Friction and degradation of rock joint surfaces under shear loads

The morpho‐mechanical behaviour of one artificial granite joint with hammered surfaces, one artificial regularly undulated joint and one natural schist joint was studied. The hammered granite joints underwent 5 cycles of direct shear under 3 normal stress levels ranging between 0.3 and 4 MPa. The regularly undulated joint underwent 10 cycles of shear under 6 normal stress levels ranging between 0.5 and 5 MPa and the natural schist replicas underwent a monotonics shear under 5 normal stress levels ranging between 0.4 and 2.4 MPa. These direct shear tests were performed using a new computer‐controlled 3D‐shear apparatus. To characterize the morphology evolution of the sheared joints, a laser sensor profilometer was used to perform surface data measurements prior to and after each shear test. Based on a new characterization of joint surface roughness viewed as a combination of primary and secondary roughness and termed by the joint surface roughness, SR_s, one parameter termed ‘joint surface degradation’, D_w, has been defined to quantify the degradation of the sheared joints. Examinations of SR_s and D_w prior to and after shearing indicate that the hammered surfaces are more damaged than the two other surfaces. The peak strength of hammered joint with zero‐dilatancy, therefore, significantly differs from the classical formulation of dilatant joint strength. An attempt has been made to model the peak strength of hammered joint surfaces and dilatant joints with regard to their surface degradation in the course of shearing and two peak strength criteria are proposed. Input parameters are initial morphology and initial surface roughness. For the hammered surfaces, the degradation mechanism is dominant over the phenomenon of dilatancy, whereas for a dilatant joint both mechanisms are present. A comparison between the proposed models and the experimental results indicates a relatively good agreement. In particular, compared to the well‐known shear strength criteria of Ladanyi and Archambault or Saeb, these classical criteria significantly underestimate and overestimate the observed peak strength, respectively, under low and high normal stress levels. In addition and based on our experimental investigations, we put forward a model to predict the evolution of joint morphology and the degree of degradation during the course of shearing. Degradations of the artificial undulated joint and the natural schist joint enable us to verify the proposed model with a relatively good agreement. Finally, the model of Ladanyi and Archambault dealing with the proportion of total joint area sheared through asperities, a_s, once again, tends to underestimate the observed degradation. Copyright © 2001 John Wiley & Sons, Ltd.