洲头咀沉管隧道模袋砂围堰施工变形实测与数值模拟

Measurement and Numerical Simulation of Deformation of the Cofferdam with Geotextile Sand-Bags in the Zhoutouzui Immersed Tunnel Project

文章以广州市洲头咀沉管隧道模袋砂围堰工程为背景,通过现场动态实测,揭示了模袋砂围堰施工过程变形发展规律;结合既有研究成果,明确了围堰位移突变的内在机理,提出了围堰设计施工主要控制指标,且通过精细化有限元模型计算验证。研究结果表明:模袋砂围堰施工过程不同位置位移响应规律和程度不尽相同,局部位移出现突变,其中竖向受到的影响较水平向显著;围堰位移突变主要因模袋砂抽水使浮容重逐渐变为干容重,增加了上层模袋砂对下层模袋砂的压力,使模袋砂压扁而发生较大位移;受上部模袋砂围堰的作用,地基土经历竖向压密→侧向挤压→局部滑动三个阶段,使围堰最大水平和竖向位移出现在顶部与坡脚。数值计算表明,围堰未发生滑移破坏,且未形成贯通的塑性区而处于稳定状态,与理论分析的结论一致,围堰应以堆高为主控指标。用常规基坑支护结构位移限值评判模袋砂围堰力学状态存在不足,同时围堰的坡率设计应充分考虑模袋加筋作用。

The deformation development law of cofferdam with geotextile sand bags is explored by field measurement based on the construction process of the Zhoutouzui immersed tunnel in Guangzhou. Based on the existing research results, the deformation saltation mechanism is determined, the main control indexes for design and construction of the cofferdam are proposed and the rationality is verified by a refined FEM calculation. The results show that： 1） the response law and degree of deformation are different at different positions during the construction of the geo- textile sand bag cofferdam, with sahation of the local displacement and a larger influence in the vertical direction than in the horizontal direction; 2） the main reason for deformation sahation of the cofferdam is that the buoyant unit weight turns into a dry unit weight due to the dewatering of the geotextile sand bag, which increases the overlying pressure of the upper geotextile bag to the lower ones and flattens them and causes large deformation; and 3） the foundation soil experiences three phases of vertical pressure consolidation, lateral extrusion and local sliding, which cause the maximum vertical and horizontal displacements to occur at the top and foot of the cofferdam. The numerical calculation results show that： 1） it is stable since no sliding failure occurs at the cofferdam and no cut-through plastic zone is formed, which is consistent with the results deduced by theoretical analysis; 2） the stack height of cofferdam should be taken as the control index; and 3） there are some weak points in using the displacement limit value for traditional foundation pit support structure to judge the mechanical state of the cofferdam with geotextile sand bags, and the design for the slope ratio of the cofferdam should fully consider the reinforcement effect of the geotextile sand bags.