Study On Mechanical Properties And Propagation Failure Law Of Sandstone Caused By Cross Fracture

Cross-fractured rock mass is a common rock type with obvious geometric distribution and distinct primary and secondary division in jointed rock mass.In the process of oil and gas exploitation,due to the application of external load,the failure of complex rock strata and the instability of fractured rock mass seriously affect the smooth progress of engineering.Therefore,it is very important to study the mechanical properties and failure laws of rock mass with cross fracture.Based on the similarity principle,sandstone samples containing different primary and secondary crack lengths and dip angles of primary and secondary cracks were made in this paper,and uniaxial compression tests were carried out.During the test,DIC system was used to observe the failure process of the samples,and the mechanical properties,failure modes and crack initiation and propagation of rock mass with cross cracks were explored.The main conclusions are as follows:(1)The stress-strain curve of the sample with cross cracks will have a stress recovery stage,because in the early loading stage,the prefabricated crack with a long projection length along the horizontal direction will be pressed to closure due to the increase of stress level,and then the sample will be damaged and appear macro cracks.Since there is another prefabricated crack,Therefore,in the late loading period,another crack with a shorter projection length along the horizontal direction bears the stress,and thus a bimodal curve appears.(2)The peak strength of the samples with cross cracks is less than that of the intact and single cracks.The strength of the sample increases with the increase of the dip Angle of the primary crack.With the increase of the inclination Angle of the primary crack,the elastic modulus of the sample presents a "U" shaped distribution law,which firstly decreases and then increases.The peak strength and elastic modulus of the samples are controlled by the crack length and crack Angle,but the crack Angle has a more significant effect.(3)Tensile failure occurs in most samples,with fewer surface blocks;The failure of the sample is controlled by a prefabricated crack with a long projection length along the horizontal direction,and the tensile crack of the airfoil is generated and destroyed by the end of the prefabricated crack with a long projection length along the horizontal direction.With the increase of the dip Angle of the secondary crack,the crack opening degree increases,and the failure surface of the sample is wider..(4)Through the DIC digital image system and PS software rendering,it is found that the surface crack of the sample starts from the end of the prefabricated crack;At the initial loading stage,the crack initiation occurs at the end of the prefabricated crack with short horizontal projection length.With the increase of stress level,the crack development stops at a certain distance from the top of the sample.At this time,the crack initiation occurs at the end of the crack with long horizontal projection length,and develops until it runs through the sample with the increase of stress level.A more severe failure occurs after the intersection of the airfoil cracks generated by the two prefabricated crack ends.(5)PFC2D numerical simulation application has a good match with uniaxial compression test;By reducing the crack dip gradient,the failure mechanism of the sample was verified.It is satisfied that the failure of the sample is controlled by a crack with a longer projection length along the horizontal direction,and the other crack plays an induction role in the initiation and development of the crack.