Stress-seepage Coupling Deformation Mechanism Of Deep Foundation Pit In The Process Of Dewatering

With the rapid economic growth,the pace of urbanization in China is accelerating,and traffic congestion problems are gradually emerging.In order to solve the traffic congestion problem caused by urbanization,the country put the construction projects of subways in many cities on the agenda,and followed by a series of deep underground pit problems.Unlike conventional commercial,residential and other civil building foundation pits,subway pits have typical features of longitudinal planar dimensions that are much larger than lateral dimensions.This paper selects Midas GTS NX to analyze the coupled seepage deformation mechanism of foundation pit dewatering support stress based on the Jinfu station deep foundation pit of Chengdu Metro Line 6#.Through the actual monitoring and control data on site,the deformation evolution mechanism of the foundation pit retaining structure and surrounding strata is studied in detail during the interaction process of the seepage and stress in the foundation pit dewatering support.The conclusions are as follows:(1)The modified Mohr-Coulomb constitutive model is chosen to illustrate the strain evolution mechanism of the underground continuous wall during the foundation pit excavation process,which can be accurately reflected in the excavation process of the foundation pit and the mechanics of deformation of the foundation pit near the soil caused by the unloading stress of the soil.(2)According to the construction process of foundation pit excavation,the displacement of the underground continuous wall,the ground subsidence,the supporting force,the cofferdam bending moment,the force of the lattice col umn and the mechanical behavior of the uplift pile at the bottom of the foundation pit are analyzed in detail.During the construction of the foundation pit,the moving deformation mode of the underground continuous wall gradually changes from the original cantilever mode to the “bow shape”.And the maximum displacement value of the underground continuous wall appears near the construction surface of the foundation pit,showing a deep foundation pit with a longitudinal dimension much larger than the lateral dimension.The maximum displacement of the longitudinal and lateral directions of the foundation pit mainly depends on the stiffness of the longitudinal and lateral directions of the foundation pit.(3)During the construction of the foundation pit,the suppo rt mainly provides lateral stiffness,and there is mutual coordination of the axial forces between the supports to ensure the safety and stability of the foundation pit construction.With the gradual increase of the excavation depth of the foundation pit,the distribution pattern of surface settlement behind the underground continuous wall gradually changes from “inverted triangle” to “groove shape”.The maximum vertical displacement position is 0.4 to 0.5 times the excavation depth of the underground retai ning wall and does not change with the depth of the excavation.As the excavation depth of the foundation pit is gradually deepened,the bottom soil uplift value is expressed as an elastic bulge pattern with a large middle and small sides.(4)By analyzing the field measured data,it is found that during the deep foundation pit excavation process,the maximum horizontal offset of the retaining pile gradually decreases with the increase of the excavation depth,and the maximum value appears near the excavation surface.The deep horizontal displacement of the retaining pile is expressed as “big belly”,which indicates that the lateral displacement of the retaining pile during the construction of the foundation pit mainly lies in the relative restraining stiffness of the longitudinal and lateral underground retaining piles of the foundation pit.