Shear Strength Criteria for Rock, Rock Joints, Rockfill, Interfaces and Rock Masses

Summary

Although many intact rock types can be very strong, a critical confining pressure can eventually be reached in triaxial testing, such that the Mohr shear strength envelope becomes horizontal. This critical state has recently been better defined, and correct curvature, or correct deviation from linear Mohr-Coulomb has finally been found.

Standard shear testing procedures for rock joints, using multiple testing of the same sample, in case of insufficient samples, can be shown to exaggerate apparent cohesion. Even rough joints do not have any cohesion, but instead have very high friction angles at low stress, due to strong dilation.

Great similarity between the shear strength of rock joints and rockfill is demon-strated, and the interface strength between rockfill and a rock foundation is also addressed.

Rock masses, implying problems of large-scale interaction with engineering structures, may have both cohesive and frictional strength components. However, it is not correct to add these, following linear Mohr Coulomb (M-C) or non-linear Hoek-Brown (H-B) standard routines. Cohesion is broken at small strain, while friction is mobilized at larger strain and remains to the end of the shear deforma-tion. The criterion ‘c then tan ϕ’ should replace ‘c plus tan ϕ’ for improved fit to reality. In all the above, scale effects need to be accounted for.