Supported Excavations

Sheet Pile Retaining Walls

and
Bracing Frame Retaining Walls
 Types of Retaining Wall

• Gravity walls

• Embedded walls
 Embedded walls

• Steel Sheet Pile construction

• Soldier/King pile construction

• Bored Pile construction

• Diaphragm wall construction

Soldier/King pile walls
Bored Pile walls

Diaphragm walls
 Types of embedded Sheet pile walls
Sheet walls may be split into 3 groups, each with its
separate method of analysis. The groups are
• Cantilever wall
• Walls with a single strut or anchor (anchored or tied wall)
• Walls with multiple struts (braced or propped wall)
 Extent and depth of investigation

For piling work, the number of boreholes, or other form of

investigation, should be adequate to establish the ground
conditions along the length of the proposed piling and to
ascertain the variability in those conditions. The centres
between boreholes will vary from site to site but for retaining
wall structures should generally be at intervals of 20 m to 200
m along the length of the wall. Closeness of position to the
proposed pile line and spacing is particularly important for
river walls and where glacial deposits with a high degree of
variability prevail. For embedded sheet pile walls particular
attention to ground levels and the position of the boreholes is
important for the relevance of information for the designer.
 Geotechnical parameters
Suitable parameters for use in the geotechnical design of embedded
retaining walls have to be selected:
• Derived values: obtained by theory, correlation or empiricism from
test results”. Test results may be converted into derived values by
use of correlations (such as that between SPTs and UU triaxial
tests; cone penetration resistance and angle of shearing resistance

• Characteristic values: as “a cautious estimate of the value affecting

the occurrence of the limit state”. A cautious estimate is an
approximate calculation or judgement that is careful to avoid
problems or dangers.

• Conservative value: of the properties of the soil as it exists in situ...

properly applicable to the part of the design for which it is
intended”
 Soil types and descriptions
Design and construction of sheet piles requires correct description
of the soil types:

1. coarse grained - cohesionless soils: granular materials such as

sand, gravel, weathered rock, filling etc.;

2. fine grained - cohesive soils: clays and silts. Under certain

conditions chalk and other similar materials can be treated as
cohesive soils;

3. mixed soils: combinations of groups 1 and 2 such as sand with

clay, or sand with silt.
 Ground model
A ground model from the ground investigation showing the strata
levels is needed for geotechnical design.

Typical Cross-section

Tabulated
characteristic
values
Typical properties of coarse grained soils
Typical properties of fine grained soils
 Chemical Analysis
Influence of corrosion on the durability of steel needs to be
thoroughly identified in the ground investigation report (GIR).
Although the chemical analysis of soil and leachates may be
provided in the GIR - expert interpretation for the durability of
steel may be required for the design of embedded steel piles in sites
polluted by industrial waste against:

1. Loss of section thickness due to corrosion versu max Bending

Moment.

2. Also in this instance, so that the correct decisions for selection

of appropriate sealants for watertightness performance and
protective coatings can be taken by the designer for the
durability required
 Groundwater and seepage
Measurement of groundwater conditions, the level of the
water table, and their variation with time is a vital part of any
site investigation. The effect that water has on the engineering
properties of soil should be clearly understood and carefully
considered during the site investigation period. In addition to
the tests on individual soil samples, the direction of seepage,
upwards or downwards, should be determined before any
decision is reached on the design of a sheet piled retaining
wall together with a system incorporating reliable drainage.
Information required for design of embedded sheet pile
walls
Having determined the nature of the ground within the site from the
Ground Investigation Report and ascertained the individual soil
properties, it is desirable to release certain basic information to the
piling designer to ensure the best possible arrangement in terms of
strength and economy. The minimum details should include the
following:
• historical records covering the previous development of the site,
particularly the location of old foundations and other buried
structures;
• copies of relevant site drawings showing the projected retaining
wall / site boundaries and proximity of waterways, buildings, roads
and services;
• environmental restrictions – noise and vibration if relevant;
• surcharge and loadings temporary and permanent;
• serviceability limitations;
Information required for design of embedded sheet pile
walls
• durability or design life of structure;
• fire resistance requirements;
• sustainability issues for selection of materials; • details of ground
water levels, flooding and tidal range;
• clear brief on stage construction, design excavation levels or
design bed or dredged levels and profile of submerged ground levels
where relevant;
• design wave levels and berthing loads for Marine structures and
information relevant to design to BS 6349-2 [xv], requirements for
protection of steel or cathodic protection and maintenance
preferences for instance;
• information pertaining to control of watertightness for sealed
walls;
• requirements for impermeability performance or seepage for flood
control.
Failure mechanisms of Embedded Retaining Walls

• Collapse of side walls due to rotation.

• Heave due to water pressures.

• Seepage carrying fines into base of excavation.

• Global failure resulting from deep-seated slip failure of the

soil and ensure that the proposed pile toe passes through
the critical slip plane.

• Anchor failure: anchor walls should be located outside

potential slip planes.
 Performance of Sheet Retaining Walls
For steel sheet pile walls durability and driveability in the ground
conditions are an important feature of the design. EN 1990
requires structures to be designed to sustain all likely actions and
influences likely to occur during their execution and use and to
remain fit for use.
For all sheet walls the following must be considered in design and
must have adequate:

• Resistance: The overall stability of the soil/wall system

• Durability: The structural strength of the wall

• Serviceability: The possibility of damage to adjacent structures,

and services in the ground, due to wall construction
 General design considerations
An earth retaining structure must be designed to perform adequately
under two particular sets of conditions:
1. those that can be regarded as the worst that could conceivably
occur during the life of the structure (ultimate limit state, ULS)
and
2. those that can be expected under normal service conditions
(serviceability limit state, SLS).

ULS: include instability of the structure as a whole including the soil

mass, failure of the structure by bending or shear and excessive
deformation of the wall or soil to the extent that adjacent structures
or services are affected.
SLS: involves a consideration of the deformation of the structure and
movement of the ground to ensure that acceptable limits are not
exceeded.
 General design considerations
The design situations should include the following where appropriate:
1. Applied loads and any combinations of loadings.

2. Geometry of the problem including unexpected changes to height

between the surface and excavation base levels.

3. Material characteristics.

4. Groundwater variations.

5. Environment and installation.

Rankine Lateral Earth Pressures
 Surcharge, Concentrated and Linear loading
It is common in the UK to design embedded retaining walls to withstand a minimum
10 kPa uniform distributed load surcharge acting behind the wall, however, the
following methods are recommended for assessing the additional horizontal earth
pressures that bear on a sheet pile wall owing to the presence of a selection of surcharges
with finite dimensions
 Rankine Lateral Earth Pressure (LPE) on
Cantilever Walls

Direction of
wall movement

Excavation

Active pressures

Passive
pressures
 Rankine Active and Passive pressures
Direction of
wall movement

 ´v
Active

´h

 ´v
Passive

 ´h

Assumptions:
- Wall is frictionless
- Principal stresses are vertical and horizontal
Active and Passive LEP
 Rankine Lateral Earth Pressures
Go through this scenario calculations of the LEP for total, water and
soil (effective) pressures as part of your revision knowledge.
 Rankine Active and Passive pressures
For most walls the long term, fully drained, condition
governs the stability.
Use effective stress strength criterion with c’ = 0, f’ = f’cs
The effective lateral stresses on the wall are then

1  sin f 
ACTIVE  h   v  K a  v
1  sin f 

1  sin f 
PASSIVE  h   v  K p  v
1  sin f 
Sheet pile wall penetrating different soils

Supported
Excavation depth, H

Embedment
Embedment depth, d
 Analysis methods
• The Soil Structure Interaction (SSI) calculation of embedded sheet
piles can be very complex due to the amount of data and the
sophistication of the structure. However, modern computer
software packages provide the engineer with the opportunity to
carry out a simple Limit Equilibrium design (LEM) or a
sophisticated Finite Element (FE) analysis.

• When the structure is such that there will be little or no stress

redistribution, as can be expected for a cantilever wall, limit
equilibrium calculations are considered sufficient to calculate
minimum pile length and bending moments.
 Design situations of cantilever wall
Fixed earth conditions:
The assumption of fixed earth conditions is fundamental to the design
of a cantilever wall where all the support is provided by fixity in the
soil. Increased embedment at the foot of the wall prevents both
translation and rotation and fixity is assumed. The stability of an
embedded cantilever wall can be verified by assuming “fixed earth”
conditions. The wall, which is assumed to rotate about the fixed point
“O”, relies on the support of the ground to maintain horizontal and
moment equilibrium.
 Design situations of cantilever wall
Free earth support walls:
A wall designed on free earth support principles can be considered
as a simply supported vertical beam. The wall is embedded a
sufficient distance into the soil to prevent translation, but is able to
rotate at the toe, providing the wall with a pinned support at “O”. A
prop or tie near the top of the wall provides the other support. For a
given set of conditions, the length of pile required is minimized, but
the bending moments are higher than for a fixed earth support wall
 Free Earth Support Walls
• When analysing an anchored sheet piles, a free-earth-
support system is assumed:
• The wall is hinged at its base, hence it can rotate about this
point.
• The pile is rigid.
• Passive soil above the anchor assumed as active
• There will be no reaction and the sheet pile is supported by
the passive pressure in front of the face and the anchor.

Take moments
at the anchor
 Anchored sheet pile wall – effects of Anchors

Free earth support Fixed earth support Fixed earth support

with no reaction with reaction at end of with reaction at both
sheet pile end of sheet pile and
base of excavation
Location of ground anchorage behind the sheet
pile and failure plane
 Some considerations
• The effect of toe fixity is to create a fixed end moment in the wall, reducing the
maximum bending moment for a given set of conditions but at the expense of
increased pile length.
• When a retaining wall is designed using the assumption of fixed earth support,
provided that the wall is adequately propped and capable of resisting the applied
bending moments and shear forces, no failure mechanism relevant to an overall
stability check exists.
• It is important to note that when designing the pile length to free earth support in
the ULS case then in reality in the SLS case a fixed or partially fixed condition
may occur.
• Fixed earth conditions may be appropriate where the embedment depth of the wall
is taken deeper than that required to satisfy lateral stability, e.g. to provide an
effective groundwater cut-off or adequate vertical load bearing capacity.
• However, where driving to the required depth may be problematic, assumption of
free earth support conditions will minimise the driven length and ensure that the
bending moment is not reduced by the fixity assumed.
 Cantilever wall stability

Geometry Pressure Diagram

Active

x Passive
d

Passive
Point of
rotation
Active
 Cantilever wall stability
Design calculations are required to determine the depth of
penetration, d, of the wall.
Because the depth of the point of rotation is also unknown 2
equations are required to obtain a solution.
These are moment and force equilibrium

SF = 0

SM = 0
 Cantilever wall stability

Pressures

hKa gd (xH)
hKpgd x

h Kp gd (xH)

h  Ka g d d h  Kp g d (d  H)
 Cantilever wall stability

Pressures Forces

PA1
hKa gd (xH)
hKpgd x
PP1
h  Kp g d (x  H)

PA2 PP2

h  Ka g d d h  Kp g d (d  H)
 Cantilever wall stability
1
PA 1  K a g d (x  H) 2
2
1
PP 1  KP g d x2
2
1
PA 2  K a g d x (d  x)  K a g d (d  x) 2
2
1
PP 2  K p g d ( x  H ) (d  x)  K p g d (d  x ) 2
2

Force Equilibrium leads to

PA1 + PP2 - PP1 - PA2 = 0

This gives a quadratic equation with terms in x2 and d2

 Cantilever wall stability
Moment equilibrium gives

x  H  d  x  x d  x 
PA 1  3   PA 2  2   PP1 3  PP 2  2 
     

A cubic equation involving terms in x3 and d3

The cubic equation can be solved mathematically using a

calculator or by graphical plotting of assumed x values and
calculated d values.
 Simplified
By making point x as the bottom of the pile thus eliminating one unknown, the system
reduces to this:
Pressures Forces

PA1
hKa gd (xH)
hKpgd x
PP1

Note: with this simplification the calculated x must be increased by

20% to obtain the d i.e. 1.2x
 Cantilever wall limit state equilibrium
By assuming the simplified arrangement of point of rotation, the unknown x can be
calculated by equating the forces and moments in the horizontal direction, re-arrange
the equations and solve for x. Then increase the x by 20% to get the d which is
expressed by this equation:

1.2 H
d
Kp Note: refer to handout notes for full derivation of the d equation
3 1
Ka
1
And since Ka =
Kp
Substituting Ka into previous formula for d can also be expressed as:

1.2 H
d
K p 1
3 2
 Cantilever wall serviceability

• Movements of the wall is critical to the deformation of the

surrounding ground.

• Because of excessive movement of the wall will fail to

meet serviceability requirements well before ultimate
failure.

• It is thus assumed that the pile is rigid i.e. no deflections,

thus only movement is rotation.
 Cantilever wall serviceability
• However, considerable movement of the wall is required to
mobilise the limiting passive stresses
• The movements required to reach the active and passive
conditions depend on the soil type.
• For example, for retaining walls of height H the
movements required are

SAND Active 0.001H

Passive 0.05H - 0.1H
CLAY
Normally Consolidated Active 0.004H
Passive large
Over-Consolidated Active 0.025H
Passive 0.025H
 Cantilever wall serviceability

• Movements of the wall are associated with settlement of

the supported soil

• Because of excessive settlements the wall will fail to meet

serviceability requirements well before ultimate failure

• To control the settlements the earth pressures are factored

• There are two main methods of doing this, based on the

different wall movements to reach limiting conditions.
 Serviceability Design Method 1
Method 1 for sands and normally consolidated clay
The Factored Moment Method (FMM)

• Assume sufficient movement occurs to allow active

pressures to fall to their minimum limiting value

• Factor the effective passive pressures by 2 (i.e. FS=2).

• The pressure diagram looks similar to that for limiting

equilibrium but the passive earth pressure coefficient Kp is
reduced by half
1.2 H 1.2 H
d d
Kp Kp
3 1 3 1
Ka FS p K a
 Design Method 2

Method 2 for over-consolidated clay

The Factored Strength Method (FSM)

• Both active and passive pressures require similar wall

movements and both are factored.
tan f 
tan f 
*

Ff
• A factor is applied to tan f’, so that

• The factored value of f* is then used to calculate new

values of the earth pressure coefficients Ka and Kp
1.2 H
d
K p 1
3 2
 Cantilever wall design – Tutorial exercise
3 o
Consider a wall with H = 1 . 8 , g d = 1 9 k N / m , f ´ = 3 0
For limiting equilibrium method f’ = 30, Ka = 0.3333, Kp = 3
1.2 H
d
Use: Kp and d = ? m
3 1
Ka

For serviceability method 1 (FMM) f’ = 30, Ka = 0.3333, Kp = 3/2

1.2 H
d
Use: 3
Kp
1 and d = ? m
FS p K a

For serviceability method 2 (FSM) F =1.3, f’ = 30, f* = 23.95,

1.2 H
Ka = 0.423, Kp = 2.366
Use: d 
3 K 2 1
p and d = ? m

Q: Which method do we adopt for design?

 Cantilever wall design - example

• For a retained height of 1.8 m a total of 4.2 m (average) of

sheeting is required.

• It is also recommended that the depth of penetration be

increased by 20% to allow for uncertainties in the analysis.

• It is evident that cantilever walls are not suitable for large

heights of supported soil and when settlements in the
surrounding soil must be minimised
 Optimal design of d

• Decision must be made between the FMM and FSM

required.

• Calculate the required passive resistance for horizontal

equilibrium of the sheet pile.

• Calculate the fraction ratio with the available passive

resistance.

• Take the more economical depth d

Cantilever wall - effects of surcharge
s

 v   s  g d z

 h  K a (  s  g d z)
 Cantilever wall - effects of water

Water
Table
Water

Effective stresses must be used when evaluating the lateral

stresses  h  K  v
Pore water pressures will cancel
 Cantilever wall - effects of water

 v   v  u and  h  K  v
Force due to water is now different on the two sides of the wall and
this must be taken into account when considering equilibrium
 Cantilever wall - structural strength
The maximum moment in the wall must be determined to
size the wall thickness

Free body H
diagram of
top of wall

F
M
Maximum moment occurs where F = 0
Structural design of sheet pile sections
• Section classification
• Combined bending, shear, and compression
• Member buckling
• Shear buckling
• Connections and Dimensions of washer plate
• Shear resistance of flange
• Fatigue
• Corrosion
• etc
 Braced excavation

• Recommended for deep excavations • Poor at controlling seepage

• Recommended in sensitive clay • Enhance bottom heave
• Thus used in built environment • Working space obstruction
 Cofferdams
• The purpose of a cofferdam is to exclude soil and/or water from an
area in which it is required to carry out construction work to a depth
below the surface.

• For basement construction the designer should always consider

incorporating the cofferdam into the permanent works.

• Considerable savings in both, time and money, can be achieved by

using the steel sheet piles as the primary permanent structural wall.

• Where control of ground movement is a specific concern the use of top

down construction should be considered. This will ensure that
movement at the top of the wall is restricted with the introduction of
support at ground level prior to excavation starting.

• Further it will also remove the possibility of secondary movement

occurring when the lateral soil loading is transferred from the
temporary supports, as they are removed, to the permanent structure.
Cofferdams: showing spacing of struts
 Cofferdams with unbalanced loading
• Method A – the removal of soil from the landward side;
• Method B – the use of “fill” on the water side of the cofferdam;
• Method C – the use of external anchorages to the landward side;
• Method D – the use of raking struts inside the cofferdam.
Recommended lateral earth pressure diagrams braced excavation

For frictional soils

For cohesive soils (b) OC clay and (c) NC clay

Heave: Base failure mechanism
 Sheet pile driving methods in pictures
• Hydraulic hammer
Circular cofferdam
Driving methods in cohesive soils