The strength characteristics of rocks at depth can be, in the first instance, best revealed by conducting triaxial tests in the laboratory. From the available literature, triaxial test data of a large number of rocks has been used to from an empirical non-linear strength criterion, which relates the failure strength, σ1 of rocks to the isotropic stress, σ3 (which is γ.H in the case in-situ condition). The criterion is based upon Mohr-Coulomb approach and its applicability has been examined for a wide range of isotropic stress state from 0.1 MPa to 700.0 MPa (i.e. the equivalent depths of about 5 m to 2800 m from the surface, with the average density of rocks being 2.5 kN/m3), for the rocks of various origins and compressive strengths
The criterion proposed by Ramamurthy (1986) and Ramamurthy et al. (1985) is based upon Mohr-Coulomb theory and it takes care of non-linear behaviour of intact rocks as well. The investigators observed linear failure envelopes for test results of selected eighty rocks from the earlier published literature and four sandstones tested by them on a plot between log[σ1-σ3)/σ3] and log (σc/σ3) on ordinate and abscissa, respectively, throughout the range of σc/σ3. The slope of the envelopes, α, has been observed to be varying in a narrow range, an a constant value of 0.8 has been suggested by them for all the rocks. The variation observed in the values of B was from 1.8 to 3.0. In the present work, more triaxial data of various rocks from the published literature has been included to establish the validity of the criterion by suggesting possible modifications. The data has been selected, specially for the rocks tested either in a very low or very high range of the confining pressure to cover a wide range of isotropic stress state, i.e. from brittle to ductile.
To project a conceptual view of the nonlinear strength criterion, the various possible existing conditions in Eqn.(l) are discussed. As the value of B is positive and greater than 1, this equation implies that the deviatoric stress at failure is always more than the compressive strength and is a constant unaffected by the confining pressure. Practically, this stage may occur in a rock under very high confining conditions when (σ1-σ3) does not increase any further by increasing σ3, i.e. ductile condition.
(4) α > 1 and B > 1: This condition of the straight line, passing through the point (l,y) where y > 1, and having a slope greater than 1, in the plot suggests that with increasing confining pressure, there is an exponential decrease in the value of (σ1-σ3) throughout the range of σc/σ3, which is not possible.
(5) α < 1 and B > 1: The envelope represented by this condition is a straight line passing through a point (1,y) where y > 1.
