Water permeability, water retention and microstructure of unsaturated compacted Boom clay

Three classes of experiments are considered in this paper to provide information for two artificially prepared Boom clay fabrics: mercury intrusion/extrusion tests; main wetting/drying paths; and water inflow/outflow transient (permeability) tests. These tests, which are usually treated separately, are joined in a common reference frame to provide information about the morphology of the porous medium and factors influencing Boom clay unsaturated hydraulic states with reference to water retention curves and relative water permeability values. The main objective is to interpret mercury intrusion porosimetry results in order to define an entrance pore size region at ca 130 to 180 nm separating intra-aggregate and inter-aggregate zones. This pore size region is further associated to a delimiting zone in the retention curve separating regions of ‘intra-aggregate governing suction’ at gravimetric water contents lower than 13–15% (gravimetric water content is not affected by mechanical effects) and ‘inter-aggregate governing suction’ (gravimetric water content is sensible to mechanical actions). This water content is further used to define a threshold zone around a relative water permeability of kw/kws=0.01 delimiting a zone of greater water relative permeability from others that present a restricted flow in a generalised Darcian sense. All these results are consistent with the existence of two main pore size regions: an intra-aggregate porosity with quasi-immobile water that is little affected by loading processes and an inter-aggregate porosity for which the loading mechanism results in a reduction of interconnected macropores affecting free water. Testing results show that intra-aggregate water represents between 54 and 59% of the total volume of water in soil in a low-porosity packing compacted at a dry unit weight of 16.7 kN m−3, whereas it corresponds to ca 28 and 38% in the case of a high-porosity packing compacted at a dry unit weight of 13.7 kN m−3.