Research On Fracture Cracking Rules Of Jointed Rock Specimens Based On Discontinuous Deformation Analysis

As a common structural form in the surrounding rock of underground caverns,there are many micro-fissures and holes featured by fracture discontinuity inside the fractured rock.The rock mass is composed of rocks and discontinuous structural planes.Many studies have shown that the discontinuous fracture surface in the rock affects the stability of the entire rock system,and plays a leading role in the destruction of the surrounding rock in the actual project.Therefore,it is very necessary to carry out a research on the rupture mechanism of rock fractures.This paper will proceed from two parts--the laboratory test and DDARF numerical simulation,analyze the fracture phenomena of the rock-like specimens with built-in three-dimensional fissures under uniaxial compression conditions with the help of theoretical study,and obtain the crack initiation sequence,expansion phenomena and failure modes of rock specimens of different joint types as well as the mechanical properties during the destruction of the test piece.Therefore,the research content of this paper mainly includes:(1)Making a rock-like test piece that can be used to replace a rock structure based on the characteristic parameters of the study object with similar theory as a guideline.The test pieces is made of cement mortar.To pre-insert a PVC sheet with a width of 15mm into the mold to realize joint penetration.There are 16 kinds of joint arrangements in total according to the number of joints,angles,positions and types of joints.(2)A uniaxially loaded laboratory test of a rock-like specimen with a joint and horizontal orientation of 30°,45°and 60°is carried out.It is found that the failure wing crack of the specimen is connected to the tip of the original joint and is parallel to the direction of the axial force to penetrate the upper and lower ends of the test piece,and the cracking mode is cleavage disruption.Through comparison,it is found that when the joint dip angle is 30°,it is shown as tensile failure,and at this time,the fracture phenomenon of the specimen is obvious.The DDARF numerical simulation is performed on these three angle specimens to get crack propagation law under axial force,and it is found that when the joint dip angle is 30°,the cleavage disruption of the specimen is the most serious,which is consistent with the results of the indoor test.(3)Uniaxial compression tests are performed on specimens containing one,three,and five parallel joints,and it is found that bridge joints occurred between adjacent joints.With the increase of joints,the cleavage disruption of specimens becomes more and more serious.Therefore,when the number of joints is 5,the cleavage disruption phenomenon is most obvious.The DDARF numerical simulation was performed on the specimens containing 2 to5 joints,to get the fracture propagation mode and the expansion sequence of the parallel joints specimens.When the number of joints is 5,the cleavage disruption of the specimens was more thorough,which is consistent with the results of the indoor test.(4)For uniaxial compression tests on intact specimens and single joint specimens containing five different joint locations,it can be found by observing the fracture phenomena and mechanical properties of the specimens that the destruction of the rock-like specimens is brittle failure.When the joints are located in the middle position,the cleavage disruption phenomenon is the most obvious with the largest peak strength at the time of destruction and the strongest influence of cleavage disruption.Comparing the failure characteristics of intact specimens and joint specimens,it can be found that the cleavage phenomenon of intact specimens is more obvious than that of joint specimens,and the peak strength of intact specimens is greater,so the cracks have a reducing effect on the strength of rock.By observing the DDARF numerical simulation results of the specimens with different joint locations,it is found that the expansion of the wing crack propagation is featured by Primary and secondary crack propagation supplementation with the same expansion form as that of the indoor test.(5)For uniaxially loaded indoor test on cross-section joint test specimens,no matter how to change angle,the cracks in the splitting wing parallel to the axial direction are generated on the joints of?(28)45~?,and rock bridges also occur between the two intersecting joints.When?(28)45~?and?(28)45~?,the cleavage disruption of the rock-like specimens are the most serious with the biggest peak strength and the largest influence.Compared with the single“X”joint test piece,the cleavage disruption phenomenon of the double“X”joint test piece is more serious,and the peak strength at the time of destruction is reduced and it is more likely to occur cleavage disruption,which can better verify the conclusion that the fracture has a reduction effect on the rock intensity.The DDARF numerical simulation of the cross-joint test piece gives the rule of the expansion of the fissure,that is,it always extends along the joint angle of 45°,and there is a phenomenon of bridging between adjacent joints,which is highly consistent with the results of the indoor test.