Study On The Scale Effect On The Shear Strength Of Soil-rock Mixture

Soil-rock mixture is a kind of special geological material between blocky rock mass and homogeneous soil mass.Its particle size distribution has multi-scale characteristics.The mechanical properties of soil-rock mixtures have obvious structural and size effects,which makes it very difficult to estimate the strength parameters of soil-rock mixtures at engineering scale.In this paper,the scale effect of soil-rock mixtures in Xuzhou area is studied by means of large-scale direct shear test and numerical simulation,taking into account the gradation effect of strength characteristics of soil-rock mixtures,in order to provide reference for engineering practice.The introduction part introduces the research background of soil-rock mixture,and after summarizing the research progress and shortcomings of soil-rock mixture at home and abroad,establishes the research content and technical route of this paper.In the second chapter,fractal theory is used to study the particle size distribution of soil-rock mixture.The natural gradation curve of soil-rock mixture was obtained,and the curvature coefficient,non-uniformity coefficient and gradation fractal curve were calculated.The fractal structure of natural gradation of soil-rock mixture was verified,and the fractal dimension D=2.3.It is pointed out that the traditional soil mechanics description method is difficult to reveal the complexity and discontinuity of soil-rock mixtures.Fractal dimension and maximum particle size are used as the gradation description method to solve the scale effect of soil-rock mixtures.In the third chapter,the large direct shear apparatus is used to solve the problem of shear gap effect on shear strength of soil-rock mixture,and then the particle size effect on shear strength of soil-rock mixture is obtained.With the increase of shear gap,the shear strength of soil-rock mixture firstly decreases,then varies stably and finally decreases.The cohesion and internal friction angle are negatively correlated with the increase of shear gap.Based on the variation of cohesion and internal friction angle,when the fractal dimension of gradation is 2.3,the reasonable shear gap is between 0.2 and 0.4 times the maximum particle size.At this situation,the particles are in relatively stable rolling and sliding,and are less constrained by the boundary of shear box.Considering the size effect of shear gap,experiments have been carried out to study the particle size effect on strength soil-rock mixture.The results show that the particle size can enhance the shear strength of soil-rock mixture,and the average enhancement coefficient is 0.07 at 100-300 kPa.After dimensionalization,there is an exponential relationship between the cohesion ratio and the maximum particle size ratio,and a logarithmic relationship between the tangent ratio of internal friction angle and the maximum particle size ratio.In the fourth chapter,the conclusion of physical experiment is verified and explained at the micro level by the discrete element software PFC2 D,and simulation tests under different gradations and normal stresses were carried out.Simulated experiments of shear gap under different gradations show that when the fractal dimension of gradation is unchanged,there is a quantitative relationship between the suitable shear gap and the maximum particle size(D=2.3);when the maximum particle size is unchanged,the suitable shear gap decreases with the increase of the fractal dimension of gradation(D=2.5).The contact force distribution of soil-rock mixture particles under different shear gap shows that with the increase of the gap widths,the force chain concentrates from the "overall distribution" to the "local concentration" of the shear plane,indicating that the shear strength is mainly formed by the particles in the shear band.As the gap width continues to increase,the force chain shows an irregular distribution once again.In addition,the particle size effect on the shear strength of soil-rock mixture is positively correlated with the normal stress.With the increase of particle size,the contact force of particles in the soil-rock mixture increases gradually,forming a stable strong chain structure.In the fifth chapter,the strength parameters of soil-rock mixtures under engineering scale are estimated,analyzed and discussed around the innovative point of this paper.Based on the laboratory test results,a method for estimating the strength of soil-rock mixtures at engineering scale is proposed for the first time.The strength size effect formula of soil-rock mixtures proposed in this paper can estimate the problem of strength estimation of coarse-grained soil,soil-rock mixtures and broken rock masses through examples with different scale characteristics.Chapter 6 summarizes the research results and points out the work and research prospects that need to be improved.