Research On Horizontal Displacement Control Technology Of Deep Foundation Pit Of Subway Station In Soft Soil

Since the incoming of the 21 st century,a significant improvement in the economy is observed,and the proportions of urbanization are increasing rapidly.Due to the limited urban space,underground space projects have been put on the agenda for reasonable city planning.It has been proved that difficulties exist in underground space engineering based on the deformation control of deep foundation pits during the excavation process,which is particularly prominent in the typical soft soil strata in coastal areas in China.The execrable geological conditions have a significant impact on the stability of the foundation pit.Effective safety precautions should be implemented in the construction process in time,otherwise,the risks concerning safety and economic cost will be increased.If a collapse disaster takes place in the foundation pit,casualties could happen.Based on the foundation pit project of the station of Shaoxing Metro Line 2,using the Midas GTS NX finite element numerical simulation method,this paper emphatically analyzes the deformation characteristics of the foundation pit under the support mode of "diaphragm wall support",analyzes the sensitivity of the main factors affecting the deformation of the foundation pit,and puts forward the corresponding optimization countermeasures.The research contents and results are as follows:(1)By analyzing the measured deformation data of the foundation pit,the horizontal displacement of the diaphragm wall,the deformation law of the external surface settlement of the pit,and the influence of the servo system on the deformation of the foundation pit are studied.The results show that in the early stage of foundation pit excavation,the horizontal displacement of the wall is small,and it is in a cantilever mode;With the increase of excavation depth,the maximum horizontal displacement of the diaphragm wall gradually extends downward,showing an inward convex pattern,and the maximum horizontal displacement is between 40 mm and 60mm;The surface settlement of foundation pit is changed from triangle mode to groove mode,and the maximum surface settlement is between 50 mm and 80 mm.Maximum horizontal displacement of diaphragm wall δ Hm is in the range of(0.05%,0.37%)H,and the average value is about0.21% H;Maximum horizontal displacement buried depth H δ Hm is within(0.69,0.94)H,surface settlement δ The VM range is(0.01%,0.5%)H,and the average value is about0.26% H.The axial force servo system can effectively restrain the horizontal displacement of the foundation pit retaining structure.After the application of the servo system,the deformation of the foundation pit accounts for about 20% of the total deformation.(2)The validity of the method is proved by comparing the simulation data of the 3D finite element model with the measured data.By using the single variable analysis method,the influence of foundation pit foundation reinforcement depth,foundation reinforcement depth,and pre-axial force is studied.The results show that:(1)with the increase of wall thick Ness,the horizontal displacement of the diaphragm wall gradually decreases,and at800 mm,its horizontal displacement meets the design requirements;(2)Compared with the horizontal displacement,the embedded depth of the diaphragm wall has a greater impact on the ground settlement.When the embedded depth increases to 43 m,the settlement decreases by 15%;(3)The horizontal displacement and surface settlement decrease with the increase of the reinforcement depth of the foundation pit bottom,and the influence range of surface settlement is<40m(2.5H);(4)The pre-applied axial force of the steel support has a great influence on the horizontal displacement of the diaphragm wall and the ground settlement.The greater the axial force,the smaller the horizontal displacement,and the smaller the ground settlement.The actual project should apply1.2～1.5 times the design axial force.(3)Through the analysis of sensitivity factors,the order of the effect of each influencing factor on the deformation of the foundation pit is obtained as follows: the pre axial force of steel support>the reinforcement thick Ness of pit bottom soil>the thick Ness of diaphragm wall>the embedded depth of diaphragm wall;Considering the project cost and construction difficulty,the restraining effect of each influencing factor on the deformation of foundation pit is: pre-applied axial force>embedded depth of diaphragm wall>thick Ness of diaphragm wall>reinforcement thick Ness of pit bottom soil.To sum up,the above two aspects,increasing the pre-imposed axial force of steel bracing has the best effect and the lowest cost on restraining the deformation of the foundation pit.(4)According to the excavation characteristics of the foundation pit and various factors affecting the deformation of the foundation pit,the corresponding optimization is carried out for the foundation pit,and the corresponding optimization scheme is given:when the prestress value is adjusted to 1.5 times,the stability of the foundation pit is enhanced,and it has good economic benefits,and the comprehensive cost is the highest.