The response of the Europier Terminal Building to the excavation of the T2B basement at Heathrow Airport

Abstract

The new Terminal 2 Concourse B (T2B) is a satellite pier which forms part of the remodelling of the Eastern Campus at Heathrow Airport. The T2B concourse includes a 15m deep, 280m long, 100m wide (maximum) basement. The substructure was constructed using both top-down and bottom-up techniques with excavation taking place between one metre thick diaphragm walls embedded between 5m and 7m below formation level. The diaphragm wall retained cuts were supported at a single high level by either a stiff ground slab (top-down) or temporary multi-strand ground anchors (bottom-up). The 3-storey Europier Terminal Building was constructed in 1995 and is of steel frame construction. It is a repetitive modular building, approximately 280m in length in 39 equal bays, using a minimum number of standardised components including glazing. It contains four structural movement joints, approximately equally spaced throughout the length of the structure. The new works were located in close proximity to the Europier Terminal Building with the diaphragm walling taking place within 10m of its foundations; the building was to remain operational throughout basement construction. The ground conditions are typical of the London Basin comprising Made Ground overlying River Terrace Deposits (the Taplow Gravel), which is underlain by the London Clay Formation. The Lambeth Group and Chalk underlie the London Clay Formation at depth. At Heathrow, the groundwater level is typically 2-3m below existing ground level. This paper presents the response of the Europier Terminal Building to the excavation of the T2B Basement, compares the observed movements to those anticipated and discusses the risk mitigation strategy adopted and its efficacy. The T2B underground works were undertaken with minimal impact on the Europier Terminal building which remained operational throughout; the measured movements were less than anticipated in the impact assessment and the sensitive features, the large glass panel glazing units, were unaffected.