Study On Stability And Safety Risk Assessment Of Metro Deep Foundation Pit In Sandy Soil Layer

Sandy soil layer is widely distributed in rivers,lakeside and coastal basins in China,especially in the Yangtze River Delta,Pearl River Delta and coastal economically developed areas.It is a sensitive stratum.In the construction process,due to the action of dynamic load,vibration liquefaction phenomenon occurs,which brings great harm to surface buildings and underground structures.In the foundation pit engineering,the existence of sandy soil stratum will affect the stress situation of foundation pit,which is manifested as the increase of excavation deformation,thus threatening its stability and safety.In this paper,through the study of literature,orthogonal test and regression analysis and other means to analyze the factors affecting the stability of the foundation pit,relying on the actual project,from the stability of deep foundation pit and the safety risk assessment of deep foundation pit,the safety and stability of deep foundation pit in sandy soil layer is studied.The research contents and results are as follows:(1)Through the orthogonal test method,the stability of foundation pit is quantified as the maximum value of horizontal displacement of retaining wall,surface settlement and uplift of pit bottom,which is taken as the test index.According to the literature,the depth of foundation pit,groundwater level,insertion ratio,soil reinforcement coefficient,the number of steel supports and the strength of retaining wall with high grouting degree are selected as the influencing factors of foundation pit stability.The orthogonal test of six factors and five levels is designed to analyze the influence of each factor on each assessment index and comprehensive stability,and finally the order of influence magnitude is obtained.(2)Combined with the results of orthogonal test in(1),the effect diagram of six influencing factors corresponding to each assessment index is obtained.According to the effect diagram,the one variable nonlinear regression model of the assessment index under the condition of single factor is established.On this basis,the multiple nonlinear regression model of horizontal displacement of retaining wall,surface settlement and uplift of pit bottom under the coupling effect of various factors is further obtained.The goodness of fit of the regression model established in this paper is 0.981,0.959 and 0.988 respectively,which indicates that 98.1%,95.9% and 98.8% samples can be explained,and the fitting degree is high.Through the regression model,the maximum values of horizontal displacement of retaining wall,surface settlement and heave of pit bottom can be calculated when the foundation pit is excavated to the bottom,so as to obtain the stability of the foundation pit at this time.(3)Combined with the research contents of(1)and(2),the influencing factors of safety risk of deep foundation pit excavation construction are identified from three aspects of geological environment,design parameters and monitoring parameters.Based on analytic hierarchy process(AHP)and extenics theory,a risk assessment model is established.Taking a practical project as an example,the safety risk level of construction and excavation of deep foundation pit in sandy soil layer is evaluated.The evaluation results are consistent with the actual situation,which shows that the evaluation model has high credibility.