| The geological structure in western Sichuan is complex,and macroscopic and meso-fractured rocks are often encountered in related underground engineerings.the fracture structural plane controls the safety and stability of the structure,and the importance of studying the properties of fractured rocks is self-evident.Because of its heterogeneity and discontinuity,it is difficult to analyze complex strength changes and overall failure behavior by using traditional stress yield criteria and failure criteria.Energy method can show the damage process of rock failure in the form of dissipated energy.the main research and application of energy method is to establish rockburst criterion,but there are few studies on the fracture process of fractured rock by energy method.For this reason,this paper mainly uses rock-like materials to prefabricate fractured rock to study the effects of fracture density and length on rock fracture mechanism and energy conversion process.Because it is difficult to control multi-fracture dip angle in laboratory test,PFC2D simulation is used to supplement the indoor test on inclination angle.Finally,laminated sandstone samples in engineering are collected and compared with prefabricated samples to analyze their energy characteristics.The main achievements and conclusions of this paper are as follows:First of all,PET film is used to simulate fractures in rock,four kinds of fracture lengths are prepared,and four different fracture densities are prepared for each length.Sixteen square rock samples with fractures and one sample without fractures are prepared.The full stress-strain curves of 17 samples are obtained by uniaxial compression test,and all the mechanical parameters(including peak strength,elastic modulus and residual strength)are calculated from the sample data.The final fracture state of rock is recorded and the fracture sketch is drawn.Through the observation,analysis and comparison of the fracture sketch,it is found that the failure degree of the sample is mainly tensile failure and the failure degree of the sample increases with the increase of fracture density.In order to intuitively analyze the gap between the fracture specimen and the non-fracture sample,the decrease ratio of the parameters is calculated,which is respectively fitted with the law between the fracture density and the fracture length parameters.When the fracture length is 1cm,the decrease ratio increases gradually with the increase of fracture density,and accords with the linear law.Secondly,the uniaxial compression curves of all specimens are further treated by the theory of energy dissipation,and the energy evolution curve is combined with the fracture process and the damage characteristics of the full stress-strain curve.Then,the energy evolution curve is divided into six stages:pore,fracture compaction stage,stable damage stage,damage stationary stage,unstable fracture development stage,accelerated failure stage and post-failure stage.The relationship between the peak energy index of all samples and the fracture length and density parameters is drawn.It is found that when the fracture length is1cm,the peak energy density decreases with the increase of fracture density and accords with the linear law.When the fracture density is 10%(cm-1),the peak energy density decreases with the increase of fracture length and accords with the linear law.In order to solve the problem that the prefabricated fracture can not control the dominant inclination angle of the prefabricated fracture in the laboratory test,the parameter calibration of the prefabricated cylindrical standard sample is numerically simulated by using the PFC2D particle flow program.The density and length of the fractures are fixed,the fracture arrangement is arranged in parallel with each other,the distance between the fracture is fixed,and the dip angle is in the range of 0?90°.Six kinds of angles are simulated with 15°as the dividing angle,and the dip angles of all fractures are changed uniformly.The energy evolution curve is drawn by the energy tracking function,and the relationship between the peak energy and the fracture dip angle is drawn.It is found that the peak energy value increases with the increase of the fracture dip angle,and accords with the linear law.Finally,in order to compare the energy difference with the prefabricated samples tested in laboratory,the dip angles of 7 bedding samples are selected,and the range of self-defined bedding dip angle is 0?90°.The uniaxial compression test is carried out,and the mechanical parameters(including elastic modulus and peak strength)are calculated.The relationship between mechanical parameters and bedding dip angle is plotted.The results show that the law is a"U"curve,and the valley bottom corresponds to the bedding dip angle of 61°.The uniaxial compression curves of all specimens are further treated by the theory of energy dissipation,and the energy evolution curve is combined with the fracture process and the damage characteristics of the full stress-strain curve.The energy evolution curve is also divided into six stages:pore fracture compaction stage,stable damage stage,damage steady stage,damage sudden increase stage,unstable fracture development stage and failure stage.The law of peak energy and bedding dip angle is also similar to the"U"law.The energy evolution curves of layered rational sandstone and cylindrical prefabricated samples are compared and analyzed.The results show that the energy characteristics of prefabricated rocks and sandstone are obviously distinguished in the stage after pore fracture compaction.The energy storage property of sandstone is better than that of rock-like samples. |
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