Environmental Engineering Reference
In-Depth Information
Figure 6.3.
Diagrammatic stress displacement behaviour.
Figure 6.4.
Ring shear tests on sand-bentonite mixtures (from Skempton, 1985; after Lupini et al., 1981).
sliding shear when the effects of the silt is overridden by the predominance of clay particles;
and a transition between these two conditions. These results were summarized by Skempton
(1985) and are reproduced in Figure 6.4.
Skempton (1985) presented results of field residual and ring shear tests on a range of
soils. These are reproduced in
Figure 6.5
.
These figures show that “sliding shear” with complete reorientation of the clay particles
is likely to occur only where the clay fraction (finer than 0.002 mm) exceeds 50% of the
total soil, and that for less than about 25% clay fraction, “turbulent” or “rolling” shear
occurs without the influence of clay particle alignment. Note that Figure 6.5 only applies
to clays with a Plasticity Index/Clay Fraction (PI/CF) ratio of 0.5 to 0.9.
Apart from the clay fraction, the mineralogy of the clay also has an effect on residual
strength. This is particularly so when the clay fraction is large. This reflects the fact that the
different clay minerals have different particle shape and different interparticle bonding.
Most clay minerals, e.g. kaolinite, illite, chlorite and montmorillonite are platey structures,
and are therefore subject to alignment when sheared. Montmorillonite has a particularly