a b s t r a c tThe behaviour of soil, and in particular compacted clay fill, can have significant implications on the safe and reliable operation of man-made infrastructure. The mechanical behaviour of soil (e.g. volume change and shear strength) is widely recognised as being associated with the microstructural arrangement (fabric/structure). In the case of high plasticity clays, despite the large amount of research carried out, soil microstructure and its evolution along mechanical and hydraulic paths are still not well understood. This makes incorporation of microstructural analysis difficult in engineering practice and highlights the need for further research. A comprehensive microstructural analysis of Maryland clay, a high plasticity residual soil, based on mercury intrusion porosimetry tests, is presented in this paper. Experimental results obtained from undisturbed, reconstituted and compacted specimens subjected to different hydraulic and mechanical paths are described. As with mechanical investigations, the reconstituted state is proposed to be used routinely as a reference state for comparison of undisturbed and compacted soil. The microstructural evolution of the compacted clay, prepared on the wet side of standard Proctor optimum water content, with an initially high void ratio, is examined along the main drying path. Importantly, a monotonic suction increase from the as-compacted state is shown to have negligible effect on the distribution of macro-pores. However, a new insight is provided based on the evolution of the dominant micro-pore entrance diameter which is shown to reduce with increased suction. This micro-pore entrance diameter is shown to correspond with the theoretical suction back-calculated from a simple capillary tube model, up to a limit. It is observed that, under oedometric conditions, the as-compacted microstructure is erased during saturation (soaking) and resembles the reconstituted microstructure. For this particular material and preparation conditions, it is demonstrated that a bimodal microstructure is not recovered on drying from a saturated state.

The microstructure of compacted clay plays an important role in its hydraulic and mechanical behaviour. However, little experimental evidence is available for high-plasticity clay compacted on the wet side of the standard Proctor optimum water content. The results of an oedometric and microstructural investigation (mercury intrusion porosimetry) on reconstituted and compacted highplasticity clay are presented. The microstructural results indicate that a bimodal pore size density function is not limited to clays compacted on the dry side of the standard Proctor optimum. The compacted clay prepared on the wet side can display a bimodal pore size distribution dependent on the level of compaction, current void ratio and degree of saturation. The bimodal distribution of the compacted clay can be lost by compression to high vertical stress accompanied by an increase in the degree of saturation. In this case the compression path ultimately approaches the normal compression line of the reconstituted saturated material. The results demonstrate that the observed behaviour of the compacted clay can be interpreted with reference to the reconstituted clay.

This note presents a novel approach for controlling the degree of saturation during onedimensional compression of unsaturated soils. This technique offers a simple and versatile way to study the hydro-mechanical response of unsaturated soils as well as the unsaturatedsaturated soil transition. By using a multi-stage approach, the same specimen can be used to evaluate the compressibility of unsaturated specimens at different degrees of saturation which may reduce the long testing periods commonly required for unsaturated soil testing. The experimental results described in this paper shows that the proposed technique is capable of controlling the degree of saturation within reasonable limits and provides an interesting approach to analyse the coupled hydraulic and mechanical behaviour of unsaturated soils.

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