of anchor and steel material
Td : Anchor power Generally, the type of anchor is determined by comparing
Fsp : Design safety factor the tension strength of steel material with the skin frictional
c : Cohesion resistance between the ground and grout as well as the
l : Length of sliding surface allowable adhesive stress between a tendon and grout.
b : Width of slice
W : Weight of slice 3) Calculation of the fixed length of an anchor
u : Pore water pressure For the design anchor power to meet the allowable anchor
pullout force, the length of contact between the ground and

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grout must be compared with that between the tendon and most unfavourable possible conditions that might be expected
over the period of the anchor’s useful life.
grout. Whichever is longer should be used as the fixed
length.

Tab ＝ πD・τab ……………… （1.7）

Tab : allowable adhesive force (kN/m)
D : diameter of tendon (m)
τab : allowable adhesive stress between tendon
and grout (kN/m)

1
Tag ＝ ・πD・τag ………… （1.8）
f
Tag : skin friction force (kN/m)
f : safety factor
D : diameter of anchor (m)
τag : skin frictional resistance (kN/m2)

ls ＝ Td ／ τab （ ls＜ lsa） ………… （1.9）

ls ： required length of tendon (m)
lsa： standard length of tendon
Td： design anchor power (kN/m)
τab： allowable adhesive stress between tendon
and grout (kN/m)

In the preliminary design stage, the locations, directions,

intervals and angles of anchors should be considered. Corrosion
countermeasures should be taken to give protection under the

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(3) Criteria for selection of countermeasures
A1-6 COUNTEMEASURES WORKS AGAINST ROAD Adequate measures for preventing road collapse should be
COLLAPSE (FP) selected in consideration of the causes, mechanism and scale of
(1) General the anticipated road collapse, embankment materials, and
Generally, road collapse results from (1) the toe failure of an foundation conditions. Generally, the following guidelines
embankment slope, (2) scouring of the surface of an should be followed for selection.
embankment slope, (3) rising pore water pressure within an 1) In principle, a standard embankment slope (gradient and
embankment, (4) a gradient steeper than the standard gradient, or, height) should be designed, especially where sufficient land is
in some cases, (5) settlement of an embankment’s ground available to meet the full width of the base of the embankment. If
foundation. Therefore, countermeasures for road collapses sufficient space is not available, a retaining structure such as a
consist mainly of slope protection and drainage works. retaining wall should be considered.
2) The surface of an embankment should, in principle, be
(2) Classification of Countermeasures protected by closed turf. Selection of a type should take into
Table A1-6.1 shows the classification of countermeasures for account the susceptibility of embankment materials to
road collapses weathering or erosion.
Table A1-6.1 Classification of Countermeasures against road 3) Separate drainage work to deal with surface water runoff and
collapses groundwater inside the embankment is essential, for both
CLASSIFICATION TYPE OF WORK construction effectiveness and the long-term stability of the
Earth Work Earth Work Embankment embankment.
Hydroseeding
Vegetation Vegetation
Re-Vegetation
Surface Drainage Drain Ditch and Cascade
Water Drainage Subsurface Horizontal Drain Hole
Drainage Culverts
Pitching Work Stone Pitching
Slope Work
Crib Work Crib Work
Soil Nail
Anchoring Anchoring
Ground Anchor
Gabion Wall
Stone Pitching Wall
Wall and Concrete Block Wall
Resisting Retaining Wall Retaining Wall (Gravity Type)
Structures Retaining Wall (Supported Type)
Crib Wall
Pile Wall

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be considered. Table A1-6.2 gives the recommended standard
embankment slope for different embankment materials. These
ＳＴＡＲＴ can only be applied where the foundation ground has sufficient
bearing capacity.
Causes
of embankment failure
Furthermore, for high embankments consisting of different kinds
of materials, a standard gradient suitable to each material should
be applied to each slope. The stability of the foundation ground
Embankment
Standard slope Water materials of the embankment should be reviewed prior to construction.

No Can Yes No
Susceptible
Yes Table A1-6.2 Recommended Standard Embankment Slopes
standard slope be
secured ? Surface
Groundwater
to erosion?
Height Gradient
water
Embankment Materials
(m) (V:H)
Well graded sand, gravels and sand or <5m 1:1.5 ~ 1:1.8
silt mixed with gravels (GW, GP, GM,
5 ~ 15 m 1:1.8 ~ 1:2.0
GC)
Horizontal Vetegation or Vegetation or
Retaining wall Embankment Drainage ditch
drain hole Slope work No work Poorly graded sand (SP). < 10 m 1:1.8 ~ 1:2.0
< 10 m 1:1.5 ~ 1:1.8
Rock masses (including muck).
10 ~ 20 m 1:1.8 ~ 1:2.0
Figure A1-6.1 Flow chart for selection of countermeasures for <5m 1:1.5 ~ 1:1.8
Sandy soils (SM, SC), hard clayey soil
road collapse and clays (CL, ML). 5 ~ 10 m 1:1.8 ~ 1:2.0

(4) Embankment Soft clayey soils <5m 1:1.8 ~ 1:2.0

1) Purpose Note) Height of embankment is the vertical height from the toe
Embankments are used at the toe of unstable or potentially to the top of the embankment.
unstable slopes to balance the driving force of additional loading.
The insertion of a sand layer at an embankment at certain
intervals for drainage purposes as shown in Figure A1-6.2 is
2) Design considerations sometimes conducted to prevent the failure of an embankment
The main considerations for embankments chiefly concern slope.
stability analysis as well as the selection of slope gradient in
proportion to embankment materials. In selecting embankment
materials, their strength and deformation characteristics should

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Embankments with Sandy Soil Liable to Erosion by Lowering of the groundwater level inside an embankment is
Rainwater effective to not only reduce rain-induced disasters but also as an
As this type of soil is characterised by poor water retention as earthquake-proof measure.
well as poor nutrition, protection of the slope surface with top
soil, etc. as part of planting is necessary. Meanwhile, facilities
with a sufficient drainage function are required to prevent slope
erosion by rainwater during construction. For general
embankment work, the introduction of a higher cross grade at the
central part during construction to create a downward inclination
towards the top of the slope and the introduction of a drainage
ditch at the top of the slope is recommended.

Embankments on Inclined Ground

In regard to embankments on inclined ground, embankments Figure A1-6.2 Groundwater Drainage Facilities and Drainage
filling a valley, half-bank and half-cut and boundary of cut and Layer for the Embankment on Inclined Ground
embankments, spring water from the ground often infiltrate the
embankment, destabilising the embankment slopes. In such cases,
the introduction of a drainage layers inside the embankment is Widening of Embankments
recommended to reduce the water pressure in the embankment in When the widening of an existing road is planned by
addition to the introduction of sub-surface drainage to prevent constructing an additional embankment(s) at the side(s) of the
the infiltration of groundwater to the embankment (Figure A1- road, the construction of the new embankment(s) by the bench
6.2). cutting of the existing embankment slope(s) as shown in Figure
As some inclined ground has a soft layer(s) which makes an A1-6.3 (a) is preferable.
embankment vulnerable to failure, it may be necessary to There are many examples of road collapse due to rain or an
conduct investigations on the foundation ground and to design an earthquake at those sites where a narrow embankment has been
embankment which is similar to Mass movement. added to inclined ground. When the slope stability of the ground
The shear strength of soil near the ground surface is often low is secured, much cutting volume should be introduced as shown
because of weathering and other reasons and, therefore, it is in Figure A1-6.3 (b). If possible, the bottom parts of these cut
preferable for the ground to be excavated in the form of bench slopes should be made into a horizontal surface above which an
cutting as deep as possible prior to the construction of an embankment is constructed. The widening of an embankment
embankment. The minimum width and minimum height of bench with long-term stability can be successfully achieved in this
cutting should be 1 m and 0.5 m respectively. manner.

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Figure A1-6.3 Widening of Embankment

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APPENDIX V-2

INVESTIGATION

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A2-1 BASIC CONCEPT OF INVESTIGATION
Appendix V-2 Investigation describes the standard method for
investigation and monitoring through instrumentation for each of
six classified disasters. The locations where abnormal condition
were found and prevention measure works required by Disaster
Inventory Inspection or Regular Inspection shall be investigated
in detail or monitored.
The investigation shall be to know the Primary Factor and the
Contributory Factor.
In the course of a series of procedure as above described,
selection of the investigation method and establishment of the
investigation plan are made, taking into account generally the
objectives, scale of failure, geological conditions, surface and
topographic conditions, access to the site, overall
countermeasure policy, restriction in terms of budget and time,
etc. Engineers concerned are expected to draw flexible and
appropriate judgments concerning the urgency and safety, since
conditions of actual sites vary widely,
The site reconnaissance is the basic investigation to know the
primary factors and the contributory factors.
The mass movements are normally large scale and moving slow,
therefore detailed investigation other than the site reconnaissance
may be important.
The sauces of debris flows are normally far from the road,
therefore detailed investigation other than the site reconnaissance
may be important.

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Table A2-1.1 Check Points of Site Investigation

Type of Disasters Primary / Contributory Factors Detailed Study
Collapse (CL) Primary Factor: highly weathered surface, colluviums, rock conditions of Confirm depth of highly weathered zone or lose layer covers
fissures having certain regularity, daylight, erosion weak soil, weak rock slope
when dip in water (slaking), trace of collapse
Contributory Factor: rainfall, earthquake, cut off at the base of slope
Rock Fall (RF) Primary Factor: rock conditions of fissures having certain regularity Confirm number and size of loose rocks, if necessary.
Contributory Factor: earthquake, rainfall, vibration by construction works
Rock Mass Failure Primary Factor : hard rock conditions of fissures having certain regularity, Confirm width and opening speed of crack, if necessary.
(RM) daylight structure, vertical fissures
Contributory Factor: earthquake, opening of fissures, vibration by
construction works, freeze / fusion of water in fissures
Mass movement (LS) Primary Factor : thick weathering zone, colluvium on slope, daylight Preliminary Study is made to estimate the scale and
mechanism of landslide, followed by establishment of the
Contributory Factor: ascent of groundwater in sliding mass, cut off at the
necessary investigation and monitoring plan. To confirm depth,
toe of sliding mass, embankment on the top of sliding mass
volume, speed of landslide for designing of the
countermeasure
Debris Flow (DF) Primary Factor : debris deposit on stream, insufficiency of section of cross Check if there is any drawing around the road concerned. If
stream, collapses on upper stream necessary, to confirm thickness of sediments and gradient
along stream.
Contributory Factor: rainfall
Embankment (EB) Primary Factor : rainwater overflow on road surface, erosion at the end of Check if there is any data and record during construction. If the
embankment toe, embankment is critical stage, detailed study is made for
Contributory Factor: rainfall, earthquake, cut off at the base of slope designing of the countermeasure.

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Table A2-1.2 Outline of Preliminary Study and Detailed Study

Item Item Description Application
Preliminary Study Interpretation of topography Interpretation of wide area around the slope concerned. Large- Implemented always
scale topographic map and stereoscopic aerial photos
Collection of existing data Past disaster history, boring log, geological map, groundwater
Site reconnaissance In-depth survey of the slope concerned and its surrounding area
Range from the ridge to channel in certain cases
Simplified measurement Simplified measurement with marking and batten plate
Survey Made when no existing survey drawings exist or are available.
Detailed Study Geophysical survey Seismic refraction survey, resistivity imaging method, etc. Implemented if judged
Applicability and profile line length are reviewed with reference to
the transverse direction. necessary as a result
Boring investigation Basically, all-core boring is made, including sampling. of detailed survey
Survey to be made to a point below the estimated slide surface,
collapse surface, and bed rock.
Sounding Dynamic probing, Swedish sounding, etc.
Borehole logging Borehole television, electrical method, PS logging, etc. depending
on geological condition
In-situ test Standard penetration and borehole horizontal loading tests in
boreholes depending on geological condition, including sampling
in test pit, adit, and shaft
Laboratory test Rock and soil tests to be made as required
Instrumentation Measurement of ground surface slide movement (ground surface
extensometer), measurement of borehole movement (probe-
and Monitoring inclinometer), groundwater survey (piezometer, Casagrande
type), precipitation survey (rain gauge), periodical ground Site
reconnaissance

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A2-2 SITE RECONAISSANCE (2) Site Reconnaissance

(1) Basic Concept of Site Reconnaissance Following items shall be investigated, estimated and examined at
Site reconnaissance consists fundamentally of visual and careful the site. Also the site reconnaissance results shall be entered in
confirmation of any deformation of the ground surface in plan and cross-sectional views of the scale of about 1/100～
addition to geological, topographical, vegetative, and hydraulic 1/500 if available.
condition by engineering geologists or geotechnical engineers 1) The geology and its structure of the slope. Observation is
with the information such as sketch and photos. made while paying attention to the hardness of geology,
Main purpose of the site reconnaissance is to find the primary separation surface, weathering, and looseness of rock mass.
factors. With attention paid to discontinuity of joint, etc., and its type,
direction, spacing, continuity, and nature, determination is
The primary factors of landslides are shown on Table A2-2.1
made on whether the ground is excavated or dip slope.
2) Microtopography on the slope. Slope height, slope length,
Table A2-2.1 Ground Condition and Primary Factors of Disasters
gradient, position of the knick line, small steps, presence of
Ground Primary Factors
open cracks or faulting, etc.
Condition
Topographic 1 Over hung 3) Presence of seepage location, water-holding condition of
Conditions 2 Water channel surface soil, vegetation, etc and drainage facilities.
3 Trace of landslides
4 Existence of clacks on the ground surface
4) Range and depth of unstable matter through the site
5 Damage of cut slopes and natural slopes reconnaissance by surface deformation with fracture and
Geological 1 Weathering elevation, faulting and cave-in, distribution of grooved
Conditions 2 Conditions of fissures having certain regularity, topography, etc.
such as bedding stratification, foliation and joints
3 Existing of colluviums
5) Mechanism of the disaster.
4 Boulder stones which is not firmly embedded in the ground 6) Existing countermeasure condition. Fracture and squeezing,
Water 1 Water Spring peeling and deterioration of concrete, cut and corrosion of
Conditions 2 Erosion net, rope, and wire, loosened anchor.
3 Channels
7) Positional relationship between the disaster and road. The
pattern is estimated on the basis of overall evaluation on the
location, scale, pattern, and failure route of the phenomena
possibly affecting the road as well as effectiveness of existing
countermeasures.
8) Any other anomaly or deformation surrounding area.

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When there is any deformation that may indicate possible
collapse, simplified measurements are made. They include the
measurement of a distance between marked pins and nails
provided on both sides of fracture and step, observation of
mortar provided to repair fracture for any deformation, etc as
shown on Figure A2-2.1.

Figure A2-2.1 Example of Simplified Measurement

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i) Irregular arrangement of contour lines
A2-3 INVESTIGATION FRO MASS MOVEMET ii) Presence of horse-shoe shaped type cliffs or plateau type
(1) Checkpoints of Survey slopes, or presence of dense contour lines below gentle
Mass movements tend to occur frequently in areas with specific slopes
geology or geological formations such as Tertiary mudstone or iii) Regularly arranged ponds, swamps and marshes
tuff zones or Mesozoic and Palaeozoic metamorphic rocks zones, iv) Presence of irregular contour lines in front of steep cliffs,
and such areas frequently indicates unique topographic features or presence of separate small hills
due to past successive mass movement activity. The relation v) Abnormal curves of such as gorges, streams, abnormal
between the topographical features of mass movements and the curves and changes in the grade of road
properties of sliding mass in the process of mass movement vi) Presence of terrace paddy field, debris flow deposit
movement have been generally recognized. Mass movements are
also frequently induced by small-scale earthworks in areas where (2) Survey on Stability of Mass movement
the topographical features of mass movements are apparent. Surveys are mainly performed by borings and sometimes the
Such mass movement zones can be frequently determined by excavation of test pits is also performed to study the mass
aerial photographs. movement mechanism from the viewpoint of soil mechanics and
A survey on mass movements is conducted to obtain the to examine the soil mechanical characteristics in detail of sliding
following types of information when a road is crossing at a mass surface clay.
movement site or at a site where the possibility of a mass For soil and geological survey, at least four borings should be
movement is recognised. made and three of these should be on and across the sliding
i) Extent and scale of sliding block and one should be on the upper slope of a block. It is
ii) Direction and velocity of sliding and presence of a scarp important to plan and actually make a boring at least 5 m into the
iii) Stability of a mass movement and changes of the safety bedrock in order to distinguish the sliding mass from the bedrock.
factor due to earthwork and other causes Also, where the mass movement area is wide and the distribution
iv) Mutual relationship between a mass movement and of bedrock is quite irregular, it is also important to perform
contributory factors elastic wave exploration and to provide an auxiliary course of
v) Location of installed measuring instruments traverse to allow a review in the traverse direction.
The entire picture of a mass movement is clarified based on the From the mechanical point of view, this survey is performed to
above information and a suitable route is selected or adequate determine the hardness of the landslide mass and the soil texture,
countermeasures are implemented by investigating the causes and also to determine the strength of soil mass required for the
and mechanism of the mass movement. There will be a high stability analysis of the slope in the mass movement area and for
potential of mass movement occurrence requiring detailed designing countermeasures with the results of sounding, standard
surveys if the following symptoms are found in the survey areas penetration tests, in-situ tests, and tests for physical and
after reading the topographic maps and aerial photographs. mechanical properties.

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The survey results are then compared to the results of the field
survey and topographic and geologic map readings, and then the Groundwater should be surveyed by continuous observation
characteristics and extent of mass movement activity are since it often has seasonal fluctuations. In addition, vegetation in
examined. the mass movement area is closely related to the distribution of
the groundwater zone and, this, must be compared to the results
(3) Survey on Contributory Factors of Mass movement of field survey.
It is well known that mass movements frequently occur during
periods of heavy rain and that mass movement movement 2.3.4 Survey on Extent and Movement of Mass movement
becomes active as the groundwater level rises. Information This survey is performed to examine the extent, direction of
concerning the mechanism and possibility of mass movements movement, and the mechanism of mass movements in detail
can be obtained by determining the groundwater conditions in when any sign of mass movement motion such as slide scarps or
the mass movement area (fluctuation in ground water level, flow cracks are found or when there is any possibility of the
of groundwater, runoff path, current speed, quality and occurrence of mass movements in the future.
temperature of groundwater, etc.) to make it possible to examine Measurement or monitoring survey may involve monitoring of
the quantity and location of effective drainage work as movement or survey on sliding surface. Monitoring of
countermeasures based upon the state of groundwater. movement is undertaken in order to confirm the extent of sliding
Groundwater surveys can be generally classified into and to obtain material for forecasting future activation of mass
groundwater distribution surveys and groundwater pressure movements and measures the amount of fluctuation of
surveys. The contents of the survey are as shown in Figure A2- inclination and expansion or contraction of the slope surface.
3.1, and the scope of survey should be determined according to The method of measurement involves the use of such devices as
the purpose. invar wire extensometer (Figure A2-3.2) and ground
inclinometer or a simplified method using displacement stake or
Measurement of groundwater level displacement plate (Figure A2-3.3).
Groundwater
pressure survey Measurement of pore water pressure
Electric prospecting
Groundwater Preliminary Water quality analysis survey
survey survey
Water temperature survey
Groundwater
distribution Plane survey Groundwater
survey tracing
Detailed
survey Groundwater
Vertical survey in bore hole
survey
Pumping test
Figure A2-3.1 Groundwater Survey

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(Figure A2-3.4) in the bore hole to measure changes in the
internal stress in a ground or bore hole incline angle in order to
grasp the condition of the sliding surface. The point at which the
pipe strain gauge type or insertion type underground
inclinometer is embedded is, in principle, all bore holes but in
some cases, this may be limited to the bore holes along the main
measurement line.
Survey of the sliding surface is undertaken when it is clear that
the mass movement block has been moving from the results of
the site reconnaissance or measurement of ground movement.
Figure A2-3.2 Schematic Diagram of Invar Wire Extensometer

Figure A2-3.3 Simple Displacement Plate

Sliding surfaces may be investigated through examination of the

boring core or measurement of the sliding surface. Core
examination is employed to verify evaluation through
measurement or when the sliding surface cannot be evaluated
through measurement survey. Survey through measurement of
the sliding surface is undertaken by embedding rigid polyvinyl
chloride pipe equipped with strain gauge (pipe strain gauge type
inclinometer) or an insertion type underground inclinometer Figure A2-3.4 Underground inclinometer

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A2-4 INVESTIGATION FOR DEBRIS FLOW

A2-4.1 Checkpoints of Survey

A debris flow is a phenomenon in which soil at the bottom of a
valley or upstream moves as a result of heavy rain or earthquake
and flows in hydraulic bores out of the valley. There are 4 main
types of debris flows as shown in Table A2-4.1.
Survey on debris flow is focused on the following points and
involves investigation of materials on past damage, site
reconnaissance and interpretation of aerial photographs.

i) Possibility of a debris flow occurrence

(frequency of occurrence, weather conditions, etc.)
ii) Scale and characteristics of the debris flow
(peak amount of discharge, the flow velocity, wave height,
volume of soil, maximum gravel diameter, etc.)
iii) Area likely to be inundated by the debris flow
iv) Presence of existing facilities for erosion control or
forestry protection and its configuration and size.

In the event a planned road would cross over a mountain stream

by bridge or culvert where the occurrence of debris flow is
expected, the peak amount of discharge, the flow velocity,
maximum wave height and maximum gravel diameter of the
debris flow should be investigated as material for reviewing
whether or not the debris flow will pass without damaging the
bridge or culvert.

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Table A2-4.1 Classification of Debris Flow

Category Description Schematic Drawing

Fluidisation of Fluidisation of sediment and gravel deposited on the bed of

deposited sediment and a steep stream due to the supply of a large quantity of water
Sediment deposited
gravel on the riverbed due to a downpour or rapid thawing on the riverbed

Road

Fluidisation of Fluidisation of sediment produced by a hillside failure as

sediment produced by the structure is broken up while sliding down the slope and Hillside landslide
a hillside failure mixed with water

Failure
Collapse by natural Fluidisation of sediment forming a natural check dam(s) Road
Natural check dam formed
check dam which is created by the blockage of a mountain stream by by deposited sediment
originating from a failure
failed sediment and which is then eroded by overflow or
which collapses Road

Landslide

Fluidisation of mass Fluidisation of a cohesive soil mass with a high water

movement soil mass content originally produced by a landslide

Road

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In the event there has been no rainfall that has caused debris flow
A2-4.2 Survey on Occurrence of Debris Flow in the relevant area, the maximum amount of rainfall at which
(1) Identification of Locations where Debris Flow may occur such debris flow did not occur shall be used as the temporary
In depressed topography to which rain water gathers, in rainfall conditions on the occurrence of debris flow, and other
mountain streams where water flow exists during rainfall, for information such as topography and soil properties should also
mountain streams with 15 degrees or more gradient of the river be used in the determination.
bed and more than 5 ha of catchment area upstream from the
point at which the gradient of the river bed is 15 degrees, with
consideration focused on cases in which there is sediment at the
river bed that could become debris flow, locations at which such
debris flow may occur need to be investigated.
Moreover, catchment areas of less than 5ha upstream from a
point at which the gradient of the river bed is 15 degrees where
relatively large failure at a mountainside can be expected due to
the soil properties, spring water or history of failure may be
considered as areas where debris flow can be expected. The
above survey should be undertaken through rough interpretation
of topographic map and aerial photographs firstly and should be
revised through site reconnaissance.

(2) Estimation of Frequency of Debris Flow

Existing materials on disaster records, hearings and site
reconnaissance should be utilized to determine the times and
frequency of occurrence of recent debris flow in the relevant
mountain stream.

(3) Estimation of Rainfall Conditions Causing Debris Flow

Areas should be divided based on similarity of rainfall conditions,
and materials on rainfall that caused debris flow in the past,
heavy rains that did not lead to such occurrence should be
collected in order to determine the distinction between rainfall
conditions that lead to such debris flow and those that do not
lead to such debris flow.

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A2-4.3 Survey on Estimation of Scale, Character and also begin where the width of the flow suddenly increases and
Inundation Area of Debris Flow the gradient of a river bed is 15 degrees or less, where the width
(1) Estimation of Scale and Character of Debris Flow of the flow becomes narrower or where the gradient of river bed
In order to review the scale and positioning of countermeasures suddenly becomes gentle.
for debris flow, the scale of the debris flow (discharged soil In general, most mountain stream has experienced debris flow in
volume), peak amount of discharge, the flow velocity and other the past. For this reason, when the exit of a valley is investigated,
such factors need to be estimated. it is possible to know what scale of debris flow with what
Estimation of the discharged soil volume and the maximum grain diameter of gravels and what extent of discharge and
size of debris flow may be undertaken by investigating the sedimentation have occurred in the past.
volume of river bed sedimentation and grain size distribution
through site reconnaissance, but the peak discharge, the flow
velocity and unit weight of debris flow are estimated with
reference given to experimental and theoretical research on
debris flow and empirical formulae obtained from field
observation of debris flow.
The flow velocity of a debris flow ranges from several meters to
10 meters per second in the case of flow having large gravel
content (gravel type debris flow). In debris flow in volcanic
areas (mud flow type debris flow), depending on water content,
the velocity may range from small up to 15 to 20 meters per
second. Moreover, with respect to the wave height of a debris
flow, the height is generally 3 meters or less. If there are records
of debris flow that have occurred close to the targeted mountain
stream, such materials should be used as reference in making
estimations after confirming the similarity of the topography and
geology.

(2) Estimation of Inundation Area of Debris Flow

The debris flow area and its inundated area downstream from the
point at which such flow may occur are estimated. Debris flows
are assumed to become sedimentation covering the flat land at
the bottom of valleys and alluvial fans in the present topography.
While in general, the point at which sedimentation begins is
assumed to be the exit of the valley, partial sedimentation may

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A2-5 OTHER INVESTIGATION A2-5.1 Geophysical Survey
Based on the site reconnaissance, plan of the detailed study shall When compared with boring investigation providing spot data,
be determined. geophysical survey is more advantageous for investigation of
Boring investigation is effective for clarification of the geology slope failure requiring urgent countermeasure in its ability to
and geological structure. Boreholes can also be used as survey obtain data on underground structures over a wide area within a
holes for observation of groundwater and as dynamic relatively short period. However, data obtained from geophysical
observation hole to estimate the collapse depth. On the other survey remain to be specific physical properties of the ground.
hand, geophysical survey proves effective for investigation of the Besides, the resolution of geophysical survey varies depending
degree of weathering and looseness of rock mass and, when used on geological conditions and operational restrictions. Therefore,
together with boring investigation, enables understanding of the interpretation of the geology and geological structure is essential
sectional structure of unstable slope. In any case, transfer or by geophysical engineers. In order to implement geophysical
temporary set up of the machinery in the site of steep slope with survey, objectives should be clearly defined and the appropriate
possibility to collapse is mostly not easy. Therefore, the study method should be selected according to the applicability of each
plan must be developed taking into account the applicability of physical quantity and their limit for each objective. After
investigation methods to the site concerned and investigation implementation, data obtained from site reconnaissance and
costs. As described above, the necessity to implement Detailed boring investigation are combined for overall geological analysis.
Study should be determined after thorough review of above General practices applied to the site investigation of road slope
factors. are seismic prospecting (refraction method) and electric
This chapter introduces the following survey methods as exploration (resistivity method). Their planning, implementation,
standard approaches in Detailed Study. Specific principle and and analysis that require attention are described below.
procedure of each method are described in ASTM and technical
(1) Seismic Refraction Survey
documents, so that this chapter deals with considerations mainly
for implementation. Considerations for operation of seismic refraction survey on
the road slope
i) Normally, dynamite is used to generate the elastic wave. The
Boring investigation
stacking method is less applicable in sites where the arrival
Geophysical survey Seismic refraction survey distance of wave is short, the operation efficiency is
Resistivity image profiling deteriorated excessively, and there is much noise in the
Others Sounding neighbourhood of the road. Because of considerable time
required for the application procedure to obtain approval for
Sampling
the use of dynamite, it is essential to take the careful
Laboratory test preparatory steps. The person in charge of handling of
In-situ test and borehole logging explosive must take the responsibility everyday for transport

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from the storage place to the site, storage, and consumption. vii) Analysis is difficult in locations with complicated
Therefore, the operation schedule must be determined taking geological structure.
into account the process of this responsible person. viii) The velocity varies greatly even when the degree of
ii) Generally, site conditions of the slope cause the time for weathering or looseness is almost equal if the
transfer of persons and material/equipment. To ensure the groundwater exists or the condition is dry.
efficiency and safety, preparatory in-depth negotiation and ix) The rate of propagation is in almost the same order
mutual understanding of case-by-case actions are essential among soft rocks, fracture crushed zone, and weather
among operation team members. rock, making their identification impossible if the line length
iii) Depending on objectives (depth, etc.) of investigation, is short.
blasting should be made with the amount of charged x) In order to estimate the nature of crack in rock mass and
explosive appropriate to the geological condition of the that of fracture crushed zone from the elastic wave
section covered by observation. It is essential that the velocity, analysis considering geological observation results
blasting plan is developed by the well-experienced is necessary.
prospecting engineer. Since blasting in water is more xi) Values obtained from seismic prospecting are obtained on
effective than in soil, blast points should be provided in the the basis of assumption that the rock is an elastic
valley as far as site situations allow. During blasting, utmost body. Therefore, they do not necessarily indicate conditions
care must be taken to prevent scattering stones from contributory to stabilization of the slope.
damaging properties in the neighbourhood.
(2) Resistivity Image Profiling (computer controlled multiple
Cautions for application of seismic refraction survey on slope electrode technique)
stability Conventional horizontal electrical profiling and vertical electric
i) In V-shaped valleys and steep slops, the depth of sounding are applicable to a location that is symmetrical in terms
weathered layer and bed rock tends to be estimated of topography and geology and as much flat as possible. In line
shallower than practical. with the progress of exploration and computer technologies, the
ii) Any hard (rapid in propagation) rock body or boulders two-dimensional exploration method has recently been applied in
projecting from the slope or natural slope may readily increasing numbers. This method proves more favourable and is
cause misunderstanding of the structure of velocity layer. expected to enable analysis with higher accuracy in investigation
iii) Any lower layers with low velocity cannot be detected. of structures more complicated in topography and geology.
iv) Intermediate layers may not be detected when thin. Resistivity of the ground varies depending on the component
v) It is essential that the line length is five- to six-fold of the rocks or grain composition of layer, void ratio, resistivity of pore
prospecting depth. water, porosity, and content of conductive materials (clay
vi) Low-velocity layers (fault, fracture crushed zone) are minerals, etc.). Accordingly, this method produces wide-varying
difficult to detect when their thickness is 3 m or less. resistivity values even when the layer is uniform geologically.

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The two-dimensional exploration method consists of determining concerned in the length larger by 1/2 to 1-fold than the
the true resistivity distribution under the ground in the form of a exploration depth.
two-dimensional sectional view through computer-aided inverse ii) Analysis error can be decreased by providing lines normal to
analysis of measurement data obtained with electrodes only contour lines.
arranged on the surface line. A method performing measurement iii) When a remote electrode is provided in the pole-pole array
enclosing the investigation area with boreholes or adits is also method, it is recommended to distance such electrode from
called a resistivity tomography, which is expected to produce the line terminal by more than ten times the maximum
results with much higher accuracy. electrode spacing. In the case of four-pole method, the
remote electrode is not necessary. In any case, minimizing
the earth resistance of electrode will contribute to
enhancement of the exploration accuracy.
iv) The resolution of a structure that can be analysed is
approximately equivalent to the minimum electrode spacing
around the ground surface and is deteriorated as the depth
increases.
v) Because of noise contained in measurement data and
deterioration of the analysis accuracy (reliability) around the
section concerned, a ghost of underground structure that
actually does not exist under lines may appear in the analysis
Figure A2-5.1 Example of analytical results of the resistivity section. This must be taken into account during
imaging method
interpretation of analysis section.
vi) The performance of instruments varies greatly, so that the
Considerations for operation of the resistivity image profiling appropriate one must be selected according to the target
on the road slope exploration depth.
i) The electrode spacing (minimum electrode), exploration vii) To insert electrodes into the ground, smooth insertion should
depth, and line length are to be planned with due be ensured without resistance through contact with the
consideration of objective of the investigation and site ground. When the ground surface is dry sand, water should
conditions. The electrode spacing is related to the resolution be sprayed around electrodes.
of exploration. Namely, smaller the spacing, the higher the
resolution is. Normally, the spacing is set to about 1/10 to Cautions for application of resistivity image profiling on
1/15 of the exploration depth, and the arrangement of slope stability
electrodes should be appropriate to the targeted exploration
i) Investigation is made on the apparent resistivity distribution
depth. Because of the principle of exploration and analysis,
of the ground, determining the thickness of clayey soil and
it is necessary to set lines on both sides of the section

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gravel layers and weathered zones as well as the shape of A2-5.2 Other Site Investigations
bedrock. Based on the result, judgment is made on the The standard investigation conducted in Detailed Study is
thickness and pattern of landslide layer. Besides, this method classified roughly into three types. They are 1) sounding, 2)
is often applied to detect high-velocity and low-velocity laboratory test of samples, and 3) in-situ test/borehole logging.
seams that are difficult to recognize by seismic prospecting. Considerations for planning and implementation of sampling,
ii) The accuracy of the method is deteriorated when there is a
soil test, and in-situ test are described below.
transmission line or railway near the lines. Any location with
metallic buried structures or the underground cable causing Sounding
the stray current should be avoided because they may cause i) Sounding is a simplified method to investigate the soil
decisive adverse effects on measurement. composition and characteristics. Except for the case of
iii) Electrical exploration should be combined with other standard penetration test, direct observation of core cannot be
approaches, such as the seismic prospecting or boring made. This enables simple and inexpensive investigation and
investigation, to ensure overall interpretation of the geology. supplements the boring investigation.
ii) Standard Penetration Test (SPT) : When the core recovery of
all-core boring to search the slide surface is extremely low
during boring, the standard penetration test is made in a 50
cm pitch, taking samples from all of layers in the depth
direction for confirmation.
iii) Dynamic probing (Macintosh test) : This test is extremely
simple with a lightweight (total weight of 10 kg) instrument.
Since penetration is manual, its application is limited because
the N value of applicable soil is 5 or less. As rapid
investigation of about 3 to 5 m is possible, the instrument
should be carried around during survey.
iv) Swedish sounding (Weight sounding test) : This type of
sounding is applicable to all soils not containing boulders and
gravel. The limit depth is about 10 – 15 m. As the instrument
is as heavy a 100 kg in total weight, this is not suitable for
transport and test on the slope.
Sampling
i) The prerequisite for stable analysis of the slope is obtaining
of the undisturbed sample.
ii) In the site where disaster occurs actually on the road slope, it
is extremely difficult in many cases to take the sample in an

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undisturbed condition for the strength test from boreholes by constant necessary for design of the bearing capacity and
using the sampler. prevention piles.
iii) For soft heavily-weathered rocks or loose rock mass or layers ii) Borehole television and televiewer:
with many fractures where the core recovery with normal Boring is generally sampled in the disturbed condition from
core barrel (Double core barrel or Triple core barrel) is the deteriorated rock mass portion. When a borehole camera
extremely deteriorated, Mazier sampler is used. is used, the condition before disturbance can be observed.
iv) Where the shear test for stable analysis is necessary on The camera enables direct observation of the directionality
relatively soft soil that does not contain any large gravel, the and dirt nature of the weak layer and slide surface that exerts
thin-wall sampler or Denison type sampler is applicable. critical effects on the stability of slope. Therefore, this is an
v) When sampling using test pits, adits, and shafts is possible, effective investigation method for clarification of the rock-
undisturbed samples can be taken. If possible, in-situ tests mass fracture mechanism. However, this is expensive and
and laboratory tests using samples thus taken should be made. not yet easily applicable in Malaysia though the price has
Laboratory test lowered in these days. Besides, the site develops jamming in
i) It is relatively rare to use values determined from the soil test borehole readily as described above. Consequently, this
as they are in the design of countermeasure. Therefore, there method should be planned according to the importance and
is not much necessity to perform this test. necessity of investigation.
ii) In the Detailed Study on landslide to estimate the shear iii) Electrical methods:
strength of slide surface, the triaxial compression or box Boring is basically all-core boring. Because of low core
shear test using undisturbed sample is made if necessary. recovery, actually, geological information may remain
iii) The objective of the test is to determine the basic nature of insufficient in spite of boring investigation. In such a case,
soils that make up the slope. Using samples from boring and borehole logging (electric type, etc.) will prove effective in
standard penetration test, tests for physical properties, such determining geological characteristics. Electric borehole
as natural water content and grain size analyses, are made for logging can be made from the inside once the PVC pipe with
each layer. slit is installed in boreholes. Besides, this method is free
from jamming concerning collapse of hole wall as described
In-situ test and borehole logging
above and thus useful. Boring diameter should be
Grounds developing landslide are mostly exposed to noticeable determined after thorough review on the outside diameter
loosening and lateral pressure, which often causes jamming of and length of applicable logging probe, borehole diameter
core tube and instrument in boreholes. Safety measures such as and PVC pipe inside/outside diameter allowing insertion
protection of hole wall with a casing, etc. are necessary. after drilling, verticality of borehole, and whether or not the
i) Pressuremeter test: slide soil mass is in motion.
Depending on the type of countermeasure, the borehole
horizontal loading test is made to determine the ground

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