Civil Engineering Reference
In-Depth Information
after construction. Notice the critical importance of changing pore pressures with time
and their influence on stability. The examples were for excavated slopes where pore
pressures decreased during undrained excavation. In man-made compacted soils the
initial pore pressures are negative because the fill is unsaturated and so the initial states
at B and B
are more or less the same for cut and fill slopes.
21.5 Influence of water on stability of slopes
Water influences slope stability in several fundamentally different ways and these are
illustrated by commonly observed failures. Firstly, slopes may fail well after completion
of excavation due to dissipation of negative excess pore pressures and swelling and
softening of the soil, as discussed in Sec. 21.4. Secondly, slopes in river banks, lakes
and trenches may fail if the external water level is quickly lowered. Thirdly, slopes
often fail after periods of heavy rainfall.
Free water in a river or lake, or in a water-filled trench, applies total stresses
w
to a
soil surface, as shown in Fig. 21.6. These total stresses help to support the slope which
may fail if the support is removed. (In practice temporary excavations for piles and
retaining walls are supported by a slurry of bentonite clay, or some other natural or
artificial mud, with unit weight greater than that of water.) Notice that after undrained
excavation the pore pressures in the soil may not be in equilibrium with the free water
in the excavation.
Slope failures after rainfall, or after changes in the groundwater conditions, are
due to increases in the pore pressures which lead to reductions in effective stress and
strength. (Notice that the soil remains saturated while pore pressures change and
there is no question of the rainwater lubricating the soil - this is an entirely false
interpretation.) In order to calculate the pore pressures in a slope under steady state
conditions it is necessary to draw a flownet as described in Chapter 14.
Figure 21.7 shows flownets for steady state seepage towards an excavation.
In Fig. 21.7(a) there is a drain at the toe of a slope, while in Fig. 21.7(b) the excavation
is partly filled with water. In both cases there is impermeable rock below the soil and
the water table far from the slope is near the ground surface as shown. The pore pres-
sure in the soil anywhere in each flownet can be determined from the equipotentials,
as described in Sec. 14.3.
Often slopes fail with a shallow slip plane parallel with the surface, as shown in
Fig. 21.3(b). In Fig. 21.7(a) the flowlines near the toe become nearly parallel to the
slope, while in Fig. 21.7(b) they are normal to the slope which is an equipotential.
σ
Figure 21.6
Loads on slopes from water in the excavation.
