考虑盾构掘进扰动的砂卵石地层结构性演化及其力学响应特征分析

In recent years, it has attracted much attention on the scientific safety problems caused by shield tunneling in sandy cobble stratum, such as palm collapse and surface subsidence, with some limits on their research methods. Considering the particularity structure of sandy pebble soil (the size and shape of particles have self-similarity, and the particle size distribution conforms to the fractal characteristics) and the "scale effect" of strength, a quantitative description equation of mechanical properties of sandy pebble soil with fractal dimension (gradation) as the basic parameter is established. In this research, based on the experimental data in literature and triaxial test, these data were used to establish a quantitative description equation of mechanical properties of sandy pebble soil with fractal dimension (grading) as the basic parameter. Then, the single particle crushing test was carried out. Based on the experimental results, a particle breakage model related to particle bed pressure, gradation (fractal dimension) and size of pebble was established. Moreover, the shield excavation process is simulated, which takes account of the influence of soil particle gradation, fragmentation, and shape (achieved by simulating pebble particles with particle clusters). The interaction mechanism between the cutter head and the stratum and the evolution of the mesoscopic fabric of the sandy pebble soil are studied. And an evaluation model of sand - pebble formation stability is proposed. Furthermore, the evaluation model is verified by an experiment on the stability of sand and pebble formation under impact disturbance. The effective implementation of the project can provide evaluation criteria for the stability of sandy cobble stratum (especially "delayed subsidence") caused by shield construction, as well as the scientific theoretical basis for grouting reinforcement.

近年来，砂卵石地层盾构施工所引起的掌子面坍塌、地表沉降等安全问题备受关注，而其有关研究存在一些局限。由于砂卵石土结构的特殊性（颗粒的尺寸、形状具有自相似性，粒度分布符合分形特征）和强度的“尺度效应”，本项目基于文献分析和试验数据，辅以三轴试验，建立以分维数（级配）为基本参数的砂卵石土力学性能指标的定量描述方程；进行单颗粒破碎试验，建立与颗粒床压力、级配（分维数）以及卵漂石尺寸相关的颗粒破碎判断模型；进行盾构开挖过程的数值模拟，研究土颗粒级配、破碎、形状（通过颗粒簇模拟卵石颗粒实现）的影响，揭示刀盘与地层的相互作用机理以及砂卵石土细观组构演化，提出砂卵石地层稳定性评价模型，并通过冲击扰动的砂卵石地层稳定性试验进行验证。本项目的有效实施，可为盾构施工引起的砂卵石地层稳定性（尤其是“滞后沉降”）界定提供系统的评价标准，并对其进行注浆加固提供科学方案依据。

针对砂卵石地层盾构隧道施工所引起的地表塌陷、滞后沉降等问题，课题组充分调研砂卵石地层结构特点、盾构施工掘进参数、管线沉降规律，采用侧限压缩实验、坍落拱实验、砾石土振动响应实验、数值模拟以及理论分析等手段研究砂卵石土的力学特性以及刀盘切削特征，进而分析地层滞后沉降发生机理，结果显示：.1、自然沉积状态下形成的砂卵石地层，其粒径分布近似满足分形结构，且分形维数集中在2.3到2.5之间，土体颗粒之间的接触数满足正态分布。.2、对于重塑砂卵石土，在侧限压缩状态下，粒径分布的分形维数在2.4到2.5之间时承载能力最优，但成拱能力较弱，极易形成垂直状的贯通性坍塌。.3、在动载荷作用下，重塑粗颗粒土相对于砂土，对荷载有明显的放大作用。对于粒径分布分形维数为2.4的粗粒土，上层土体的动土压力振幅较下层土体相差20倍以上；当特定频率的正弦动载荷持续作用，土压力振幅将出现周期性变化。.4、盾构机刀盘切削砂卵石土时，盾构机最大推力比最小推力大650T。刀盘推力相对于掌子面侧向土压力超过一定范围将破坏土体原先的胶结状态，土体稳定性减弱，滚刀或刮刀切削土体时产生的振动将加剧这一趋势。在地铁运营其中，管线下沉、列车运营载荷、路面行车荷载均可能进一步减弱管线上方的土体的胶结作用，增加滞后沉降风险。.5、为减少滞后沉降发生的概率，可采取以下措施：a、在盾构施工过程中，盾构机的推力应尽可能与掌子面上的土压力保持一致；b、注浆加固应尽可能扩大注浆范围，合理控制注浆压力并多次注浆，使得扰动土体的土颗粒再次有效胶结；c、在隧道运营期，应尽量减少对隧道附近砂卵石地层的振动，距隧道较近的路面应尽量避免重载通行，可在隧道结构上设置减震机构，减小列车通行所产生的振动。.综上所述，本项目的研究有效的揭示了砂卵石地层盾构施工所引起的滞后沉降发生的机理，对提升隧道施工与运营的安全、减少滞后沉降发生的概率具有指导意义。