Predicting the stand-up time of temporary London Clay slopes at Terminal 5, Heathrow Airport

The paper describes the results of finite element analyses of the temporary slope geometries in London Clay at London Heathrow Airport's Terminal 5. The aims of the analyses were to examine the times before failures developed, and to identify the failure mechanisms involved. The brittle behaviour of the London Clay was modelled, and the effects of progressive failure were taken into account. The possible presence of tectonic shears with their strength close to residual was considered by comparing analyses with and without a tectonic shear zone. The predicted time to failure of the slopes and the form of the failure, whether shallow or deep-seated, were determined by a combination of the assumed permeability profile and whether or not allowances were made for increases in permeability as the clay swelled, the average surface suction, the in situ K₀ profile, the depth of excavation, and whether or not a low-strength tectonic shear surface was present in the slope. The analyses fall into the category of Class A predictions, and were used in the assessment of how long temporary slopes to deep excavations could be left open before backfilling, and how the slopes should be monitored.