Total and Effective Stress - Elements of Soil Mechanics - page 87

Fig. 3.15 Pressure bulbs of shear stress.

3.5.6 Shear stresses

In normal foundation design procedure it is essential to check that the shear strength of the soil (see
Chapter 4 ) will not be exceeded. The shear stress developed by loads from foundations of various shapes
can be calculated. Jürgenson obtained solutions for the case of a circular footing and for the case of a
strip footing (Fig. 3.15) . It may be noted that, in the case of a strip footing, the maximum stress induced
in the soil is p/ π , this value occurring at points lying on a semicircle of diameter equal to the foundation
width B. Hence the maximum shear stress under the centre of a continuous foundation occurs at a depth
of B/2 beneath the centre.

Shear stresses under a rectangular foundation

It is sometimes necessary to evaluate the shear stresses beneath a foundation in order to determine a
picture of the likely overstressing in the soil.
Unfortunately a large number of foundations are neither circular nor square but rectangular, but Figs
3.15a and 3.15 b can be used to give a rough estimate of shear stress under the centre of a rectangular
footing.

Example 3.7: Shear stress induced by foundation loading

A rectangular foundation has the dimensions 15 m × 5 m and exerts a uniform pressure
on the soil of 600 kPa. Determine the shear stress induced by the foundation beneath
the centre at a depth of 5 m.

Solution:

Strip footing

z
B = =

5
5

1 0 .