Experimental Study On Failure Characteristics Of Granite Instability Under Different Stress Environments

The rapid development of urbanization and national economy,greatly increased the demand for high-speed rail,subway,highway,also put forward higher requirements of underground engineering and surface engineering,so the research on issues related to work more and more attention of scholars.The influence of different ground stress field on the mechanical properties of rock has been one of the key research fields.There are more and more engineering problems in complex stress environment such as high temperature,high pressure,extreme cold and disturbance,which often bring great difficulties to the smooth progress of the project.Under the laboratory conditions,it is very important to carry out a systematic and comprehensive simulation test of the mechanics of rock specimen under different stress conditions.In order to research the influence of the system and try to comprehensive factors on the mechanical properties of rocks,the granite samples are more common as the research object,the influence of different shapes and different loading rates and different moisture content and different loading cycles on the properties of granite force were studied.A total of 11 groups of laboratory tests were set up,and 96 samples of various granite samples were processed and 111 samples were analyzed.The main conclusions are drawn.(1)The distribution of the RA/AF scatter plot shows that the change of tensile failure at any rate is the most important failure mode of the specimen,regardless of the rate of change.With the increase of loading rate,the total number of tensile,shear,and tensioning shear cracks in the specimen increases continuously,and the tensile crack increases faster than the shear crack.(2)From the statistical data of rock fragments,it is concluded that the loading rate increases,and the fragmentation tends to be fragmented,and the degree of rock failure is more fragmented.The specific value of the ratio of length to thickness,aspect ratio and width to thickness ratio decreases.(3)Granite with different moisture content has significant influence on its physical and mechanical properties.The test shows that the greater the water content is,the lower the strength is,because water plays a role in deterioration of mechanical properties of granite samples to a certain extent.The strength of dry granite increased by 1/4~1/3 than that of natural and saturated granite.(4)SEM micrograph shows that there are some natural defects in granite: dissolved pore,joint surface and porous mica.These natural defects largely determine the strength of granite.After the test,the fracture morphology of the clastic is varied under the complex actual stress,but it is formed by the tensile,shear and tensile shear composite effects.(5)Equal displacement,constant upper and lower limits,cyclic loading and unloading test,the parameters of the cycle unit are:loading rate 0.9mm/min,unloading rate 0.9mm/min,cyclic amplitude 0.3mm.Under this test condition,Different samples,with the number of cycles increased the peak strength gradually increased 5 times to 10 times the strength increased significantly after 10 cycles of strength increase smoothly,after 20 cycles remained unchanged,the cyclic loading and unloading without deterioration of rock strength but the sample peak compressive strength is reduced,because the test medium the displacement of cyclic loading in elastic limit within the specimen,such as displacement and unloading cycle and make specimen internal material pressing density,strength increase.In the same loading and unloading test of the same sample,the cyclic peak strength decreases with the increase of cycle times,and the rate of decrease is smaller and smaller.Finally,it will remain unchanged after 10 cycles.(6)In the destruction type of granite rock burst pits,flaky and plate coarse crumbs mostly come from the surface of blasting craters,and fine and particulate powder crumbs mostly come from rock burst pits.(7)In the same experiment,the first cycle of acoustic emission signal intensity is larger than that of second times,and there is magnitude difference.In different tests,the scatter density of RA/AF increases with the increase of cycle times,and the number of tensile cracks increases faster than the number of shear cracks.(8)The amplitude of cyclic loading and unloading as: when the amplitude is less than 0.6mm,the sample number continued to increase subsequent bond internal damage remains unchanged,without damaging the rock specimen;when the amplitude is higher than 0.8mm,with the number of samples to unload bond failure is increasing rapidly,the final sample failure.