Study On The Stability Of Construction For Deep Foundation Pit Beside The Existing Subway Station In Mulk Layer

With the deepening of urbanization and constantly supplement and consummates ofmetro line, the construction cases of new subway stations which are adjacent to the existingstations are growing under special coastal geological condition. Sea filling silt has “Threehigh and three low” property, which will result in the main and enclosure structuredeformation of the exiting station and deep foundation pit become extremely complex in theexcavation of deep foundation pit beside the existing subway station. In this complexcondition, stability control of the exiting station and deep foundation pit is an importantdifficulty that the decision makers have to confront.In order to analyze the effects of spatial-temporal effects and fluid-structure interactioneffects to the construction of deep foundation pit, this article took Qianhaiwan Station ofShenzhen Metro Line11as background which is closeto the exiting station of Metro Line5and1. By adopting digital photogrammetric technology and man-made marks methods, thelateral unloading mechanical characteristics of sea filling silt in different consolidationpressure and unloading rate condition were studied. And by adopting numerical simulationmethod, the spatial-temporal effects of standard section of concrete bracing at theopen-cutting section and fluid-structure interaction analysis and3-D numerical simulation oncover and excavation top-down construction of engineering adjacent to subway stations wereanalyzed. In the end, combined with the engineering main sections, qualitative andquantitative analysis between open cut method and cover and cuttop down method wereanalysed. The main conclusions are as follows:(1) By adopting digital photogrammetric technology and methods man-made marks,experiment on the mechanical characteristics of lateral unloading for filling sea sludge wasresearched in different consolidation pressure and and unloading rate. The results show that,the sample deformation can be divided into three pieces. It is compression shear zone andtensile shear zone and compression zone. In different consolidation pressure, lateraldisplacement has exponent relation to time of unloading, and deviator stress has hyperbolicrelation lateral strain, whick shows apparent normalization characteristic. It shows thatDuncan-Chang model can accurately describe the unloading behaviour of filling sea sludge.And in different unloading rate, while doubled the unloading rate, the lateral deformation willbe almost doubled for active region of foundation pit.(2) The results of the spatial-temporal effects during the foundation pit excavation forfilling sea sludge layer prove that there are negative exponential relation between longitudinaland transverse segmentation number of pit excavation and the horizontal displacement of the pile. The more numble of segments and layer, the better stability of foundation pit. What ismore, the excavation effect of lateral segments is the best, then is lateral and portrait segments,and only portrait segments is the end. For this project, the lateral segments of far away formtunnel shoule be excavated firstly, and the layer thickness of vertical shoule be kept from1.5m to2m. When excavation depth is sludge formation within8m and sandy sticky clayfrom14m to17m, the deformation rate of pile is the best.(3) The results of2-D fluid-solid coupling analysis on the foundation pit adjacent tosubway stations in the top-down section show that the calculation results with consideringfluid-solid coupling get closer to the truthful data than that without consideration. Thedeformation of the existing station is based on hump of the major structure during thetop-down section construction of deep foundation pit of Metro Line11. And the existingstation of line5and1have apparent arching effect to deep foundation pit.(4) The3-D stability analysis results of cover and cut section for “layer and section”excavation show that, even though bench excavation will lead to the soli in after excavationsection flowing to the first excavation section, the deformation of enclose structure for deepfoundation pit and major structure of existing station can be controled in allowed band. Thisshows that the stability of enclose and major structure can be ensured when adopting supportsand protections system of “steel shotcrete, campshed, uplift pile and CCTDM”.(5) The result of the numerical simulation of different working conditions show that thedisplacement were enlarged that horizontal displacement of enclosed structure was60%~100%， vertical displacement of existed structure was80%， bottom heave was300%,which would be very bad for construction safety control. NATM would be better thancover-excavation method in side of economic efficiency aspect. But when consideringsociety、 application and disaster defense, it would chose the opposite method. Finallycover-excavation method would be chosen on scientific evaluation of fuzzy comparison.