Experimental Study Of Effects Of Fracture Geometrical Characteristics On Mechanical Properties And Crack Propagation Laws Of Rock Masses

Rock strength and deformation properties are influenced to a large extent by fracture geometrical characteristics and stress condition. Besides, the instability and failure of fractured rock masses are ultimately caused by the extension and convergence of structural planes in rock masses. In this study, pure cement specimens with pre-existing flaws were made as a rock-like material to simulate brittle rock. Effects of fracture geometrical characteristics(inclination angles, connectivity rates, openings, spacings, sets and shapes) and confining pressures on mechanical properties of rock masses with fractures were investigated systematically by conducting uniaxial and axial compression tests. At the same time, the crack propagation and failure mechanism of fractured rock masses under uniaxial compression were analyzed by using FLAC3 D, a finite element numerical simulation software, and the Vic-3D(video image correlate-3D) system, a non-contact and full-field strain measurement system. The main works in this paper and conclusions have been obtained are presented as follows:(1) Analyses of rock mechanical properties and crack propagation mechanism. Associating with fracture geometrical characteristics and rock failure process, mechanical constitutive models of fractured rock masses and crack propagation mechanism were theoretically analyzed.(2) Effects of fracture geometrical characteristics and confining pressures on peak strengths and elastic moduli of specimens containing flaws were investigated systematically by conducting uniaxial and axial compression tests. Under uniaxial compression, the variation curve of peak strengths of specimens containing flaws with different fracture inclination angles is roughly V-shaped with the minimum located at 45°. With the increasing of fracture inclination angle, elastic moduli generally increased and only had a slight fall at the minimum peak strength point. When it comes to axial compression, the variation rule is nearly the same as the uniaxial condition except that the corresponding minimum changed to 30°. By comparing the fitting function curves of the variation rules between reduction coefficients of rock mechanical parameters defined in this paper and different fracture geometricalcharacteristics, the sensitivities of rock mechanical parameters at different fracture geometrical characteristics can be ranged as follows: sets > spacings > connectivity rates > openings > shapes. From the perspective of the specimen failure mode, specimens under uniaxial compression mainly take on splitting tensile mode or mixed mode(shear and tensile), while specimens under axial compression all take on shear mode. Some of the specimens under axial compression damaged twice and the residual strengths increased substantially due to the confining pressure, which indicates the plasticity increased.(3) By using the Vic-3D system and FLAC3 D numerical simulation, four types of crack were observed in uniaxial compression tests, i.e., tensile crack, wing crack, anti-wing crack and shear crack, among which wing cracks seem to dominate the crack propagation. Specifically, the stress concentration around the flaw tips caused the cracks to initiate from the tips of the flaws, to propagate parallel to the loading direction and to coalesce between flaws or flaw and free surface, cutting through the rock bridges, which ultimately led to the failure of specimens.