Impact Analysis About The Scale Of Deep And Large Foundation Pit On The Subway Structure Deformation

With the increase of metropolitan cities’subway network density and the emergence of large buildings in city center, more and more deep and large foundation pits have been excavated near the subway station, and the deformation of subway structure caused by the excavation project has made an increasingly severe influence on the operation and safety of subway. The scale of foundation pits is the most concerned problem of designing and constructing, finding out the influence it has been made on subway structure deformation will show practical significance. This dissertation focuses on the impacts of foundation pits’geometric dimensions, including:the depth of excavation, the acreage of excavation, and the net distance between foundation pit and subway structure, which has been made on the deformation of subway structure.Through the typical case of Beijing Subway Line No.8Gulou deep and large foundation pit project, an overall three-dimensional finite element model, including station structure, section open cut structure and layer, was built using ANSYS finite element software. The construction process simulation has8steps, including the excavation of foundation pit, underground structure construction, ground structure construction. Also the following calculation conditions were chosen:(1)6groups of parameters were selected evenly according to the gap between the bottom and station structure bottom from15m to Om, and the excavation depth correspondingly from14.2m to29.2m;(2) keep the long side length, overall shape and net distance of foundation pit unchanged,5groups of parameters were selected according to the excavation acreage from21103m2to23891m2with the increase of short side length;(3)4groups of parameters were selected according to the clear distance from3.5m to15.5m.Based on the calculation results, existing station main structure and open cut structure’s lateral, vertical, longitudinal deformations under each calculation condition were given, and following rules were got:1. Excavation depth:when the gap between the bottom and station structure bottom is from15m to2.5m, the influence ratio of excavation depth on lateral, vertical deformation are0.0498mm/m,0.1206mm/m respectively; when the gap is from2.5m to Om, the influence ratio will be0.1589mm/m,0.1599mm/m. The relationship between the buried depth and the excavation depth has a great influence on the subway structure deformation. When excavation depth is close to or exceeds the buried depth of subway structure, the deformation increases markedly, mainly manifested as vertical deformation which is also downward. In deep and large foundation pit excavation, if the excavation depth exceeds the subway structure buried depth, the deformation of subway structure must be strictly controlled, especially the vertical deformation.2. Excavation acreage:when the excavation acreage changes by26.4%, the structure’s lateral, vertical deformation changes89.2%and100.1%respectively. When it changes by the same proportion, the influence on deformation from excavation acreage is bigger than excavation depth and clear distance. In the design and construction of foundation pit engineering, if the excavation acreage is large, the plan which has a larger excavation acreage must be chosen carefully, since the new plan may cause a big increase on subway structure deformation.3. Clear distance:when the clear distance is from5.5m to3.5m, lateral and vertical deformation changes are0.0097mm and0.5579mm respectively. When the clear distance between foundation pit and subway structure is short, the vertical deformation increases markedly, but the lateral deformation changes a little, so vertical deformation which has a clear distance less than5.5m must be strictly monitored. When the clear distance is15.5m(more than the excavation depth14.2m), lateral and vertical deformation volumes are0.9726mm and0.7864mm respectively, the deformation decreases markedly when the clear distance between foundation pit and subway structure exceeds the foundation buried depth. So it can be speculated that the main influence range of the excavation of deep and large foundation pit on peripheral subway structure is concerned about the subway structure foundation buried depth.