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物，包括交通枢纽，如主要的地下车站，都有在超固结粘土地层中建造的重要地下室结构。地下室结构设计中的一个关键不确定因素是，由于粘土长期膨胀的趋势，在最低地面接触楼板下积聚的土压力。随着土壤被开挖形成地下室，地下室下粘土的卸载导致最初的不排水土壤反应，其中总应力的减少在粘土试图隆起时作为张力传递到土壤孔隙水。由于剪切力的卸载以及雨水或地下水渗入土壤时的膨胀，开挖下方的粘土会发生初始隆起。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定状态条件，粘土发生膨胀。在低渗透性粘土中，这可能需要几十年的时间，并且大部分膨胀可能在基底结构完成后很久才发生。地下室板通常设计为与地面接触，以避免难以形成可能发生膨胀的空隙。长期的粘土隆起和孔隙水压力（如果不允许板下或通过板排水）则直接加载混凝土板的基础。因此，需要设计大型地下室结构，以适应粘土的长期隆起。地下室底板的弯曲刚度决定了其下方积聚的压力，更灵活的底板允许发生一些土壤膨胀，这可能会减少压力的积聚。较硬的板将减少隆起，但以更大的有效土压力为代价。最终膨胀压力取决于土壤刚度和运动，这可能很难确定。这种倾向是保守的，尽管这会导致深板，这会产生一个更硬的结构，然后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在于估计粘土的刚度，以确定将发生的土壤应变和运动。小应变时土壤的高刚度很重要，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升沉估计。在非常低的应力下，土壤的刚度也很难确定，从实验室测试中获得的关系可能会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粘土基准模型的重要方法，但是，很少尝试测量基础板内的垂荡压力和相关结构反应，或者在施工结束后很长时间内对持续变化进行长期测量。在伦敦等城市中，地下室结构正变得越来越深（最近的案例高达35 m深），结果是估计的膨胀压力和设计板深度越来越大。更好地理解膨胀是如何发生的，以及在地面接触板下建立的压力将在设计和成本方面产生显着的效率。本项目拟通过对作为维多利亚站升级工程一部分而建造的伦敦粘土中的大型开挖进行仪器测量，初步研究膨胀隆起与底板压力之间的关系。将安装仪器来测量土壤位移、孔隙水压力和底板荷载的变化，并在施工期间和施工后不久对其进行监测。将进一步向EPSRC提出申请，以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.