SUBSIDENCE CONTROLLING AND CONSTRUCTION SCHEME OPTIMIZATION OF CUTTING ACROSS AND UNDERPASSING AN EXISTING METRO TUNNEL UNDER COMPLEX ENVIRONMENTAL CONDITIONS
Abstract：The Dongzhimen metro station of Beijing Capital Airport line cuts across and underpasses the existing tunnel of subway line 13. This station has two storeys. The upper storey cuts across the existing line 13 subway tunnel，and is built by open-cut method. The least distance between the upper storey and the existing tunnel is only 0.33 m. The bottom storey underpasses the existing tunnel very closely，and is driven by so-called “drift- pile-beam” method. Around this construction area，there are three adjacent foundation pits in the east，west and northwest directions. Among them，the northwest foundation pit of 28 m in depth is the deepest pit in Beijing. All the constructions may induce excessive deformations. Moreover，the underpassed segment of the existing tunnel has two deformation joints which are the most unsubstantial parts of the existing tunnel.The existing line besides each deformation joint may move in the inverse direction，which can make the rails divorced from roadbed. Therefore，differential subsidence control of these two deformation joints and deformation control of retaining walls of these three pits are of great importance during this station construction，but there are few similar examples to be consulted. In order to guarantee normal operation of subway line 13 and foundation pits safety，construction sequences of this station is systematically simulated by using computer program FLAC3D(fast Lagrangian analysis of continua in 3D). The authors put forward an optimized construction scheme that the upper base pit and bottom storey are excavated alternately. In this way，the hunch caused by the upper pit excavation counteracts the subsidence caused by the bottom storey excavation. The numerical simulation results can be provided as the basis for design and construction of this station.
