Macro And Micro Compression-damage Coupling Mechanism And Its Constitutive Model Of Weakly Cemented Rock

Weakly cemented rock is a special type of rock widely distributed in the western China.The engineering disasters such as large deformation,support difficulties,and surrounding rock instability frequently occur in the weakly cemented formation,due to the characteristics of late diagenesis and poor cementation of this rock.In addition,the sensitive mechanical behavior of weakly cemented rocks under different stress levels is a common fundamental problem commonly encountered in geotechnical engineering in western China.Therefore,it is of great significance to guide relevant engineering design and improve the accuracy of engineering disaster prediction,by revealing the mechanical mechanism of deformation and failure of weakly cemented rocks under different stress levels.In this work,all studies were supported by the National Natural Science Foundation of China,which entitled.Firstly,the multi-dimensional non-uniform deformation and failure characteristics of siltstone under uniaxial compression were carried out,using the Digital Image Correlation,acoustic emission,and CT scanning technology.Then,the compaction effect and strain energy conversion characteristics of siltstone under different confining pressures were analyzed,by conducting triaxial compression tests.Subsequently,the quantitative study of siltstone compaction-damage under complete stress cyclic load was conducted.Thereafter,the mechanism of confining pressure influence on macro and micro fractures of weakly cemented rocks was revealed.Finally,a new compressiondamage constitutive relationship was established.The main research results obtained are as follows:(1)The one-dimensional macro mechanical behavior(1D)and characteristic stress of siltstone was studied by analyzing the relationship between acoustic emission ringing counts,wave velocity,and volumetric strain.The evolution law of two-dimensional(2D)non-uniform deformation field,fracture field,and zonal failure characteristics have been analyzed through DIC technology.The classification criteria of macro-meso-crack criteria proposed,and the spatial distribution characteristics of microcracks at different scales were discussed,by reconstructing the three-dimensional(3D)cracks.The transformation characteristics of the main and secondary bearing areas during the pre-peak and post-peak stages were revealed.(2)The confining pressure effect on the volumetric dilatancy stress,elastic modulus,Poisson’s ratio,and wave velocity were analyzed,by the triaxial compression tests of siltstone under different confining pressures.The volume expansion characteristics and Poisson’s ratio of weakly cemented rocks were compared with the igneous,metamorphic,and sedimentary rocks,and it was found that weakly cemented rocks have significant confining pressure compaction effects.The influence of confining pressure on the energy distribution of weakly cemented rocks at different stages was revealed,by studying the characteristics of storage,dissipation,and release during the loading of confining pressure and deviator stress.(3)The complete cyclic loading and unloading tests of siltstone under different confining pressures were designed to study the compaction and damage law of weakly cemented rocks.Firstly,the strength and deformation laws of siltstone during the loading and unloading of confining pressure and deviatoric stress are analyzed.Then,the real-time wave velocity response characteristics during the full stress cyclic loading and unloading process were studied,and the correlation between wave velocity and stress level during the full stress cyclic loading and unloading process and the theoretical model of wave velocity evolution were established.Finally,the compression damage law of weakly cemented rocks under triaxial compression was clarified,the rock elastic wave velocity porosity model was modified,and porosity inversion analysis was conducted.(4)Firstly,the distribution of macro micro cracks in siltstone under different confining pressures was analyzed by using CT three-dimensional reconstruction technology.It is found that with the increase of confining pressure,the distribution of micro cracks that have not penetrated were more complex.Then,the influence of confining pressure on the evolution of micro fracture mode and micro fracture energy release scale in the progressive failure process of siltstone was studied,revealing the macro and micro fracture mode of weakly cemented rocks under low confining pressure and high confining pressure.Finally,the evolution mechanism of micro and micro fracture morphology of siltstone under natural state,uniaxial compression,low confining pressure and high confining pressure was discussed by SEM.(5)Based on the equivalent medium theory,the effective compressibility coefficient of weakly cemented rock was introduced and the rock fissure compaction coefficient was established.A fracture pressure density model considering the amount of rock fracture closure and the degree of compaction difficulty was proposed.The Kachanov damage variable was introduced and a weak cemented rock damage degree based on Weibull distribution was proposed.Finally,the model validation results show that the new constitutive model can accurately describe the entire deformation process of weakly cemented rock,and can also represent the compaction and damage characteristics in various deformation stages under different confining pressures.