Lateral Earth Pressure - Elements of Soil Mechanics - page 213

Fig. 7.28 Effect of an inclined drain on seepage forces.
adjacent equipotentials entering the drain will be equal to each other (in a manner similar to the upstream
slope of an earth dam).
Owing to the seepage forces, an additional force, P w , now acts upwards and at right-angles to the failure
plane. From the flow net it is possible to determine values of excess hydrostatic pressure, h w , at selected
points along the failure plane (see Fig. 7.27a ). If a smooth curve is drawn through these h w values (when
plotted along the failure plane), it becomes possible to evaluate P w (see Example 7.11) .
The resulting force diagram is shown in Fig. 7.27b . In theory the polygon of forces will be as shown in
Fig. 7.27c but, as seepage will only occur once the soil has achieved a drained state, the operative strength
parameter is φ ′ , with c ′ generally being assumed to be zero.
The seepage flow net for the inclined drain in Fig. 7.26e is shown in Fig. 7.28. Such a drain induces
vertical drainage of the rain water and it is seen that the portion of the flow net above the drain is abso-
lutely regular and, more important, that the equipotentials are horizontal. This latter fact means that,
within the soil above the drain, the value of excess hydrostatic head at any point must be zero. The failure
plane will not be subjected to the upward force P w and the pressure exerted on the back of the wall can
only be from the saturated soil.
7.10.2 Differential hydrostatic head
When there is a risk of a groundwater level developing behind the wall then the possible increase in lateral
pressure due to submergence must be allowed for. This problem will occur in tidal areas, and quay walls
must be designed to withstand the most adverse difference created by tidal lag between the water level
in front of and the groundwater level behind the wall. As there is no real time for steady seepage condi-
tions to develop between the two head levels, the effect of possible seepage forces can safely be ignored.
Example 7.11: Thrust due to saturated soil
A vertical 4 m high wall is founded on a relatively impervious soil and is supporting soil
with the properties: φ ′ = 40°, c ′ = 0, δ = 20°, γ sat = 20 kN/m 3 .
The surface of the retained soil is horizontal and is level with the top of the wall. If
the wall is subjected to heavy and prolonged rain such that the retained soil becomes