Study On The Influence Of Karst Distribution On Deep Foundation Pit Excavation In Water-rich Sandy Soil Strat

With the development of underground spaces in China,deep foundation pit projects have increasingly high requirements for the stability of the surrounding environment.As the Foshan area is located in a tectonic fracture zone,there are a large number of hidden karsts that pose a serious threat to the construction and operation of the metro.Few scholars have studied the influence of karst location on foundation pit engineering.This paper aims to investigate the impact of karst distribution on deep foundation pit excavation in water-rich sandy soil areas using three-dimensional non-linear finite element software.Additionally,a parametric study is conducted for the reinforcement of support structures.The main research contents of this paper are summarized as follows:(1)On-site soil extraction is conducted for indoor geotechnical tests to obtain soil parameters,including basic physical tests,indoor triaxial tests,and standard consolidation tests.These tests provide reliable soil parameters for subsequent finite element numerical simulations.This paper studies the impact of pit excavation on the enclosure structure and surrounding environment by comparing and analyzing the numerical simulation data of pit excavation at Diejiao Station with the on-site monitoring data.The results indicate that the vertical displacement of the top of the diaphragm wall exhibits a rising trend with the precipitation and excavation in the pit.Precipitation and excavation in the pit cause the top of the diaphragm wall to slope toward the inner side of the pit in the horizontal direction.The lateral deformation of the embedded wall is large in the middle and small at the ends,and it becomes larger with the increase of excavation depth.Moreover,the location of the maximum lateral displacement shifts downward.With the precipitation and excavation of the pit,the settlement curve of the surface exhibits a notch type,and it becomes larger with the increase of excavation depth.The leakage of the diaphragm wall causes the groundwater level behind the wall to gradually drop,reducing the overall stability of the pit.(2)Based on the three-dimensional model of foundation excavation at Diejiao Station,this paper investigates the effect of the solidification effect of the support structure on foundation excavation.The results indicate that solidification significantly limits the surface settlement of the soil outside the wall and the deformation of the diaphragm wall.In-wall reinforcement has a significant effect on the deflection of the diaphragm wall,and its reinforcement effect gradually increases with the excavation of the pit.Out-wall reinforcement has a smaller effect on the deflection of the diaphragm wall but a larger effect on the surface settlement outside the wall,and this effect gradually weakens with the excavation of the pit.Increasing the modulus of the solidification can reduce the wall deformation and surface settlement.The optimal reinforcement depth is 1.1 H(i.e.,28.2 m),where H is the excavation depth.(3)This paper investigates the influence of karst distribution on the excavation of the foundation pit based on the three-dimensional model of foundation excavation at Diejiao Station.The results demonstrate that at different depths(Z-direction),the rock layer at 25 m from the ground and the soil layer at 14 m from the ground are the locations where the karst has the greatest influence on the surface settlement outside the wall.In different locations(XY direction),the karst located inside the pit near the diaphragm wall in the rock layer has the greatest influence on the lateral deformation of the wall.The karst located in a certain range outside the pit in the soil layer has a significant effect on the ground settlement behind the wall.The karst located inside the diaphragm wall in the rock layer has the greatest influence on the bending moment of the wall.