Civil Engineering Reference
In-Depth Information
occur as the effective stresses move from B
towards C
in Fig. 21.4. In the design
of temporary excavations the important question is not so much the undrained sta-
bility but how quickly the pore pressures will increase. In Fig. 21.4(b) and (c) the
broken lines A
C
→
represent the drained case in which pore pressures remain
constant.
If a slope fails the total stresses change as the angle and height reduce as shown in
Fig. 21.5(a). Figure 21.5(b) shows stress paths for a steep slope failing during undrained
excavation. The effective stress path is A
→
B
and this ends on the critical state line
where the undrained strength is
s
u
. The total stress path would like to continue to X,
corresponding to the initial slope angle
i
x
, but cannot; therefore the slope geometry
changes and the mean slope angle
i
c
and height
H
c
correspond to total stresses at B.
Figure 21.5(c) shows stress paths for a slope that fails some time after excavation.
The state immediately after excavation is B and B
and failure occurs at C and C
when the pore pressure is
u
f
. Subsequently, as the pore pressures continue to rise, the
effective stresses move along C
→
D
down the critical state line and the total stresses
move more or less along C
D due to unloading (i.e. reduction) of the shear stress
as the slope angle decreases. The slope will reach a stable state when the pore pressure
is the final steady state pore pressure
u
∞
.
These analyses and the stress paths shown in Figs. 21.4 and 21.5 are simplified and
idealized but they illustrate the essential features of the behaviour of slopes during and
→
Figure 21.5
Stress and pore pressure changes in failing slopes.
