| When a tunnel passes through layered surrounding rock with a gentle dip angle in cold regions,the occurrence of multiple freeze-thaw(F-T)cycles can easily cause fatigue damage to the rock bedding surfaces and seriously affect the rock stability.One of the engineering problems that need to be solved is to determine the deformation and failure mechanisms of such surrounding rock under F-T cycles.This paper is based on National Natural Science Foundation of China(Grant No.52108367)and the Science and Technology Department of Guangxi Zhuang Autonomous(Grant No.AD21238018).Combined with a tunnel project in the Qinghai-Tibet Plateau,study on the deformation and failure characteristics of layered tunnel surrounding rock with gentle dip angles after freeze-thaw cycles was carried out by using the methods of literature investigation,laboratory test,theoretical analysis and numerical simulation.The purpose of this study is to provide some theoretical guidance for the safety construction and support measures of layered tunnel surrounding rock with gentle dip angles in the corresponding cold region.The main work and achievements are as follows:(1)Based on a tunnel engineering background,similar sandstone materials are configured with quartz sand,gypsum,cement and other raw materials.The similar tunnel model box and its F-T cycle and loading test scheme are designed.The research results determined the similar sandstone material dosage with water-binder ratio of 0.686,sand-binder ratio of 3.489 and paste water ratio of 0.299.According to the temperature conditions in the tunnel area,the number of outdoor freeze-thaw cycles was converted into the number of indoor freeze-thaw cycles,and the equivalent number of indoor freeze-thaw cycles for 5 years was determined to be 40.05,and the test was set to 40.In the test,the tunnel model of layered structure was built with gentle dip angle(bedding angle is 0°,10°,20° respectively).(2)The mechanical response of tunnel lining structure and the progressive failure process of surrounding rock under long-term F-T action were studied by physical model test.The results show that the carrying capacity of the model decreases with the increase of bedding angle and the number of freeze-thaw cycles.At the same time,the bending moment and axial force evolution characteristics of the layered tunnel surrounding rock with gentle dip angles are consistent with the deformation characteristics,which accords with the actual mechanical and deformation characteristics of the tunnel under different F-T cycles.The deformation mode of layered tunnel surrounding rock with gentle dip angles conforms to the characteristics of asymmetric deformation,and the lining is partially empty under the action of F-T cycles.(3)A constitutive model was developed to study damage in transversely isotropic rock masses caused by F-T cycles.The model was based on the Weibull distribution function,the macroscopic bedding plane damage variable and a new F-T damage variable.The model was verified by the results of uniaxial and triaxial compression tests of Carboniferous slates at various bedding angles that corresponded to different numbers of F-T cycles.(4)Based on the secondary development environment of finite element software,the constitutive model is embedded in the FLAC3 D numerical analysis software.By establishing tunnel numerical calculation models for surrounding rock with three kinds of bedding angle(0°,10°,20°)and verifying the practicability of the numerical models based on the test results,the horizontal and vertical displacements of surrounding rock at the tunnel entrance section under the ground stress condition at the depth of 400 m are finally calculated.The research results accurately reflect the long-term deformation characteristics of the layered surrounding rock tunnel with gentle dip angle in the cold region,and can provide some theoretical guidance for the application of the corresponding tunnel support measures in the cold region. |
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