Geomechanical Behaviour Of Biaxial Loaded Limestone

Nowadays, there are too much research literatures on the strength and failure mechanism of rock, but few of them put sight on the geomechanical behaviour of biaxially loaded rock. Among the previous limited research of biaxial compression, the ready-made uinaxial loading apparatus were usually employed to setup biaxial loading apparatus by adding a perpendicular loading frame. Anisotropic stiffness of two loading frames always exists, so the research results are not comprehensive; the research are not systemic and most current failure criteria ignore the affection of intermediate principal stress.In order to solve the above problems and try to explore geomechanical behaviour of rock material, a new Isotropic Biaxially Loading System (IBLS) which has higher loading capacity than previous apparatus, has been designed and established by Henan Polytechnic University of China and McGill University of Canada. IBLS can be used to investigate bigger rock or concrete samples by both uniaxial and biaxial loading conditions than previous investigations.By IBLS, the investigation of both 75mm and 100mm cubic limestone specimens was implemented. In order to generalize the results of the test, uniaxial and triaxial compression tests on the cylinder specimen were conducted according to the related standards of ISRM.Based on the test results, it is found that the biaxial strength of limestone is more than its uniaxial strength, and the intermediate principal stress can obviously affect the strength of rock. With the increase of intermediate principal stress, the biaxial strength ascends at first and then descends; when the value of intermediate principal stress gets 70~80% of uniaxial strength, the biaxial reaches its peak value, after that the biaxial biaxial declines but the biaxial result is still larger than uniaxial strength.Also because of the existing of lateral confine the failure mode of rock changes: biaxial failure of limestone is tension-shear failure. The rock first spalling is from free surface for the reason of tension and with the specimen is undermined gradually at last the specimen collapses for the reason of shear. Meantime the failure is accompanied with obvious premonition and is helpful in actual engineering by predicting the appearance of biaxial failure of rock. After the rapid loading test of 100mm cubic specimen at the velocity of 5MPa/s, the biaxial strength drops about 10% compared with the biaxial strength at the velocity of 1MPa/s. Scale affects the biaxial no more than 5%. On the contrary, the shape of specimen affects the biaxial remarkably, for example, the uniaxial strength of 75mm cubic specimen is 1.67 times of that ofΦ50×125mm cylinder.