Terzaghi has developed the bearing capacity equation on the assumption that water table is at great depths. If water table is present close to the foundation, some modification is necessary. This is done as per the followings.
Case 1 Water table above footing base
The weight of soil below water table is reduced due to buoyancy. The influence of water table on bearing capacity is incorporated in the general bearing capacity equation with the help of Fig.1.
Zw1 = Depth of water table below ground level
a = Height of water table above base of the footing
Df = Depth of foundation.
B = Width of foundation
= Submerged unit weight of soil
= Soil unit weight
RW1 = Correction factor for depth term
RW2 = Correction factor for width term
If the water table is at a depth ZW1 such that 0 < ZW1 < Df then, the bearing capacity equation is written as:
Where RW1 can be obtained from:
RW1 = ½ (1 + ZW1/Df) ———- (2)
Case 2 – Water table below footing base
In this case, soil above the base of the footing is moist and below base may be fully or partially submerged. If the water table is at the base then the bearing capacity equation is written as:
If the water table is at a depth B below the case of the footing then the bearing capacity is not affected and no correction is required. If the water table is at an intermediate depth say, ZW2 below the base of the footing where 0<ZW2<B, then the bearing capacity equation is Written as:
Where, RW2 can be obtained from the equation:
When the water table at base, ZW2 = 0 and RW2 = 0.5 and when ZW2 = B, RW2 = 1. This means when water table is at base the weight term in the bearing capacity equation is reduced by half and when water table at great depth ( i.e. greater than width of foundation below the base of foundation) water table will have no influence in the bearing capacity.