Study On Axial Force Preloading And Control Technology Of Steel Support In Subway Station Foundation Pit

The strutted retaining structure is widely used in deep foundation pit engineering because of its good control effect on excavation deformation.Although there is a relatively complete theoretical foundation and mature construction technology,there is little research on the pre-axial force range of the steel pipe.For areas with better soil conditions,The large pre-axial force given by the relevant specification increases the internal force of the supporting structure,which not only causes material waste,but sometimes artificially increases the engineering risk.Based on this situation,select the standard section of the foundation pit engineering of Ruyihu North Station of Zhengzhou Rail Transit Line 4 as a research example.Explore the reasonable range of pre-axial force supported in the steel pipe.The foundation pit of the example project adopts the underground continuous wall and the internal support.There are four internal supports in the standard section,the first and third supports are made by cast-in-place reinforced concrete,and the second and fourth supports are made by steel pipes.Through the field test and finite element simulation method,the internal force and deformation of the supporting structure under different pre-axial force is studied.The main research contents and conclusions are as follows:(1)Six test sections are selected in the foundation pit of the example project,and different pre-axial forces are applied to the second and fourth steel pipes inner support in each test section.The variation laws of the internal support axial force,the deformation of the underground continuous wall and the surface settlement under different pre-axial loads are compared and analyzed.The test results show that in the deep part of the foundation pit,the third reinforced concrete inner support plays an important role in limiting the deformation of the foundation pit;The deep horizontal displacement of the underground continuous wall near the crawler crane working area is large;the maximum deep horizontal displacement of the supported soil behind the wall is smaller than the underground continuous wall;the underground continuous wall and the surface within 2m range are in a raised state.(2)Starting from the engineering practice,the finite element model of the inner support of the solid element steel tube and the inner support of the beam unit are respectively established.The difference between the two internal support stiffnesses under the same load is compared and analyzed.The results show that the stiffness of the inner support of the beam unit is much larger than the stiffness of the inner support of the steel tube in actual engineering.It is suggested that when the incremental method calculation and finite element simulation use the inner support of the beam unit steel,the stiffness should be appropriately reduced.(3)Through the field test and finite element simulation,the influence range and amplitude of changing the pre-axial force in the steel pipe on the internal force and deformation of the supporting structure are analyzed.The results show that changing the pre-axial force has a great influence on the inner support of the steel pipe and the axial force of the adjacent upper reinforced concrete.At the same time,the local deformation of the wall near the steel pipe changes.(4)The finite element simulation is carried out with a pre-axial force of 0.1~0.8 times of the design axial force,respectively.By comprehensively comparing the internal support axial force and the horizontal displacement of the underground continuous wall under various schemes,it is considered that: Foundation pit engineering with the similar engineering geological conditions and form of support to the example project,the reasonable pre-axial force of the steel pipe support is 0.2~0.3 times of the designed axial force.(5)On the basis of the theory of "geometrical distribution method",the method for estimating the reasonable pre-axial force of the steel pipe is improved.The "stiffness distribution method" in which the boundary points of the soil layers between adjacent inner supports are distributed according to their stiffness is proposed,and the actual engineering is taken as an example to prove that the "stiffness distribution method" is superior to the "geometry distribution method".