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108 Applied Petroleum Geomechanics

k 2 ¼ UCS min =UCS max
where, q // is the strength with the bedding parallel to the sample axis; q t is
the strength with the bedding perpendicular to the sample axis; UCS min is
the minimum strength at any orientation.

3.4 Rock failure criteria

3.4.1 Rock failure types
A rock fails when the surrounding stress exceeds its tensile, compressive, or
shear strength, whichever is reached first. There are several failure types
depending on rock lithology, rock microstructures, and applied stresses.
Jaeger and Cook (1979) described rock failures at various confining pres-
sures, as shown in Fig. 3.17. In unconfined compression (Fig. 3.17A),
irregular longitudinal splitting is observed. With a moderate amount of
confining pressures, the rock failure is characterized by a single plane of
fracture, inclined at an angle of less than 45 degrees to the direction of s 1 ,as
shown in Fig. 3.17B. This is a typical shear failure under compressive
stresses, and a shear displacement along the surface of the shear fracture is
generated. If the confining pressure is increased so that the rock becomes
fully ductile (Jaeger et al., 2007), a network of shear fractures accompanied
by plastic deformation appears, as shown in Fig. 3.17C. The second basic
type of failure is tensile failure, which appears typically in uniaxial tension.
Its characteristic feature is a clean separation with no offset between the
surfaces (Fig. 3.17D). If a slab is compressed between line loads as shown in
Fig. 3.17E, a tensile fracture appears between the loads.
There are various failure (strength) criteria applied to compare the
stresses to rock strength to determine whether the rock fails or not. For the
porous media the effective stress concentrations near an underground

Figure 3.17 Rock failure types. (A). splitting; (B). shear failure; (C). multiple shear
fractures; (D). tensile failure; (E). tensile failure induced by point loads.