Soil-water retention behaviour of fine/coarse soil mixture with varying coarse grain contents and fine soil dry densities

An interlayer soil identified in the French conventional rail track corresponded to a mixture of fine soil and coarse grains. To investigate the role of fines in the soil-water retention property of such mixture, different coarse grain contents fv and dry densities of fine soil ρd-f were considered. The filter paper method was applied to measure the matrix suction. Mercury intrusion porosimetry tests were performed for the microstructure observation of fine soil. In terms of gravimetric water content of fine soil wf with matrix suction Ψ, the soil-water retention curve (SWRC) was significantly affected by ρd-f for Ψ < 715 kPa, while independent of ρd-f for Ψ > 715 kPa. Interestingly, this threshold Ψ corresponded to a delimiting pore diameter of bi-modal microstructure of fine soil, which separated micro-pores from macro-pores. In terms of degree of saturation Sr with Ψ, the SWRC was significantly affected by ρd-f in the full suction range, while independent of fv. These findings help better understand the results on samples with the dry density of mixture ρd kept constant: an increase of fv resulted in a decrease of ρd-f and the suction changed accordingly. In that case, both fv and Ψ affected the mechanical behavior.