Study On The Progressive Failure Mechanism Of The Surrounding Rock Of Tunnel Constructed In Soft Rock

Tunnel collapse is one of the frequently occurring disasters in tunnel construction, and it leads to great damages, such as the delay in the carrying out the projects, and the loss of life and property, and also poses great latent threat to the safety of the tunnels. Therefore, the research, centering on the failure mechanism of the collapse of tunnels, is of great help and necessity for the design and construction of the tunnels.Basing on the NSFC (National Natural Science Foundation of China) funded project "Research on the Progressive Failure Mechanism of Tunnels Constructed in Soft Rock and the Prediction of Their Loosening Load", this thesis focuses on the progressive failure mechanism of surrounding rocks (grades IV-VI) of road tunnels, by combining statistical analysis, model test and numerical simulation with theoretical analysis.Besides introduction and conclusion, this thesis consists of five chapters.The beginning part is a general literature review and a brief introduction. Chapter One of the thesis examines many instances of the collapse of road tunnels, railway tunnels, hydraulic tunnels by means of extensive reading of files and field researches, classifies these collapses into several types, and analyzes the main influencing factors attributing to the collapse.Basing on these detailed statistic analysis, the succeeding chapter puts forwards the scheme and method of model tests on the progressive failure mechanism of the surrounding rocks of tunnels. Furthermore, by describing a series of model tests, which deal with the influences of the strength of surrounding rock, the cross section and buried depth of tunnels, and the surface water on the failure mechanism of tunnels, this chapter summarizes certain key traits of different types of collapse in terms of the progressive process, the laws of the stress of surrounding rock and their displacement during the collapse.By using DEM software PFC which simulates the whole process of the collapse, the third chapter provides certain supplements to the scheme of model tests, and makes a special analysis of the failure mechanism of the collapse of small-distance tunnels and of the work faces of tunnels.In Chapter Four, a detailed analysis which combines the statistical analysis, model test with numerical simulation leads to the research on the effects of influencing factors on the mechanism of collapse of the tunnels and a comprehensive summary of the progressive failure mechanism of tunnels caused by excavation.The fifth chapter puts forwards the theory concerning the dynamic pressure arch of surrounding rock based on progressive failure, and applies this theory to the analysis of model tests, such as, analyzing the changes of the dynamic pressure arch during the tests, and explaining the changes of the stress of the surroundings of tunnels perfectly. And the numerical simulation is also used to add accuracy and reliability to the theoretical analysis.In the final section comes the conclusion, which summarizes the whole thesis and points out its inadequacies that call for further studies.