The Influence of Geological History on Preferred Particle Orientation and the Observed Anisotropy of Over Consolidated UK Mudrocks

One of the major aspects of mudrocks which influences their engineering behaviour is anisotropy, especially that of strength and stiffness. Anisotropy is caused by an underlying preferential alignment in particle orientations. Many factors contribute to the production of enhanced preferred particle orientations including: sedimentation process; particle shapes; bioturbation; burial depth; tectonism; weathering; and aging; in short the material's entire geological history. Initial structure is formed by sedimentation processes during deposition. These are then modified by post-depositional events. With burial, the particles of sediments tend to rearrange to allow for a decrease in void ratio which is combined with an expulsion of water from the soil. Much of the southern UK is underlain by mudrocks of Mesozoic and Cenozoic age displaying some degree of preferred particle orientation and hence anisotropy in their engineering behaviour. The degree of particle orientation is quantified by analysis using environmental scanning electron microscope imagery. Anisotropy in engineering behaviour has been quantified by a range of laboratory and field, static and dynamic, methods that allow the anisotropic elastic behaviour of the mudrocks to be investigated at very small strains. Generally good agreement is observed between four fully independent methods for evaluating the elastic GVH stiffness mode. The results of both the image analysis and the laboratory testing build up a picture of microstructure anisotropy that results from the total geological history of each of the mudrocks investigated.