pond to the normal and shear stresses on the failure plane, which must
therefore be obtained indirectly, e.g. by the construction of a Mohr circle
plot (Fig. 2.5(c)) or ‘p–q’ plot (not shown). The stress–strain response of
soils plotted from triaxial tests is essentially curvilinear, i.e. there is little
elastic response to load, as shown in Fig. 2.6.
Coarse soils such as sands derive their shear strength essentially from
particle interlock and internal friction, and are therefore termed cohesion-
less (c0) or frictional soils. When loaded under conditions of no
drainage, saturated clays may appear to possess cohesion only. Clays are
frequently identified in generic terms as cohesive soils (c0,0). Soils
of intermediate type, including the majority of ‘cohesive’ soils, will exhibit
both cohesion and internal friction (a ‘c–’ soil). The shearing behaviour
of each type can be represented through the Mohr–Coulomb relationships
(equations 2.6(a) and 2.6(b)), giving the example failure envelopes illus-
trated in Fig. 2.5.
The measured shear strength of frictional soils is controlled largely
by density, a higher density giving a greater angle of shearing resistance,
(Fig. 2.6(a)).
Most engineering problems occur with fine cohesive soils, and arise
from the nature of the clay particles. Because of their low permeability
and strong affinity for water, natural clay soils usually occur in a saturated
or near-saturated state. High porewater pressures are generated by
changes in external loading conditions, including construction operations,
and are very slow to dissipate. There is a clear relationship between shear
strength and increasing water content: at high water contents the cohesive
forces between clay particles rapidly weaken, resulting in very much
reduced shear strengths.
The important factor influencing the shear strength and consolida-
tion characteristics (Section 2.3.3) of a saturated clay is stress history
rather than density. If the present in situeffective stresses are the greatest
to which the clay has historically been subjected the clay is described as
ENGINEERING CHARACTERISTICS OF SOILS 51
Fig. 2.6 Soil stress–strain response curves