Continuous failure state direct shear tests

Summary The paper deals with a new testing procedure aimed at determining the failure envelopes for peak and residual strength in direct shear tests. In a continuous failure state direct shear test the specimen is maintained in a state of permanent sliding while the shear and normal stress are being steadily changed. First of all, the specimen is brought to a state of failure in the conventional manner at a chosen constant normal stress. Then under monotonously increasing shear displacement the normal stress is continuously adjusted so that a straight line is produced in the shear stress-displacement plane. Both an increase in the stresses and a decrease is possible. The proper selection of the inclination of the straight line may involve a stress path which corresponds closely to the failure envelope of the specimen. In the case of smooth joint surfaces or in the residual strength state of rough surfaces it is possible to determine exactly the failure envelope with the aid of a single test specimen. The paper also describes a newly developed shear test apparatus suitable for combination with sophisticated servo-controlled loading machines generally available in rock mechanics laboratories.