Geotechnical Engineering

DHARM

296 GEOTECHNICAL ENGINEERING

Normal pressure or deviator stress

s

Strain Î

(a)

Pore water pressure

Strain Î

u

(b)

+

Fig. 8.42 Stress-strain relationships for a normally consolidated clay

Normal pressure ordeviator stress

s

Strain Î

Power water pressure

Strain

Î

+

Fig. 8.43 Stress-strain relationships for an overconsolidated clay

Effect of rate and nature of shear strain
Clays are often sensitive to the rate and manner of shearing. Usually standard rates of shear-
ing are adopted for proper comparison. A strain of about 0.10 to 0.15 cm/min., is considered
standard in strain-controlled direct shear. However, it is not common that strain is controlled
in nature or in construction operations.
It is observed that shear strength increases somewhat with increased rates of strain. If
the loading is not at a uniform rate but is effected in increments, much greater shearing resist-
ance is developed; however, the failure in such a case is observed to occur rather suddenly. The
increase in shear strength could be as much as 25% with increase in rate of strain from a very
slow rate; this increase would be as high as 100% or more if the loading is by increments.
If there is interruption of strain, the shear stress could decrease steadily by a creep in
saturated clays; but in the case of sands, this will not have any significant effect on shearing
stress.
Also, greater shearing displacements are associated with smaller rates of shearing strain
and vice versa. This is also in contrast to the behaviour of sand for which these factor do not
appear to materially affect the results.

Sensitivity of clays

If the strength of an undisturbed sample of clay is measured and its strength is again meas-
ured after remoulding at the same water content to the same dry density, a reduction in strength