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

Abstract

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.

Measured short-term subsurface ground displacements from EPBM tunnelling in London Clay

“…Given the relatively high average advance rate of the TBM, this suggests that the ground permeability was higher than usually expected for London Clay and therefore the ground response was partially drained. This correlates well with the fact that the lower part of the tunnel is located in the London Clay sub-units B1 and A3ii, which are much more permeable than unit B above them (Standing & Burland, 2006;Hight et al, 2007;Wan & Standing, 2014b). Other features specific to the site, such as the presence of claystones (Wan & Standing, 2014b) and fissures in the London Clay, may also contribute to more rapid drainage.…”

“…Typical normalised modulus decay curves determined from undrained triaxial compression tests on London Clay samples (e.g. Hight et al, 2007) suggest that this level of vertical compressive straining could result in a 25-65% reduction in the undrained secant modulus.…”

“…Typical normalised modulus decay curves determined from undrained triaxial compression tests on London Clay samples (e.g. Hight et al, 2007) suggest that this level of vertical compressive straining could result in a 25-65% reduction in the undrained secant modulus.Corresponding incremental transverse horizontal strains developed after the westbound and eastbound tunnel construction are presented in Figs 17(a) and 17(b), respectively. In general, compressive horizontal strains developed above and below the tunnel construction, while extensional horizontal strains developed either side of it.…”

Stabilisation of excavated slopes in strain-softening materials with piles

“…The material properties are summarised in Table 1 and are based on the work of Kovacevic (1994) and Potts et al (1997), who simulated a realistic failure time and mechanism that agreed with field data for cutting slopes of the same dimensions in London Clay. The adopted permeability values are from subsequent work of Hight et al (2007). It is well established that the conventional FE solution of strain-softening problems, in which strain localisation occurs within a zone of limited thickness, can lead to numerical instability and significant mesh dependency of the solution (Galavi & Schweiger, 2010;Summersgill et al, 2017aSummersgill et al, , 2017b.…”

“…(b) The impact of DL on the weighting function distribution Table 1. Soil model parameters (Kovacevic, 1994;Potts et al, 1997;Hight et al, 2007) …”