SHEAR STRENGTH OF WEAKNESS PLANES IN ROCK

The influence of a single plane of weakness on shear strength in direct shear loading is examined. It is shown that total shear strength is determined by fundamental shear strength (cohesion) and internal friction in solid bridges and by joint friction along the separated parts of the weakness plane. If neither of the frictional resistances is mobilized before total loss of cohesion occurs, maximum strength is controlled solely by the tensile strength of the solid bridges. A criterion of failure in direct shear is developed for this case and the theory is compared with results of model tests. It is also shown that deformation requirements for the full realization of joint and internal friction are generally different and hence the two types of frictional resistances should be treated separately in evaluating total shear strength. © 1969.