Study On Stability And Support Parameter Optimization Of Metro Foundation Pit In Composite Stratum

Traffic congestion in China has become a serious problem in recent years,and many cities have chosen to build subways to alleviate the situation.As an important part of subway construction,it is very difficult to construct metro foundation pit,especially in composite stratum.Excavation surrounding environment is very complex,once the accident will cause bad social impact;At the same time,the resource waste caused by the conservative foundation pit support has also attracted people’s attention.This paper is based on the foundation pit of Shuanglong Station of Liuzhou Metro Line 1,the stability and support parameter optimization of foundation pit are studied by means of field monitoring,numerical simulation and theoretical analysis.The main research contents are as follows:(1)Based on the field monitoring data and FLAC3 D numerical simulation results,the rules of horizontal deformation of supporting pile,surface settlement and deformation of deep soil outside the pit in the construction process of subway foundation pit are summarized.The existing analytical solution is improved to make it applicable to the calculation of horizontal displacement of soil caused by any deformation mode of retaining wall based on the differential thought and superposition principle,then the rationality and engineering practicability of the improved theoretical solution are verified.(2)The combined method of point estimation and strength reduction method is used to analyze the reliability of the three main working conditions of subway foundation pit,and the influence laws of foundation pit geometry parameters and soil parameters on the failure probability of foundation pit is discussed.The results show that the depth of bedrock has a significant effect on the failure probability of the foundation pit.When the depth of bedrock is greater than the critical depth,the influence of bedrock on the failure probability is not obvious.With the increase of the depth of the retaining pile,the failure probability decreases,but it is unnecessary to increase the depth of the retaining pile too much.When the internal friction Angle is between 10° and 20°,the failure probability is sensitive to the change of the internal friction Angle,so the accuracy of the mean value of the internal friction Angle should be ensured in engineering design.The failure probability increases with the increase of the coefficient of variation of cohesion and internal friction Angle,but by contrast,the coefficient of variation of cohesion has a more obvious effect on the failure probability.(3)The design variables,optimization objectives and constraint conditions of the optimization design of the foundation pit supporting structure were determined.Five factors and five levels of orthogonal tests were designed,and the mechanical responses of 25 different supporting schemes were calculated.The grey correlation analysis-combined weighting method are used to process the test results of orthogonal test,and a group of optimal support parameter combinations are determined.Compared with the original scheme,the optimized scheme reduces the project cost by about 13.5%,the maximum horizontal displacement of supporting pile by about 27.0%,and the maximum surface settlement by about 18.1%.