Geotechnical Engineering

DHARM

392 GEOTECHNICAL ENGINEERING

below the water table. The consolidation characteristics are determined for representative soil
samples; the compression index and the coefficient of consolidation are evaluated and these
are utilised in the settlement computation. As far as possible undisturbed samples must be
used for this purpose.

11.2.2 Stresses in Subsoil Before and After Loading

The initial and final values of the intergranular pressure, i.e., the effective stresses in the
subsoil before and after loading from the structure, have to be evaluated, since these are nec-
essary for the computation of the settlement.

When there is no horizontal variation in the strata, the total vertical stress at any depth
below ground surface is dependent only on the unit weight of the overlying material. The total
stress, the neutral pressure and the effective stress at the mid-depth of the compressible stra-
tum may be computed and used. However, if the thickness of the stratum is large, these values
must be got at least at the top, middle and the bottom of the stratum and averaged.

The values of neutral pressures and hence the intergranular pressures depend upon the
conditions prior to the loading from the structure. The four possible basic conditions are:

1. The simple static condition


2. The residual hydrostatic excess condition


3. The artesian condition


4. The precompressed condition
   Sometimes combinations of these cases may also occur.

The Simple Static Condition

This is the simplest case and the one commonly expected, the neutral pressure at any
depth being equal to the unit weight of water multiplied by the depth below the free water
surface. It may be noted that the neutral pressure need not be determined in this case since
the intergranular pressure may be obtained by using the submerged unit weight for all the
zones which are below the water table.

The Residual Hydrostatic Excess Condition

A condition of partial consolidation under the preloading overburden exists if part of the
overburden has been recently placed. This kind of situation exists in made-up soil or deltaic
deposits. The hydrostatic excess pressure would have been only partially dissipated in the
compressible clay stratum. The remaining excess pressure is referred to as the ‘residual hy-
drostatic excess pressure’.

Any structure built above such a stratum must eventually undergo not only the settle-
ment caused by its own weight but also the settlement inherent in the residual hydrostatic
excess pressure.

The Artesian Condition

The water pressure at the top of the clay layer normally depends only on the elevation of
the ground water surface, but the neutral pressure at the bottom of the clay stratum may
depend upon very different conditions. A pervious stratum below the clay stratum may extend
to a distant high ground. In such a case the water above it will cause a pressure at the bottom
of the clay stratum which is referred to as the ‘artesian condition’. If the artesian pressure