Heathrow Terminal 5

The majority of the below-ground construction at London Heathrow Airport’s Terminal 5 (T5) was carried out in deep open-cut excavations made in the London Clay Formation underlying Terrace Gravels. There was a need to adopt slopes that were as steep as possible, to minimise the excavation and backfill volumes, but which would be stable for periods of up to 6 months. Temporary slope geometries were initially selected on the basis of experience and precedents for other, usually shallower, short-term slopes cut in the London Clay Formation.

GCG was commissioned to carry out finite element (FE) analyses of the temporary slope geometries at T5. The aims of the analyses were to examine the times before failures developed and to identify the potential failure mechanisms. Prior to carrying out the FE analyses, the GCG team specified extensive ground investigations at T5 to characterise in detail the London Clay Formation. Appropriate soil models were chosen and associated model parameters derived. The models were then calibrated using two cases of slope failures local to T5. The FE analyses showed that the slopes were stable in the short term immediately after excavation. However, the time to failure of the slopes and the form of the failure were determined by a combination of various factors, such as the assumed permeability profile, the average surface suction, the in situ ground stresses, the depth of excavation and the presence of the low strength tectonic shear zone(s). The results of the analyses suggested the usefulness of suction measurements, together with displacement monitoring, as a means of monitoring and controlling temporary cut slope behaviour, so that they were eventually used to guide the monitoring of the slopes.

References

Kovacevic, N., Hight, D. W. & Potts, D. M. (2004). Short-term failures in London Clay: Two case studies. Proc. Skempton Memorial Conference Advances in Geotechn. Engng, Vol.3, London UK, Thomas Telford, 1-14.

Kovacevic, N., Hight, D. W. & Potts, D. M. (2007). Predicting stand-up time of temporary London Clay slopes at Terminal 5, Heathrow Airport. Geotechnique, Vol. 57, No.1, 63-74.