GCG was a key member of the design team for Heathrow Terminal 5, providing specialist geotechnical input on site investigation, earthworks (including soil stabilisation and temporary cut slope stability), pile design in swelling ground, and predictions of ground and tunnel movements.
A central focus of our work was the design, supervision, and interpretation of a detailed site investigation to characterise the in situ stiffness, strength, and expansibility of the London Clay. This involved rotary coring from two adjacent boreholes—one for detailed geological logging and the other for laboratory testing—and a third co-linear borehole for high-quality thin-walled tube sampling. Sample quality was assessed using simultaneous suction measurements (via the Imperial College suction probe) and shear wave velocity testing (via bender elements). Soil stiffness was further evaluated through locally instrumented triaxial tests with shear wave velocity monitoring. The co-linear boreholes also supported in situ geophysical testing, including crosshole and downhole shear wave velocity, natural gamma, and electrical resistivity measurements.
GCG was also commissioned to perform finite element (FE) analyses of the temporary slope geometries at T5. These analyses aimed to assess time-to-failure and identify potential failure mechanisms. Prior to modelling, we specified an extensive ground investigation to characterise the London Clay Formation in detail. Appropriate soil models were selected and calibrated using data from two local slope failures. The FE results indicated that while the slopes were initially stable post-excavation, long-term stability was influenced by factors such as permeability profiles, surface suction, in situ stress conditions, excavation depth, and the presence of low-strength tectonic shear zones.
Our findings demonstrated the value of integrating suction measurements with displacement monitoring to manage temporary slope behaviour—an approach that was ultimately adopted for slope monitoring at the site.
Publications
Hight, D.W., Gasparre, A., Nishimura, S., Minh, N.A., Jardine, R.J. & Coop, M.R. (2007). Characteristics of London Clay from the Terminal 5 site at Heathrow Airport. Géotechnique 57, No. 1, 3-18.
Jardine R.J., Potts D.M., Fourie A.B. & Burland J. B. (1986). Studies of the influence of non-linear stress-strain characteristics in soil-structure interaction. Géotechnique 36, No. 3, pp 377-396.
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.
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