天然沉积结构性软黏土地层盾构施工引起的长期地层位移机理研究

It has been well documented that the mechanical behavior of natural soft clays differs substantially from that of the corresponding reconstituted soils due to the effects of soil structure. The mechanical behavior of natural soft clays will be altered significantly while shield tunneling as the soil structure will be destroyed in this process, accompanying with the rise of excess pore pressure. While the dissipation of excess pore pressure, soil consolidates and additional long-term settlement occurs. Therefore the influence of soil structure by tunneling disturbance on the mechanic behavior of natural soft clays is the key factor that controls the mechanism of long-term settlement induced by shield tunneling. In this study, Chandler’s sensitivity framework quantifying the effect of soil structure and the influence of stress path representing the effect of tunneling are introduced to incorporate to investigate the disturbance of soil structure by shield tunneling. Based on such an innovative approach, the following issues are investigated in this study: 1) establishing the quantitative relationship between stress path and stress sensitivity, which representing the influence of shield tunneling; 2) investigating the change law in long-term soil settlement near tunnel and surface displacement; 3) understanding the mechanism of long-term settlement induced by shield tunneling. It is expected that this study will break the bottleneck of the traditional study of long-term effect by shield tunneling focusing on the surface displacement. The research results obtained in this study can provide a theoretical foundation for assessing the potential differential settlement of underground infrastructure and accurately evaluating the long-term safety of underground tunnel.

天然沉积软黏土往往受到土结构性的影响，在其地层中进行盾构施工将会不可避免的引起隧道周围土体结构破损，土体力学性状发生改变，产生超静孔隙水压力，而随着超静孔隙水压力消散土体固结将会进一步产生长期地层位移。因此，明确盾构施工扰动引起的土结构性损伤对天然沉积软黏土力学性状改变的影响是分析盾构施工引起的长期地层位移问题中亟需解决的关键科学问题。本项目以土结构性定量评价体系为基础，耦合盾构施工扰动对天然沉积软黏土力学性状的改变作用，基于理论研究、室内试验和数值分析的手段，建立反映盾构施工影响的不同应力路径下土结构性定量变化规律，明确盾构施工引起的土体长期变形与长期地面沉降之间相互关系，探索长时间跨度下考虑土结构性损伤影响下盾构施工引起的长期地层位移机理。研究成果预期突破传统长期效应研究停留在地面沉降的局限，对于地铁隧道的长期安全运营以及对邻近地下基础设施潜在危害的防控具有重要的理论指导意义。

我国东部沿海地区分布着深厚的软黏土，在这样特殊地基中进行盾构施工往往引起过大地面沉降和长期沉降问题。其中一个重要原因为盾构施工扰动将会引起的土结构性劣化从而使得软黏土中盾构施工引起的地面沉降和长期沉降难以预测。本研究针对这一特殊工程问题开展了深入研究，提出了不同应力路径对天然沉积软黏土力学性状劣化作用定量评价方法和力学参数的修正方法；建立能够考虑盾构开挖影响的天然沉积粘性土结构性本构模型；并且基于数值模拟，明确了土结构性损伤对于地面沉降影响规律，揭示了长期地层位移影响机理。主要得到以下成果：1）基于Chandler应力灵敏度指标体系为基础，耦合初始含水率影响，构建了应力灵敏度随应力水平的幂函数定量表示；以此为基础结合Liu结构性土压缩模型提出了天然沉积软黏土屈服后阶段压缩曲线的计算表达式；2）基于应力灵敏度随应力水平定量演化规律，以修正剑桥模型为基础，导入小应变模量，弹性非线性，次加载固结比等土体特性，构建了适用于盾构施工的土体本构模型；3）基于有限元软件ABAQUS对本构模进行二次开发，构建数值模型，探讨了盾构施工引起的土结构性劣化对沉降曲线模拟结果的影响，明确了天然沉积软黏土盾构引起地面沉降演化机理为：盾构扰动引起土结构性劣化使得隧道周围土体上施加更大的应力，得到更大的剪应力和更窄的最大剪应力带，同时能够模拟由于结构性损伤引起的土体变形软化特性，从而使得计算地面沉降曲线变得“深”而“窄”接近现场实测数据；4）土结构性对盾构施工引起的超静孔隙水压力分布有着显著影响，在距隧道中心水平大于1倍隧道直径范围内不考虑土结构性影响的误差为10%左右，在实际工程中若不考虑盾构施工引起的土结构性劣化机制，将会极大的低估超静孔隙水压力消散后作用在隧道上的应力约28%-40%，从而使得设计隧道处于危险状态。