The stiffness of natural London Clay

Abstract

An investigation of natural London Clay is reported involving advanced triaxial, hollow cylinder apparatus (HCA) and dynamic testing techniques. Significant anisotropy was revealed at all scales of deformation, and the framework of cross-anisotropic elasticity was found to apply broadly to the initial elastic behaviour. The stiffness parameters obtained by independent techniques generally exhibited good agreement, with the greatest deviation being seen in the Poisson's ratios, which fell far from the values usually assumed in conventional foundation analysis. Probing tests established the limits to the elastic domain over a range of depths, showing that these scaled in proportion to the mean effective stress level, as did those of a second kinematic surface that surrounded the elastic domain. Once engaged, this second surface signified a new pattern of strain increment directions, faster elastic-plastic stiffness decay with strain, and also a greater dependence of behaviour on recent stress history. However, the two kinematic surfaces cover a relatively small proportion of the admissible stress space, and behaviour at larger strains is both anisotropic and strongly non-linear, features that affect profoundly the soil displacements induced by geotechnical construction in this deposit.

Effect of anisotropy in ground movements caused by tunnelling

“…15b). from high quality measurements of the cross-anisotropic stiffness properties of intact London Clay measured in an independent study for Heathrow Airport T5 (Gasparre et al, 2007). These data show that London Clay exhibits pronounced stiffness anisotropy at small strain levels.…”

“…The solutions are then evaluated through comparisons with data from the Jubilee Line Extension (JLE) project, involving open-face shield tunnel construction beneath a well-instrumented site in St James Park (Nyren, 1998). This is a very well instrumented and documented case site, with extensive supporting data on cross-anisotropic stiffness parameters for London clay reported by Gasparre et al (2007). The JLE test section has been extensively analyzed by others using FE analyses and many have reported problems in predicting far-field deformations, and hence, provides an interesting opportunity to assess the capabilities of the proposed analytical solutions.…”

“…The results show that 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 both the isotropic and cross-anisotropic analytical solutions produce very good fits to the measured ground displacements. Using the high quality measurements undertaken by Gasparre et al (2007), it can indeed be concluded that cross-anisotropic stiffness parameters have only a small influence on predictions of the far-field ground deformations caused by tunneling in London Clay. The analytical solutions achieve comparable levels of agreement with measurements of the surface settlement trough that are conventionally fitted using an empirical Gaussian distribution function.…”

“…Figure 5 shows non-linear secant stiffness measurements of E v , E h and G vh from drained hollow cylinder (HCA) uniaxial load tests on natural London Clay (unit B2) as functions of strain level (Gasparre et al, 2007). The data show that London Clay is strongly anisotropic at very small strain levels (true elastic range).…”

“…More recently Gasparre et al (2007) have presented results from a comprehensive and definitive laboratory investigation of the stiffness properties of natural London Clay using block samples obtained during the excavations for Heathrow Terminal 5. Their test program included measurements of small strain elastic properties (based principally on wave propagation data using triaxial devices equipped with bender elements), limits on the reversible elastic response (referred to as the Y 1 yield condition) through drained and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4 undrained triaxial stress probe tests, and measurements of the degradation of secant stiffness parameters with strain level (using local strain measurements in triaxial and hollow cylinder devices).…”