11.1 INTRODUCTION
Foundations of all structures have to be placed on soil. The structure may undergo settlement
depending upon the characteristics such as compressibility of the strata of soil on which it is
founded. Thus the term ‘settlement’ indicates the sinking of a structure due to the compres-
sion and deformation of the underlying soil. Clay strata often need a very long time—a number
of years—to get fully consolidated under the loads from the structure. The settlement of any
loose strata of cohesionless soil occurs relatively fast. Thus, there are two aspects—the total
settlement and the time-rate of settlement—which need consideration.
If it can be assumed that the expulsion of water necessary for the consolidation of the
compressible clay strata takes place only in the vertical direction, Terzaghi’s theory of one-
dimensional consolidation may be used for the determination of total settlement and also the
time-rate of settlement.
Depending upon the location of the compressible strata in the soil profile relative to the
ground surface, only a part of the stress transmitted to the soil at foundation level may be
transmitted to these strata as stress increments causing consolidation. The theories of stress-
distribution in soil have to be applied appropriately for this purpose. The vertical stress due to
applied loading gets dissipated fast with respect to depth and becomes negligible below a cer-
tain depth. If the compressible strata lie below such depth, their compression or consolidation
does not contribute to the settlement of the structure in any significant manner.
There is the other aspect of whether a structure is likely to undergo ‘uniform settle-
ment’ or ‘differential settlement’. Uniform settlement or equal settlement under different points
of the structure does not cause much harm to the structural stability of the structure. How-
ever, differential settlement or different magnitudes of settlement at different points under-
neath a structure—especially a rigid structure is likely to cause supplementary stress and
thereby cause harmful effects such as cracking, permanent and irreparable damage, and ulti-
mate yield and failure of the structure. As such, differential settlement must be guarded against.
11.2 DATA FOR SETTLEMENT ANALYSIS
A procedure for the computation of anticipated settlements is called ‘Settlement analysis’. This
analysis may be divided into three parts. The first part consists of obtaining the soil profile,
Chapter 11
SETTLEMENT ANALYSIS
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