DHARMSETTLEMENT ANALYSIS 393
below the clay remains constant, it will cause only slight increase in the intergranular pres-
sures and has no tendency to cause consolidation. However, decrease in artesian pressure,
say, due to driving of artesian wells into the pervious stratum below the clay, may cause large
amounts of consolidation.
It can be demonstrated that the basic consolidation relationships are not changed by
the presence of artesian pressure.
The Precompressed Condition
Clay strata might have been subjected in past ages to loads greater than those existing at
present. This precompression may have been caused in a number of ways—by the load of
glaciers of past ages, by overburden which has since been removed by erosion or by the loads of
buildings that have been demolished. The existence and the amount of precompression, char-
acterised by the preconsolidation pressure, are of greater interest than the cause.
The construction of a structure on a precompressed stratum causes recompression rather
than compression. It has been seen in chapter seven that recompression causes relatively
smaller settlements compared to those caused in virgin compression. This aspect, therefore,
has also to be understood in the settlement analysis.
The stresses in the subsoil after loading from the structure can be obtained by comput-
ing the stress increments at the desired depths under the influence of the loading from the
structure; the nature as well as the magnitude of the loading are important in this regard. The
concepts of stress distribution in soil and the methods of determining it, as outlined in chapter
ten, would have to be applied for this purpose. The limitations of the theories and the underly-
ing assumptions are to be carefully understood.
11.3 Settlement
The total settlement may be considered to consist of the following contributions:
(a) Initial settlement or elastic compression.
(b) Consolidation settlement or primary compression.
(c) Secondary settlement or secondary compression.11.3.1Initial Settlement or Elastic Compression
This is also referred to as the ‘distortion settlement’ or ‘contact settlement’ and is usually
taken to occur immediately on application of the foundation load. Such immediate settlement
in the case of partially saturated soils is primarily due to the expulsion of gases and to the
elastic compression and rearrangement of particles. In the case of saturated soils immediate
settlement is considered to be the result of vertical soil compression, before any change in
volume occurs.
Immediate Settlement in Cohesionless Soils
The elastic as well as the primary compression effects occur more or less together in the
case of cohesionless soils because of their high permeabilities. The resulting settlement is
termed ‘immediate settlement’.
The methods available for predicting this settlement are far from perfect; either the
standard penetration test or the use of charts is resorted to.