Civil Engineering Reference
In-Depth Information
Figure 21.4
Stress and pore pressure changes in a stable slope.
As shown in Fig. 21.4(b), the pore pressure immediately after construction
u
i
is
less than the steady pore pressure
u
∞
and so the initial excess pore pressure
u
i
is
negative (i.e. the level of water in the standpipe is below the phreatic surface, as shown
in Fig. 21.4(a). As time passes the total stresses remain unchanged at B (because the
geometry of the slope remains the same) but the negative excess pore pressures dissipate
and the pore pressure rises. The effective stress path is B
→
C
and this corresponds
to swelling and a reduction in mean normal effective stress, as shown in Fig. 21.4(b)
and (c). The final state at C
corresponds to a steady state pore pressure after swelling
u
∞
; in the example shown
u
∞
=
u
0
but the arguments would be the same if
u
∞
was
different from
u
0
, which would be the case if the excavation was drained of water.
The slope will fail if the states of all elements along the slip surface reach the critical
state line: if B
reaches the critical state line the slope fails during undrained excavation
and if C
reaches the critical state line the slope fails some time after construction. The
distance of the effective stress points B
or C
from the critical state line is a measure
of the factor of safety of the slope and Fig. 21.4 demonstrates that the factor of safety
of a slope decreases with time.
This means that the critical time in the life of a slope is in the long term when
the pore pressures have come into equilibrium with the steady state seepage flownet.
Consequently, a permanent slope should be designed for the long-term, fully drained,
condition. Temporary slopes that are required to stand for very short periods are often
designed as undrained, but remember that just because a slope or a trench is stand-
ing now does not mean that it will still be stable in 10 minutes time. Slopes and
excavations are very dangerous; many people are killed by trench failures which
