Soil Mechanics 2 Worked Examples

An 8 m thick layer of silty clay is overlaying a gravel stratum containing water under artesian pressure. A stand-pipe was inserted into the gravel and water rose up the pipe to reach a level of 2 m above the top of the clay (Fig. 2.12).

The clay has a particle specific gravity of 2.7 and a natural moisture content of 30 per cent. The permeability of the silty clay is 3.0 x 10~8 m/s.

It is proposed to excavate 2 m into the soil in order to insert a wide foundation which, when constructed, will exert a uniform pressure of 100 kN/m2 on to its supporting soil.

Determine (a) the unit rate of flow of water through the silty clay in m3 per year before the work commences, (b) The factor of safety against heaving: (i) at end of excavation (ii) after construction of the foundation. (Take unit weight of water as 10 kN/m3.)

Solution

(a) Assume that GWL occurs at top of clay. Head of water in clay = 8 m Head of water in gravel = 10 m .". Head of water lost in clay = 2m q = Aki

Consider a unit area of one square metre, then:

+6 -2

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