Study On Safety Assessment Of Support Structure For Deep Excavation In Wuhan

With the increasing of the construction scale of high-rise buildings year by year, more and more deep excavation engineering appears and some accidents happens occasionally, which results not only huge economic losses, but also bad impacts on society. Therefore, the safety control of the deep excavation engineering has become an urgent issue to be solved.Firstly, the study situation of the technology and management of deep excavation engineering at home and abroad was reviewed. Moreover the 50 deep excavations with accidents in Wuhan since 1994 was gathered and analyzed.36 of these deep excavations with soil nails was especially analyzed and the reasons of the accidents were analyzed and sumerized. The result indicates that more than 80% of the accidents are related to the surface water etching after rainfall and the leaking from sunken pipe culvert in the soil mass.The undisturbed soil samples were gathered from an excavation accident site for the following tests:characteristics test, no-expansion saturation test, single direction expansion saturation test and the single direction expansion saturation test with dewatered samples. The test results indicate that the clay in the surface of the slope will easily dewater in high temperature in the summer in Wuhan, the numerous slight cracks (especially vertical cracks) in the soil mass easily occurred in. When encountering the rainfall, the rainwater will infiltrate into the soil mass rapidly through those vertical cracks, and the soil mass will expand and be softened. The intensity parameters obtained from tests with this soft soil were input into the "TianHan" software to re-check the stability for two failure slopes. It was point out design defects in the accidental excavation engineerings and the suggestion to improve design the deep excavation was presented.The causal relationships among the accidents of soil nail excavation, the design defects and the construction defects are analyzed in the thesis. Based on Fault Tree Analysis Method and the rating method in "Standard for Reliability Evaluation of Civil Buildings", the safety grade appraised system for the soil nail excavation is established and applied to three engineering projects. Finally, a typical accident excavation of is chosen to study the tradeoff selection process between the controllable accident risk and construction safety performance. The Analytic Hierarchy Process and Entropy Weight Method are applied to make quantitative analysis for decision-making behavior. The advantages and disadvantages of two multi-objective risk decision methods are analyzed. The research results indicate Entropy Method is more scientific and objective when the appraisal index is more qualitative and more quantitve factor.