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Failure Test of N-type Combination Jointed Rock Mass Under Uniaxial Compression
Received date: 2018-07-26
Revised date: 2019-03-12
Online published: 2019-08-19
The rock mass contains a large number of complex fracture forms，which have a great impact on the strength and stability of the rock mass，making the rock mass show obvious anisotropy，and thus affecting the safety and stability of civil engineering structures.The common crack forms in rock mass are parallel，cross and intermittent and complex form of these three crack combination forms，and so on.While previous researchers’ study of fractured rock mass were more focused on the common fracture forms such as parallel，intersection and discontinuity.But in practical engineering，the rock mass cracks are mostly complex cracks formed by different combination of fracture forms，such as parallel and intersection joints forming N-shaped joints，and the stress and failure characteristics of N-type joints are more complex than that of single fracture form，and have a greater influence on strength and deformation of rock mass.Aiming at the stability problem of surrounding rock in underground engineering such as tunnels and roadways， the stability of rock mass with N-shaped fractures combined with parallel and cross fractures were studied，taking the N-type joints formed by the combination of 45 degree inclined main joints and two parallel joints as the research object，and the multi-repeated failure modes of tension-shear and compression-shear of rock mass under the combination of parallel and cross-fissures were studied.The influence of different primary and secondary joint angles on rock mass strength and failure mode was analyzed，which provided theoretical support for the design，construction and safety evaluation of underground geotechnical engineering.Uniaxial static load test and ultrasonic test were carried out on the cube specimen of cement mortar with side length of 150 mm，strength of 10 MPa and prefabricated with N-type joint，to obtain their stress strain，failure forms and ultrasonic data.Combined with fracture mechanics theory，it is concluded that the strength and deformation characteristics，the ultrasonic wave velocity attenuation law and failure mode （Ⅰ，Ⅱ crack type crack and extend the features） of N type joint rock mass.The results show that firstly the crack types of N-type composite jointed rock mass have wing crack and secondary inclined crack in sequence，and their propagation paths tend to be in the direction of main stress eventually，which is different from the crack development law with single jointed rock mass.There is a nonlinear relationship between stress intensity factor at the tip of the main joint and the load P and fracture angle θ 0.When θ 0 is small and P is close to the peak strength，the specimens mainly propagate in mode Ⅱ crack shape，while mode Ⅰ crack morphology is more obvious when θ 0 is larger.Secondary the average ultrasonic wave value of the specimens with joint angles ranges from 2.463~2.699 km/s，which is quite different from the ultrasonic wave velocity of the complete specimens （2.725 km/s），indicating that the existence of joints accelerates the internal damage of the specimens.When the inclination angle of the main joint is constant，the angle between the main and secondary joints and the number of joints has some influence on the physical properties of the specimen.The attenuation rate of each group of joint specimens is from 0.9% to 9.6%, the velocity attenuation of joint specimens with angle of 15 and 90 is the fastest，while that of joint specimens with angle of 60 is the slowest.Thirdly the constitutive relationship，peak strength and failure characteristics of composite jointed rock-like mass all show nonlinear characteristics.The distribution law of peak strength basically obeys M-type distribution，which is different from U-type distribution with single joint.Among them，the quasi-brittle failure occurs at the angles of 15，30，45，75，90 degrees and the complete specimen，while the brittle failure occurs at other angles，this may be caused by discrete of the specimens.
Bing SUN , Yu LUO , Jiehui XIE , Sheng ZENG . Failure Test of N-type Combination Jointed Rock Mass Under Uniaxial Compression[J]. Gold Science and Technology, 2019 , 27(4) : 548 -556 . DOI: 10.11872/j.issn.1005-2518.2019.04.548
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