Fracture Criteria for Fissured Clays under Mixed-Mode Loading

Stiff clays forming part of earth dams and natural slopes contain fissures or cracks in their structure. These cracks are subjected in the field to a combination of normal and shear stresses (mixed-mode type of loading). Under tis type of loading, the cracks concentrate stresses at their tips that cause the failure of the intact clay surrounding the cracks. This failure is in the form of secondary cracks that propagate from the tips of the original cracks. Six fracture criteria exist to determine the direction of secondary cracks propagating in brittle materials. This study presents a review of three of these fracture criteria that are applicable to materials such as stiff clays. Using uniaxial and triaxial compressive tests on samples of stiff kaolinite clay with a pre-existing crack, the direction of crack propagation in the samples was best predicted by the Maximum Tangential Stress Criterion for sharp cracks.