Study On Micro-contact Evolution And Shear Mechanical Model Of Fractured Rock Surfaces

The shear strength and long-term stability of fractured rock mass are largely depended on the micro contact features of the fractured surfaces. In order to reveal the relationship between the contact status and the shear contact mechanics from the micro level of scope, creep shear experiment was carried out with two kinds of fractured rock samples. The fractures were induced into the rock samples by modified shear and Brazilian split tests, respectively. By means of laser scanning, acoustic emission and CT detection, the micro contact mechanism and the macro mechanical characteristics were studied including the wear down of the asperities, the grow tendency of the micro voids and cracks and the evolution of the surface damage. With respect to the discrete distribution of the micro contacts on the fractured surfaces, fractal principles were adopted and equations were derived to feature to contact status of the fractures. Besides, the contact mechanics model of the interlocks of the asperities was developed. The work in this paper could be as the theoretical basis to depict the micro evolution process and macro mechanical behaviors of the fractured surfaces.The main research achievements are listed as follows.(1) The overall morphology of the fractured rock surfaces were observed and analyzed quantitatively. Two types of fractures were induced in rock samples by means of modified shear and indirect tension. The morphology of the fractured surfaces was reconstructed using the data obtained by laser scanning. The waviness and roughness of the fractured surfaces were analyzed quantitatively based on fractal theory.(2) By means of creep shear experiments, the macro mechanical behavior of the fractured rocks was studied. Shear loadings were induced step by step, thus, the displacement and the shear strength of the fractured rocks were obtained. Based on the elaborations of the micro formation mechanism, the macro mechanical characteristics of these two kinds of fractured surfaces were revealed.(3) By means of ’CT slice method’, the damage modes of these two kinds of fractured rocks in creep shear experiments were summarized. The main law of the damage process are summarized as the shear damage of the fractured surfaces, the wear down and transfer of the micro asperities, the formation of the sub-surface cracks, and the grow and evolution of the two-wing fractures. Thus, the micro contact structures of the fractured surfaces, the spatial morphology and 3D damage status within the fractured rocks were demonstrated.(4) With acoustic emission technology, the acoustic frequency and the energy released in the creep shear experiments were captured and analyzed. For rocks damaged by indirect tension, the energy peak of the acoustic emission appears in the middle and late loading stages. While for rocks damaged by modified shear, low ebb of acoustic energy would appear in the middle loading stage. It was found that the damage locations on the fractured surfaces tend to be belt-like distributions during the creep shear.(5) The distribution status of the contact areas on different cross sections with different vertical position was extracted and analyzed. Results show that, for the rock surfaces fractured by indirect tension, the two-wing contact areas tends to distribute as island shapes, while for the rock surfaces fractured by modified shear, the contact morphology shows an irregular coastline shape. Based on the evolution characteristics of the micro contacts, the relationship between the contact area and the shear strength of the fractured rocks was established.(6) Based on the principle of fractal geometry and contact mechanics, the fractal contact model of fractured rock surfaces in creep shear was built. The contact area of the fractured surfaces evolves and becomes larger with the increase of normal force. The contact of modified shear fractured surfaces was macrowaved areas, while the contact of indirect tension fractured surfaces tends to be discrete micro asperities. Through this study, the variation law of the fractal dimensions with regard to the contacts on fractured surfaces was obtained. Moreover, the relationship between the creep shear loadings and the fractal dimensions was finally derived and validated.