Study On Mechanical Properties And Acoustic Emission Characteristics Of Underlying Sandstone Under Bolt Action

The sedimentary rock with layered structure in nature accounts for about two-thirds of the land area,and the layered structure is widely distributed in China.Therefore,the stability of layered rock is often encountered in the construction of underground space in China’s geotechnical engineering.Under the action of layered structure,the mechanical properties of layered rock are obviously anisotropic,and the failure mechanism of rock mass is obviously different from that of other rock masses.In order to solve this problem,the paper takes the anchored layered sandstone as the research object,and carries out the indoor basic rock mechanics test research on its mechanical properties and acoustic emission characteristics.The main research contents and conclusions are as follows:（1）In terms of uniaxial compression test of anchored layered sandstone:The compressive strength,elastic modulus E and E_80-20 of non anchored and anchored layered sandstone show a trend of first increasing and then decreasing with the increase of bedding angle,and the volume expansion phenomenon occurs during deformation and failure;Poisson’s ratio decreases first and then increases with the increase of bedding angle.The modulus and Poisson’s ratio of anchored rock samples are higher than that of non anchored rock samples;The failure modes are split failure along the weak plane,shear failure along the oblique bedding plane and slip failure along the weak plane of bedding.When the bedding angle of sandstone increases from0°to 60°,the failure mode changes from brittle fracture to shear failure along the weak plane of bedding;In the process of energy evolution of layered sandstone,the released energy increases continuously in the compaction stage,and the energy dissipation energyη_d is higher than the energy storage rate _sη.In the elastic stage,the released energy remains basically stable,the energy storage rate _sηgradually exceeds the energy dissipation rateη_d,and then it occupies a dominant position.Until the release energy in the plastic stage increases rapidly in exponential form,the growth rate _SW（?）of elastic strain energy shows a steady growth trend,and its fluctuation amplitude increases with the increase of bearing stress until the instability failure of sandstone occurs.（2）In terms of uniaxial cyclic loading and unloading test of anchored layered sandstone:The compressive strength of rock samples increases first and then decreases with the increase of bedding angle,and the residual strain shows a stable decreasing trend,and finally tends to 0.In 0°,30°and 45°layered sandstones,the residual strain of anchored sandstones is generally higher than that of non anchored sandstones in the same stress level cyclic loading and unloading,and this phenomenon is opposite in 60°and 90°layered sandstones;The elastic modulus of rock samples tends to increase continuously.In the same bedding,the elastic modulus of anchored layered sandstone is larger than that of non anchored layered sandstone,and the elastic modulus increases with the increase of stress level;The failure mode is similar to that of uniaxial compression test;The difference is that under high stress level,cyclic loading and unloading will promote more crack damage in rock samples,but the effect of bolt can inhibit the development of cracks to a certain extent.In the energy distribution,the input energy and plastic strain energy will gradually decrease with the increase of the number of cycles,and the input energy will decrease significantly in the second cycle.In the whole cycle loading and unloading process,rock samples are mainly stored by elastic strain energy,and the proportion of elastic strain energy increases with the increase of cycle times.（3）In terms of acoustic emission characteristics during uniaxial loading of anchored layered sandstone:The high count and high energy of AE mainly occurred in the stage of plastic damage;G-R formula is introduced to discuss the change of B value.It is found that the b value of anchored and non anchored sandstones with different bedding angles increases first and then decreases in the process of disguised failure.The b value of anchored sandstones is generally higher than that of non anchored sandstones in the whole compression process.The anchoring effect can inhibit the development of internal fractures in sandstones;The fractal characteristics of acoustic emission events in rock space are studied by using the cylindrical covering method.It is found that the fractal dimension D of acoustic emission events in the failure process of anchored and non anchored layered sandstone ranges from[2,3].With the increase of bedding angle,the D value first increases and then decreases,which is consistent with the change of sandstone energy storage and compressive strength.In the same bedding angle,the D value of anchored sandstone is greater than that of non anchored sandstone.