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Ground Stability, Foundations and Substructures 79
take up the settlement. Designing a foundation is a combined function of both the site
conditions and the characteristics of the particular structure.
Bearing pressures
The intensity of pressure on subsoil is not uniform across the width or length of a foundation
and decreases with depth below the foundation. In order to determine the probable
behaviour of a soil under foundations, the engineer needs to know the intensity of pressure
on the subsoil at various depths. This is determined by Boussinesq’s equation for the stress
at any point below the surface of an elastic body, and in practice is a reasonable approximation
to the actual stress in soil.
By applying the equation, the vertical stress on planes at various depths below a point
can be calculated and plotted as shown in Figure 3.5. The vertical ordinates at each level
d1, d2, etc. represent graphically unit stress at points at that level. If points of equal stress
A, B and C are joined, the result is a bulb of unit pressure extending down from L. If this
operation is repeated for unit area under a foundation, the result is a series of bulbs of equal
unit pressure, as illustrated in Figure 3.6. Thus the bulb of pressure gives an indication of
Point load L
d 1
C
d 2
B
d 3
A
Pressure
bulb
Figure 3.5 Vertical stress distribution.
Pressure p
per unit area
B
B
0.95p
0.9p
0.8p
0.7p
0.6p
0.5p
B
0.1p
0.2p
B
0.3p
0.4p
Figure 3.6 Bulbs of pressure under a strip foundation.