Fig. 8.7 Variation of the coefficient Nc with depth (after Skempton, 1951).

8.3.4 Summary of bearing capacity formula

It can be seen that Rankine's theory does not give satisfactory results and that, for variable subsoil conditions, the slip surface analysis of Fellenius provides the best solution. For normal soil conditions, Equations (6)-(9) can generally be used and may be applied to foundations at any depth in c—<f> soils and to shallow foundations in cohesive soils. For deep footings in cohesive soil the values of Nc suggested by Skempton may be used in place of the Terzaghi values.

8.3.5 Choice of soil parameters

As with earth pressure equations, bearing capacity equations can be used with either the undrained or the drained soil parameters. As granular soils operate in the drained state at all stages during and after construction, the relevant soil strength parameter is ft.

Saturated cohesive soils operate in the undrained state during and immediately after construction and the relevant parameters are cu and </>u (with oa generally assumed equal to zero). If required, the long-term stability can be checked with the assumption that the soil will be drained and the relevant parameters are c' and ft (with c' generally taken as equal to zero) but this procedure is not often carried out.

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