基坑工程首道内支撑采用P-CFST的工程实例研究

针对北京地铁17号线某盾构竖井基坑工程开挖深度大、作业空间小的难点，围护结构首道支撑位置采用新型装配式钢管混凝土（简称P-CFST）支撑结构，扩大了支撑间距，便于基坑开挖、出土和支撑架设作业. 利用ABAQUS软件建立三维有限元模型，开展基坑开挖全过程数值模拟. 在工程实施过程中，对支撑轴力、围护桩水平位移、桩顶水平位移和地表沉降进行系统监测，保证了P-CFST支撑和钢支撑组合支护下的基坑施工安全，研究盾构竖井围护结构变形的空间效应、地表沉降曲面形态、不同位置处的支撑轴力关系等. 由模拟和监测结果的分析表明：围护桩同一深度上变形呈现抛物线形状或“盆形”，空间效应对盾构井围护结构变形的影响主要发生在距离基坑阴角小于8 m的范围内；基坑附近地表沉降等值线形状经过“圆弧形”-“陀螺形”-“梯形”变化，最大地表沉降位置经历由近及远、再向基坑靠近的移动过程；首道P-CFST支撑轴力对地层开挖、支撑架设等工况的影响更加敏感，大于架设深度更大的2、4道钢支撑轴力. 盾构竖井基坑工程内撑式围护结构首道支撑选用高刚度、高承载力的P-CFST内支撑，扩大了设计间距，围护结构和周围地层变形得到了有效控制.

Case study on braced excavation with P-CFST for top internal support

A prefabricated concrete-filled steel tube (P-CFST) bracing was applied for the top internal support of supporting structure aiming at the difficulties of deep excavated depth and narrow working space in the excavation engineering of a shield shaft for Line 17 of Beijing Subway. The interval among bracings were enlarged by using P-CFST bracing resulting in being convenient for excavation and bracing erection. The whole process of excavation and support was simulated with a three-dimensional finite element model by ABAQUS. Systematic monitoring for axial force of bracing, deformation of fender piles, displacement of pile top and ground settlement were accomplished in implementary process of excavation. The security of excavation in combined support of P-CFST bracing and steel bracing was ensured. The spatial effect on deformation of supporting structure, surface of ground settlement and axial force of different bracing was analyzed. The simulation and monitoring results indicated that the deformation in same deep of supporting structure manifested shape of parabola or basin, and scope of spatial effect on deformation of supporting structure was less than 8 m. Contour line of ground settlement near foundation pit changed from arc to trapezoid. The location of biggest settlement first moved further and then moved back. Axial force of P-CFST bracing was more sensitive than steel bracing under influence of excavation and bracing erection. Design interval among bracings can be enlarged by applying high-powered P-CFST bracing in first internal support, and deformation of supporting structure and stratum may be effectively controlled.