Analysis of the micro to macro response of clays to compression

An investigation of clay microstructure and its evolution under one-dimensional (1D) and isotropic compression is presented for different clays. Data from the literature are compared to original results on two Italian clays, obtained using scanning electron microscopy, image processing, mercury intrusion porosimetry and on-purpose swelling tests. The effects of composition and loading history on clay microstructure, as well as its changes along the compression path (pre- and post-gross yielding) are analysed and a conceptual model of microstructure evolution is proposed for the clays under study. Normally consolidated clays at early virgin compression, either natural or reconstituted, are found to possess an open fabric of random to low orientation, complying with a prevailing inter-aggregate and a smaller intra-aggregate porosity, whose size and distribution depend on composition. Under 1D compression, either in the field or in the laboratory, the inter-aggregate porosity is lost, at a rate dependent on composition and loading history, and the dominant intra-aggregate micropore is progressively reduced. Accordingly, perfectly oriented stacks of domains are recognised which, however, embed preserved random particle arrangements even at large pressures, resulting in an increase of average orientation up to the reach of a steady orientation degree. Isotropic compression causes faster microstructure evolution, although large pressures are required to change 1D-induced fabric orientation.