Question

Immediately after a fill is placed, the excess pore water pressure in an underlying saturated clay is equal to:

a. 0

b. 100 lb/ft2

c. The hydrostatic pore water pressure

d. The unit weight of the fill times its thickness

e. Half of Δσ_z

Answer 1

1. The fill will apply a load(due to its own weight) on the clay present beneath it, therefore the vertical stress increase on the surface of saturated clay is equal to unit weight of fill times it's thickness.

2. Saturated clay has water particles and clay soil grains present in it, as soon as the load is applied the load is taken by the element which has high stiffness

3. Clay soil has very low permeability, therefore the water present in clay is in a confined condition.

4. The stiffness of confined water is very large as compare to that of soil grained structure

5. Hence all applied load is initially taken by water only which results in increase of pore water pressure by an amount= unit weight times thickness of fill.

Therefore, the correct answer is option (d)

Also note that:-

Immediately after a fill is placed the total load of a fill is taken by water, as the pore water pressure increases the water will try to escape out and this expulsion of pore water under the weight of fill is known as consolidation.

With time this pore water pressure decreases an effective stress on soil grain increases.