Study On Deformation Of Transfer Station Deep Foundation Pit Dewatering Excavation In Water-rich Sand And Pebble Stratum

The sand and pebble strata is widely distributed in Sichuan Province and other areas in China.The engineering properties of this stratum are highly specific,with high diving content,fine permeability,poor self-stabilisation and regional variability,and in order to ensure the safety of the construction and the surrounding buildings,the foundation pit project of this stratum has high requirements in terms of deformation.According to the fifth phase of the Chengdu Urban Rail Transit Plan(2024-2029),a large number of new metro lines will be built in the next few years,which means that a large number of foundation pits will emerge.At present,there are few studies on the deformation law of foundation pit projects in the area of water-rich sand and pebble layer,and there are limited engineering examples for reference,so the study on the deformation law of foundation pit projects in this area has certain engineering practical value.In this paper,based on the deep foundation pit project of Fengxihe Station of Chengdu Metro,on the basis of fully understanding the engineering geological conditions of the study area,combined with monitoring data and based on the flowsolid coupling theory,a three-dimensional numerical model was established using COMSOL Multiphysics finite element simulation software to analyse the overall deformation law of the deep foundation pit in the water-rich sand and pebble layer under the precipitation and excavation conditions.The details and conclusions are as follows:(1)Research was carried out on the pumping test at Fengxi River Station,and the laws governing the change in pumping volume and water level in observation wells over time in the study area,as well as the permeability coefficient of the sand and gravel layer,were obtained.(2)The monitoring data of the seven monitoring items in the deep foundation pit at Fengxi River Station were summarized and analyzed,mainly focusing on the mechanism of the influence of excavation and precipitation on the structure of the foundation pit itself and the surrounding soil,and the deformation characteristics of the foundation pit during construction were summarized.(3)Based on field survey data and monitoring data,a three-dimensional numerical model was constructed using finite element numerical software,and the overall deformation characteristics of the deep foundation pit in the highly permeable sand and gravel layer under precipitation and excavation conditions were analyzed based on the theory of fluid-solid coupling,including the evolution characteristics of the seepage path,the deformation characteristics of the surrounding soil and retaining structure.The simulation results were compared with the monitoring results,and it was found that the deformation trends of the two were consistent,with the simulation values slightly larger than the monitoring values,thus verifying the reliability of the simulation.(4)Based on the established finite element model,the effects of the permeability coefficient of the sand and gravel layer,the thickness of the retaining structure,and whether to consider coupling on the deformation of the deep foundation pit were discussed.The results showed that increasing the permeability coefficient would exacerbate the deformation of the foundation pit,and the thickness of the retaining structure has limited control on the deformation of the foundation pit.Considering the coupling analysis,the surface settlement,horizontal displacement,and lateral displacement of the retaining structure were all greater than those of the non-coupled analysis,and the amount of uplift at the bottom of the pit was lower than that of the non-coupled analysis.