Geotechnical Engineering

DHARM

LATERAL EARTH PRESSURE AND STABILITY OF RETAINING WALLS 461

H 2 = depth of fill above water table (taken to be moist),

γ = moist unit weight, and

γ ′ = submerged or effective unit weight.

KHa2g¢

H

gw1H

H 1

H 2 Moistsand ( )g

Saturated

sand

(effective

unit wt. : )g¢

KHa1g¢

KHa2 1 g KHa2g¢ gw1H

KHo2g (^2) f
2
ff 1 (< 2 )
(a) Partly submerged backfill (b) Lateral pressure for partly
submerged backfill
(c) Partly submerged backfill
with different friction angles
above and below the water table
Fig. 13.10 Effect of partial submergence on lateral earth pressure
If the angle of internal friction below the water table is different from that above the
water table (the former will usually be less than the latter), the corresponding values of Ka
should be used in the respective zones. (It may be noted that Ka-values bear reciprocal rela-
tionship with φ-values while Kp-values bear direct relationship with them). At the water table,
a slight but sudden increase of pressure should be expected depending upon the difference in
the values of active pressure coefficients for the respective φ-values. These conditions are illus-
trated in Fig. 13.10 (c).
13.6.5 Effect of Uniform Surcharge
The extra loading carried by a retaining structure is known as ‘surcharge’. It may be a uniform
load (from roadway, from stacked goods, etc.), a line load (trains running parallel to the struc-
ture), or an isolated load (say, a column footing).
Let us see the effect of a uniform surcharge on the lateral pressure acting on the retain-
ing structure, as shown in Fig. 13.11.
In the case of a wall retaining a backfill with horizontal surface level with the top of the
wall and carrying a uniform surcharge of intensity q per unit area, the vertical stress at every
elevation in the backfill is considered to increase by q. As such, the lateral pressure has to
increase by Ka.q.
Thus, at any depth z, σh = Kaγ.z + Kaq
Figures 13.11 (b) and (c) show two different ways in which the pressure distribution
may be shown. In Fig. 13.11 (c), the uniform surcharge is also considered to have been con-
verted into an equivalent height He, of backfill, which is easily established , as shown.