Shear Strength of Soils - Elements of Soil Mechanics - page 131

tests, and conduct an effective stress analysis. For this analysis the shear strength of the soil, as we have
already seen, can be expressed by the equation:

= ′ + ′

′

τ σ tan φ

p

There have been many cases of slips in clay slopes which have afforded a means of checking this pro-
cedure. Knowing the mass of material involved and the location of the slip plane, it is possible to deduce
the value of the average shear stress on the slip plane, τ , at the time failure occurred. It has often been
found that τ is considerably less than τ p especially with slopes that have been in existence for some years.
Figure 4.40a shows a typical stress-to-strain relationship obtained in a drained shear test on a clay.
Normal practice is to stop the test as soon as the peak strength has been reached, but if the test is con-
tinued it is found that as the strain increases the shear strength decreases and finally levels out. This
constant stress value is termed the critical, or constant volume, strength, τ cv , of the clay. If the strain
increases significantly, the clay will reach a state of lowest strength known as the residual strength. The
strength envelopes from the three sets of strength values are shown in Fig. 4.40b .
Residual strength tests can be carried out in the ring shear apparatus, which was developed in the
1930s. A thin annular soil specimen is sheared by clamping it between two metal discs, which are then
rotated in opposite directions. The apparatus did not become popular, mainly because of the concentra-
tion at the time on the study of peak values, so readily obtained from the triaxial test, but probably also
because the ring shear apparatus was complicated and it took a long time to carry out a test.
As a result of Skempton's work in the 1960's, interest in the determination of soil strength after large
displacement was re-established and, in 1971, Bishop et al . redeveloped the ring shear apparatus
(Fig. 4.41) , which is now considered as the most reliable means for determining residual strengths of
cohesive soils.

Peak strength Peak strength

φ′ peak

Critical strength

Critical strength

φ′ cv

Residual
strength

Residual
strength

φ′ R

Strain Effective normal stress

(a) (b)

Fig. 4.40 The peak, critical and residual strengths of clays.

Fig. 4.41 Ring shear test sample (after Bishop et al ., 1971) .