One-dimensional compression of a natural clay: structural changes and mechanical effects

Part of a research programme concerning the influence of structure on the mechanics of clays is presented. The material is a stiff overconsolidated natural clay, strongly bonded by diagenetic processes. Its one-dimensional compression and consequential structural changes are compared with those of the same clay when reconstituted. The structural changes are evaluated using scanning electron microscopy. The results of oedometer tests are discussed, some carried out in a purpose-built constant-rate-of-strain oedometer which enables the behaviour of the natural clay to be studied well beyond yield. The variations in swelling index, stiffness and coefficients of consolidation and permeability are compared pre- and post-yield for the two clays. Both clays are found to have significantly different states and fabrics at all stages of compression. Post-yield the structural changes in the natural clay are initially a destruction of the original fabric, and then a rearrangement of the clay particles into a systematic packing of honeycomb and perfectly oriented fabrics, in contrast to the reconstituted clay in which a random fabric develops. Despite the very different structures of the two clays, the trends of compression and consolidation behaviour are similar.