Shear fracture energies of stiff clays and shales

Abstract

Fracture mechanics analysis of shear band propagation requires knowledge of the material’s shear fracture energy and its related properties such as the characteristic slip displacement. Yet these properties of stiff clays and shales have not been investigated systematically. This work characterizes and analyzes the shear fracture energies and characteristic slip displacements of various stiff clays and shales based on their triaxial compression data from the literature. A methodology originally developed for hard rocks was adopted for this purpose. Results show that the shear fracture energies of stiff clays and shales generally increase with the effective normal stress on the slip plane, varying by orders of magnitude—approximately from 4×101 to 7×103 J/m2—in the range of effective normal stresses from 102 to 105 kPa. An empirical equation is presented for a first-order estimate of the shear fracture energy under a given effective normal stress. The characteristic slip displacements at the laboratory scale are calculated to be smaller than 6 mm, and they appear independent of the effective normal stress. Compared with their nominal values calculated without considering the change of normal stress in triaxial tests, the shear fracture energies are approximately 70% of the nominal values, whereas the characteristic slip displacements are nearly identical to the nominal ones.