Earth and water pressures on the base of ground-contacting slabs within deep basement structures

深层地下室结构内接触地面板底部的土压力和水压力

• 批准号: EP/K02521X/1
• 负责人: Joel Smethurst
• 金额: $ 12.76万
• 依托单位: University of Southampton
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2013
• 资助国家: 英国
• 起止时间: 2013 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FK02521X%2F1
• 关键词: Earth; water; pressures; base; ground

Many large buildings in cities around the world, including transport hubs such as major underground stations, have significant basement structures constructed within an overconsolidated clay formation. A key uncertainty in the design of the basement structures is the earth pressures that build up underneath the lowest ground-contacting floor slab due to the tendency for long-term swelling of the clay.As soil is excavated to form the basement, unloading of the clay beneath the basement results in an initially undrained soil response, in which the reduction in total stress is transferred to the soil pore water as a tension as the clay tries to heave. Initial heave of the clay beneath the excavation occurs on unloading due to shear, and from swelling as rain or ground-water infiltrates into the soil. In the longer term, swelling of the clay takes place as the non-equilibrium pore pressures and suctions generated during excavation continue to equilibrate to a long-term steady state condition. In low permeability clays, this can take decades, and much of the swelling may take place long after the basement structure is complete.Basement slabs are often designed to be ground contacting, to avoid the difficulty in creating a void into which swelling can occur. Long-term clay heave and pore water pressures (if no drainage beneath or through the slab is allowed) then load the base of the concrete slab directly. It is therefore necessary to design large basement structures to accommodate the long-term heave of the clay.The flexural stiffness of the basement slab dictates the pressures that build up underneath it, with more flexible slabs allowing some soil swelling to take place that likely reduces the build up of pressure. Stiffer slabs will reduce heave, but at the cost of greater effective earth pressures. The final swelling pressure is dependant on the soil stiffness and movement, which can be difficult to determine. The tendency is to be conservative, although this results in deep slabs, which create a stiffer structure that then has the potential to attract more load from the swelling soil.The difficulty in determining the final swelling pressure is primarily in estimating the stiffness of the clay to determine the soil strain and movement that will occur. The high stiffness of the soil at small strain is important, and models that match stiffness to the likely strain level in the soil tend to produce better estimates of heave. The stiffness of soils at very low stresses can also be difficult to determine, and relationships obtained from laboratory testing may give unrealistically high void ratios at very low soil stresses.Field measurements have proved an important means of benchmarking models for clay soils, however, there have been few, if any, attempts to measure the heave pressure and associated structural reactions within the base slab, or to take long-term measurements of continued change long after construction has finished.Basement structures in cities such as London are becoming ever deeper (recent cases are up to 35 m deep), with the result that estimated swelling pressures and design slab depths are increasingly large. A better understanding of how swelling takes place, and the pressures that build up beneath ground-contacting slabs will to produce significant efficiencies in design and cost. This project proposes to investigate the relationship between swelling heave and base slab pressures, initially in the short-term, through instrumentation of a large excavation in London Clay being constructed as part of the Victoria Station upgrade. Instrumentation will be installed to measure soil displacements, changes in pore water pressures and base slab loading; and to monitor them during and shortly after construction. A further application will be made to EPSRC to continue to monitor and investigate long-term changes.

世界各地城市中的许多大型建筑，包括交通枢纽，如主要的地下车站，都有大量的地下室结构，建造在过度固结的粘土地层中。地下室结构设计中的一个关键不确定性是，由于粘土长期膨胀的趋势，在最低接触地面的楼板下形成的土压力。当挖掘土壤形成地下室时，地下室下面的粘土卸载会导致最初不排水的土壤反应，在粘土试图隆起时，总应力的减少作为一种张力传递到土壤孔隙水中。开挖下面的粘土在卸荷时由于剪切而发生初始隆起，并在雨水或地下水渗入土壤时发生膨胀。从长期来看，当开挖过程中产生的非平衡孔压和吸力继续平衡到长期稳定状态时，粘土就会发生膨胀。在低渗透粘土中，这可能需要几十年的时间，而且大部分膨胀可能发生在地下室结构完成后很长一段时间。底板通常被设计为与地面接触，以避免产生可能发生膨胀的空隙的困难。长期的粘土隆起和孔隙水压力(如果不允许在板下或通过板的排水)，然后直接加载混凝土板的底部。因此，有必要设计大型的地下室结构，以适应粘土的长期隆起。地下室板的弯曲刚度决定了在其下面积累的压力，更灵活的板允许发生一些土壤膨胀，这可能会减少压力的积累。更坚硬的楼板将减少隆起，但代价是更大的有效土压力。最终膨胀压力取决于土体的刚度和运动，这可能很难确定。这种方法倾向于保守，尽管这会产生较深的板，从而产生更坚硬的结构，从而有可能从膨胀土中吸引更多的荷载。确定最终膨胀压力的困难主要是估计粘土的硬度，以确定将发生的土壤应变和运动。土在小应变时的高硬度是很重要的，而将硬度与土壤中可能的应变水平相匹配的模型往往会产生更好的隆起估计。土壤在极低应力下的硬度也很难确定，通过实验室测试获得的关系式可能会在极低的土壤应力下得出不切实际的高孔隙率。现场测量已被证明是粘性土模型的重要基准方法，然而，很少有人尝试测量底板内的隆起压力和相关的结构反应，或者对施工完成后很长一段时间内的持续变化进行长期测量。伦敦等城市的地基结构越来越深(最近的案例深达35米)，结果是估计膨胀压力和设计板深度越来越大。更好地了解膨胀是如何发生的，以及在与地面接触的楼板下积累的压力将在设计和成本方面产生显著的效率。该项目建议通过对作为维多利亚站升级工程一部分的伦敦粘土的大型挖掘进行仪器测量，初步在短期内研究膨胀隆起和底板压力之间的关系。当局会安装仪器，以量度土体位移、孔压和底板荷载的变化，并在施工期间和施工后不久进行监察。将进一步向EPSRC提出申请，以继续监测和调查长期变化。

项目成果

Behaviour of over-consolidated clays beneath deep excavations, PhD Thesis

深基坑下超固结粘土的行为，博士论文

• 发表时间: 2020
• 作者: Procter, M