Geotechnical Engineering

DHARM

546 GEOTECHNICAL ENGINEERING

(iii) The effects of many soil characteristics which are likely to influence the bearing
capacity are ignored.

(iv) The codes do not indicate the method used to obtain the bearing capacity values.
(v) The codes assume that the bearing capacity is independent of the size, shape and
depth of foundation. All these factors are known to have significant bearing on the values.

(vi) Building codes are usually not up-to-date.

However, the values given in codes are used in the preliminary design of foundations.

14.5 Analytical Methods of Determining Bearing Capacity

The following analytical approaches are available:

1. The theory of elasticity—Schleicher’s method.


2. The classifical earth pressure theory—Rankine’s method, Pauker’s method and Bell’s
   method.


3. The theory of plasticity—Fellenius’ method, Prandtl’s method, Terzaghi’s method,
   Meyerhof’s method, Skempton’s method, Hansen’s method and Balla’s method.
   Some of these methods will be discussed in the following subsections.

14.5.1 The Theory of Elasticity—Schleicher’s Method
Based on the theory of elasticity and Boussinesq’s stress distribution, Schleicher (1926) inte-
grated the vertical stresses caused by a uniformly distributed surface load and obtained an
expression for the elastic settlement, s, of soil directly underneath a perfectly elastic bearing
slab as follows:

s = K · q · A

E

()1−ν^2
...(Eq. 14.1)
where K = shape coefficient or influence value which depends upon the degree of stiff-
ness of the slab, shape of bearing area, mode of distribution of the total load and the position of
the point on the slab where the settlement is sought;

q = net pressure applied from the slab on to the soil;

A = area of the bearing slab;

E = moduls of elasticity of soil; and

ν = Poisson’s ratio for the soil.

It may be noted that settlements are not the same at all points under an elastic slab,

while settlements are the same under all points of a rigid slab. If, in Eq. 14.1, E
()1−ν^2

is

designated as a constant, C, Schleicher’s equation reduces to:

s = K · qA

C

.

...(Eq. 14.2)

The maximum settlement occurs at the centre of circular and rectangular bearing areas
and the minimum value occurs at the periphery of the circle or at corners of the rectangle.

Schelicher’s shape coefficients, K, are given in Table 14.2.