Stress-Strain Behavior of Jointed Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 62

(3.51)

Naturally appearing discontinuities with uneven surfaces usually exhibit highly irregular
shapes and sliding-up angles i which may vary considerably from place to place. Thus,
nonlinear failure criteria were formulated (Ladanyi & Archambault 1970, Barton 1973,
Schneider 1975, Leichnitz 1981, Erban 1986, Saeb 1989, Maksimovic 1992, Haberfi eld
& Johnston 1994, Kulatilake et al. 1995, Seidel & Haberfi eld 1995, Maksimovic 1996,
Zhao 1997a, Zhao 1997b). Fig. 3.13 (blue line) shows qualitatively such a nonlinear
criterion for peak shear strength in which the friction angle φ D is a function of σ n :

(3.52)

One of the most often used failure criteria for the shear strength of rough discontinui-
ties is the following approach proposed by Barton (1973):

(3.53)

JRC stands for “joint roughness coeffi cient”, which varies from 0 (completely smooth
and planar) to 20 (rough and uneven). A qualitative scale of values for this parame-
ter based on surface profi les suggested in ISRM (1978e) was presented by Barton &
Choubey (1977).

JCS referred to as “joint wall compression strength” is defi ned as the unconfi ned com-
pressive strength of discontinuity surfaces, depending on their degree of weathering.
This parameter controls the stress level

σ n on which the asperities on the discontinuity
surfaces are sheared off. In the case of a completely smooth and planar discontinuity
(JRC = 0) Equation (3.53) takes on the form of the linear τ res - σ n relationship (3.48). The

φ IR' * , depending on the degree of
weathering of the sheared off rock material, is also denoted as the “basic friction angle”
(Barton 1973).
The residual shear strength of natural discontinuities with uneven surfaces may also
be described by a discontinuous failure criterion as in the case of a regular sawtooth
discontinuity (cf. Fig. 3.12). However, because the stress level

residual friction angle of the discontinuity surfaces

σ ns defi ning the transition
from the sliding mode to the shearing-off mode (blue dashed line in Fig. 3.13) cannot
be clearly specifi ed in practice, the residual shear strength is usually described by a linear
criterion (red line in Fig. 3.13):

τ res * = φ IR' * . σ n ⋅ tan (3.54)