Short-term behaviour of a basement structure in London Clay and conducting oedometer, Rowe Cell and triaxia tests on block samples obtained from deep excavations across central London

Abstract

Excavations into over consolidated clay are becoming increasingly deep around the world to provide functional areas such as transport hubs. These basement structures pose significant engineering challenges since there are often uncertainties relating to ground movements on and around the site. Field instrumentation was installed at Victoria Station Upgrade (VSU) to measure: ground deformations; earth and water pressures; and slab strains during and after construction. The basement structure is overdesigned as a standalone basement as the future end use of the land above the basement is to develop the site. It was found that the structure limited vertical ground movements (heave) at the approximate centre of the basement to ~10mm (0.1% of excavation depth, H) which is considerably less than the 0.2-0.25%H typically found on similar projects. The conclusion would propose that, contrary to suggestions by Burland and Hancock (1977), small ground movements can be achieved. Additionally, high quality undisturbed block samples were obtained (with support from industrial sponsor Mott Macdonald) from three sites across Central London. These sites are all deep excavations into over consolidated clays and never before have block samples been obtained from these depths (>30m). The materials collected were Lambeth Group Upper Mottled Beds and London Clay Units A2 and B2. These high quality block samples have been tested in their intact states using Rowe cell, oedometer and advanced triaxial apparatus. It was found that the average insitu effective stress, 𝑝’0, was effective at normalising stiffness degradation when plotted against strain. When compared with data obtained by Mott Macdonald on Crossrail sites and previously established bounds, stiffness degradation sits within the expected envelopes.

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