Strength and failure modes of rock mass models with non-persistent joints

Most problems faced by the practicing rock engineer involve the evaluation of rock mass strength and deformability. The theoretical evaluation of the mechanical properties of fractured rock masses has no satisfactory answer because of the great number of variables involved. One of these variables, the influence of which over rock mass behavior is poorly documented, is the degree of fracture persistence. This paper presents the results of biaxial tests performed on physical models of rock with non-persistent joints. The failure modes and maximum strengths developed were found to depend on, among other variables, the geometry of the joint systems, the orientation of the principal stresses, and the ratio between intermediate stress and intact material compressive strength (σ₂/σ_c). Tests showed three basic failure modes: failure through a planar surface, stepped failure, and failure by rotation of new blocks. Planar failure and stepped failure are associated with high strength behavior, and small failure strains, whereas rotational failure is associated with a very low strength, ductile behavior, and large deformation.