| With the rapid development of the city,the probability of surface traffic congestion in the city is also higher and higher.At the same time,the expansion of traffic routes on the ground space is also facing the problem of land shortage.The development and utilization of underground space provides a new idea to solve the problem of urban ground traffic congestion.The development methods mainly include the construction of subway and underground expressway.Complex environmental conditions,high construction safety requirements and strict deformation control requirements are the major challenges faced by underground space development and utilization projects.Therefore,based on a crossing-river deep excavation by cut and cover method,which is located in Beijing North Canal,deformation and supporting force of foundation pit by means of using two methods including analysis of the foundation pit monitoring data and analysis of the foundation pit numerical simulation calculation.Then the parameter sensitivity analysis of these change laws was done in by numerical simulation method.Finally,the support design scheme in the foundation pit is optimized and explored by numerical simulation,which provides a reference for the construction design and research of the crossing-river foundation pit project in the future.The main research work and conclusions are as follows:(1)According to the monitoring data,the monitoring data of the top displacement of diaphragm wall,the lateral displacement deformation of diaphragm wall,the settlement deformation outside the foundation pit and the axial force of the support in the foundation pit are analyzed.It is determined that the lateral displacement of diaphragm wall is " medium convex " shape.It is found that the top of diaphragm wall is uplifted,and most of the first layer of reinforced concrete support is in tension state,other four layers of steel support are in compression state.(2)Based on the data of foundation pit investigation report and foundation pit construction design,a three-dimensional layered and segmented excavation foundation pit model is established.According to the analysis of numerical simulation results,the lateral displacement deformation characteristics of diaphragm wall are consistent with the monitoring analysis.It is determined that the uplift deformation of foundation pit bottom is "n" shape,and the surface settlement deformation outside the foundation pit is "groove" shape,the uplift displacement of the top of diaphragm wall are consistent with the monitoring analysis,and the first layer of reinforced concrete support is finally in tension state,which is consistent with the monitoring analysis.Through the comparative analysis of the measured value and the numerical simulation value of ZQT2-2(diaphragm wall lateral displacement deformation)monitoring point,it shows that the calculation result of the numerical model is of some reference.(3)The control variable method is used to design 47 groups of foundation pit models with different parameters.The statistical analysis of 47 groups of numerical simulation results is carried out.According to the definition of sensitivity coefficient,the statistical results are fitted linearly,and the parameter sensitivity analysis is carried out based on the fitting results,the absolute value of sensitivity coefficient of the maximum lateral deformation of the diaphragm wall and the maximum uplift deformation at the bottom of foundation pit and the maximum surface settlement deformation outside the foundation pit and the first and third and fourth and fifth support axial force relative to the friction angle of the soil body is the largest,and the absolute value of sensitivity coefficient of the second support axial force relative to the initial water level elevation is the largest.The sensitivity coefficient relative to the friction angle in the soil body is a negative value(negative correlation),the sensitivity coefficient relative to the initial water level elevation is a positive value(positive correlation).(4)According to the optimization objective of support design,the parameters of support design optimization are determined.Based on these optimization parameters and the foundation pit model in Chapter 3,four support schemes are designed.According to the analysis of the calculation results of the four schemes,the optimal design of support can be carried out from three aspects: reducing the diameter of steel support,appropriately increasing the horizontal spacing of steel support and properly moving down the vertical position of the first layer of reinforced concrete support.Combining with these three aspects of support design can achieve the goal of reducing the amount of engineering materials and increasing the convenience of foundation pit construction,as well as maintaining the stability and safety of the foundation pit project.It is preliminarily determined that the scheme of the first layer support moving down is the best optimization scheme.There are 51 figures and 29 tables and 59 references in the paper. |
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