Study On Mechanical Response Mechanism And Failure Precursor Identification Of Microfractured Rock Under Compression Deformation

With the depletion of open-pit resources,underground exploitation of mineral resources has become a common mining mode.In underground excavation project,high stress will make macro-scale cracks gradually close under the local geological structural stress,mining depth,and other factors,resulting in the microcracks gradually show the influence of surrounding rock failure.Compared with the intact continuous rock and the rock with macro-cracks,the mechanical behavior and engineering response of microfractured rock in complex geological environment will change significantly.It is helpful to deepen the understanding of the mechanical behavior of microfractured rock to study its failure.As we all know,rockburst and roof collapse are easy to occur in surrounding rock engineering under high stress.The results show that before the dynamic disaster of surrounding rock,the local failure of free face rock such as rock spalling and ejection will appear successively.Local dynamic failure of surrounding rock is the premise of large-scale structural disaster of surrounding rock.Therefore,it is of great scientific value to study the surface fracture of surrounding rock for rockburst prediction of underground surrounding rock engineering.In this paper,various research methods were adopted to study the comprehensive evaluation of microcracks distribution,the mechanical response and energy control behavior of rock by the method of preparing microcracks at high temperature,and the local fracture and precursor information identification of surface rock.The main research results are as follows:(1)The distribution characteristics of cross-scale pores inside the microfractured rock were evaluated,and the evolution law of four kinds of pores were studied by NMR test.Based on the SPRS,the evolution process of the MH WA was obtained,and then the physical mechanism of the change in porosity "first increase,then decrease,finally increase" was characterized.To make the research results more engineering guidance,the microfractured rock was divided into different rock pore grades.The concept of average pore,which can represent the contribution of various scale pores to the overall pore distribution,was proposed.It was found that the porosity and average pore show synchronous changes.Based on the theory of pore structure fractal dimension,the fractal characteristics of crossscale microcracks were analyzed.The variation law of fractal dimension of pore in different rock pore grades was explored,and the cross-scale distribution of rock pores was described quantitatively.It was found that the mesopores and macropores of microfractured rock have significant fractal characteristics.The higher the rock pore grade is,the larger the fractal values of these pores of granite are,and the more complex the pore structure is.(2)The comprehensive effects of rock pore grades on rock mechanical properties were studied.When the rock pore grade is low,the microcracks show the closing effect and the mechanical properties are strengthened.When the rock pore grade is higher,the microcracks shows widening effect and the mechanical properties of rock are weakened.There are a lot of microcracks in Ls4 grade sandstone,which lead to abnormal changes in mechanical parameters and compactness.Meanwhile,considering that the mechanical properties are the macroscopic form of the overall response of pores,the internal correlation mechanism between each pore structure and macroscopic mechanical properties of rock was revealed by analyzing the nonlinear correlation between pore fractal dimension and mechanical parameters.(3)Based on the dissipation and release mechanism of energy,the distribution and transformation law of each strain energy in the deformation process of microfractured rock were analyzed.In combination with the energy storage parameter of rock "storing releasable strain energy in an approximate undamaged state",it was found that the relationship between energy storage capacity and mechanical properties(uniaxial compressive strength and elastic modulus)is not significant.Based on the theory of dissipated energy release,the energy index for evaluating the critical instability of microfractured rock was proposed.In this paper,the EII model considering energy self-control behavior was established.Based on this model,it is known that during the energy absorption,storage,and release process,there is the self-excitation and inhibition phenomena of strain energy,which were verified.Meanwhile,a piecewise constitutive model considering postpeak stress drop was constructed,and the concept of post-peak strain softening factor n was proposed.It was found that post-peak stress attenuation rate of rock is significantly dependent on n,and the reasonable n was determined.Finally,by comparing the model values with the test values,it was found that this model can reasonably reflect the characteristics of the stress-strain curves of the microfractured rocks.(4)Based on the AE monitoring in the rock mechanics tests,the variation characteristics of AE information in time domain and frequency domain of microfractured rock were analyzed.This paper focuses on the advantages and disadvantages of using AE parameters directly to identify the failure of microfractured rock.It was found that this method has limitations in identifying the failure precursors of the microfractured rock with many mesopores and macropores.Therefore,according to the order and chaos theory of cracks growth,the variation law of correlation dimension based on AE time sequence was explored.It was found that when the correlation dimension reaches the minimum value,it is the effective precursor information of the overall failure of microfractured rock.(5)In this paper,the predicting difficulty and non-negligible points on the local failure of free face rock in chamber surrounding rock were demonstrated,and the true triaxial failure test of microfractured rock was further carried out.The rock failure mechanism was systematically studied(such as mechanical property,cracks type,fracture mode,and cracks scale).Based on the predicting difficulty of surface rock failure,this paper proposed to use the ITJR as the key parameter for quantitative evaluation of surface rock failure by combining with the information processing of temperature field matrix.The infrared advance prediction method of surface rock stripping and ejection failure under true triaxial stress was finally constructed,and the infrared warning process of surface local failure using ITJR precursor method was discussed in underground surrounding rock engineering.