During construction of the Jubilee Line Extension (JLE), London, large varuatuins in tunnelling volume loss, from 1.1 to 3.3%, were measured between Waterloo, south of the Thames, and theare north of St. James’s Park (about 2 km). A detailed investigation was undertaken in James’s park two years after construction to establish reasins for the differences of 1.2 to 3.3% observed north and south of the park. Three primary causes are identified: tunnelling method and controol; differences in clay cover from past erosion and; divisions within the London Clay with markedly different geotechnical characteristics – in particular the permeaility. A key point is the necessity to control construction operations very closely when tunnelling through clays containing water-bearing silt and sand partings. This case study highlights the importance of understanding the engineering geology, even in London with decades of tunnelling experience and well characterised ground conditions. The message is equally applicable to many cities where large subsurface infrastructure projects are planned.
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