Civil Engineering Reference
In-Depth Information
Compression and swelling
8.1 Introduction
As soils are loaded or unloaded isotropically (i.e. with equal all-round stresses) or
anisotropically they will compress and swell. As we saw in Chapter 6, volume changes
in saturated soils involve rearrangement of grains together with breakage of weak sand
grains and swelling or compression of clay grains accompanied by seepage of water
as shown in Fig. 8.1. Volume changes occur in unsaturated soils as air is expelled.
Compression of unsaturated soil is important mostly during compaction of excavated
soil in to embankment fill. This is described in Chapter 26.
To account for seepage flow it is necessary to consider the relative rates of loading
and drainage as discussed in Sec. 6.10; this is equally true for laboratory tests and
for loadings of structures in the ground. In laboratory tests the sample may be loaded
undrained and then allowed to consolidate under constant total stress; this is the basis
of the conventional incremental loading oedometer test described in Chapter 7. In this
case measurements of effective stress can only be made at the end of consolidation
when all the excess pore pressures have dissipated (unless the excess pore pressures
are measured separately). Alternatively, the loading could be applied at a continuous
rate and the excess pore pressures measured. It is simplest, however, to load samples
fully drained at a rate that is slow enough to ensure that any excess pore pressures are
negligible so that effective stresses can be determined. I will consider the behaviour of
soil during incremental and continuous loading consolidation tests in Chapter 15; for
the present I will consider only fully drained states where excess pore pressures are
zero. The idealized behaviour described in this chapter is based on experimental data
given by Atkinson and Bransby (1978) and by Muir Wood (1991).
8.2 Isotropic compression and swelling
The general behaviour of soil during isotropic compression and swelling is illustrated
in Fig. 8.2. This shows soil in which the grains are loosely packed, initially at
p
0
at
O compressed to A, unloaded to B and reloaded through C to D where the grains are
more densely packed. This behaviour is similar to that illustrated in Fig. 3.12 and C is
a yield point.
Soil compression is primarily caused by rearrangement of the grains and so the
stiffness will increase from loose states (where there are plenty of voids for grains
