Research On The Response Of Support Structure Of Special-shaped Ultra-deep Foundation Pit With Forward And Reverse Combined Construction Method

In this paper,relying on the second-stage foundation pit project of Changsha International Financial Center,on the basis of studying the research results of scholars at home and abroad and the projects relied on by detailed investigation,and combining with the on-site monitoring data,the monitoring data of the second-stage foundation pit process are analyzed and studied firstly,and then the forward and backward construction method of the central island used in the foundation pit is simulated by means of the finite element software MTDAS GTS NX.The measured data are compared with the numerical simulation results.After checking the correctness of the model,the stress characteristics of the floor structure and the settlement characteristics of the basement structure of the central island are further analyzed.At last,the influence of different reverse working techniques on the settlement of surrounding high-rise buildings and basement structure of central island is compared and analyzed,And its influence on the axial force of each floor.Come to the fo11 owing conclusions:(1)Due to the: embedding of high-rise buildings,the retaining structure is irregular,and the second-stage construction area is concave,which makes the horizontal displacement of retaining piles change from conventional two-dimensional movement(one horizontal+one vertical)to three-dimensional movement(two horizontal+one vertical).The horizontal displacement of retaining piles in irregular sections is firstly affected by the superposition effect of active earth pressure in the rear large area,That is,the midpoint of each straight line segment in the irregular section shows the maximum displacement;At the same time,the displacement of the point which is closer to the maximum displacement point in the straight line segment is larger.The horizontal displacement of pile body is affected by many factors in the second stage excavation.The displacement of pile top is first affected by the overturning trend of stratum soil in the total area outside the foundation pit.Secondly,it is affected by the overturning trend of local soil,and finally by the amount of earthwork left in the excavation area.At the same time,because the movement on the horizontal plane changes from onedimensional to two-dimensional,the displacement of points will also be affected by other points(2)With the excavation of the remaining soil in Phase II construction,when the soil in the "convex area" and the high-rise building are located in different orientations of the foundation pit,their settlement trends are different.Each monitoring point is affected by the settlement trend,and the settlement change law is different.(3)The construction and erection of structural beams and slabs can restrain and level the horizontal displacement of retaining piles,and restrain the settlement of highrise buildings and surrounding soil.As for the horizontal displacement of pile top,when its position is symmetrical with respect to the maximum point in the center,there is an obvious convergence trend in the second stage when the first stage displacement is quite different,Pay attention to the situation that the change rate of small value is too fast,and the displacement of large value exceeds the monitoring alarm value.(4)After the construction,the axial force distribution characteristics of each floor are generally large and small,and the axial force of the floor made in the later stage is obviously smaller than that of other reversed floors.At the same time,the stress concentration of axial force is obvious near the external corner of foundation pit and the reserved hole area of spiral driveway in basement.(5)During the second stage soil excavation of the basement of the central island,the position of the maximum settlement value will move along.with the construction,and finally appear in the editing and proofreading position.With the construction,the uplift value of the central island gradually moves down from the top floor with the construction,and finally the maximum uplift value appears on the floor.(6)Compared with different technological processes,it is found that the settlement value of high-rise buildings shows that it is better to construct basement floor first.The basement settlement of central island accelerates in the later structural construction,and differential settlement occurs.The axial force of floor is greatly affected by the process changes.