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.

Let,

Z_{w1} = Depth of water table below ground level

a = Height of water table above base of the footing

D_{f} = Depth of foundation.

B = Width of foundation

Î³_{sub} = Submerged unit weight of soil

Î³ = Soil unit weight

R_{W1} = Correction factor for depth term

R_{W2} = Correction factor for width term

If the water table is at a depth Z_{W1} such that 0 < Z_{W1} < D_{f} then, the bearing capacity equation is written as:

Q_{ult} = cN_{c} + Î³[Î³Z_{W1} + (D_{f} – Z_{W1}) Î³_{sub}]N_{q} + ½ Î³_{sub} B NÎ³

Q_{ult} = cN_{c} + Î³D_{f} N_{q} R_{W1} + ½ Î³_{sub} B NÎ³ ———- (1)

Where R_{W1} can be obtained from:

[Î³ D_{w} + (D_{f} – Z_{W1})Î³_{sub}] = Î³ D_{f} R_{W1}

[Î³ Z_{w1} + (D_{f} – Z_{W1})Î³_{sub}]/Î³ = D_{f} R_{W1}

Z_{W1} + ½ (D_{f} – Z_{W1}) = D_{f }R_{W1}

½ Z_{W1} + ½ D_{f} = D_{f} R_{W1}

D_{f}R_{f1} = ½ Z_{W1} + ½ D_{f}

R_{W1} = ½ Z_{W1}/D_{f} + ½ D_{f}/D_{f}

R_{W1} = ½ (1 + Z_{W1}/D_{f}) ———- (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:

Q_{ult} = cN_{c} + Î³D_{f} N_{q} + ½ Î³B NÎ³ R_{W2} ———- (3)

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, Z_{W2} below the base of the footing where 0<Z_{W2}<B, then the bearing capacity equation is Written as:

Q_{ult} = cN_{c} + Î³D_{f} N_{q} + [1/2 Z_{W2}Î³ + Î³_{sub }(B – Z_{W2})]NÎ³

Q_{ult} = cN_{c} + Î³D_{f} N_{q} + ½ Î³ B NÎ³ R_{W2} ———- (4)

Where, R_{W2} can be obtained from the equation:

Z_{W2} Î³ + Î³_{sub}(B – Z_{W2}) = BÎ³ R_{W2}

Z_{W2} + Î³_{sub}/ Î³(B – Z_{W2}) = BR_{W2}

Z_{W2} + ½ B – ½ Z_{W2} = BR_{W2}

½ *Z_{W2} + 0.5B = BR_{W2}

BR_{W2} = 0.5(Z_{W2} + B)

R_{W2} = 0.5(1 + Z_{W2}/B) ———- (5)

When the water table at base, Z_{W2} = 0 and R_{W2} = 0.5 and when Z_{W2} = B, R_{W2} = 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.