Effect of time on the evolution of soil behaviour: new advanced triaxial creep cell experiments

延长现有基础设施的使用寿命是当前土木工程项目可持续建设的主要方面之一。例如，在城市重建项目中重新使用地基，或者提高现有防洪堤的高度，以应对气候变化导致的海平面上升。这类工程的设计要求是量化地基土能承受多大的高负荷，以便在现有地基上建造更高的防洪堤或更重的新结构。在基础的再利用和防洪堤防的抬高中，现有的结构已经存在了很长一段时间（几十年），在此期间，基础土壤已经受到了不断发展的各向异性和与时间相关的固结过程（由外加荷载产生的土壤中多余孔隙水压力的消散）和蠕变过程（有效应力变化不大的土壤的持续应变）。固结和蠕变过程都会随时间引起额外的地面运动。这些地面运动的结果是孔隙比的减小，从而导致土壤的强度和刚度随时间的增加。正是这种土壤力学特性的增强，使现有基础系统的应用负荷增加。然而，目前关于基础土的刚度或强度与蠕变或荷载持续时间的增加程度或幅度的指导很少。土壤行为随时间变化框架的这一方面定义不清，但在土木结构的可持续性方面具有重要意义。这项研究对于解决现有基础设施的可持续建设和重建的设计需求是非常及时的。它依赖于由先进的三轴蠕变装置组成的新实验设备的发展，并将使该装置能够适当地进行第一次实验。探索蠕变对粘土强度和刚度影响的长期实验之前尚未进行过，并且缺乏关于该主题的实验数据阻碍了研究和工业解释或量化由于蠕变过程而导致的工程性能改进的能力。项目合作伙伴奥雅纳对支持和合作研究岩土工程问题有浓厚的兴趣，并提供土壤样本、沉降记录和一些结构的荷载估计，这些结构建在同一土壤上，经历了非常大的长期沉降行为。这种投入将为项目提供积极和相关的焦点。实验行为数据库可以由设计用于长期准确稳定地监测土壤样品蠕变的单元生成，这将使本构模型得以校准，以帮助使用数值分析方法进行设计。该项目的主要目的是量化从蠕变时期对强度和刚度的改善，以更好地了解时间对土壤行为演变的影响，这在以前的研究中是明显缺失的。这是研究土壤随时间变化行为的一种新方法，值得注意的是，由于缺乏时间和长期精确稳定测量样本量变化的能力，以前的研究无法研究显著蠕变时期的影响。帝国理工学院岩土工程实验室在他们的专业知识和设施方面是独一无二的，能够应对这一挑战。

Extending the life of existing infrastructure is one of the main aspects of sustainable construction in current civil engineering projects. Examples include the re-use of foundations in urban re-development projects, or raising the height of existing flood embankments to account for sea-level rise due to climate change. The design requirement in such projects is to quantify how much higher load the foundation soil can take in order to enable the construction of a higher flood embankment or a heavier new structure on existing foundations.In both foundation re-use and raising flood embankments the existing structure will have been in place for a long period of time (several decades) during which the foundation soil has been subjected to evolving anisotropy and to time-related processes of consolidation (dissipation of excess pore water pressures within the soil generated by the applied load) and creep (continued straining of the soil with insignificant change in effective stress). Both consolidation and creep processes cause additional ground movements dependent upon time. The consequence of these ground movements is the reduction in the void ratio which leads to the strength and stiffness increase in the soil with time. It is this enhancement of soil mechanical properties that enables an increase in applied load on existing foundation systems. However, there is currently little guidance on the degree or magnitude of increase in stiffness or strength of the founding soil with respect to the duration of creep or loading. This aspect of the time-dependant framework of soil behaviour is poorly defined, but has significant implications in terms of the sustainability of civil structures.The proposed research is very timely in addressing design needs for sustainable construction and redevelopment of existing infrastructure. It relies on the development of new experimental equipment consisting of an advanced triaxial creep apparatus, and will enable the apparatus to be properly commissioned for the first experiments to be performed. Long term experiments exploring the effect of creep on the strength and stiffness of clay have not been performed before and the lack of experimental data on this topic is hindering the ability of both research and industry to account for or quantify improvements in engineering properties due to creep processes.Project partners, Arup, have a strong interest in supporting and collaborating in research into geotechnical engineering issues and are providing soil samples in addition to settlement records and load estimates from some structures founded on the same soil that have experienced very large and long-term settlement behaviour. This input will provide am active and pertinent focus for the project.The database of experimental behaviour which can be generated by a cell designed to monitor soil sample creep accurately and stably in the long-term will enable constitutive models to be calibrated to aid design using numerical analysis methods. The principal aim of the project is to quantify the improvement to strength and stiffness from periods of creep to gain a better understanding of the effect of time on the evolution of soil behaviour which has been notably absent from previous studies. This is a novel approach in the study of the time dependant behaviour of soil and it is of note that previous studies have been unable to study the effect of significant periods of creep, due to the lack of time and capability for accurate and stable long term measurement of sample volume change. The Imperial College Geotechnics Laboratory are unique in their expertise and facilities to be able to tackle this challenge.