Study On Damage Mechanism Of Water On Disintegration Mudstone Under Unloading Path

In China's coal mine roadways,the roof and floor rock is mainly mudstone and sandstone,the short-term and long-term instability of mudstone engineering has caused widespread concern among engineers.Mudstone strength is the main factor affecting the stability of mudstone engineering.The external cause of the deterioration of mudstone strength is engineering disturbance and temperature and humidity change,the internal cause is the unique crystal structure of clay components contained in mudstone.During the whole life cycle of the mudstone engineering,the effects of temperature,hydro,mechanical,and chemical fields cause the mudstone strength weakened to varying degrees,with varying degrees of internal damage.In this paper,the disintegration mudstone and shale in the roadway of Yangquan Sijiazhuang coal mine are taken as the research object,combined with the stress environment and geological conditions of the excavation of coal mine roadway.Laboratory experiments,numerical simulations and theory are used under the coupling of multiple fields of THMC,based on the combination of analysis methods,the basic theoretical research on the progressive damage mechanism and failure law of mudstone is carried out from different scales,so as to provide reference for the stability of the water-filled roadway.The research contents of this paper include four aspects:?1?Based on the mesoscopic damage of mudstone from different water quality and chemical composition solutions,the characterization of internal pores and fissure evolution process is carried out to reveal the mechanism of macroscopic strength reduction caused by meso damage;?2?Research on the strength reduction law under different hydraulic stresses path and water content;?3?Based on the temperature and humidity effects of mines,the mechanical response characteristics of mudstone under different temperature and humidity water vapors are studied;?4?Based on the previous experiments,the damage and failure law of mudstone caused by meso-parameter and unloading parameter?unloading rate,time?was further studied by numerical simulation.Through the research of the thesis,the following conclusions were obtained:?1?The progressive damage characteristics of shale under different water contents were studied from the microscopic scale by high-precision micro-CT system.The research shows that the water absorption of shale is a process of space-time evolution.With the increase of water-bubble time,the water absorption of mudstone is generally from the outside to the inside.However,if there is a weak surface such as the bedding plane,it will be preferentially entered along the weak surface;Using the quantitative evaluation of minerals by scanning electron microscopy to analyze the weak surface as the water seepage channel is due to the existence of scale differences between the internal particles,resulting in the existence of pores in the cementation,water easy to infiltrate;Through the CT study of the number of pores of mud shale,the law of crack length,area and quantity changes,and these parameters are equivalent to the damage coefficient.The damage coefficient and water content were found to be in accordance with the logarithmic distribution rule.?2?Based on the theory of chemical kinetics and ion exchange and adsorption,it is proposed that OH^-concentration change rate,Ca²⁺,Mg²⁺ion exchange rate and pore area change are used as evaluation indexes,and the degree of damage to mudstone caused by hydration is determined dynamically.A comprehensive analysis of the amplitude of variation of three parameters shows that alkaline water damages mudstone greatly.?3?Different stress paths and different water content experiments were carried out using a three-axis servo machine.The results show that there are five stages in the three-axis unloading stress path:initial compaction stage,elastic stage,stress sudden increase stage,yielding stage,failure stage,and the occurrence of stress sudden increase shorten the yielding section and advance the timing of the failure,resulting in a decrease in peak strength and peak strain.With the increase of water content,the compacted stage increases continuously,and the equivalent damage coefficient is approximated by the crack density of the compacted stage,the damage coefficient increases logarithmic with the increase of water content;the elastic stage gradually decreases,and the yield stage gradually increases.It shows that the plasticity and deformation capacity of mudstone increase.With the increase of water content,the compressive strength and elastic modulus of rock decrease continuously,and the poisson's ratio increases continuously.When there is confining pressure,the strength obeys the linear reduction law,while without confining pressure,the strength obeys the exponential reduction law;as the water content increases,the post-peak strength decline rate gradually becomes slower,indicating that the mudstone transforms from brittle to plastic;with the increase of confining pressure,the residual strength increases gradually and decreases slowly,indicating that mudstone changes from brittleness to plasticity.With the increase of water content,the cohesion force decreases greatly,and the internal friction angle decreases little.At the same time,the internal friction angle before and after the peak has not big difference,which provides a basis for the numerical simulation to study the reduction law of rock mass strength.?4?By comparing the energy curves of different water content in unloading state and triaxial loading state,it is found that the total energy,dissipative energy and maximum elastic energy of the two rocks decrease with water content increased.The energy absorbed in the unloading state is less than the loading state,which means the rock can be destroyed by the smaller energy in the unloading path.It also can be found that the stress path and water content will have a certain degree of influence on the damage and failure of mudstone.The weakening coefficient is used to quantitatively characterize the loading stress path and the weight of water content on the damage of mudstone.The influence factor P of unloading confining pressure is put forward,and the stress path of unloading confining pressure and the weight of water content on mudstone damage are determined.?5?A self-made temperature and humidity system was used to simulate the underground environment.The research shows that the higher the temperature,the stronger the thermal motion of water vapor molecules,the stronger the diffusion and adsorption capacity,the longer the saturation time of the mudstone and the lower the strength;The water absorption curve and the strength curve are close to the liquid water,but the strength dispersion degree is smaller.?6?Based on the fractal theory,the relationship between the fractal dimension and the water content is studied between the multi-scale and multi-parameters from the microscopic pore volume of the shale,the macroscopic fracture length of the shale,macro-disintegration fragmentation and the macroscopic fragmentation of the mudstone.With the increase of water content,the fractal dimension of the number of pores and the length of the crack increases gradually,indicating that the degree of damage increases gradually.Comparing the macroscopic and mesoscopic fractal dimensions,it is found that the macroscopic dimension is large,indicating that from microscopic to macroscopic,there exists a scale effect.The larger the scale,the greater the potential damage probability and the higher the damage degree.The fractal dimension is not completely positively correlated with the peak strength.The mathematical model of fragmentation is used to explain the fractal dimension,which is related to the number and probability of fragmentation.?7?Using the particle flow software PFC,FISHTANK secondary development is carried out on the basis of the original data flow,and the micro-fracture evolution and macro-fracture process of the intact,pre-fractured digital core under different unloading rates and unloading times are realized and studied.The results show that with the increase of unloading rate,the number of cracks decreases and the strength decreases;when the unloading timing is earlier,the mudstone enters the unloading pressure state in advance,resulting in lower ultimate strength.Compared with the intact core,the pre-fractured core has a higher rate of strength reduction and a smaller number of cracks at the same unloading rate and unloading timing,but the ratio of tensile cracks is more.