Experimental Study On High-speed Shear Rheological Properties Of Brittle Granular Flow

Large-scale geological disasters such as landslide debris flow,rock collapse,volcanic debris and avalanches can be regarded as dense granular flow.The occurrence of such natural disasters often causes huge disasters and has attracted people’s attention.Many hypotheses have been proposed to explain the hypermobility of such geological disasters,however,no consensus position has emerged in more than a century,and debate rages on.In this paper,the shear weakening characteristics of different mineral particles are studied by using the high-speed friction experiment instrument of the Institute of Geology,China Seismological Bureau,Beijing,the rotating shear test of Daguangbao limestone,quartz sand,Touzhai basalt,fluorspar and carborundum sand with different normal stress and different shear rate was carried out.The thickness of the sample was increased by improving the stator and rotor of the instrument,this makes the instrument possible to study the shear particle flow instead of the previous interfacial friction.The visualization of the sample container makes it possible to use the velocity distribution from the particle to the high-speed camera.The main conclusions of this study are as follows:(1)The peak shear resistance(shear strength)of different materials increases with the increase of normal stress,and the normal stress is 0.3-1.0 MPa at the same shear rate of 2m/s for different materials in high-speed rotating shear test.Two kinds of flow behaviors of friable granular materials can be observed,The steady-state shear resistance seems to be independent of normal stress and converges to about 200 k Pa.(2)For the rotating shear test of different materials,under the same normal stress of 1MPa and shear velocity of 0.001-2m/s.Two kinds of flow characteristics of friable granular materials can be observed.(3)Changing the container of the experimental sample,replacing the white opaque teflon ring with the crystal transparent ring,to realize the visualization of the experimental process,and using the high-speed camera to film the samples in the experimental process,the sample image is processed by Particle image velocimetry(PIV)and the velocity distribution is calculated.The glass beads were sheared at 0.3-0.7 MPa and the shear rate was 0.1 m/s.The results show that the velocity distribution along the thickness direction of the glass bead is almost linear under different normal stress.(4)A new phenomenon of the flow behavior of friable particles is found by studying the steady friction stress ratio of the experimental data with different normal stress and different shear velocity.Under the condition of high normal stress,after a long time of rapid shear,the granular material is broken and the shear strength is reduced.The experimental results of the given shear rate under different normal stresses show that the behavior of shear stress bifurcates.When the combination of normal stress and shear rate is less than the critical value,the shear resistance is stable and there is no shear weakening behavior.When the combination of normal stress and shear rate is greater than the critical value,the granular material decreases with the accumulation of shear strain,and the shear weakening behavior appears.