Experimental Study On Mode ? Fracture Character And Control Mechanism Of Anchorage Sandstone

Jointed rock mass is a complex rock mass which is often encountered in rock engineering. Brittle failure under low stress happened in underground engineering is usually caused by crack propagation of rock mass. Anchor reinforcement measures are often used to make sure the reliability and safe operation of underground engineering and Slope engineering ?n the engineering practices, however it's hard to validate the effect of anchor by present testing method, and the traditional strength theory can't make action principle clear either,while the applied fracture mechanics is an effective method.Consequently, applying rock fracture mechanics theory to study crack propagation and anchorage mechanism of rock mass engineering is of theoretical and practical application significance. ?n this article, the fracture parameters of anchored sandstone and not anchored sandstone were obtained by three-point bending fracture tests, the toughening mechanism of anchor were studied by using net stress intensity factor theory, deformation law of the upper part of crack tip was analyzed by Digital Speckle Method, anchoring mechanism of crack arrest was revealed based on crack propagation characteristics, and made the following conclusions:(1) By selecting the double K fracture criterion with the initiation condition, and determining the critical load, effective crack length,fracture toughness and other parameters under the conditions of initiation and instability, the double K criterion is introduced into the stability evaluation of anchorage rock structure, and the three-point bending fracture test of anchored rock is carried out.(2) Calculated and analyzed anchor sandstone fracture mechanics parameters, for unanchored specimen ,crack initiation stress and unstable fracture toughness reduce with the growth of specimen; with the height of specimen is fixed, crack initiation stress and unstable fracture toughness will keep a constant,and crack initiation stress and unstable fracture toughness reduce firstly then increase with the growth of height of specimen; for anchored specimen,the difference of anchor material has little effect on crack initiation stress and unstable fracture parameters, with the increase of preload and anchor position down, the fracture and instability fracture toughness corresponding increase. (3) The crack propagation law is analyzed by the opening displacement curve and the deformation field evolution, and sandstone crack propagation process can be divided into linear elastic stage before the crack, crack steady extension state after the crack, the unstable expansion stage and residual damage stage of post-peak value.?n the absence of preload, the anchor has a significant effect on the expansion of the fracture after the large deformation has occurred. With the increase of the preload and the anchoring position, the crack suppression effect is more obvious. (4) The anchor toughening mechanism of crack arrest is discussed based on the theory of fracture mechanics: anchored specimen improves whole anchor crack and instability critical value by reducing net crack tip stress intensity factor of rock; closing resistance from anchor transverse action can inhibit the propagation of crack, crack extension is hampered when crack reach anchor bolt.