Civil Engineering Reference
In-Depth Information
Critical state strength of soil
9.1 Behaviour of soil in shear tests
In simple terms the strength of amaterial is the maximum shear stress that it can sustain;
materials loaded just beyond the maximum stress will fail. Failure may be sudden and
catastrophic leading to a complete loss of strength (which is what happens when you
break a biscuit, which is brittle) or it may lead to a very large plastic straining (which
is what happens if you mould plasticine, which is ductile). For most soils, failure of
slopes and foundations involves large straining without complete loss of strength and
failing soil structures can usually be stabilized by unloading them.
The essential features of soil strength can most easily be seen in ideal shearing tests,
as illustrated in Fig. 9.1. The shear and normal effective stresses are
τ
and
σ
and,
at a particular stage of the test, there are increments of strain
v
. These are
similar to the conditions in the direct shear box test and the simple shear test described
in Chapter 7 and in soil in thin slip surfaces that occur during failure of slopes as
described in Chapter 21. The conventional direct and simple shear tests are, however,
not ideal because the stresses and deformations are likely to be non-uniform and the
states of stress and strain are not completely defined by the measurements on only one
plane. Although a shear test is not ideal for measuring soil properties it is, however,
convenient for demonstrating the basic characteristics of soil strength.
Typical stress-strain curves for soils on the wet side of critical (i.e. normally consol-
idated or lightly overconsolidated clays or loose sands) marked W and for soils on the
dry side (i.e. heavily overconsolidated clays or dense sands) marked D, tested drained
with constant
δγ
and
δε
σ
, are shown in Fig. 9.1(b) and the corresponding volumetric strains are
shown in Fig. 9.1(c). (Remember the distinctions between the wet side of the critical
line and the dry side, discussed in Sec. 8.4.) The behaviour shown in Fig. 9.1 is typical
for normally consolidated or overconsolidated clays as well as for loose or dense sands.
Soils on the wet side compress as the shear stresses increase while soils on the dry side
dilate (expand) after a small compression. Both ultimately reach critical states at which
the shear stress is constant and there are no more volumetric strains. Soils on the dry
side reach peak shear stresses before reaching the critical state. Remember that strength
is the maximum shear stress which a material can sustain so for soil there is a peak
strength and a critical state strength. There is also a residual strength which will be
discussed in Sec. 9.2. At any stage of shearing the angle of dilation
ψ
(see Sec. 2.7) is
