Bearing Capacity and Shallow Foundations - Elements of Soil Mechanics - page 285

Table 9.3 Undrained shear strength of cohesive soils.

Consistency c u (kPa) Field behaviour

Hard > 300 Brittle

Very stiff 300-150 Brittle or very tough

Stiff 150-75 Cannot be moulded in fingers

Firm 75-40 Can just be moulded in fingers

Soft 40-20 Easily moulded in fingers

Very soft

< 20

Exudes between fingers if squeezed

In the case of variable soil conditions the analysis of bearing capacity is best carried out using a finite
element analysis, although it can be carried out using some form of slip circle method, as described earlier
in this chapter. This approach can take time and designs based on one of the bearing capacity formulae
are consequently quite often used.
For the case of a foundation resting on thin layers of soil, of thicknesses H 1 , H 2 , H 3 , . . . H n and of total
depth H, Bowles ( 1982 ) suggests that these layers can be treated as one layer with an average c value c av
and an average φ value φ av , where

c H c H + + c H

3

+ + c H

1 1 2 2 3 n n

c = av

H

H tan φ + H tan φ + H tan φ

3

+ + H tan φ

− 1 1 1 2 2 3 n n

φ = tan av

H

Vesic ( 1975) suggested that, for the case of a foundation founded in a layer of soft clay which overlies
a stiff clay, the ultimate bearing capacity of the foundation can be expressed as:

q = c N + γ

u u cm z

where c u = the undrained strength of the soft clay and N cm = a modified form of N c , the value of which
depends upon the ratio of the c u values of both clays, the thickness of the upper layer, the foundation
depth and the shape and width of the foundation. Values of N cm are quoted in Vesic's paper.
The converse situation, i.e. that of a foundation founded in a layer of stiff clay which overlies a soft clay,
was studied by Brown and Meyerhof ( 1969) , who quoted a formula for N cm based on a punching shear
failure analysis.
At first glance a safe way of determining the bearing capacity of a foundation might be to base it on
the shear strength of the weakest soil below it, but such a procedure can be uneconomical, particularly
if the weak soil is overlain by much stronger soil. A more suitable method is to calculate the safe bearing
capacity using the shear strength of the stronger material and then to check the amount of overstressing
that this will cause in the weaker layers.

9.9 Estimates of bearing capacity from in situ testing

9.9.1 The plate loading test

The test procedure was introduced in Section 6.6.5 . In the test an excavation is made to the expected
foundation level of the proposed structure and a steel plate, usually from 300 to 750 mm square, is placed
in position and loaded by means of a hydraulic loading system or kentledge. During loading the settle-
ment of the plate is measured and a curve similar to that illustrated in Fig. 9.13 is obtained.