部分排水条件下体变与孔压耦合作用时软黏土的抗剪强度特性及稳定性分析

The shear strength of soft clay exhibits a strong time-dependent characteristic associated with the consolidation process. However, in the conventional types of shear testing, the failure mechanism under undrained conditions is developed without volumetric strain but with pore pressure only. The failure mechanism under fully drained conditions is developed without pore pressure but with volumetric strain only. These mechanisms are different from the failure mechanism of soft clays subjected to volumetric strain and pore pressure coupled interactions, which would be derived from simultaneous consolidating and shearing in practice. For this reason, a new type of shear testing under partially drained conditions is proposed. The volumetric contraction and pore pressure dissipation characteristics during consolidation are considered, together with the significant strain rate-dependent characteristic of soft clay. This is implemented by simultaneously controlling the volumetric to axial strain ratio and the axial strain rate. The corresponding triaxial testing apparatus is developed. An experimental study is carried out to investigate the effect of volumetric strain and pore pressure coupled interactions on shear strength. A progressive shear strength theory is established to reflect the strength change from undrained state to fully drained state. Meanwhile, a model testing apparatus is developed to carry out the shear testing during consolidating. It is used to investigate the time-dependent characteristic of shear strength. By adopting the consolidation theory and the proposed progressive shear strength theory, the time-dependent process of shear strength is predicted. On this basis, a method is developed for analysing the whole life cycle, from short-term to long-term, stability of soft ground.

软黏土抗剪强度随固结过程发展具有时变性强的特点。常规实验类型中，不排水剪切实验无体变、仅孔压作用下的破坏机制，以及完全排水剪切实验无孔压、仅体变作用下的破坏机制，与实际工程中软黏土地基固结同时剪切，即体变和孔压耦合作用下的剪切破坏机制并不一致。为此，针对软黏土在固结过程中体积收缩、孔压消散的特点，考虑其抗剪强度具有显著的应变率依赖性，采用体变与轴向应变增量比和轴向应变速率双控的方法，提出部分排水条件下软黏土剪切实验的新类型，研制相应的三轴仪，开展实验研究，揭示体变和孔压对抗剪强度的耦合作用机理，建立能够反映软黏土从不排水状态过渡到完全排水状态的渐变抗剪强度理论。同时，研制室内模型实验装置，开展固结过程中的剪切实验研究，揭示抗剪强度的时变特性。在此基础上，将固结理论与渐变抗剪强度理论结合，预测抗剪强度的时变过程，建立软黏土地基从短期稳定性过渡到长期稳定性，即全寿命周期稳定性的分析方法。

针对软黏土在固结过程中抗剪强度的时变性问题，研制了体变与轴向应变增量比和轴向应变速率双控的三轴仪，以及固结过程中可以随时开展剪切试验的系统装置，通过试验研究，揭示了不完全排水条件下体变与孔压的耦合作用对抗剪强度特性的影响机制和规律。结果表明，在不同固结压力作用下，固结完成后的不排水抗剪强度与有效应力呈现出传统的线性关系，但是，在某一固结压力作用下，固结过程中的不排水抗剪强度却随有效应力的增长呈非线性增长，而且，在不同固结压力作用下，固结压力越大，固结过程中达到相同的有效应力时所对应的不排水抗剪强度越大。固结过程中的不排水抗剪强度并不仅仅取决于剪前固结有效应力，还与剪前孔隙比相关，孔压消散速率小于变形速率是导致固结初期、剪前固结有效应力较小时，不排水抗剪强度较快增长的主要原因。在现有基于修正剑桥模型理论框架提出的各向异性软土本构模型的基础上，考虑剪前初始应力比，推导了软黏土抗剪强度的预测公式，通过计算结果与试验结果的对比验证了其适用性，并结合现有的地基应力分析方法和固结理论，建立了软黏土抗剪强度时变过程的理论预测方法。在此基础上，依据塑性极限分析的上限定理，建立了软黏土地基从短期稳定性过渡到长期稳定性，即全寿命周期稳定性的快速分析计算方法，通过与实际工程案例的对比分析，验证了该计算方法的适用性。所揭示的不完全排水条件下的抗剪强度特性是对传统认知的有益补充和完善，所建立的稳定性分析方法可为软土地基上的工程设计计算提供理论依据。