As noted previously, shearing resistance is determined by effective,
i.e. interparticulate rather than total, stress level. A more appropriate form
of equation (2.6a) is therefore
fcntan (2.6b)
wherecandare the shear strength parameters expressed in terms of
effective stresses, and nis the effective normal stress (equation (2.1)).
Equations (2.6a) and (2.6b) are represented by the failure envelopes
AB and CD respectively on Fig. 2.5(a). Any combination of normal
and shearing stresses represented by a point above the appropriate
Mohr–Coulomb envelope represents a failure state for the soil; a point
below represents a sustainable stress condition.
The envelopes AB and CD may be determined from the results of
laboratory shear tests, e.g. by a triaxial shear test (Fig. 2.5(b)), in which the
applied principal stresses 1 and 2 ( 3 ) acting on a cylindrical specimen
enveloped in a rubber membrane within a water-filled cell (cell pressure,
3 ( 2 )), are controlled in three orthogonal directions. It is an indirect
shear test in that the inclination of the failure plane is not predetermined.
The vertical ( 1 ) and horizontal ( 3 ( 2 )) principal stresses do not corres-
50 EMBANKMENT DAM ENGINEERING
Fig. 2.5 Failure envelopes and the Mohr–Coulomb criterion
