Skempton’s Theory for Cohesive Soil

Skempton 1951 suggested a bearing capacity theory for saturated clay for which ɸ = 0. Skempton gives N_c, the bearing capacity factor on the basis of theory, laboratory tests and field observations. It was found that the value of N_c increased with the increase in D_f/B ratio. The expression for N_c proposed by Skempton is given below.

For Strip footings,

N_c = 5(1+0.2D_f/B), with a maximum limiting value of 7.5   ———- (1)

For square and circular footings,

N_c = 6(1+0.2D_f/B), with a maximum limiting value of 9.0   ———- (2)

For rectangular footings,

N_c = 5(1+0.2D_f/B) (1+0.2B/L) for D_f/B ≤ 2.5   ———- (3)

7.5(1+0.2D_f/B) (1+0.2B/L) for D_f/B > 2.5   ———- (4)

For ɸ = 0 condition, the net ultimate bearing capacity is given by:

Q_net = c_uN_ç   ———- (5)

Alternatively, the graph given in Fig.1 above can be used to find the bearing capacity factor N_c. Where, N_c (rect) = N_c(sq)[0.84 + 0.16B/L] . As per Skemption, if the shear strength of the soil does not vary more than ± 50 % of the average value for a depth 2/3B below the footing, the average value of c_u can be used in the above equation.