A 5m depth of sand overlies a 6m layer of clay, the water table being at the sur

Question

A 5m depth of sand overlies a 6m layer of clay, the water table being at the

surface; the permeability of the clay is very low. The saturated unit weight of the

Figure 3.3 Example 3.1.

Response of effective stress to a change in total stress 77

sand is 19 kN/m3 and that of the clay is 20 kN/m3. A 4m depth of fill material of

unit weight 20 kN/m3 is placed on the surface over an extensive area. Determine the

effective vertical stress at the centre of the clay layer (a) immediately after the fill

has been placed, assuming this to take place rapidly and (b) many years after the

fill has been placed.

Explanation / Answer

Buoyant unit weight of sand ¼ 20 9:8 ¼ 10:2kN=m3

Buoyant unit weight of clay ¼ 19 9:8 ¼ 9:2kN=m3

At 5m depth:

0

v

¼ ð3 17Þ þ ð2 10:2Þ ¼ 71:4kN=m2

At 9m depth:

0

v

¼ ð3 17Þ þ ð2 10:2Þ þ ð4 9:2Þ ¼ 108:2kN=m2

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