Bimodal water retention curves and segmented relative permeabilities of municipal solid waste

The water retention curve (WRC) and relative permeability are of great importance for performing saturated–unsaturated seepage analysis in soils. The bimodal WRC can better reflect the difference in water retention capacity between macropores and micropores in dual-porosity media compared with the unimodal WRC. Traditional testing methods cannot effectively measure the macropore region in municipal solid waste (MSW) as water is discharged rapidly under gravity. In this study, an implementation framework for determining the bimodal WRCs and segmented relative permeabilities of MSW was proposed, and it was applied on the synthetic sample under sequential levels of overlying stresses. First, a calculation method for dividing the size boundaries of macropores and micropores was proposed from the perspective of energy analysis. Then, the computed tomography scanning combined with the maximal inscribed spheres algorithm was used to obtain the WRC data points for the macropore region, while the traditional pressure plate test was used to obtain the WRC data points for the micropore region. Finally, a modified Van Genuchten model was proposed to fit these data points to yield the bimodal WRCs, and the segmented relative permeabilities were further obtained. In addition, the bimodal probability density curves of pore-size distribution were obtained.