Total and Effective Stress - Elements of Soil Mechanics - page 86

These diagrams are known as bulbs of pressure and constitute another method of determining vertical
stresses at points below a foundation that is of regular shape, the bulb of pressure for a square footing
being obtainable approximately by assuming that it has the same effect on the soil as a circular footing
of the same area.
In the case of a rectangular footing the bulb pressure will vary at cross-sections taken along the length
of the foundation, but the vertical stress at points below the centre of such a foundation can still be
obtained from the charts in Fig. 3.12 by either (i) assuming that the foundation is a strip footing or (ii)
determining σ z values for both the strip footing case and the square footing case and combining them
by proportioning the length of the two foundations.
From a bulb of pressure one has some idea of the depth of soil affected by a foundation. Significant
stress values go down roughly to 2.0 times the width of the foundation, and Fig. 3.13 illustrates how the
results from a plate loading test (see Chapter 6) may give quite misleading results if the proposed founda-
tion is much larger: the soft layer of soil in the diagram is unaffected by the plate loading test but would
be considerably stressed by the foundation.
Boreholes in a site investigation should therefore be taken down to a depth at least 1.5 times the width
of the proposed foundation or until rock is encountered, whichever is the lesser.
Small foundations will act together as one large foundation (Fig. 3.14) unless the foundations are at a
greater distance apart (c/c) than five times their width, which is not usual. Boreholes for a building site
investigation should therefore be taken down to a depth of approximately 1.5 times the width of the
proposed building.

Plate loading test Proposed foundation

Firm soil

Soft layer

Fig. 3.13 Plate loading test may give misleading results.

Fig. 3.14 Overlapping of pressure bulbs.