Experimental Study On The Effect Of Linear And Non-linear Gradient Geo-stress On Stress Wave Propagation And Attenuation In Rock

Considering the coupling effect of self-gravity,excavation and unloading,the geo-stress of underground engineering rock mass changes significantly,and varies with the spatial location in a gradient form,and rock(mass)is often in a complex environment of gradient geo-stress occurrence.The gradient geo-stress causes the impedance of rock wave to change in a gradient form,as a result,the propagation and attenuation characteristics of stress wave in rock was changed.To investigate the rock stress wave propagation characteristics under the different forms of gradient static stress,the stress wave propagation test device with gradient static stress was used to impose linear and nonlinear gradient static stress loads on rock long specimens respectively.In order to explore the influence mechanism of gradient static stress on the propagation attenuation of rock stress wave,the characteristics of rock stress wave shape,propagation velocity and wave impedance under different gradient loading conditions were analyzed,an empirical model of the relationship between the amplitude of rock stress wave and propagation distance,propagation time and loading gradient is established.The conclusions are as follows:(1)To study the axial static deformation characteristics of the rock under the corresponding loading conditions,design different load schemes to simulate the complex gradient stress and environment,linear and nonlinear gradient static stresses are applied to rock specimens respectively.The results show under the linear gradient static stress,the axial static strain of rock decreases exponentially as the stress wave propagation distance increasing,and the axial strain increase as the stress gradient increasing.Under the nonlinear gradient static stress condition,the increase rate of axial static strain slows down gradually as the stress wave propagation distance increasing.As for the same measuring point,the axial strain increase gradually with the non-linear stress gradient.(2)Under linear and nonlinear gradient static stress conditions,two columns of compression waves can be collected within the strain collection time,and there is a section of compression platform between the two columns of compression waves whose strain value fails to recover to the initial state,and the characteristics of rock stress wave are "compression wave and compression platform reciprocating".The variation of rock compression platform is affected by the gradient static stress loading method.Under linear gradient static stress condition,the compression platform value of measuring point decreases as the stress wave propagation distance increasing,but the decreasing rate slows down.Besides,the compression platform value decreases with the increase of linear stress gradient,and the reduction rate also slows down gradually.Under the nonlinear gradient static stress condition,the compression platform gradually decreases with the increase of the stress wave propagation distance,the decreasing rate gradually slows down.With the increase of the nonlinear stress gradient,the compression platform decreases gradually,and the decreasing rate slows down gradually.(3)Under linear and nonlinear gradient static stress conditions,stress waves propagate at different velocities at different spatial positions of rock specimens.With the increase of stress gradient,the longitudinal wave velocity of rock specimen increases with the distance from the free end,but the differences exist under different stress gradient conditions.Under linear gradient static stress condition,the longitudinal wave velocity of rock increases with the increase of the distance of measuring point,but the increase rate slows down gradually.At the high linear gradient static stress stage,the longitudinal wave velocity decreases gradually.The longitudinal wave velocity of the whole specimen increases gradually,but the increase rate of wave velocity decreases gradually.Under the nonlinear gradient static stress condition,the longitudinal wave velocity increases gradually with the increase of the distance from the measuring point,but the increase rate slows down gradually.At the high linear gradient static stress stage,the longitudinal wave velocity decreases first and then increases gradually.The longitudinal wave velocity of the whole specimen increases gradually,but the increase rate of wave velocity slows down gradually.(4)Under linear and nonlinear gradient static stress conditions,the amplitude of rock stress wave decays with the increase of propagation distance and propagation time.The attenuation of stress amplitude on time and space scales is essentially affected by uniform attenuation and non-uniform attenuation factors.The attenuation characteristics of rock stress amplitude are affected by the loading mode of gradient static stress: Under linear gradient static stress condition,with the increase of linear gradient static stress,under low linear gradient static stress condition,the linear space attenuation coefficient of stress wave increases gradually,and decreases at high linear gradient static stress stage.The variation trend of amplitude time attenuation coefficient is similar to that of space attenuation coefficient.Under the nonlinear gradient static stress condition,with the increase of the nonlinear gradient static stress,the nonlinear spatial attenuation coefficient gradually increases and the increase rate gradually slows down,and gradually decreases at high nonlinear gradient.The time attenuation coefficient increases rapidly at first and then becomes stable at the high nonlinear gradient static stress stage.