Geotechnical Engineering

DHARM

506 GEOTECHNICAL ENGINEERING

P Active earth thrusta

Large

batter

Weight

W

To e Heel

Passive

resistance

Pp

Sliding resistance T
Bearing force N
Fig. 13.49 Forces acting on a gravity retaining wall
The active thrust acts to push the wall outward. This outward motion is resisted by
sliding or shear resistance along the base of the wall and by the passive resistance of the soil
lying above the toe of the wall. The active thrust also tends to overturn the wall around the toe.
The overturning is resisted by the weight of the wall and the vertical component of the active
thrust. The weight of the wall thus acts in two ways: it resists overturning and it causes fric-
tional sliding resistance at the base of the wall. This is the reason for calling the wall a ‘gravity’
retaining wall.

A gravity retaining wall, together with the retained backfill and the supporting soil, is a
highly ‘indeterminate’ system (Lambe and Whitman, 1969). The magnitudes of the forces that
act upon the wall cannot be determined from statics alone and these will be affected by the
sequence of construction and backfilling operations. Hence, the design of such a wall is based
on an analysis of expected forces that would exist if the wall started to fail, that is, to overturn
or to slide outwards.

Considering the patterns of deformations observed from experiments, an approach to
the design of gravity retaining walls may be stated. First, trial dimensions for the wall are
chosen; next, the active thrust on the wall is determined under the assumption that the active
pressure is fully mobilised; then the resistance offered by the weight of wall, the frictional
resistance at the base of the wall, and the passive resistance, if any, at the toe of the wall, are
determined. Finally, the active thrust and total resistance are compared and it is ensured that
the resistance exceeds the thrust by a suitable safety factor. Consider a gravity retaining wall
as shown in Fig. 13.50.

Let W represent the weight of the wall per unit length perpendicular to the plane of the
figure, acting through the centre of gravity of the cross-section of the wall. Let the active
thrust on the wall be Pa acting at an angle δ with the normal to the back face of the wall. For