| This paper takes two foundation pits of highway tunnels located on the same site respectively above Shenzhen Metro Line 11 and Shenzhen Metro Line 5 as the background.And the following study is carried out:(1)Before the construction of the site above Metro Line 5,to predict the risk of slope excavation on the underlying tunnel of Metro Line 5 and take effective protective measures.Therefore,based on the improved calculation method of the effect of slope excavation on the underlying metro tunnel,Monte Carlo method is used to calculate the probability of risk of different levels of tunnel structure.The results show that the improved calculation method has wider application and is more practical than the original calculation method which is only limited to rectangular excavation of foundation pit;without any protective measures,the probability of tunnel structure of Metro Line 5 in an unsafe state will be as high as 72.64%,of which 65%is in a very unsafe state.(2)To verify the effectiveness of the protective measures,it is important to accurately predict the displacement of tunnel of Metro Line 5 after the foundation pit above Metro Line 5 is decided to be constructed with protective measures.And it is a key step to accurately determine the physical and mechanical parameters of the soil layer of the foundation pit in the process of accurately predicting the displacement of the underlying metro tunnel caused by the foundation pit excavation.Also,the two foundation pits are located in the same site,so the physical and mechanical parameters of the soil layers should be consistent,and the construction of the foundation pit above Metro Line 5 will not start until the completion of the construction of the foundation pit above Metro Line 11.Therefore,on the basis of determining the physical and mechanical parameters of the soil layers by the back analysis of the measured vertical and lateral displacement of tunnel of Metro Line 11 before the foundation pit excavation,the multi-objective optimization and decision of the physical and mechanical parameters of the soil layer are carried out by combining NSGA2 genetic algorithm and Critic-TOPSIS method.And the best scheme of the combination of the physical and mechanical parameters of the soil layer is obtained.The results show that the errors between the measured value and the calculated value of the vertical and lateral displacements of the two subway tunnels after the excavation of the foundation pit are 2.61%,5.35%,7.927%and 5.934%less than that of the initial scheme respectively.It proves the effectiveness of the optimization and decision method for the soil parameters of the foundation pit above or near the metro tunnel,and provides an accurate combination of physical and mechanical parameters of soil layers for the following theoretical calculation analysis and numerical simulation of tunnel of Metro Line 5.(3)To explore the effect of foundation pit excavation on the side tension pile of tunnel of Metro Line 5,and to solve the problem that the research method in this field is only limited to the 3D numerical analysis which is only for single project and is complex,has much work in modeling,based on Mindlin solution,the vertical displacement of tension pile caused by the friction of soil around the pile is derived by the Pasternak foundation beam model and the displacement coordination principle;the vertical displacement of tension pile caused by the shear deformation of soil outside the pile surface is derived by the shear displacement principle of soil and the balance condition of pile;By the equilibrium condition of the pile micro-element segment and Hooke’s Law,the vertical displacement of the tension pile caused by the axial deformation of the pile is derived.The total vertical displacement of the tension pile is obtained by the superposition of the above three vertical displacements.At the same time,the axial force of the tension pile on the side of the metro tunnel caused by the excavation of the foundation pit is calculated.Finally,the calculation method of total vertical displacement and axial force of tension pile is verified by comparing the numerical model verified by the practical project,and the effect of the different parameters involved in the calculation method on total vertical displacement and axial force of tension pile is analyzed.The results show that the method of calculating the vertical displacement and axial force of the tension pile on the side of the metro tunnel caused by the foundation pit excavation is reasonable and reliable,which is suitable for practical projects.(4)To study the effect of the tension pile construction on the vertical displacement of tunnel of Metro Line 5 and the control effect of the side friction of the tension pile on the vertical displacement of tunnel of Metro Line 5 during the excavation of foundation pit,and to solve the problem that the research method in this field is only limited to the 3D numerical analysis which is only for single project and is complex,has much work in modeling,the vertical displacement caused by the tension pile construction are derived by Mindlin solution and two-stage method;based on the principle of pile-soil displacement coordination and the principle of soil shear displacement,the vertical displacement of the metro tunnel and surrounding soil caused by the tension pile construction are derived.The total vertical displacement of the metro tunnel caused by the tension pile construction is obtained by the superposition of the above two vertical displacements.In the same way,the total vertical displacement of the metro tunnel controlled by the tension pile during the foundation pit excavation is derived,and the two calculation methods are verified by the practical project.Finally,the effect of the different parameters involved in two calculation method on two calculation values of the vertical displacement of the metro tunnel is analyzed.The results show that two calculation values of the vertical displacement of the metro tunnel in this paper are in good agreement with the measured values and the finite element simulation values,which proves that the two calculation methods are suitable for practical projects.(5)Although the foundation pit above Metro Line 5 has been constructed according to the initial scheme,the cost of foundation pit is large.So it is necessary to optimize the scheme to provide a useful reference for the design of the foundation pit above the similar metro tunnel in the future.Therefore,based on NSGA2 genetic algorithm,the Pareto optimal solution set is obtained by taking 5 key design parameters of the foundation pit controlling the vertical displacement of the underlying metro tunnel as the optimal design variables,the maximum vertical displacement of the tunnel and the variable value of construction cost of foundation pit as the optimization objectives.Combining the expert scoring-COWA operator-game theory combination weighting method and TOPSIS method,the optimal scheme is selected from Pareto optimal solution set,and the effect on the change of the subjective weight of expert scoring on the design variables,objective function values and some other parameters of the best scheme are analyzed.The results show that the maximum vertical displacement of the tunnel in the best scheme is 23.3%less than that in the initial scheme,and the variable value of construction cost of foundation pit is 36.9%less than that in the initial scheme.It proves the effectiveness of the optimization design method for the foundation pit above the metro tunnel. |
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