The Non-dilatant Frictional Resistance of Fresh Tension Fractures in Sandstones and Marbles

The aim of this study is to investigate the influence of origin and mineralogy on the dilation-corrected (non-dilatant) frictional resistance of rock joints. The investigated rocks were two pure, dense, high strength and low porosity calcite marbles and two high strength, low porosity sandstones. Samples with different surface roughness were obtained by artificial splitting of prismatic blocks and tested in direct shear, using a multi-stage testing procedure with sample repositioning at their original position prior to changing the normal load. The tests were conducted at a shear displacement rate of 1.0 mm/min, under constant normal stresses in the range 0.005–2.0 MPa. Detailed shear and normal displacements were recorded and the instantaneous dilation angle over the whole shear displacement was accurately calculated. The results are expressed as dilation-free (non-dilatant) frictional resistance, corresponding to the basic friction angle of essentially flat and naturally textured surfaces, i.e., surfaces with their granular texture exposed but without any macroscopic interlocking and roughness causing dilation during shear. The results demonstrate a clear influence of mineralogy on the textural frictional resistance, which was found to be approximately 35° for sandstones and 40° for marbles. In addition, the dilatant component at peak shear strength was found to decrease logarithmically with normal stress. Also, the influence of grain size on the dilatant and the non-dilatant shear strength is discussed. It is believed that the results of the study provide a better understanding of the origin of shear strength of rock joints.