Contact pressure is the actual pressure transmitted from the foundation to the soil. In all the foregoing discussions it has been assumed that this contact pressure value, p, is uniform over the whole base of the foundation, but a uniformly loaded foundation will not necessarily transmit a uniform contact pressure to the soil. This is only possible if the foundation is perfectly flexible. The contact pressure distribution of a rigid foundation depends upon the type of soil beneath it. Figures 4.13a and 4.13b show the form of contact pressure distribution induced in a cohesive soil (A) and in a cohesionless soil (B) by a rigid, uniformly loaded, foundation.
On the assumption that the vertical settlement of the foundation is uniform, it is found from the elastic theory that the stress intensity at the edges of a foundation on cohesive soils is infinite. Obviously local yielding of the soil will occur until the resultant distribution approximates to Fig. 4.13a.
For a rigid surface footing sitting on sand the stress at the edges is zero as there is no overburden to give the sand shear strength, whilst the pressure distribution is roughly parabolic (Fig. 4.13b). The more the foundation is below the surface of the sand the more shear strength there is developed at the edges of the foundation, with the result that the pressure distribution tends to be more uniform.
In the case of cohesive soil, which is at failure when the whole of the soil is at its yield stress, the distribution of the contact pressure again tends to uniformity.
A reinforced concrete foundation is neither perfectly flexible nor perfectly rigid, the contact pressure distribution depending upon the degree of rigidity. This pressure distribution should be considered when designing for the moments and shears in the foundation, but in order to evaluate shear and vertical stresses below the foundation the assumption of a uniform load inducing a uniform pressure is sufficiently accurate.
A spoil heap, even if compacted, is flexible as far as the supporting soil is concerned. Most existing heaps are of non-uniform section and the stresses induced in the soil below the tip can be approximately determined by superposition in which the tip is divided into a set of equivalent layers. Each layer is assumed to act in turn on the surface of the soil and the total induced
(a) Cohesive soil
(a) Cohesive soil
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