Mudrocks (or stiff clays) cover much of south-east England. These materials were deposited from the Triassic to the Eocene in various marine environments. They have been
submitted to complex sedimentation and post-sedimentation processes, resulting in the
development of a wide range of micro and macro structures. Despite their engineering
signi cance, advanced experimental investigations of their properties are limited. From
2007 to 2012, an Imperial College research project investigated the structure and mechanical properties of British mudrocks with the help of advanced laboratory testing.
As part of this project, the research presented here focused on the stiffness and shear
strength anisotropy of mudrocks in relation to their structure. Three UK mudrocks,
deposited from the Jurassic to the Cretaceous, were selected for this project: Oxford
clay, Kimmeridge clay and Gault clay.
High quality block and rotary cored samples were tested in two Hollow Cylinder
Apparatus (HCA), which control independently four stress parameters, among which
the intermediate principal stress factor (b) and the orientation of the major principal
stress (α ). The present study concentrated on the effects of α on the stiffness and
strength properties. A set of six tests was performed on each soil: specimens were re-
consolidated to their estimated in-situ stresses before being sheared undrained at various α values while controlling b. The results were interpreted together, taking account of
an earlier comprehensive study on London Clay (Nishimura, 2006; Anh-Minh, 2007).
The value was found to affect both the strength and stiffness properties, at various degrees for each mudrock. While the considered constitutive model influences the
recorded degree of shear strength anisotropy, a good correlation was observed between
the latter and the micro-structure anisotropy, as quantified by Wilkinson (2011)'s associated work. Qualitative assessment also revealed the strong influence of the macro-
structure on the strength anisotropy.