Civil Engineering Reference
In-Depth Information
24.11 Summary
1. Retaining walls are used to support slopes that are too high or too steep to remain
stable if unsupported or to limit ground movements. There are a number of dif-
ferent kinds of retaining wall. They can fail in different ways including slipping in
the soil, failure of the wall itself and failure of props or anchors.
2. As a wall moves away from the soil the horizontal stresses are active pressures
and as it moves towards the soil they are passive pressures. For drained loading
on smooth walls these are
σ
a
=
σ
v
tan
2
45
◦
−
φ
1
2
σ
v
=
K
a
(24.3)
σ
p
=
σ
v
tan
2
45
◦
+
φ
1
2
σ
v
=
K
p
(24.4)
where
K
a
is the active earth pressure coefficient and
K
p
is the passive earth pressure
coefficient. For undrained loading on smooth walls active and passive pressures are
σ
=
σ
−
K
au
s
u
(24.9)
a
v
σ
=
σ
+
K
pu
s
u
(24.10)
p
v
where
K
au
and
K
pu
are earth pressure coefficients for undrained loading.
3. For walls retaining excavated slopes pore pressures rise with time and the safety
deteriorates, but for walls retaining coarse grained fill the excess pore pressures
developed in the foundations during constructionwill generally decrease with time.
4. The depth of the toe of a wall below the base of the excavation must be sufficient to
ensure overall stability (with an appropriate margin of safety). Overall stability is
examined by considering the statical equilibrium of the forces due to the active and
passive earth pressures and the loads in props and anchors. Different calculations
are required for cantilever and propped walls.
Worked examples
Example 24.1: Calculation of active and passive earth pressures
Figure 24.16 shows a
10 m high wall retaining layers of sand and clay. The active and passive total stresses
in the drained sand are
σ
a
=
σ
a
+
=
σ
z
K
a
+
u
u
=
(σ
z
−
u
)
K
a
+
u
σ
p
=
σ
p
+
=
σ
z
K
p
+
u
u
=
(σ
z
−
u
)
K
p
+
u
tan
2
(45
◦
−
1
2
φ
c
) and
K
p
tan
2
(45
◦
+
1
2
φ
c
) and, for
φ
where
K
a
=
=
=
30,
K
p
=
1/
K
a
=
3. The total active and passive stresses in the undrained clay are
σ
=
σ
−
K
au
s
u
a
z
σ
=
σ
+
K
pu
s
u
p
z
