An 11 m-deep basement structure in London SW1 was left vacant from 1968 to 1989. The basement heaved significantly during this period due to the lack of a superstructure, providing a unique opportunity to study the development of long-term heave in London Clay. May (1975) presented the monitoring results from 1968-73 and excerpts of site data collected after 1973 have also been circulating informally in the industry since the 1990s. However, the full set of monitoring data remains hitherto unpublished.

This paper was initially drafted in the early 1990s when three of the authors (Nicholson, Chapman, and Solera) were working together with Arup. The paper somehow never got published as people and circumstances changed. More recently, the first author (Chan) started postgraduate research on heave and pressure beneath slabs in excavations in over-consolidated clays, using the heave monitoring data from the draft paper to complement centrifuge test results. For this reason, it was decided that the draft paper should be revised and published for the benefit of the wider industry.

This paper provides a comprehensive case history of the site, publishing further heave data to June 1989, giving a total of 21 years of heave monitoring. Further site investigation data and calculations are included for comparison. The data show that the presence of a basement did not significantly reduce the shear strength of the clay beneath it. The coefficient of consolidation of the clay was 38 – 52 m²/year and long-term heave was still ongoing 21 years after the end of excavation.