In Situ Direct Shear Test Of Gravity Dam Body Bedrock And Simulation Of Crack Propagation

With the increasingly prominent role of gravity dams in the national economy,the safety issues of gravity dams have become particularly important.Exploring the mechanical properties of the dam body and bedrock,as well as potential crack propagation issues after dam construction,can help understand the load-bearing mechanism of gravity dams and ensure their safety.Based on the in-situ direct shear test results of Poyue Reservoir in Guangxi,this paper studies the mechanical characteristics and failure mechanism of the dam bedrock interface in the direct shear test;At the same time,based on the concrete fracture mechanics and the extended finite element method,the direct shear test of Poyue Reservoir and the crack propagation simulation of Poyue Reservoir 5 # dam block with initial cracks are carried out.Finally,the water volume weight overload method is used to simulate the potential crack propagation of concrete gravity dams.The specific research contents and conclusions are as follows:(1)The shear stress-strain displacement curves of the specimens under different normal stresses all exhibit three stages: the stable expansion stage of initial internal defects under stress,the unstable expansion stage of microcracks gradually expanding to form macroscopic cracks,and the cracking stage of separation between concrete and rock mass.The normal stress,the composition and content of rock mass,and the weathering degree of rock mass have significant effects on the fracture energy and shear strength of samples: the fracture energy increases with the increase of the normal stress or the content of gray sandstone,and decreases with the increase of the content of argillaceous siltstone or the weathering degree;The shear strength of concrete dam bedrock increases with the increase of gray sandstone content in bedrock,and decreases with the increase of argillaceous siltstone content or bedrock weathering degree.The failure modes of the dam bedrock specimen under different lithological characteristics are mainly manifested as cementation surface failure,rock mass failure,and mixed type failure.(2)The simulation model of direct shear test based on fracture theory and extended finite element method is established to study the fracture process of the sample and reveal its fracture evolution mechanism.The numerical simulation reproduces the crack propagation,penetration and cracking process,which is consistent with the failure process of the in-situ direct shear test,and reveals the fracture evolution mechanism of the direct shear test: because the tensile stress at the loading end is higher than the tensile strength of concrete and bedrock,the cracks first occur at the loading end,and then because the strength of bedrock is less than the strength of concrete and there are initial defects in the bedrock,The crack extends from the area with high tensile stress at the crack tip to the weak part inside the rock mass,and then forms a through passage when the crack reaches the non-loading end of the test block.Finally,as the shear load increases,the concrete test block and the bedrock separate from each other.On this basis,the extended finite element method was applied to the simulation of crack propagation in gravity dams.The study found that the initial crack propagation changes of 58.5m high concrete gravity dams were relatively small under the design flood level,check flood level,and full load reservoir water level conditions.(3)Based on the water unit weight overload method,the critical overload coefficient of the 5 # dam block of the Poyue Reservoir is studied.Under the critical overload coefficient,the initial crack models at different positions,lengths,and angles are simulated for crack propagation,exploring the propagation law of potential cracks in the dam and providing reference for the stability of the project.The results indicate that during the crack propagation process,the stress concentration zone will move towards the crack tip along the crack propagation path.When the tensile stress at the crack tip reaches the tensile strength of the material,the crack continues to expand;When the stress concentration zone moves to the crack tip and the tensile stress at the crack tip is less than the tensile strength of the material,the crack stops expanding.This law can provide reference for the study of crack propagation mechanism in dams.Under the critical overload coefficient of 1.4,the total length of initial crack propagation at the dam heel is greater than that at the upstream inflection point of the dam;The total length of initial crack propagation increases with the increase of initial crack length;The total length of initial crack propagation gradually decreases with the increase of initial crack angle.When the initial crack angle is 30 ° or 45 °,crack propagation is more likely to occur in the dam,which is a dangerous initial crack angle and requires reinforcement of the dam foundation at this angle at the dam heel.