DHARM290 GEOTECHNICAL ENGINEERING
s 31 s 32 s 33 s 11 s 12 s (^13)
t
cu
Apparent cohesion Failure envelope (common tangent)
fu=0
(Total
stresses)
s
Fig. 8.33 Undrained triaxial compression tests on
saturated sands (total stresses)
8.12 Shearing Characteristics of Clays
The understanding of the fundamentals of shearing strength is much more important in the
case of cohesive soils or clays in view of their troublesome nature with regard to stability. In
fact, the most complex physical property of clays is the shearing strength, as it is dependent on
a multitude of inter-related factors. One of the most difficult tasks is to interpret results of
laboratory shearing strength tests to the shearing strength of natural clay deposits.
8.12.1 Source and Nature of Shearing Strength of Clays
Cohesion
This is a characteristic of true clay. This is sometimes referred to as no-load shear strength
and is responsible for the strength of unconfined specimens. Cohesion in clays is a property
which varies considerably with consistency. Cohesion therefore varies with both the type of
clay and condition of clay. It is a kind of surface attraction among particles.
Adhesion
Whereas cohesion is the mutual attraction of two different parts of a clay mass to each other,
clay often also exhibits the property of ‘adhesion’, which is a propensity to adhere to other
materials at a common surface. This has no relation to normal pressure. This is of particular
interest in relation to the supporting capacity of friction piling in clays and to the lateral
pressures on retaining walls.
Viscous friction
Solid friction effects are of relatively minor importance and the effects of viscous friction are
quite pronounced. The laws of viscous friction are, in general, opposite to those of solid friction.
The total frictional resistance is independent of normal force, but varies directly with the
contact area. It varies with some power of the relative velocity of adjacent layers of fluid or
with the rate of shearing. The well-established fact that the strength of saturated clays varies
with consistency also is in accord with the concept that strength is due to viscous rather than
solid friction.
Tensile strength
In varying degrees and for different periods of time, many clays are capable of developing a
certain amount of tensile strength. This may affect the magnitude of normal stresses on failure
planes.