Fracture Failure Mechanism Of Perilous Rocks In Alpine Region Considering Temperature-stress Ineraction

With the in-depth implementation of China Western Development Strategy,the major projects such as Qinghai-Tibet Railway and Sichuan-Tibet Highway is under steady construction.The influence of geological environment factors on the construction of engineering projects in alpine regions is gradually studied and attached great importance year by year.The alpine region has severe climate and a large number of dangerous rock band.The in Instability of pelirous rock has the characteristics of short cycle,strong burst,wide range and strong destruction under the condition of cyclic freeze-thaw cycle.To study the failure mechanism of perilous rock in alpine region under the combined action of temperature and stress provides a theoretical basis for the treatment and protection of dangerous rock in cold region,and it is of great significance and value to guarantee the safety of construction and operations in alpine area.The background of this thesis is the high mountain gorge section on both sides of Suomo River in Aba Qiang Tibetan Autonomous Prefecture,Sichuan Province.Aiming at the dangerous rock belt along the line,the failure mechanism of dangerous rock in cold region is studied.By means of laboratory test,theoretical analysis and numerical simulation,the law of rock mass strength deterioration is analyzed,and the failure mechanism of perilous rock in alpine region under the combined action of temperature and stress is revealed.The research contents and conclusions of this thesis are as follows:（1）The quality and surface changes of rock samples were analyzed after freeze-thaw cycle test.Brazilian disc splitting test was carried out on freeze-thaw cycle specimens,and the load-displacement curves of Brazilian disc splitting test were analyzed.The anisotropy coefficients of tensile strength are 1.81,1.92,2.39 and 2.77 when the number of freeze-thaw cycles is 0,15,30 and 45,respectively.And it is concluded that the anisotropy coefficient of tensile strength increases with the increase of freeze-thaw cycles.Considering the superposition effect of schistosity and freeze-thaw cycles on rock fracture toughness,a prediction model of the slate’s fracture toughness was established with freeze-thaw cycles and schistosity angle as variables.The results show that the anisotropy coefficient of tensile strength increases with the increase of freeze-thaw cycles,this effect deepens the schistosity effect of stratified rocks.When the angle between prefabricated cracks and direction of schistosity is small,the fracture toughness of specimens is more significantly affected by the combined action of freeze-thaw cycle and schistosity effect with the increase of the number of freeze-thaw cycles.（2）Based on the theory of fracture mechanics,the failure mechanism of perilous rock is analyzed under four conditions:the fracture is filled with water,the water starts to freeze,the fracture is filled with ice and the ice starts to melt.Based on Paris formula,the prediction formula of fracture fatigue propagation life of the model is established,and the evolution analysis of fracture propagation process of dangerous rock is carried out through calculation,and the prediction of fracture propagation process is roughly made.The results show that the initial angle and fracture toughness of rock mass affect the critical fracture length and angle in water-filled condition.Factors such as low temperature,fracture toughness of rock mass and proportion of freezing length can lead to the change of critical cracking length under freezing condition,but have little effect on cracking angle.Ice-filled cracks occur mostly in a large area of small collapse.,while water-filled cracks are several orders of magnitude and meter-scale collapse.The crack propagation length is dominated by frost heaving force in alpine zone,and the crack path is controlled by hydrostatic pressure in the crack,which indicates that freeze-thaw cycle has great influence on fracture propagation of dangerous rock mass.（3）Based on the XFEM extended finite element method and combined with the conclusions of the previous chapters,the evolution of the collapse process of perilous rock is simulated and analyzed,and compared with the theoretical model,the errors were found to be small.The prediction model of slate’s fracture toughness in Chapter 2 was used to simulate the change of fracture extension length by changing the fracture toughness of perilous rock model,and the change rule of fracture extension length influenced by fracture toughness was analyzed.The calculation and analysis showed that the power function relationship between fracture extension length and the freeze-thaw deterioration coefficient of slate’s toughness was about（D_K）^-1.2.It is predicted that the life of perilous rock in alpine region should be more than 20 years,and timely response should be made.