Geotechnical Engineering

DHARM

COMPRESSIBILITY AND CONSOLIDATION OF SOILS 203

The volume decrease of a soil under stress might be conceivably attributed to:

1. Compression of the solid grains;


2. Compression of pore water or pore air;


3. Expulsion of pore water or pore air from the voids, thus decreasing the void ratio or
   porosity.

Under the loads usually encountered in geotechnical engineering practice, the solid
grains as well as pore water may be considered to be incompressible. Thus, compression of
pore air and expulsion of pore water are the primary sources of volume decrease of a soil mass
subjected to stresses. A partially saturated soil may experience appreciable volume decrease
through the compression of pore air before any expulsion of pore water takes place; the situa-
tion is thus more complex for such a soil. However, it is reasonable to assume that volume
decrease of a saturated soil mass is, for all practical purposes, only due to expulsion of pore
water by the application of load. Sedimentary deposits and submerged clay strata are invari-
ably found in nature in the fully saturated condition and problems involving volume decrease
and the consequent ill-effects are associated with these.

Specifically, the compressibility of a soil depends on the structural arrangement of the
soil particles, and in fine-grained soils, the degree to which adjacent particles are bonded
together. A structure which is more porous, such as a honey-combed structure, is more com-
pressible than a dense structure. A soil which is composed predominantly of flat grains is more
compressible than one with mostly spherical grains. A soil in a remoulded state may be more
compressible than the same soil in its natural state.

When the pressure is increased, volume decrease occurs for a soil. If the pressure is
later decreased some expansion will take place, but the rebound or recovery will not occur to
the full extent. This indicates that soils show some elastic tendency, but only to a small degree.
It is rather difficult to separate the elastic and inelastic compression in soils.

There is another kind of volume rebound shown by fine-grained soils. Water held be-
tween the flaky particles by certain forces gets squeezed out under compression. When the
stress is removed, these forces cause the water to be sucked in again, resulting in the phenom-
enon of ‘swelling’. Expulsion and sucking of water may take a very long time.

The process of gradual compression due to the expulsion of pore water under steady
pressure is referred to as ‘Consolidation’, which is dealt with in later sections. This is a time-
dependent phenomenon, especially in clays. Thus, the volume change behaviour has two dis-
tinct aspects: first, the magnitude of volume change leading to a certain total compression or
settlement, and secondly, the time required for the volume change to occur under a particular
stress.

The process of mechanical compression resulting in reduction or compression of pore air
and consequent densification of soil is referred to as ‘Compaction’, and it is dealt with in a later
chapter.

In sands, consolidation may be generally considered to keep pace with construction;
while, in clays, the process of consolidation proceeds long after the construction has been com-
pleted and thus needs greater attention.