Question

If the ground water table is exactly at the base of the foundation, the maximum water table reduction factor may be ............

• A. 0.25
• B. 0.50
• C. 0.65
• D. 0.75

Hint:

If the water table is at or above the base of a foundation, use a reduction factor of 0.5 to adjust the net bearing capacity. This accounts for loss in effective stress due to buoyancy.

Answer 1

The Correct Option is B

The presence of groundwater affects the bearing capacity of soils by reducing the effective stress through buoyancy. IS 6403:1981 provides guidelines for applying a water table correction factor when the water table is at or near the foundation level.
When the water table is located exactly at the base of the foundation, it exerts an upward buoyant force, reducing the soil's effective weight below the foundation. This scenario leads to a decrease in the soil's shear strength and ultimately the bearing capacity.
To account for this reduction, a correction factor — known as the water table reduction factor — is applied. According to IS code provisions, the maximum reduction factor to be applied in such a case is 0.5. This means the influence of the soil's unit weight below the foundation is halved in bearing capacity calculations.
Hence, when the water table coincides with the base of the foundation, the correct factor to be used is: \[ \boxed{0.50} \]