Retaining Walls Questions and Answers

Answer: D

Main steel goes at the top in the heel slab and at the bottom in the toe slab.

The heel slab, under the retained earth, carries the heavy downward weight of the backfill, which exceeds the upward soil pressure beneath it. It bends as a cantilever from the stem with tension on its top face, so its main bars are placed at the top.

The toe slab, in front of the stem, has little load on it but receives the largest upward soil pressure. It bends upward as a cantilever, with tension on the bottom face, so its main bars are at the bottom. The other three options put the steel on the compression face of one or both slabs.

Answer: B

The minimum factor of safety against overturning is 1.5.

A retaining wall is checked so that the moment resisting overturning about the toe is comfortably larger than the moment of the earth pressure. IS 456:2000 (clause 20.1) requires the restoring moment, counting only 0.9 times the dead load, to be at least 1.4 times the overturning moment, which works out to about 1.4/0.9 = 1.55, and design texts take the minimum as 1.5.

2.0, 2.5 and 3.0 are more conservative values a designer may choose, but none is the minimum the question asks for.