Failure Mechanism Of Dangerous Rocks In XiaJiang Section Considering Stress-Chemical Interaction

The geological environment of the Three Gorges Reservoir Area is complex,and it is a high incidence and severe disaster area.The unstable failure of dangerous rocks is characterized by strong suddenness,wide distribution,strong disaster-causing ability and concealment,etc.Once the disaster occurs,it will seriously affect the geological safety of the Yangtze River waterway and bring great losses to people’s lives and property.Therefore,it is of great practical significance and value to scientifically and accurately reveal the failure mechanism of dangerous rocks in the Xiajiang section of the Three Gorges reservoir area under the stress-chemical action,and then provide theoretical basis for judging whether the Yangtze River waterway is safe,controlling perilous rocks,and predicting disasters.This paper is based on the general project of Chongqing Natural Science Foundation "Intelligent identification and early warning technology of highstand perilous rock based on the integration of sky,space and earth".Based on laboratory tests,theoretical analysis and numerical simulation,the failure mechanism of the highstand perilous rock in the Xiajiang section of the Three Gorges Reservoir Area under stress-chemical action is analyzed.The main research contents and conclusions of this paper are as follows:(1)By carrying out uniaxial compression test and direct shear test of limestone and marl after soaking in chemical solution and dry-wet cycle,the decaying rule of uniaxial compressive strength and shear strength parameters of the two rock samples under stress-chemical action was analyzed,and the dissolution rate of limestone and marl at different p H values was calculated.The strength parameters and mass attenuation law of limestone and marl were predicted and analyzed by multivariable optimal fitting,and the shear strength parameter deterioration prediction model was established based on particle swarm optimization BP neural network(PSO-BP neural network).The results show that the strength parameters of rock samples are reduced by the increase of acidity and dry-wet cycles,and the dissolution rate of marl is greater than that of limestone.PSO-BP neural network can predict the deterioration of parameters well.(2)Based on the basic principle of damage mechanics and considering the chemical action of rock,the rock damage constitutive equation under the joint stress-chemical action was established.The parameters were solved and the equation was verified by the data of uniaxial test in Chapter 2.Based on the PFC particle flow software,the rock failure law under triaxial compression was revealed,and the change law of internal structure,crack propagation and pore evolution of limestone and marl was revealed by scanning electron microscope.The results show that the compressive shear mixed failure plays a dominant role in the triaxial failure of limestone and marl,and the failure process all goes through four stages: initiation,propagation,extension and failure.The process of rock failure can be divided into vulnerable shedding zone,damage disturbance zone and nondestructive compaction zone,which can better reflect the characteristics of rock failure.(3)In view of the special geographical environment in the Xiajiang section,the failure inducements of perilous rocks in this area are summarized as stress action,chemical corrosion,weathering,dry-wet cycle,shipping and human activities,etc.,and the development characteristics and failure modes of typical perilous rocks in the Xiajiang section are analyzed.Based on the reliability principle,the failure mode of sliding perilous rock under stress and the calculation method of instability probability were established considering the two cases that the outflow cracks is blocked or not in the fracture of perilous rock mass.Based on the finite difference software FLAC3 D,the numerical simulation was carried out to discuss the displacement in X and Z directions,maximum principal stress and maximum shear strain of perilous rock mass under 145 m water level and 175 m water level in natural condition and 162 m water level in rainstorm condition.The results show that if the outflow cracks is blocked,the instability probability of perilous rock mass will reach 33% in rainstorm conditions.Rock mass has greater instability risk in rainstorm condition than that in natural condition.In the rainstorm conditions,the displacement,maximum principal stress and maximum shear strain of the dangerous rock will increase.With the continuous decrease of the strength of the rock mass at the base and the existence of the fracture at the back edge,the rock mass is highly likely to slip and failure along the base.Therefore,it is necessary to strengthen the monitoring of the stability of the rock mass in the rainstorm conditions,and to timely plug the cracks in the rock mass and draw out the crack water,which can better protect the rock mass.