Civil Engineering Reference
In-Depth Information
pressure which is negative. As time passes the total stresses remain approximately
unchanged at B (they will change a little as the total stresses redistribute during con-
solidation, although there is no more excavation) but the pore pressures rise. The
effective stress path is B
→
C
, which corresponds to swelling and a reduction in the
mean normal effective stress. The final state at C
corresponds to a steady state pore
pressure
u
∞
.
The wall will fail in some way if the states of all elements along the slip surfaces in
Fig. 24.6(a) reach the critical state line; if B
reaches the critical state line the wall fails
during undrained excavation and if C
reaches the line the wall fails some time after
construction. The distance of the effective stress point B
or C
from the critical state
line is a measure of the factor of safety against collapse and Fig. 24.6(b) demonstrates
that the factor of safety of a retaining wall supporting an excavation will decrease with
time. This is the same as for a slope, discussed in Sec. 21.4. We could also trace the
state paths for failing walls as we did for failing slopes, but this is not really relevant
as retaining walls should not be allowed to fail.
Figure 24.7(a) shows a wall embedded in soil and retaining coarse-grained fill. In this
case the shear and normal stresses on typical elements on a slip surface both increase.
Total and effective stress paths are shown in Fig. 24.7(b). The effective stress path
for undrained loading is A
→
B
and this is the same as that in Fig. 24.6(b), but the
Figure 24.7
Changes of stress and pore pressure for a wall retaining fill.
