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

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.

The proposed research project will impact on the civil engineering and construction industry, major private clients and government providers of infrastructure, and the public at large. Providing better guidance on short-term (and potentially later on, long-term) heave movements and basement slab pressures will enable more confidence in design and more efficient and better performing underground infrastructure. In detail, the beneficiaries of the research are:
i) The designers of large basement structures, who will have access to better guidance on the efficient design and short term performance of basement slabs.
ii) Private and government clients of major underground infrastructure, through lower initial construction costs as a result of less material use, and potentially faster construction, and better long-term structural performance.
iii) Society at large, which will benefit from cheaper and better performing underground infrastructure such as metro systems, and reduced carbon emissions from more efficient use of materials such as concrete.

Potential beneficiaries will have access to the research through a programme of dissemination within the UK construction industry and the international community. The industrial collaborators will initially help steer the project and subsequently engage further beneficiaries through the direct application of the research results in their design and procurement of infrastructure. The exploitation of the research results will be through direct use of new knowledge by engineering designers in the design of new basements and understanding the performance of and providing modifications to older structures. As dissemination enables the improved design and construction of underground basements, so the benefits of the work will reach other clients and society as a whole.

The Project Partners, who are Mott MacDonald and London Underground Ltd have both been formal partners on, and have directly funded previous research at the University of Southampton. Both have made large in-kind contributions to the proposed work, including providing staff members for participation in steering group meetings, in helping with programming and installation of instrumentation on site, and in interpretation, analysis and publication of results.

Communication of the results to a wider audience beyond the steering group will occur through a number of mechanisms, including publication of scientific results in leading geotechnical journals such as Geotechnqiue; shorter articles in magazines such as Ground Engineering; and dissemination at international conferences. Regular short project updates will be posted on the University of Southampton Geomechanics Research Group web pages.