PRE-EXCAVATION GROUTING

I. Introduction

The excavation of a tunnel always involves risks of unpredicted ground conditions, including
the chance of meeting a body of high-pressure ground water. Ground water can cause problems to
the tunnel and the surrounding area, even only in smaller volumes, creating ground water leakage
and seepage. These problems can be maintained using drainage and grouting.

Grouting is the process of injecting grout materials into a borehole with the purpose of
sealing fissures and joints that intersects with the borehole. The most common grouting material is
based on cement (Emmelin et al., 2007). Other material used in grouting is micro-cement and
colloidal silica. The grout slurry is a mixture of cement and water, it may contain additives such as
superplasticizer to control its viscosity.

Grouting can be done in two different methods, post-grouting and pre-grouting. Post-
grouting, or post-excavation grouting, is done in an excavated opening. While pre-grouting, or pre-
excavation grouting, is done ahead of the face. In general, pre-grouting is easier than post-grouting,
because the difficulty and cost implications of handling a significant water inflow in an already
excavated tunnel can be extreme compared to the cost of the probe drilling and injection works
carried out ahead of the tunnel face.

In grouting, using high pressure injection may be fundamental to a good result, i.e. much
reduced water inflow and improved stability. The reasons for performing high pressure (50 to 100
bar) injection when pre-grouting ahead of tunnels is that grout pressure ground water pressure have
to be controlled. It is necessary to monitor the intended pressure and volume of the grout slurry in
order to get a good sealing of ground water around the tunnel. There are several criteria used for
deciding when to stop grouting in order to have the intended grouting results, such as stop criteria
(Houlsby, 1990). Stop criteria limits the pressure, volume, and flow of grouting. In order to control
fracture deformation, Lombardi and Deere (1993) suggested to consider a combination of injection
pressure and injected grout volume (The Grout Intensity Number or GIN-value).

II. Material microcement with a w/c ratio of 1 will

give the same marsh funnel time, but
The most common material used
a bleeding of only 0-2%.
in grouting is cementitious material.
Generally, for the grout mix, w/c ratio The properties of grout can be
starts at 3:1 and is gradually changed with different kinds of
thickened to 2:1, 1:1 and 0.5:1. The additives, such as superplasticizers
w/c ratio should be as high as possible and accelerators. Superplasticizers are
without giving too much bleeding. used to control grout viscosity,
Bleeding should be as low as possible, flowability, and dispersion.
around 2-5% bleeding maximum. A Accelerator is added to the grout in a
normal fine cement with a w/c ratio nozzle on the injection lance. In this
of 1 will give a marsh funnel time of way, it is possible to adjust the open
about 33 seconds and a bleeding of time of the grout to only around 5-20
around 10-15%. While a good minutes.
 The particle size of the grout IV. Methodology
material is selected based on the
According to Houlsby (1990), to
fissure size in the rock mass of the
perform grouting work, a set of main
tunnel. Generally, to achieve cost
steps should be followed:
effectiveness, locally available
Ordinary Portland Cement (OPC) is a. Investigation: Geology of the area and
used. This cement has an average permeability situation are investigated.
particle size of 45 μm. Micro-cement Spacing, width, inclination of joints, and
is used on fissures with a smaller size. strength of rock are among the important
Micro-cement is finely grounded geological factors. Knowledge of
ordinary cement with a large surface permeability is important for controlling
area and fine particle size of about 15 seepage. Lugeon test is the most
μm. As an alternative to cement- common way to quantify the
based materials, various solutions permeability of rock mass. The most
have been used as grouting materials important outcome of this investigation
such as colloidal silica. Colloidal silica is size of cracks.
is an aqueous dispersion of silica b. Design of grouting: Spacing and length of
particles, with a particle size of less boreholes, properties of required
than 0.015 μm. A schematic material, and injection pressure are
illustration of different cement decided using the data obtained from
particles (µm) with respect to rock investigation.
aperture is portrayed in Figure 1. c. Execution: The main work in this phase is
preparing machineries and facilities,
preparing grout mix material, drilling
boreholes, and injecting grout material
into fissures and fractures of the rock
mass.
d. Compilation: Achieving maximum certain
pressure, maximum certain injected
volume or production of pressure and
volume (P.V) are among popular refusal
points. Based on theories developed by
Gustafson & Stille (2005), it is possible to
Figure 1. Particle size distribution (Subash et estimate penetration of the grout mix
al., 2016) analytically in real time, thus required
Microcements for injection penetration length can be set as the stop
should be as fresh as possible, it criterion.
should not be older than 6 months. If e. Assessment: The procedure should be
the cement is too old, it will create assessed to identify if grout has spread
more bleeding, slower setting times, enough around the borehole and not
and a higher risk of cement lumps. more than required. The other
Also, cement should be stored in considerable issue is the stresses induced
ventilated rooms, not in the actual to the rock mass due to grouting.
tunnel. Because, the humidity in the Prolonging the procedure, the induced
tunnel will get to the cement, excess pressure may lead to larger
resulting in a pre-hydrated cement. deformation and in worse case cause
damages to the ground structures. Also,
III. Equipment the increase in the size of fracture
 aperture will lead to lower penetration
length than expected, which will affect
sealing efficiency of the grouting work.

IV.A. Execution

Pre-grouting is commonly done

from inside the tunnel. The drilling
and injection method of grouting
depends on the site condition. In
general, grouting methodology is
carried out as follows (Subash et al.,
2016):
Figure 2. Pre-grouting Methodology (Subash
1. Drilling of 40-75mm diameter hole to et al., 2016)
required length and inclination as per the
The effectiveness of pre-grouting
site condition. The pattern and spacing of
execution is controlled by some
the grout holes will be based on
parameters, such as: grout design,
groutability test.
including number of probe holes;
2. Installation of a pipe with internal
grout hole pattern and spacing;
diameter to fit the expendable packers.
overlap length; grout materials and
3. Placement of packer at the very end of
grout properties; and grouting stop
the pipe and injection of a cement-based
criteria.
grout which fills the space between the
rock and the pipe. Hardening for about 12
hours.
4. When the grout is hardened, drilling
through the pipe to feasible length.
5. Placement of packer and pressure
injection with appropriate cement grout
for penetration into the rock mass in
drilled length of the hole. Stop criteria as
per grout mix design.
6. After hardening of the injected grout, re- Figure 3. Grout Screens with Overlaps
drilling through the pipe and injected area (Association of Geotechnical &
to design length beyond last drilled Geoenvironmental Specialists, Hong Kong)
length.
Figure 3 shows an illustration of a
7. Placement of the packer in the pipe and
typical grout screens with overlaps
inject (repetition of step 5).
around an underground space. The
screen also has to cover the invert,
because residual leakage in the invert
is more difficult to observe as well as
presenting problems for drilling for
post grouting.

Grout hole length is dependent

on the excavation diameter/span,
usually around 15 m - 25 m with a
grout hole centre to centre spacing of
 1 m - 1.5 m at collaring point, and a between subsequent grout fans
lookout distance at the end of the should be 5 m or more. In extremely
grout holes of 3 - 5 m from the poor ground up to 10 m may be
theoretical tunnel surface. Control required. A large face area will require
holes are usually located within the more overlap than the face area of a
grout screen or even within the small tunnel.
excavation profile. The overlap

Figure 4. Typical Grout Pattern (Association of Geotechnical & Geoenvironmental Specialists, Hong
Kong)

It has been found from recent Norwegian Figure 5. Sources of Pressure Drop (Barton,
tunnelling projects that high pressure pre- 2003)
injection may be fundamental to a good
One of the problems in grouting is high
result, i.e. much reduced inflow and improved
conductivity contrast. To reduce the effect of
stability (Barton, 2004). The reason for this is
high conductivity contrast, dual stop criteria
that while grout is still flowing, there is a
(pressure or volume) needs to be applied.
gradient of pressure from the start of the
Using stop criteria limits the grout material
injection holes to its end. Using high-pressure
consumption, while still achieving sufficient
grouting will minimize the risk of this pressure
grout penetration and distribution.
loss, which then leads to effective grouting.
IV.B. Stop Criteria

During grouting execution, it must be

decided when to stop. The criteria for this are
called stop criteria. There are several types of
stop criteria that can be used (Emmelin et al.,
2007):

 A maximum grouting time, where

grouting is stopped when it has
proceeded for a given length of time.
 A maximum grouting volume, where
grouting is stopped when a certain
volume has been injected.
 A minimum grouting flow, where Figure 6. Example of Grouting Path Curves
grouting is stopped when the grouting (Lombardi and Deere, 1993)
flow is lower than a certain value.

In order to control fracture deformation,

Lombardi and Deere (1993) suggested to
consider a combination of injection pressure
and injected grout volume (The Grout
Intensity Number or GIN-value) in addition to
the previously set stop criteria by Houlsby
(1990). This means that with respect to GIN
number, the maximum applicable pressure
should be decided by considering the amount
of injected volume (Rafi, J., Stille, H.,
Johansson, F, 2016)
Figure 7. Proposed Limiting Envelopes for
IV.C. Grout Intensity Number (GIN) Grouting (Lombardi and Deere, 1993)
Lombardi and Deere (1993) suggest that Figure 7 illustrates five suggested limiting
in a given grouting interval, the energy envelopes for different intensities for
expended is approximately proportional to grouting. The uppermost envelope represents
the product of the final grouting pressure p a very high intensity of grouting with a GIN
and the grout volume injected V, giving a value of 2500 bar.l/m, a very high limiting
product p.V (bar.litres/m). This p.V number is pressure of 50 bar, and a very high limiting
called the Grouting Intensity Number (GIN). volume of 300 l/m. The lowest envelope
represents a very low intensity, with a GIN
By keeping the GIN value constant during
value of 5000 bar.l/m, a maximum pressure of
the grouting process for all grout intervals, a
15 bar and a limiting volume of 100 l/m. For
nearly constant reach of the grout will be
most conditions, it is recommended using the
obtained. It will also limit the volume in an
moderate intensity envelope with a given
open, wide fissure, but allowing pressures to
value of 1500 bar.l/m, a limiting pressure of
increase in zones of tighter and less groutable
30 bar, and a limiting volume of 200 l/m
fissures. Combinations of high pressure and
volume are eliminated, a condition that would The complete limiting envelope thus
only lead to huge uplift or hydro splitting consists of the limiting pressure line (in the
forces. Similarly, combinations of low range of 15 to 50 bar), the limiting volume line
pressure with a low take are eliminated, that (in the range of 100 to 300 litres per meter of
would inadequately grout the fine fissures grouted interval), and a selected GIN curve
connecting the two limit lines (with GIN values
ranging from very low to very high, for
example, 500 to 2500 bar.l/m).
 Figure 8. Grouting Process of a Single
Borehole Stage (Lombardi and Deere, 1993)

The GIN method requires close

monitoring by PC graphics of real-time curves
of pressure versus time, grout flow rate versus
time, grout flow rate versus time, and total
injected volume versus time, plus the derived
curve of penetrability (flow rate divided by
pressure, q/p) versus time. This latter curve is
of particular value in signifying the approach
of grouting refusal, or at least of diminishing
absorptions.

Figure 9. Real Time Curves of a Single Stage Grouting Process (Lombardi and Deere, 1993)

V. Quality Control moulds and stripped and immersed in lime

water until tested.
V.A. Compressive Strength
V.B. Shrinkage
This test method covers determination of
This test method determines the change
the compressive strength of hydraulic cement
in length on drying of mortar bars containing
mortars, using 2 inch or 50 mm cube
hydraulic cement and graded standard sand.
specimens. The mortar used consists of 1 part
This test method establishes a selected set of
cement and 2.75 parts of sand proportioned
conditions of temperature, relative humidity
by mass. Portland or air-entraining Portland
and rate of evaporation of the environment to
cements are mixed at specified water/cement
which a mortar specimen of stated
ratios. Water content for other cements is
composition shall be subjected for a specified
that sufficient to obtain a flow of 110±5 in 25
period of time during which its change in
drops of the flow table. Two inch or 50 mm
length is determined and designated “drying
test cubes are compacted by tamping in two
shrinkage”.
layers. The cubes are cured one day in the
V.C. Bleeding ASTM C596. Standard Test Method for Drying
Shrinkage of Mortar Containing Hydraulic
This test method determines the amount
Cement.
of expansion and accumulation of bleed water
at the surface of freshly mixed hydraulic- ASTM C940. Standard Test Method for
cement grout used in the production of Expansion and Bleeding of Freshly Mixed
Preplaced-Aggregate (PA) concrete. Grout is Grouts for Preplaced-Aggregate Concrete
placed in a graduated cylinder. Changes in in the Laboratory.
total volume and accumulation of bleed
ASTM C953. Standard Test Method for
water, if any, on the surface of the grout are
Expansion and Bleeding of Freshly Mixed
observed over a period of time.
Grouts for Preplaced-Aggregate Concrete
V.D. Setting Time in the Laboratory.

This method covers the determination ASTM D6910. Standard Test Method for
test of time of setting of hydraulic cement Marsh Funnel Viscosity of Clay
grout mixtures used in Preplaced-Aggregate Construction Slurries.
(PA) concrete using the Vicat apparatus. The
Barton, Nick. 2004. The Why’s and How’s of
time of setting, initial and final, of a sample of
High-Pressure Grouting. Tunnels and
fluid grout is determined using the Vicat
Tunnelling International: 28-35.
apparatus.
Emmelin, et al. 2007. Rock Grouting: Current
V.E. Flowability (Marsh Funnel)
Competence and Development for The
This test method provides an indirect Final Repository. Stockholm. Svensk
measurement of the viscosity of clay slurries Kärnbränslehantering AB.
using a funnel (Marsh Funnel) and a
Lombardi, G., Deere, D. 1993. Grouting design
graduated cup of specific dimensions. This
and control using the GIN principle.
test method provides a practical indicator of
International Water Power & Dam
the viscosity on a routine basis. The result
Construction.
determined using the method is referred to as
the Marsh Funnel Viscosity. Melbye, T., Hognestad, H., Dimmock, R. Pre–
Injection Technology for Mines.

Subash et al. 2016. Pre-grouting for Leakage

VI. References
Control and Rock Improvement. J Civil
Association of Geotechnical & Environ Eng Volume 6 Issue 3.
Geoenvironmental Specialists (Hong
Kong). Tunnel Construction Guidelines 05
- Guidance Note on Pre-Excavation
Grouting for Underground Construction
in Hard Rock – Principles & Design
Elements.

ASTM C109. Standard Test Method for

Compressive Strength of Hydraulic
Cement Mortars (Using 2-in. or [50-mm]
Cube Specimens).