7.1 Introduction
When a structure is placed on a foundation consisting of soil, the loads from the structure
cause the soil to be stressed. The two most important requirements for the stability and safety
of the structure are: (1) The deformation, especially the vertical deformation, called ‘settle-
ment’ of the soil, should not be excessive and must be within tolerable or permissible limits;
and, (2) The shear strength of the foundation soil should be adequate to withstand the stresses
induced.
The first of these requirements needs consideration and study of the aspect of the
“Compressibility and Consolidation of soils” and forms the subject matter of this chapter. The
second needs consideration of the aspects of shear strength and bearing capacity of soil, which
are dealt with in subsequent chapters.
The nature of the deformation of soil under compressible loads may be elastic, plastic or
compressive, or a combination of these. Elastic deformation causes lateral bulging with little
change of porosity and the material recovers fully upon removal of the load. Plastic deforma-
tion is due to the lateral flow of the soil under pressure with negligible rebound after removal
of load. ‘Plasticity’ is the property by which the material can undergo considerable deforma-
tion before failure. Clays exhibit this property to a greater or smaller degree at moisture con-
tents greater than the plastic limit. Compressive deformation occurs when the particles are
brought closer together by pressure causing volume changes in the soil. The property of a soil
by virtue of which volume decrease occurs under applied pressure is termed its ‘Compressibility’.
Since natural soil deposits are laterally confined on all sides, deformation under stress
is primarily associated with volume changes, specifically, volume decrease.
7.2 Compressibility of Soils
A soil is a particulate material, consisting of solid grains and void spaces enclosed by the
grains. The voids may be filled with air or other gas, with water or other liquid, or with a
combination of these.
Chapter 7
COMPRESSIBILITY AND
CONSOLIDATION OF SOILS
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