10.1 INTRODUCTION
Stress in soil in caused by the first or both of the following:
(a) Self-Weight of soil.
(b) Structural loads, applied at or below the surface.
Many problems in foundation engineering require a study of the transmission and dis-
tribution of stresses in large and extensive masses of soil. Some examples are wheel loads
transmitted through embankments to culverts, foundation pressures transmitted to soil strata
below footings, pressures from isolated footings transmitted to retaining walls, and wheel
loads transmitted through stabilised soil pavements to sub-grades below. In such cases, the
stresses are transmitted in all downward and lateral directions.
Estimation of vertical stresses at any point in a soil mass due to external loading is
essential to the prediction of settlements of buildings, bridges and embankments. The theory
of elasticity, which gives primarily the interrelationships of stresses and strains (Timoshenko
and Goodier, 1951), has been the basis for the determination of stresses in a soil mass. Accord-
ing to the elastic theory, constant ratios exist between stresses and strains. For the theory to
be applicable, the real requirement is not that the material necessarily be elastic but that
there must be constant ratios between stresses and the corresponding strains.
It is known that, only at relatively small magnitudes of stresses, the proportionality
between strains and stresses exists in the case of soil. Fortunately, the order of magnitudes of
stresses transmitted into soil from structural loadings is also small and hence the application
of the elastic theory for determination of stress distribution in soil gives reasonably valid re-
sults.
The most widely used theories regarding distribution of stress in soil are those of
Boussinesq and Westergaard. They have developed first for point loads and later, the values
for point load have been integrated to give stresses below uniform strip loads, uniformly loaded
circular and rectangular areas.
The vertical stress in soil owing to its self-weight, also called ‘geostatic stress’ (already
dealt with in Chapter 5), is given by:
σz = γ. z ...(Eq. 10.1)Chapter 10
STRESS DISTRIBUTION IN SOIL
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