Reassessing variations in the small-strain stiffness of London Clay

High-quality rotary core samples of natural London Clay were taken during installation of subsurface instrumentation to monitor the ground response at Hyde Park, London, to the construction of Crossrail tunnels. An extensive programme of advanced triaxial tests was performed on specimens from London Clay divisions A3 to C. Detailed analysis of their response to undrained shearing in compression from their estimated in-situ stress state provides new insights into the magnitude and changes in stiffness at small- and medium-strain levels. A consistent trend in the variation of small-strain stiffness with depth and lithological division is established for the Hyde Park research site. Differences between the stiffness magnitudes from this investigation and those reported earlier by others are shown to be a result of the conditions before and during undrained shearing, particularly the ratio of shearing axial strain-rate to the preceding creep strain-rate. This provides an explanation for significant discrepancies in small-strain stiffness values reported in recent years, which have led to uncertainties regarding which values to adopt, particularly for numerical analysis applications. Understanding the relevance of the axial shearing to creep rate ratio allows for improved triaxial testing techniques and the subsequent interpretation of the test data.