The following statements pertain to effective stress in a soil mass under hydrostatic condition
and select the correct option.
Statement 1: the effective stress at any section goes on increasing as the water table goes
down.
Statement 2: the effective stress depends upon the bulk unit weight above the groundwater
table and submerged unit weight below the water table.
According to Terzaghi's Effective Stress Principle, effective stress is given by:
Statement 1: Correct
When the groundwater table is lowered, pore water pressure decreases. Consequently,
effective stress generally increases, assuming hydrostatic conditions and conventional soil
unit weights.
Thus, lowering the water table increases effective stress in the soil mass.
Statement 2: Correct
Effective vertical stress at a depth below the groundwater table can be calculated as
Where:
gbulk = Bulk unit weight of soil above the water table
g' = Submerged unit weight of soil below the water table
h1 = Depth above the water table
h2 = Depth below the water table
The submerged unit weight is:
Therefore, effective stress depends on the bulk unit weight above the groundwater table and the submerged unit weight below it.
Important Note: The increase in effective stress due to lowering the water table assumes hydrostatic conditions. Capillary effects and other changes in soil conditions may affect the actual stress distribution.
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