Research On Evolution Characteristics And Assessment Method Of Construction Risk For Urban Deep-large Foundation Pit

With the formation of urban agglomerations,the central city has gathered a large number of people,and the ground resources are scarce.China has accelerated the development of underground space.As the entrance of urban underground space development,foundation pit engineering plays an important role in the construction of large underground complex and subway station.The construction area of foundation pit is larger and larger,and the shape is irregular,and the excavation depth is constantly reaching new heights.There are many uncertain factors in the process of deep foundation pit construction,such as dense underground pipe network,complex hydrogeological conditions,crisscross surrounding buildings and roads,dense personnel and vehicles,frequent construction accidents,which have caused serious social impact.Therefore,in order to reduce the probability of accidents in the process of deep foundation pit construction,risk assessment will be carried out before construction.Based on the national key research and development project "coupling evolution mechanism and safety evaluation system of major risks in urban underground large space construction"(National Key R&D Program of China(2018YFC0808701)),through the statistical analysis of urban deep and large foundation pit engineering data and accident cases,this paper numerically simulates the risk evolution in the construction process,sums up the risk characteristics of deep and large foundation pit construction,and puts forward some suggestions suitable for urban deep and large foundation pit construction The evaluation method and process of process risk.The main contents and conclusions are as follows :(1)Through the statistical analysis of the urban deep foundation pit engineering data,the definition of the urban deep foundation pit engineering is given.Through the statistical analysis of the accident cases of deep-large foundation pits in cities,the main accident types and risk sources are summarized.By numerically simulating the construction process of deep-large foundation pits,the risk evolution curve of the whole construction process is calculated,indicating that the overall risk level changes continuously with the construction process.Based on the above research,the construction risk characteristics of deep-large foundation pits in the city are summarized: cause complexity,dynamic evolution,unpredictability and high risk.(2)Considering the risk characteristics of urban deep-large foundation pits,an evaluation method combining Markov chain and fuzzy comprehensive evaluation method is proposed.The transfer matrix in the Markov chain and the membership and weight calculation methods in the fuzzy comprehensive evaluation method are studied.The establishment process and calculation steps of the Markov-AHP-FCEM risk assessment model are analyzed.On this basis,the aided program for risk assessment calculation is developed using the Python programming language.(3)From the four aspects of evaluation index system,evaluation process,risk level assessment,risk classification and control measures,the whole process risk assessment system before construction of urban deep-large foundation pits is explained.(4)According to the Markov-AHP-FCEM evaluation model and the entire construction evaluation process,the risk assessment of the deep-large foundation pit project of Guangzhou South Railway Station was carried out.The results showed that the overall risk level of the project was three-level risk.