A microstructural insight into compacted clayey soils and their hydraulic properties

• Exhaustive review to highlight microstructural effects on hydraulic properties
• New data on compacted clayey soils on the dry side with dominant bimodal PSD
• Microstructural model used to explain the evolution of hydraulic properties
• Effects on hydraulic properties were successfully tracked with proposed model
• Pattern of variation of microstructural state variables in compaction plane

A review of the literature is initially presented to bring into light the important microstructural effects on the hydraulic properties of different compacted clayey soils. Experimental data coming from microstructural and macroscopic studies on different compacted clayey soils with dominant multi-modal pore size distribution are analysed to provide a comprehensive picture of different phenomenological features of hydraulic soil behaviour. The data come from clayey soils compacted on the dry side (i.e., with an intrinsic and permanent aggregated structure) or alternatively with dominant coarse fraction (i.e., sand–bentonite mixture with shielding skeleton and well-developed inter-grain porosity), which undergo important microstructural changes on wetting and drying. A microstructural model, already developed to take into account microstructural aspects on water retention curves, is used to plot in the Proctor plane the microstructure set up by compaction and its evolution along hydraulic paths. The model is also used to explain the evolution of hydraulic properties (water and air permeability, water retention) along wetting and drying paths on an artificially prepared mixture of sand and bentonite with dominant granular fraction. The bentonite strongly reacts to changes in water content and allows studying the transition from a granular soil with large pores between sand grains to a low-permeability material on wetting. The key point of this simple model is the introduction of the dependence of the microvoid volume (admitting saturated aggregates) on water content, following an equivalent behavioural response to the macroscopic shrinkage curve.