(c) Wall yield by sliding forward

The differences between the various pressure diagrams can be seen in Fig. 6.27d where the three diagrams have been superimposed. It has been found that if the top of a wall moves 0.1 per cent of its height, i.e. a movement of 10 mm in a 10 m high wall, an arching-active case is attained. This applies whether the wall rotates or slides. In order to achieve the totally active case the top of the wall must move about 0.5 per cent, or 50 mm in a 10 m wall.

It can therefore be seen that if a retaining wall with a cohesionless backfill is held so rigidly that little yield is possible (e.g. if it is joined to an adjacent structure) it must be designed to withstand earth pressure values much larger than active pressure values.

If such a wall is completely restrained it must be designed to take earth pressure at rest values, although this condition does not often occur; if a wall is so restrained that only a small amount of yielding can take place arching-active conditions may be achieved, as in the strutting of trench timbers. In this case the assumption of triangular pressure distribution is incorrect, the actual pressure distribution being indeterminate but roughly parabolic.

If the wall yields 0.5 per cent of its height then the totally active case is attained and the assumption of triangular pressure distribution is satisfactory. Almost all retaining walls, unless propped at the top, can yield a considerable amount with no detrimental effects and attain this totally active state.

In the case of a wall with a cohesive backfill, the totally active case is reached as soon as the wall yields but due to plastic flow within the clay there is a slow build-up of pressure on the back of the wall, which will eventually yield again to re-acquire the totally active pressure conditions. This process is repetitive and over a number of years the resulting movement of the wall may be large. For such soils one can either design for higher pressure or, if the wall is relatively unimportant, design for the totally active case bearing in mind that the useful life of the wall may be short.

6.11 Strutted excavations

When excavating a deep trench the insertion of shuttering to hold up the sides becomes necessary. The excavation is carried down first to some point, X, and rigidly strutted timbering is inserted between the levels D to X (Fig. 6.28a).

As further excavation is carried out, timbering and strutting are inserted in stages, but before the timbering is inserted the soil yields by an amount that tends to increase with depth (it is relatively small at the top of the trench).

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