The Tensile Failure Mode And Unloading Mechanical Properties Of Thermally Treated Deep-seated Granite

The exploitation of mineral resources and the construction of underground projects are going deep,where the engineering excavation involving deep-seated rock masses differs from shallow engineering activities and is faced with two problems: on the one hand,the deep-seated rock masses are generally in a special environment of high formation temperature;on the other hand,the artificial engineering excavation is a kind of unloading behaviors different from loading.With this regard,the three-point bending test and the triaxial confining-pressure unloading test were conducted on the thermally treated granite samples.In this way,the influence law of temperature on fracture behaviors of deep-seated rock masses and the mechanical properties of these thermally treated rock masses under unloading conditions were explored.The research results showed that(1)X-ray diffraction(XRD)and scanning electron microscopy(SEM)revealed that the mineral compositions and internal microscopic physical properties of granite samples changed before and after thermal treatment,thereby causing changes in the macroscopic physico-mechanical properties.The analysis results of the physical properties provide necessary data support for future research.(2)In the three-point bending test,the tensile strength of the granite samples with pre-fabricated cracks of different angles decreased with rising temperature.Meanwhile,the temperature imposed different influences on tensile strength of the rocks in different fracture modes,among which the high temperature of 900℃ gradually exerted the most significant influence on fracture toughness of the rock featuring mode-I fracture.(3)Compared with the conventional triaxial compression test,the stress-strain curve in the confining-pressure unloading test did not show apparent peak stress points.Instead,it had an apparent yield plateau.With the gradual decreasing confining pressure in the confining-pressure unloading test,the volume of the rocks expanded,the circumferential deformation increased,and an obvious stress drop point was observed,indicative of apparent brittle failure characteristics of the rocks.(4)In the triaxial confining-pressure unloading test,the volumetric strain of the granite samples did not increase apparently in the initial stage of unloading.The volumetric strain and the confining pressure did not show the linear relationship any longer with the gradual decrease of the latter.As the confining pressure was unloaded to a certain extent,the samples exhibited abruptly growing volumetric strain till they were broken.(5)The strain-confining pressure compliance of the samples exhibited basically same changes in the triaxial confining-pressure unloading test.Compared with changes in the axial strain-confining pressure compliance,the radial and volumetric strain-confining pressure compliance were much larger,suggesting that the radial strain of the granite samples was more sensitive to changes in the confining pressure.(6)In the triaxial confining-pressure unloading test,the elastic modulus of the granite samples showed the same change trend with the Poisson’s ratio: they both changed slightly in the initial stage of unloading,while they decreased rapidly with the further reduction of the confining pressure till the rocks were damaged.