Chapter 8-Earth Retaining Structures-13

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Geotechnical and Foundation Engineering – Chapter Eight
CHAPTER EIGHT
EARTH RETAINING STRUCTURES
Learning outcomes:
Upon completing the study of this chapter, students are able to:
1. Identify various types of retaining wall systems;
2. analyse and design common retaining wall systems.
Introduction
Retaining walls (also known as earth retaining structures) are vertical or near-vertical walls
that retain soil or rock (Cudoto 1999). The purpose of a retaining wall is to resist the forces
exerted by the retained soil or rock and to safely transmit these forces to the foundation. The
retaining wall is designed for both strength and deformation (including movement of the
wall). It is important to note that the cost of constructing a retaining wall is high; therefore, it
would be appropriate to consider forming a new slope rather than building a wall if it is
feasible.
A retaining wall must fulfil fundamental requirements of stability, stiffness, durability, etc
during construction and throughout its design life (GEOGUIDE 1). In terms of limit states
design, two limit states must be considered in the design, namely, the ultimate limit state and
the serviceability limit state. According to GEOGUIDE 1, the ultimate limit state is defined
as the state at which a failure mechanism can form in the ground or in the retaining wall, or
severe structural damage occurs in principal structural elements. The serviceability limit state
is defined as the state at which specified serviceability criteria (eg. deformation or movement
of the wall) are no longer met.
Types of Retaining Walls
According to O’Rourke and Jones (1999), retaining walls can be broadly classified as
externally stabilised systems and internally stabilised systems as shown in the figure below.
For this course, we will only concentrate on the externally stabilised systems.
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The terminology for gravity and reinforced concrete retaining walls are illustrated in the
figures below.
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The types of retaining walls are also shown in the following figure (GEOGUIDE 1).
Gravity Retaining Walls
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The gravity retaining walls rely on their masses to resist the lateral force from the soil and
surcharge, if any. The design of gravity retaining walls should consider the:
 Loss of overall stability of the wall (e.g. weak soil under the wall)
 Sliding of the wall due to the lateral load (shear between the wall base and the
soil).
 Overturning of the wall due to the lateral load. (rotation of the wall about its toe)
 Bearing failure of the soil (ground bearing pressure exceeds the load-carrying
capacity of the foundation).
 For the above design, active earth pressure is assumed.
1. Mass Concrete Retaining Walls
Mass concrete retaining walls are one of the simplest forms of retaining wall and
are particular suitable for retained heights of less than 3 m. The concrete used
for the wall must have adequate durability.
2. Crib Walls
Crib walls are built up of individual prefabricated units assembled to form a
series of crib-like structures containing suitable free-draining granular infill. The
crib units and the infill are designed to act together as a gravity retaining wall.
Precast reinforced concrete units are often used to build the crib walls in Hong
Kong.
3. Gabion Walls
Gabion walls are made up of row upon row of orthogonal cages or baskets
(gabions) which are filled with rock fragments and tied together. Gabion walls
are quite easily to construct and they are particularly suitable for river works
since they are quite permeability.
Reinforced Concrete Retaining Walls
A reinforced concrete retaining wall resists bending due to earth pressures from the backfill.
However, the backfill also provides part of the stabilising force by resting on the base slab of
the wall, and hence acts together with the wall as a semi-gravity structure. Schematics of
reinforced concrete retaining walls are shown in the figure below. The design of reinforced
concrete retaining walls should consider the:
 Sliding of the wall due to the lateral load (shear between the wall base and the
soil).
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 Bearing failure of the soil (ground bearing pressure exceeds the load-carrying
capacity of the foundation).
 Structural design of the wall stem and base slab. They should be designed to
resist the bending moments and shear forces due to the earth pressure (at rest)
acting on them.
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1. L or inverted T-shaped cantilever retaining walls

These walls have a vertical or inclined slab monolithic with a base slab. The
stem of these is designed as a cantilever.
2. Counterfort retaining walls

These walls have a vertical or inclined slab supported by counterforts monolithic
with the back of the wall slab and base slab. In designing these walls, the
counterforts should be designed as cantilevers of a T-shaped cross-section and
the wall stem should be designed as a continuous slab.
3. Buttressed retaining walls

These walls have a vertical or inclined slab supported by buttresses monolithic
with the front of the wall slab and base slab. The design of these walls is similar
to that of the counterfort walls.
Cantilevered Retaining Walls
The cantilevered retaining walls are generally sheet pile walls, soldier pile walls, diaphragm
walls and bored pile walls. A cantilevered retaining wall derives its support from the passive
pressure of the soil or rock located in front of the wall. The resistance the lateral soil pressure
is generally developed by the flexural strength of the wall and of course the forces will
eventually be transmitted back to the soil. The design of cantilevered retaining walls should
consider the:
 Structural design of the wall based on calculated bending moment and shear force
(simplified model of analysis for the bending moment and shear force distribution
can be used, GEOGUIDE 1).
1. Sheet pile walls

Sheet pile walls are a series of thin sheet piles connected together and hammer
driven to soil. They are commonly made of steel. The sheet pile wall can be a
pure cantilever wall, or with internal bracing if the wall is high. For taller walls,
tieback anchors are usually employed to stabilise the wall as shown in the figures
below.
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2. Soldier pile walls

Soldier pile walls consist of vertical wide flange steel members with horizontal
timber lagging. They are usually used in the temporary works.
3. Bored pile walls

Bored pile walls are a row of bored piles which can be contiguous or closely
spaced with infilled concrete panels in between. The spacing shall not be greater
than three times the pile diameter. If the pile is constructed in steep terrain where
access is difficult, hand dug caisson may be required to construct the caisson
wall. Hand-dug caissons are usually excavated in stages of about one metre
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depth and lined with insitu concrete in each stage of excavation. For machine
bored piles, excavation can be done by drill/rotary drilling bucket or by grabs and
chisels within a steel casing.
4. Diaphragm walls
Diaphragm wall is constructed by excavation in a trench which is temporarily
supported by a bentonite slurry. When the finding level is reached, steel
reinforcement cage will be lowered into position and concrete will be placed (by
tremie) to displace the concrete as shown in the figure below.
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The above figure shows the guide wall and the hydraulic grab.
The above figure shows a circular diaphragm wall.
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Analysis and Design of Retaining Walls.
In this section, we are going to discuss briefly the basic design and analysis of retaining
walls. We will only consider the concrete gravity walls, reinforced concrete walls and the
anchored sheet pile walls. As discussed above, the design for retaining wall must consider
both the ultimate limit state and the serviceability limit state. However, in this course we will
only design for the ultimate limit state, in particular, the stability failure of the wall.
The analysis and design of retaining walls for stability failure requires the evaluation of the
structural adequacy of the wall to prevent
a) Overturning of the wall
b) Sliding of the wall at the base
c) Bearing capacity failure of the soil.
In the following discussion, we will then concentrate on the evaluation of the above three
requirements. It should be noted that the factor of safety of gravity walls is as follows:
Mode of Failure Recommended Factor of Safety for a Ten-year Return Period

Rainfall
New Walls Remedial and Preventive Works to
Existing Walls
Sliding 1.5 1.25
Rotation 2.0 1.5
Bearing Capacity 3.0 Existing value to be maintained if below
3.0
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Reinforced Concrete Retaining Wall
Unlike the gravity retaining wall which resists overturning and sliding mainly by its self-
weight, the reinforced concrete retaining wall supports the lateral force by flexural and
shear of the stem. As can be seen from the figure below, passive earth pressure may be
developed if there is soil in front of the toe area. The figure also shows a shear key which
is sometimes used to achieve more sliding resistance (more area for the passive pressure
to develop). However, one should keep in mind that the soil in front of the toe area may
be subsequently removed due to whatever reasons, hence, care should be taken when
including that area for the passive pressure calculation.
As discussed in the previous section that the soil or fill behind the wall helps to stabilise
the wall. When there is surcharge (i.e. additional loading on the soil or fill e.g. a house),
additional lateral force due to the surcharge will be applied to the wall. The usual
procedure is to treat the surcharge (w) as an additional soil depth of z =w/ or simply
evaluate the additional lateral force by the following equation:
P’= wHKa
Where P’ is the additional lateral force to the wall due to the surcharge, w is the surcharge
and H is the height of the wall.
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A
A
B
+
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Anchored Sheet Pile Wall
Sheet piles are commonly used to provide temporary supports for excavations. The piles are
usually driven by hammer. Sheet piles can be braced by strut and wales if the length of the
piles is too long. However, to provide a clear working area for wide excavation works the
use of anchored sheet pile walls may be of considerable advantage. Soil anchors are installed
by drilling a hole through the soil and insert a tie or rod which is then anchored away from
the sheet piles. The anchor rod may also be grouted to obtain the necessary bond strength in
order to support the wall.
The forces that exert on an anchored sheet pile wall are shown in the figure below. It can be
seen from the figure that water pressure exists from both sides of the wall.
For this course, we will examine an anchored sheet wall for its overturning failure and
evaluate the tie force for the anchor.
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Braced and Strutted Excavation
If excavation of earth is in a wide area, the use of braced or strutted sheet piles may be
required. Of course, the sheet piles may also be anchored as discussed above. The bracing
for sheet piles usually consists of wales running across the wall to provide intermediate
supports for the sheeting. The wales are in turn supported by struts which provide lateral
support to resist earth pressure behind the sheeting. Typical braced sheet piles are shown in
the figure below.
The analysis of braced sheet piles relies on the empirical diagrams of lateral pressure against
the braced sheets as shown in the figures below.
or 0.3H
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In analysing the forces of the struts, the usual assumption is that the struts are hinged at all
levels except at the top and the bottom ones as shown in the figure below. The forces in the
struts can then be evaluated by considering the equilibrium of the individual free body
diagram.
References
1. Tomlinson M.J. “Foundation design and construction”, 2001 Prentice Hall.

2. Liu C and Evett J B “Soils and Foundations” 5th edition, Prentice Hall.
3. D P Coduto “Geotechnical Engineering Principles and Practice 1999, Prentice Hall.
4. Geotechnical Engineering Office Geoguides 1,2 and 3; CED Hong Kong Government,
Government Publication Centre.
5. O’Rourke T.D. and Jones C.J.F.P. (1990) “Overview of Earth Retention Systems: 1970-
1990” Design and Performance of Earth Retaining Structures, Geotechnical Special
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Publication No. 25, pp 22-51. P.C. Lambe and L.A. Hansen, Eds., ASCE
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BRE 426

Geotechnical and Foundation Engineering – Chapter Eight

Types of Retaining Walls

Gravity Retaining Walls

Reinforced Concrete Retaining Walls

1. L or inverted T-shaped cantilever retaining walls

2. Counterfort retaining walls

3. Buttressed retaining walls

Cantilevered Retaining Walls

1. Sheet pile walls

2. Soldier pile walls

3. Bored pile walls

The above figure shows a circular diaphragm wall.

Analysis and Design of Retaining Walls.

Mode of Failure Recommended Factor of Safety for a Ten-year Return Period

Reinforced Concrete Retaining Wall

Anchored Sheet Pile Wall

Braced and Strutted Excavation

1. Tomlinson M.J. “Foundation design and construction”, 2001 Prentice Hall.

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