Question

Consider the following statements:

1. Effective stress in a sand layer below a lake with standing water does not alter as the water level fluctuates.

2. Regarding water table below the ground surface, any rise in the water table causes equal changes in both pore pressure and effective stress.

3. Capillary saturation will cause the effective stress to increase.

Which of the above statements are correct?
1. 1, 2 and 3
2. 1 and 2 only
3. 2 and 3 only
4. 1 and 3 only

Answer 1

Correct Answer - Option 4 : 1 and 3 only

Concept:

When a load is applied to soil, it is carried by the solid grains and the water in the pores. The total vertical stress acting at a point below the ground surface is due to the weight of everything that lies above, including soil, water, and surface loading. Total stress thus increases with depth and with unit weight.

Vertical total stress at depth z, σ_v = γ.Z

Below a water body, the total stress is the sum of the weight of the soil up to the surface and the weight of water above this. 

σv  = γ.Z+ γ​_w.Z_w

Under hydrostatic conditions, no water flow takes place, and the pore pressure at a given point is given by
u = γ_w.h , Where h = depth below water table or overlying water surface

Above the water table, when the soil is saturated, pore pressure will be negative (less than atmospheric). The height above the water table to which the soil is saturated is called the capillary rise, and this depends on the grain size and the size of pores. In coarse soils, the capillary rise is very small.

Δσ´ = Δσ - Δu

Where Δσ´ = Change in Effective stress

If both total stress and pore water pressure change by the same amount, the effective stress remains constant.

At any point within the soil mass, the magnitudes of both total stress and pore water pressure are dependent on the ground water position. With a shift in the water table due to seasonal fluctuations, there is a resulting change in the distribution in pore water pressure with depth.

Justification for statements:

1. Effective stress in a sand layer below a lake with standing water does not alter as the water level fluctuates.

For a lake with standing water when the water level fluctuates the total stress and pore water pressure will be changed by the same amount, as a result of which effective stress does not alter.

So it can be concluded that Changes in water level above ground do not cause changes in effective stresses in the ground below. A rise above ground surface increases both the total stress and the pore water pressure by the same amount, and consequently effective stress is not altered.

The above statement is correct.

2. Regarding water table below the ground surface, any rise in the water table causes equal changes in both pore pressure and effective stress.

Changes in water level below ground result in changes in effective stresses below the water table. A rise increases the pore water pressure at all elevations thus causing a decrease in effective stress. In contrast, a fall in the water table produces an increase in the effective stress.

So the above statement is incorrect.

3. Capillary saturation will cause the effective stress to increase.

As shown in the figure pore pressure due to capillary saturation is negative, which means this value will be added to the total stress to get the effective stress.

So the capillary saturation will cause the effective stress to increase.

∴ Hence statement 1 and 3 are correct.