Design of gravity walls

7.5.1 Limit states

During the design of gravity walls, the following limit states should be considered.

(1) Slip of the surrounding soil (Fig. 7.5a). This effect can occur in cohesive soils and can be analysed as for a slope stability problem.

(2) Bearing failure of the soil beneath the structure (Fig. 7.5b). The overturning moment from the earth's thrust causes high bearing pressures at the toe of the wall. These values must be kept within safe limits -usually not more than one-third of the supporting soil's ultimate bearing capacity.

(3) Overturning. For a wall to be stable the resultant thrust must be within the base. Most walls are so designed that the thrust is within the middle third of the base.

(4) Forward sliding (Fig. 7.5c). Caused by insufficient base friction or lack of passive resistance in front of the wall.

(5) Structural failure caused by faulty design, poor workmanship, deterioration of materials, etc.

(6) Excessive deformation of the wall or ground such that adjacent structures or services reach their ultimate limit state.

(7) Unfavourable seepage effects and the adequacy of any drainage system provided.

(a) Slip of surrounding soil

Fig. 7.5 Limit states for earth retaining structures (adapted from BS 8002: 1994).

7.5.2 Bearing pressures on soil

The resultant of the forces due to the pressure of the soil retained and the weight of the wall subject the foundation to both direct and bending effects.

(b) Bearing capacity failure



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