Skempton's findings on clay bearing capacity

Skempton (1951) studied the bearing capacity of saturated clay ($\phi_u = 0$) and showed that the bearing capacity factor $N_c$ is not constant. It increases with depth of foundation relative to width.

• For a strip footing: $N_c = 5\left(1 + 0.2\dfrac{D_f}{B}\right)$ for $D_f/B \le 2.5$, and $N_c = 7.5$ for larger depths.
• For a square or circular footing: $N_c = 6\left(1 + 0.2\dfrac{D_f}{B}\right)$ up to $D_f/B = 2.5$, and $N_c = 9$ beyond that.
• For a rectangular footing: $N_c(\text{rect}) = \left(0.84 + 0.16\dfrac{B}{L}\right)N_c(\text{square})$.

The net ultimate capacity is $q_{nu} = cN_c$ and the net safe bearing capacity is

$$q_{ns} = \frac{q_{nu}}{F} = \frac{cN_c}{F}$$

with $F$ usually 3, where $c = q_u/2$ is the undrained cohesion. The gross safe pressure is $q_{ns} + \gamma D_f$.