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

世界各地城市中的许多大型建筑，包括交通枢纽，如主要的地铁站，都有重要的地下室结构，建造在超固结的粘土地层中。地下室结构设计的一个关键的不确定性是由于粘土长期膨胀的趋势，在最低的与地面接触的楼板下面形成的土压力。当土壤被挖掘形成基底时，基底下粘土的卸载导致最初不排水的土壤反应，其中总应力的减少作为张力传递给土壤孔隙水，因为粘土试图隆起。开挖下粘土的初始隆起发生在卸载时，由于剪切作用，以及雨水或地下水渗入土壤时产生的膨胀。从长期来看，随着开挖过程中产生的非平衡孔隙压力和吸力继续平衡到长期稳态状态，粘土会发生膨胀。在低渗透性粘土中，这可能需要几十年的时间，而且大部分膨胀可能在基底结构完成后很久才发生。地下室板通常被设计成与地面接触，以避免产生可能发生膨胀的空隙。长期的粘土隆起和孔隙水压力（如果板下或通过板不允许排水），然后直接加载混凝土板的底部。因此，有必要设计大型地下室结构以适应粘土的长期隆起。基底板的弯曲刚度决定了它下面积聚的压力，更灵活的板允许一些土壤膨胀，这可能会减少压力的积聚。更硬的楼板将减少隆起，但代价是更大的有效土压力。最终的膨胀压力取决于土的刚度和运动，这很难确定。这种趋势是保守的，尽管这导致了深板，这创造了一个更硬的结构，然后有可能从膨胀的土壤中吸引更多的载荷。确定最终膨胀压力的困难主要在于估计粘土的刚度，以确定将发生的土壤应变和运动。土壤在小应变下的高刚度是重要的，并且将刚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。在非常低的应力下，土壤的刚度也很难确定，并且从实验室测试中获得的关系可能在非常低的土壤应力下给出不切实际的高孔隙比。现场测量已被证明是建立粘土基准模型的重要手段，然而，很少有人（如果有的话）尝试测量底板内的升压和相关的结构反应，或者在施工完成后很长一段时间内对持续变化进行长期测量。伦敦等城市的地下室结构正变得越来越深（最近的案例达到35米深），其结果是估计的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板下形成的压力，将在设计和成本方面产生显著的效率。本项目旨在研究膨胀隆起和底板压力之间的关系，最初是在短期内，通过在伦敦粘土中建造一个大型挖掘的仪器，作为维多利亚站升级的一部分。将安装仪器，以测量土壤位移、孔隙水压力变化和底板荷载；并在施工期间和施工后不久对其进行监控。将向EPSRC提出进一步的申请，继续监测和调查长期变化。

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.