Characteristics of the London Clay from the Terminal 5 site at Heathrow Airport

The innovative engineering approach adopted for the new Terminal 5 at Heathrow Airport called for an advanced investigation of the London Clay strata, including detailed in situ profiling and stress path laboratory testing on high-quality rotary-cored samples. Although the scope of the investigations exceeded that normally specified for conventional design, questions relating to the structure and anisotropy of stiffness and strength of the clay remained that could not be answered. Further research was required, and the deep excavations at the site provided the opportunity for a team from Imperial College to take multiple block samples from three depths, supplemented by two additional dedicated rotary-cored boreholes. Intensive research was performed at Imperial College on these samples, as described in three companion papers by Gasparre et al. and Nishimura et al. This overview paper integrates the findings from the recent research with those from the commercial investigation and with earlier studies to extend our understanding of the geology and key characteristics of this stiff clay, and their variation with depth. The influence of lithology, structure and destructuring is examined, and the practical implications of the work are discussed.