Research On The Mechanical Properties Of Rock-like Materials Under Load And Unload Conditions

In the process of deep mining and mining,due to the effect of excavation and unloading,the energy accumulated in the high geostress within the rock is released,causing the instability of the excavated roadway.In extreme cases,a large amount of elastic energy which generated by kinetic energy released in an instant and caused the rock mass to burst,accompanied by the ejection of the rock mass.In this paper,by designing orthogonal tests,using range analysis,linear and nonlinear regression and other comprehensive means to develop rock similar materials.True triaxial loading and unloading test of similar simulation materials were performed,and the characteristics of deformation and damage of rock similar material were analyzed.The process of germination and propagation of internal cracks in rock under different stress conditions and the distribution of acoustic emission were studied.The research results obtained in this paper are mainly as follows:(1)By designing the orthogonal test and range analysis,the degree of influence of various index factors of similar material mixtures on the mechanical parameters of similar materials was analyzed.The results show that the sand content has the greatest influence on the uniaxial compressive strength of the specimens;the content of borax has the most significant effect on the tensile strength and elastic modulus of the specimen.Other factors,such as gypsum cement ratio,the percentage of water also has a certain influence on the mechanical parameters of similar specimens.(2)By using the ratio of similar materials as the independent variable and the mechanical parameters of similar materials as the dependent variable,a multiple linear regression and projection pursuit nonlinear fitting prediction model of similar material mechanical parameters were established.The results show that there is a significant nonlinear relationship between the ratios of similar materials and mechanical parameters,and the projection pursuit regression(PPR)model has stronger processing power for this kind of nonlinear relationship than the linear regression model.(3)By analyzing the effect of initial stress levels on strain and strength of rock during the period of unloading.The results show that: a)The three-dimensional strain of the specimen will be subject to different degrees of creep,and the degree of creep will increase as the initial stress level increases.b)After the minimum principal stress is unloaded,the minimum principal strain will undergo a sharp retracement phenomenon,and the specimen released a large amount of previously stored elastic energy at the unloading surface.c)The peak strength of the specimen increases with the increasing of three-dimensional initial stress level under conditions of true triaxial loading and unloading for limestone and dolomite-like specimens.(4)The effects of different initial stress levels on elasto-plasticity and rockburst tendency of rock were analyzed by processing the elastic modulus and stress brittleness characteristics during loading and unloading.The results show that: a)in the three-dimensional initial stress loading process,the overall elastic modulus increases with the increase of the maximum principal stress.b)After excavation and unloading,as the maximum principal stress gradually increases,the ductility of limestone increases,and the possibility of sudden damage decreases,ie,the tendency of rock burst is smaller;the brittleness of dolomite increases,and a sudden burst occurs,ie,the probability of sudden destruction occurs and rock burst increases.(5)Through the analysis of the ratio of the number of acoustic emissions at the time of unloading and peak strength,the acoustic emission(RA)values,and the macroscopic morphological characteristics of the specimens during failure,the effects of unloading on the degree of damage and destruction form of limestone and dolomite were studied.The results show that: a)Under the different initial stress conditions,as the three-dimensional initial stress increases,the unloading effect on the limestone damage is enhanced,and the damage to the dolomite is weakened;b)Similar specimens for limestone,with the increase of initial stress,the number of tension cracks increases,and the failure of specimens is dominated by splitting.c)With the the initial stress field gradually increasing,the destruction patterns of dolomite similar specimens have obvious evolutionary characteristics from splitting to shear failure,indicating that the initial stress field will have an important influence on the limit values of tensile and shearing of the specimens.