The Direct Shear Test Of The Soil-Rock Mixture And PFC2D Simulation Of Its Mesoscopic Features

The soil and rock mixture widely exists in the field of hydraulic engineering,and it is also an important treatment object in the wading geological disaster prevention project.It plays an important role in China’s economic construction and development,so it is very necessary to study the mechanical characteristics of the soil and rock mixture.The mechanical response of the soil and rock mixture under shear and its deformation and failure are important research directions for its mechanical characteristics.Based on this,this paper takes the shallow surface soil and rock mixture of the Outang landslide in the Three Gorges Reservoir area as the research object.Experiments and numerical experiments are carried out to conduct systematic research to comprehensively analyze the macro-mechanical behavior and meso-change characteristics of the soil and rock mixture during the shear process.The main research contents and conclusions of this article are as follows:1)Four in-situ test points were selected from the lowest to the highest on the Outang landslide.Three in-situ samples were made at each test point,and three levels of normal stress were applied successively for in-situ direct shear test.It is found that in a certain elevation range,the particle size composition characteristics of the soil and rock mixture landslide in nature have certain laws to follow,which provides a reference for the establishment of a high degree of discrete soil and rock mixture landslide structure model.At the same time,the shape characteristics of the shear stress-shear displacement curves of all specimens in the in-situ test are basically the same.The shear process can be divided into four stages: linear deformation stage,yield stage,peak stage and strain softening stage.2)Four undisturbed samples were prepared at each in-situ test site,and a total of 16 undisturbed samples were transported back to the laboratory for indoor direct shear test,and the physical and mechanical parameters such as moisture content,natural density,particle grading were measured.Compared with the results of in-situ test,it is found that the shape characteristics of shear stress shear displacement curve and the change rule of shear strength parameters are the same in the laboratory test and in-situ test.The difference is only reflected in the difference of numerical value.Because in-situ test is not disturbed and interfered too much,the result is greater in numerical value.For the soil and rock landslide,with the change of the elevation of the test point,the water content changes little,and the rock content increases with the increase of the elevation.At the same time,the higher the landslide elevation of the soil and rock,the greater the internal friction angle φ of the soil and rock in this area,but the smaller the cohesion c,the main reason is the different rock content.3)The basic principle of PFC2 D program of particle flow is clarified,and the PFC2 D numerical direct shear test under different rock content is simulated to explore the influence of different rock content on the mechanical properties of soil and rock mixture.It is found that the cohesion c decreases with the increase of rock content,the internal friction angle φ increases with the increase of rock content,and the overall shear strength increases with the increase of rock content.Through the observation of the meso change characteristics of the soil rock mixed particle system in the shear process,it is found that the distribution and development rule of the internal cracks in the structure with different rock content are basically the same.The overall failure of the structure is caused by the joint action of shear failure and tension failure,and the number of tension cracks is significantly more than that of shear cracks.Higher rock content can enhance the interaction between particles,increase the range of shear band,and limit the relative movement between particles.The increase of rock content leads to the increase of the stability and quantity of the strong chain in the force chain network,which is the meso mechanism of the macro strength enhancement of the soil and rock mixture.Therefore,the high rock content is helpful to the stability of the soil and rock mixture structure,and has the reinforcement effect.