The influence of structure on the behaviour of London Clay

Abstract

An intensive investigation is described into the London Clay units from Heathrow Terminal 5. Intrinsic properties and composition were established, and relating the behaviour of intact and reconstituted samples allowed the effects of the clay's natural structure to be identified at all depths. Structure varied between units, but some general features emerged that have not been seen in other stiff clays. In particular, intact samples follow paths under isotropic or K0 compression that fail to provide well-defined gross yield points, or converge with the unit's intrinsic compression lines. Structure contributes to the enhanced shear strength of intact unfissured clay and affects the initial stiffness relationships. Some identifiable features of the intact units’ fabric correlated directly with their behaviour. Natural fissures within the clay, the most important mesofabric feature, had an important impact on shear strength and led to an unusual pattern of directional dependence, and particle orientation trends identified by scanning electron microscopy governed the strong elastic stiffness anisotropy. The potential for destructuration through swelling to low effective stresses was also studied and found significant for subsequent volumetric compression behaviour, but not for shearing. Hight et al. synthesise the data from this and companion papers presented by the authors, and discuss the practical consequences of the results obtained.

The undrained shear strength anisotropy of four Jurassic to Eocene stiff clays

“…The simple c′ = 0, ϕ′ = 19°criterion provided a lower bound for all samples that failed on pre-existing planar fissure discontinuities aligned at angles near to the optimum 45°-ϕ′/2 orientation from the vertical at which τ/σ′ n is greatest under active triaxial conditions. Gasparre et al (2007a) report similar patterns from triaxial tests conducted on high-quality London clay specimens from a wide depth range at the Heathrow T5 site. After normalising the peak q-p′ stresses by the Hvorslev equivalent mean effective stresses p* e , as defined by oedometer tests performed on reconstituted specimens from similar depths, they concluded that the clay's state boundary surfaces varied with depth in the similar en-echelon sequence reproduced in Fig.…”

“…Natural stiff clays often manifest directionally oriented macro-to microstructures (Hight et al, 2007;Hosseini Kamal et al, 2014). Tectonic shears and fissure discontinuities can affect their mass shear strength profoundly.…”

“…Experiments on Gault, Kimmeridge and Oxford clays formed part of the integrated study detailed by Wilkinson (2011), Brosse (2012) and Hosseini Kamal (2012); Hosseini Kamal et al (2014) presented full geotechnical profiles for each site and reported on the in situ testing undertaken at each location. The London Clay hollow cylinder apparatus (HCA) experiments were performed as part of the Heathrow Terminal 5 study; Hight et al (2007) provided full details of this site's geotechnical profile. Tables 1 and 2 summarise key aspects of the four clays' descriptions, micro-and meso-fabrics, index properties and estimated in situ effective stresses.…”

Influence of smectite content on the deformation behaviour of clay

“…However, the opposite behaviour is observed for Danish Palaeogene clays where the yield stress is often lower than the geological stress (Femern 2014a;Mortensen 2012;Krogsbøll et al 2012), as described in section 3.1.2. The Palaeogene, highly plastic London clay have been tested intensively, (Hight et al 2003;Hight et al 2007;Gasparre et al 2007b;Gasparre et al 2007a). London clay is structured with fissures, joints, bedding surfaces and shear surfaces (Hight et al 2003), which may have been created by a large overburden of clay combined with small tectonic events followed by erosion of the overburden (Hight et al 2007).…”

“…Generally, the natural London clay shows some variability between the different units, however within a unit, e.g. the unit B2(c), the clay exhibits fairly constant compression behaviour, see figure 3.4 (Gasparre et al 2007a). As may be noted from figure 3.4 the onset of yielding is not well-defined on the compression curves, as no clear onset of clay structure breakdown (distinct bend in the compression curve) can be identified on the curve (also reported by Hight et al (2007)).…”