Research On Construction Timing And Deformation Of Sub-pit Construction Of Foundation Pit Group In Jiangbei New District

With the continuous expansion of my country’s construction scale,the development and construction of underground space in a complex environment has become the norm.The simultaneous excavation of multiple adjacent foundation pits is becoming common,and the construction timing of foundation pit groups is becoming more and more prominent.This paper aims to reduce the mutual influence of the construction of adjacent foundation pits through the study on the optimization of construction timing of foundation pit groups,and form a method for analyzing the timing of deformation control of sub-pit construction,which can provide a reference for similar projects in the future.This paper mainly focuses on the underground space foundation pit group engineering of Nanjing Jiangbei New District in river floodplain.The actual measurement and numerical simulation of the deep foundation pit in a single subway section of the foundation pit group were verified,and the numerical simulation of the construction timing optimization of a single large foundation pit in the foundation pit group and two adjacent deep pits and shallow pits in the foundation pit group was carried out.The analysis and comparison of the change law of the displacement field of each time sequence,the main conclusions are as follows:(1)The measurement and numerical simulation of deep foundation pits in a single subway section of the foundation pit group show that: with the increase of the excavation depth,the horizontal displacement of the envelope structure increases overall,the maximum d isplacement position gradually moves down,and the displacement shape is "bow" shape,the depth of the maximum displacement position of the enclosure wall is around 1 times the excavation depth;as the working conditions advance,the first supporting axial force increases first and then decreases,and the second supporting axial force gradually increases.The measured and simulated foundation pit deformation laws are the same,which proves that the numerical simulation method is feasible.(2)The numerical simulation study of precipitation of a single large foundation pit in the foundation pit group shows that: in the dredging stage,at the same depth below the diving surface,the inner head of the bottom curtain is less than the hanging type;in the decompression stage,at the same depth,the inner head of the hanging type is higher than that of the bottom type,the outer head of the hanging type curtain is lower,and the outer head of the bottom type is basically unchanged.(3)The numerical simulation stud y on the optimization of the timing of the construction of a single large foundation pit in the foundation pit group shows that: the depth of the envelope structure in different parts of a single foundation pit is different,and the wall displacement law is also different.The row of piles and the underground continuous wall are basically parabolic displacement,the form of the pile displacement is kicking,and there is no displacement at the bottom of the ground wall.As the excavation depth increases,the maximum displacement increases at an accelerated rate,and the position moves downward from the top of the wall;with each construction sequence,as the excavation depth increases,the overall surface settlement outside the pit gradually with the increase,the maximum settlement is accelerated,and the curve is distributed as a “PECK” settlement trough curve;when the foundation pit with large width is excavated,the deformation shape of the pit bottom uplift is plastic,and the amount of springback below the pit bottom gradually decreases along the depth.(4)The numerical simulation of the time sequence optimization of the construction of a single large foundation pit in the foundation pit group shows that: with the increase of the excavation depth,the maximum displacement position gradually moves down.The ratio range of the maximum horizontal displacement position to the excavation depth is 0.98～1.4 in five construction sequences;with the increase of the excavation depth,the maximum ground settlement position is further away from the pit wall,and the ratio range of the maximum ground settlement position to the pit wall distance to the excavation depth is 0.65～1.53 in five construction sequences;with the increase of excavation depth,the influence range of surface settlement increases gradually.In the five time series,the ratio between the main influence area of surface settlement near the diaphragm wall and excavation depth ranges from 2.73 to 3.42,and that near the row pile ranges from 1.73 to 2.35.(5)The numerical simulation study on the optimization of the construction time sequence of the single large foundation pit in the foundation pit group shows that: when a large area foundation pit with diagonal bracing is excavated,among the five excavation time sequences,the basin excavation and island excavation have the smallest maximum ground settlement and horizontal displacement of the enclosure,the island excavation scheme has the smallest pit bottom uplift,and the basin excavation scheme has the largest effect on the control of the enclosure deformation and the surrounding environment,so the basin excavation method is recommended.(6)The numerical simulation study on the time sequence optimization of adjacent deep and shallow foundation pits in the foundation pit group shows that: construction of deep pits and shallow pits with large difference between two adjacent spaces,t he time sequence of deep and shallow alternative construction has the greatest effect on controlling the deformation of the exterior wall and common wall of the shallow pit,followed by the time sequence of first deep and then shallow,followed by the time sequence of first shallow and then deep;the time sequence of deep and shallow alternative construction is most conducive to reducing the settlement of the outer surface of the pit,followed by the time sequence of first deep and then shallow,followed by the time sequence of first shallow and then deep;when the earthwork in the lower part of the deep pit is not excavated,the time sequence of alternating shallow and deep construction has the greatest effect on controlling the heave of the shallow pit bottom;while after the earthwork in the lower part of the deep pit is excavated,the time sequence of first shallow and then deep has the greatest effect on controlling the heave of the deep pit bottom.Based on the comprehensive consideration,it is recommended to adopt the alternative deep shallow pit construction scheme.