Research On Structure Effect And Size Effect Of Mechanical Parameters Of Fractured Rock Mass

The stability of underground engineering structures often depends on the understanding and grasp of the mechanical properties of rock mass media.The existence of discontinuities in rock mass forms the complex structural characteristics,and the mechanical response of rock mass is greatly affected by the geometrical and mechanical characteristics of discontinuities.A correct understanding of the mechanical properties of fractured rock masses has important theoretical and practical significance for rationally studying the stability and safety of rock mass engineering.This paper takes the fractured rock mass as the research object,adopts the particle flow discrete element method to construct synthetic rock mass models with random fracture network characteristics,and carries out numerical loading tests on them to study the structure effect and size effect of the mechanical parameters of the fractured rock mass.The main research contents and results are as follows:(1)Synthetic rock mass models with a set of random fractures are established,and the influence of structural features such as the fracture density,the mean fracture dip angle,the distribution characteristics of fracture dip angle,and the distribution characteristics of fracture diameter on the mechanical parameters of the fractured rock masses is analyzed.This paper discusses the structure effect of mechanical parameters of fractured rock masses.The results show:the fracture density and the mean fracture dip angle have a greater influence on the mechanical parameters.The influence of Fisher distribution constant on mechanical parameters is greater when the mean fracture dip angle is 60°,but less when the mean fracture dip angle is other than 60°.The power law distribution index of fracture diameter has little effect on the mechanical parameters.The uniaxial compressive strength is the most sensitive to the geometric parameters of the discrete fracture network,and the elastic modulus is the least sensitive.(2)Representative elementary volume(REV)is the minimum rock mass size when the rock mass properties tend to be stable.According to the geological parameters of fractures,fracture network models containing three sets of fractures with different sizes are constructed.The fracture density parameters P₃₂ and P₃₁ are selected to characterize the structural characteristics of the fracture network model,and the changes of P₃₂ and P₃₁ with the model size are calculated.The results show that P₃₂ and P₃₁ exhibit obvious size effect.The rate of change and coefficient of variation are set to be less than 10%as the quantitative index of the REV size,and the geometrical REV size of the fractured rock mass is finally determined to be3m×3m×6m.(3)Combining the fracture network models and the calibrated microscopic parameters of complete rock blocks and fractures,synthetic rock mass models of different sizes are constructed.A series of uniaxial compression numerical tests are carried out on them,and the variation rules of uniaxial compressive strength and elastic modulus with the model size are calculated.The results show that the size effect of uniaxial compressive strength and elastic modulus is obvious.According to the same quantitative index,the REV size of uniaxial compressive strength is 5m×5m×10m,and the REV size of elastic modulus is 3m×3m×6m.(4)Triaxial compression numerical tests are carried out on synthetic rock mass models of different sizes under different confining pressures to calculate the variation rules of peak strength and elastic modulus with model size under different confining pressures,and the REV sizes of all mechanical parameters are obtained according to the same quantitative index.The results show that the confining pressure has a certain influence on the size effect of mechanical parameters of fractured rock mass,and the confining pressure weakens the discontinuity effect.Within the range of confining pressure studied,the REV size of peak strength and elastic modulus decreases with the increase of confining pressure.(5)Keeping the mechanical properties of rock blocks and fractures unchanged,by changing the geometric parameters of the fracture network,analyze the influences of fracture density,fracture diameter,and fracture orientation distribution characteristics on the REV size of uniaxial compressive strength.The results show that:with the increase of fracture density P₃₂,the REV size of uniaxial compressive strength tends to decrease.With the increase of fracture diameter,the REV size of uniaxial compressive strength gradually increases.The change of Fisher distribution constant K has little effect on the REV size of uniaxial compressive strength.