Experimental evaluation of scale effects on the mechanical behavior of rock joints

This article analyzes scale effects on the mechanical behavior of unfilled rock joints during shear and compression tests. The effect of scale on the shear behavior of joints is studied by performing direct shear tests on two sized replicas (10x10 and 20x20 cm(2)) obtained from three different zones of a joint surface whose dimension is 60x60 cm(2). Scale effects are found to be important when the normal stress is low (0.75 MPa); for increasing normal stresses size effects seem to disappear. A series of compression tests on the same replicas have also been carried out, considering three different dimensions (10x10, 20x20 and 60x60 cm2); contact areas are then determined through a technique using ink. The ratio between contact area and nominal area is found to reduce with increasing the dimension of the specimen.