Civil Engineering Reference
In-Depth Information
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.
3.16b
). 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 dis-
tribution 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.
Exercises
Exercise 3.1
A raft foundation subjects its supporting soil to a uniform pressure of 300 kPa. The
dimensions of the raft are 6.1 m by 15.25 m. Determine the vertical stress increments
due to the raft at a depth of 4.58 m below it (i) at the centre of the raft and (ii) at
the central points of the long edges.
Answer
(i) 192 kPa,
(ii) 132 kPa
Exercise 3.2
A concentrated load of 85 kN acts on the horizontal surface of a soil. Plot the vari-
ation of vertical stress increments due to the load on horizontal planes at depths of
1 m, 2 m and 3 m directly beneath it.
Answer
Fig.
3.17.
Fig. 3.17
Exercise 3.2.
