Civil Engineering Reference
In-Depth Information
Earth pressures and stability of
retaining walls
24.1 Introduction
Retaining walls are used to support slopes and vertical cuts that are too steep or
too deep to remain stable if unsupported. The principal characteristics of a retaining
structure are illustrated in Fig. 24.1. The wall is a structural member that acts as a beam
with various loads on either side. Slender walls are embedded into the ground below
the excavation level and they may be supported by props or anchors. Thick heavy
gravity walls derive their resistance principally from the shear stresses between the soil
and the base of the wall. During excavation (or filling on the high side) slender walls
will tend to move and bend as indicated as the earth pressures develop. Walls move
towards the passive side and away from the active side.
Retaining walls should be designed so they have a margin of safety against fail-
ure which might occur in any of the modes discussed in Sec. 24.4. There is often a
requirement to limit ground movements so nearby buildings and tunnels are not dam-
aged. Construction of a supported excavation will probably change the groundwater
conditions and water pressures contribute to the loads on a wall.
24.2 Earth pressures
Loads on retaining walls arise from the horizontal stresses in the ground (known as
earth pressure) and from any props or anchors used to support it. The earth pressure
depends principally on whether the wall is moving towards or away from the ground
and on the properties of the soil.
The development of earth pressure with displacement is illustrated in Fig. 24.2.
In Fig. 24.2(a) a wall supported by a force
P
retains soil where the horizontal total stress
is
σ
h
; obviously the stresses and the force must be in equilibrium. If
P
is increased the
wall moves towards the soil with displacements
δ
p
and the horizontal stresses increase,
as shown in Fig. 24.2(b); if
P
is decreased the wall moves away from the soil with
displacements
a
and the horizontal stresses decrease. If the movements are sufficient
the horizontal stresses reach the limiting values of the passive pressure
δ
σ
p
and the active
pressure
σ
a
. If there is no movement the horizontal stress
σ
0
is the earth pressure at
rest, corresponding to
K
0
(see Sec. 8.6).
