复杂环境条件上跨下穿同一既有地铁隧道的变形控制分析及施工方案优化

SUBSIDENCE CONTROLLING AND CONSTRUCTION SCHEME OPTIMIZATION OF CUTTING ACROSS AND UNDERPASSING AN EXISTING METRO TUNNEL UNDER COMPLEX ENVIRONMENTAL CONDITIONS

首都机场线东直门站C区下穿并上跨北京地铁13号线东直门站站后折返线,该工程东、西两端和西北角各有一个紧邻基坑,其中西北角基坑为北京市目前最深的基坑,深达28m。以上所述的施工都可能会引起折返线结构的剧烈变形,且折返线隧道结构在施工范围内有两条变形缝,变形缝是折返线结构最薄弱的部位,变形缝两侧结构差异变形过大会使折返线轨道悬空变形乃至折裂,严重威胁折返线的正常运营,如此复杂的施工环境给控制折返线结构变形控制带来很大的困难。国内外鲜有类似工程可供参考,为确保施工过程对折返线结构的影响在可控范围内,运用FLAC3D非线性大变形分析软件对该工程进行施工过程数值模拟分析和方案优化,提出采用上垮基坑分层开挖和下穿暗挖结构施工分步交替施工,用基坑卸载引起的隆起量抵消部分暗挖施工引起的沉降的方案,并把沉降缝处的结构差异沉降控制在1.1mm内,为设计、施工提供依据。

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.