Experimental Study On Mechanism Of Rock Deformation And Fracture Under Gradient Stress

Excavation of deep roadways will cause lateral unloading and vertical stress concentration.Vertical loading will be caused by strong mining.The redistributed surrounding rock stress caused by roadway excavation presents gradient change characteristic,and the gradient stress will cause structural gradient failure in the surrounding rock.The team independently developed a gradient stress loading test system.By accurately controlling the stress on different loading channels,the 150*150*150 mm~3 test block was loaded with different stresses in different regions,forming gradient stress loading in space,and studying the mechanism of gradient stress on rock deformation and fracture.The uniform stress loading test system was used to carry out the mechanism test of uniform stress on rock deformation and fracture under different loading rates.The test results show that:(1)As the loading rate increases,the failure mode of the test block becomes more complicated,from relatively simple tensile failure to mixed tensile and tensile-shear failure,and then to"X"-shaped conjugate inclined plane shear failure,and the damage degree of the test block becomes more serious.The elastic modulus,ultimate strain value and compressive strength of the test block during the failure process show the change characteristics of first increasing and then decreasing.(2)Both the AE count and energy will peak when the test block is damaged.With the increase of the loading rate,the AE signal,the peak value of the AE count and energy and the energy of the AE event inside the test block becomes more concentrated and greater.The gradient stress loading test system was used to carry out the mechanism test of the gradient stress on the rock deformation and fracture.On this basis,the influence of different stress gradient,different loading rates and different gradient stress distributions on the rock deformation and fracture mechanism was further studied.The test results show that:(1)The initiation cracks of the test block will be generated at the interface of gradient stress.The initiation cracks will expand from the top to the bottom,and the width of the cracks will gradually decrease from top to bottom.The shape of the broken part of the test block is wedge-shaped.From the low stress area to the high stress area,the transverse and axial strains in different areas increase gradiently.(2)AE count peak and energy peak are higher than the condition of uniaxial uniform stress loading.The number and energy of AE events have obvious gradient increase characteristics from the low-stress area to the high-stress area inside the test block.(3)As the stress gradient increases,the failure type of the test block changes from shear failure to tensile-shear failure,and the failure mode is more complex.The shape of the part of the failure of the test block also changes from wedge-shaped fragmentation to layered spalling,and then to blocky.The degree of damage is also more serious.As the loading rate of different loading channels increases,the integrity of the damaged part of the test block is better,and the range of the damaged area becomes smaller.(4)With the increase of stress gradient and loading rate,the maximum value of gradient stress tends to increase when the test block fails.The various stages of loading deformation and failure of the test block,from the low-stress zone to the high-stress zone,the axial strain and transverse strain increase gradiently.The larger the stress gradient and the loading rate,the more obvious the increasing trend of strain gradient.(5)With the increase of stress gradient,the peak value of AE count and energy of the test block are also increasing.As the loading rate of each channel increases,the peak value of the AE count and energy show a decreasing trend.(6)When the test block is loaded with different gradient stress distributions,cracks are first generated in the area with higher gradient stress on the free side.As the loading progresses,cracks will appear again in the higher stress area inside the test block,and the cracks will turn around.The larger the stress gradient is,the closer the fracture cracks are to the upper part of the test block from the beginning to the turning position.The strain in the region with large gradient stress is also greater,and the strain will increase gradiently from the low stress region to the high stress region.The larger the stress gradient is,the larger the AE count peak and energy peak of the test block are,and the more AE events are in the test block.There are 63 figures,11 tables and 104 references in this thesis.