Research On Optimization Method Of Generalized Size Of Inner Support Of Long And Narrow Foundation Pit

Long and narrow foundation pits are mainly used in subway stations and integrated pipe corridors which are mostly located in the bustling areas of cities surrounded by many other buildings and pipelines.Therefore,it is mostly supported in the form of internal support,which can not only achieve the stability of the foundation pit but also reduce its impact on the surroundings.The paper introduced the design method and layout requirements of the support in the foundation pit,the principle of stability of the whole foundation pit,and the method of settlement.Taking the case of a long and narrow foundation pit of a subway station in Wuhan,the finite element software ABAQUS was employed to calculate the internal forces of the internal support and pile and the settlement of the foundation pit.To be economical and secure,the restriction of settlement and safety factor were taken as constraint conditions,and the cost of foundation pit as the objective function,and then the generalized size of foundation pit design was optimized with genetic algorithm,and finally a reasonable plan was obtained.In this paper,the research was mainly conducted from the following aspects.(1)Based on the finite element calculation results of the original foundation pit design,the finite element model was calibrated by comparing the calculated value of foundation pit settlement with the actual monitoring value.(2)Taking the horizontal spacing of the internal supports,the vertical spacing,the number of gussets and the spacing of piles as design variables,a 4-factor 3-level orthogonal numerical experiment was designed,and a total of 15 sets of support schemes were calculated,of which 6 sets were Random support plan.The numerical calculation results of the internal force,settlement and safety factor of the supporting structure show that the corresponding results are obtained by the finite element calculation method,an optimization model is established,and the genetic algorithm is used to optimize the model.The results prove that this method is effective.It can be applied to the optimization of other foundation pit projects.(3)Based on the numerical calculation results,the regression function relationships between the above four factors and the settlement,safety factor,and material cost are obtained.(4)Taking the material cost as the objective function,the settlement amount and the safety factor as the constraint function,respectively,the genetic algorithm is used to carry out iterative calculation to obtain the optimal support plan: first establish an optimization model,select the design variables,and establish the objective function and constraints function.Use genetic algorithm to optimize the objective function.First,give the chromosome an initial value,calculate whether it meets the constraint conditions according to the fitting function of the sedimentation amount and the safety factor,and then calculate its fitness according to the fitted material cost function;the second step is The chromosome selects,crosses,and mutates to generate a new chromosome,judge whether it meets the constraints,and calculate the fitness;compare the fitness to determine whether it is the current optimal solution;repeat the above steps to find the optimal solution in turn.In order to make the calculated results more credible,the program was run 100 times repeatedly,the objective function was calculated each time,and the optimal result was selected as the optimization result.(5)Finally,the optimized support plan was re-calculated by finite element method,and the results showed that the optimized plan can effectively save materials and improve work efficiency under the premise of meeting the settlement and safety factor.