Vesic (1975) suggested that, for the case of a foundation founded in a layer of soft clay which overlies a stiff clay, the ultimate bearing capacity of the foundation can be expressed as:

where cu = the undrained strength of the soft clay and Ncm = a modified form of Nc, the value of which depends upon the ratio of the cu values of both clays, the thickness of the upper layer, the foundation depth and the shape and width of the foundation. Values of Ncm are quoted in Vesic's paper.

The converse situation, i.e. that of a foundation founded in a layer of stiff clay which overlies a soft clay has been studied by Brown and Meyerhof (1969) who quoted a formula for Ncm based on a punching shear failure analysis.

For other cases of more heterogeneous soil conditions there is presently no recognised method by which the bearing capacity equations can be realistically applied.

At first glance a safe way of determining the bearing capacity of a foundation might be to base it on the shear strength of the weakest soil below it, but such a procedure can be uneconomical, particularly if the weak soil is overlain by much stronger soil. A more suitable method is to calculate the safe bearing capacity using the shear strength of the stronger material and then to check the amount of overstressing that this will cause in the weaker layers. The method is shown in Example 8.7, which illustrates a typical problem that may arise during the selection of a site for a new spoil heap.

For structural foundations the factor of safety against bearing capacity failure is generally not less than 3.0, but for spoil heaps this factor can often be reduced to 2.0.

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