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

The interlayer soil identified in conventional French rail track corresponds to a mixture of fine soil and coarse grains. To investigate the role of fines in the soil-water retention property of such a mixture, different coarse grain contents f v and dry densities of fine soil ρ d–f were considered. The filter paper method was applied to measure matric suction. Mercury intrusion porosimetry tests were performed to observe the microstructure of the fine soil. In terms of the gravimetric water content of fine soil w f with matric suction ψ, the soil-water retention curve (SWRC) was significantly affected by ρ d–f for ψ < 715 kPa, while independent of ρ d–f for ψ > 715 kPa. Interestingly, this threshold ψ corresponded to a delimiting pore diameter of the bi-modal microstructure of fine soil, which separated micropores from macropores. In terms of degree of saturation S r with ψ, the SWRC was significantly affected by ρ d–f in the full suction range, while it was independent of f v . These findings help to better understand the results for samples where dry density of mixture ρ d is kept constant: an increase in f v resulted in a decrease in ρ d–f and the suction changed accordingly. In this case, both f v and ψ affected the mechanical behavior.