Rani Jaafar

Pore-Scale Model for Water Retention and Fluid Partitioning of Partially Saturated Granular Soil

AbstractA model for the water retention behavior of unsaturated granular soil is developed by extending the classic bundled cylindrical capillary representation of pore space to a geometry more closely approximating that of granular porous media. Expressions for pore-scale saturation are derived as functions of matric suction for a three-dimensional unit pore comprising angular pore space bounded by spheres in simple cubic packing order. Water retention curves are modeled by assigning a statistical distribution of pore sizes optimized to best match experimentally determined retention curves. A key model attribute is its capability to capture evolution of fluid partitioning along drainage or wetting paths by differentiating pore water retained as thin films adsorbed to particle surfaces, liquid bridges retained in wedge-shaped pores, and saturated pockets in relatively small pores. Interfacial surface tension, solid-liquid contact angle, wetting direction, and mineralogy (Hamaker constant) are treated as m...

Estimating Water Retention Characteristics of Sands from Grain Size Distribution using Idealized Packing Conditions

A new approach is presented for estimating the soil-water characteristic curve (SWCC) of granular porous media along a drainage path using the relatively simple measurements of grain size distribution (GSD) and mass-volume relationships (void ratio). GSD measured using mechanical sieve analysis is converted into an equivalent population of smooth, spherical particles. Pore size distributions representing relatively loose and relatively dense packing conditions are calculated from the geometry of idealized two-dimensional unit pores formed among particle groups randomly assembled from the simulated particle population. The Young–Laplace equation is used to quantify the amount of pore water retained in the form of thin films, liquid bridges, and completely filled pockets, thus allowing the SWCC to be modeled over the entire saturation range. The measured void ratio is used to constrain the modeled SWCC via the theoretical consideration of the work done to expand air-water interfaces throughout the matrix. Nine sets of results for sands and glass beads are used to evaluate the model’s performance. SWCCs are most effectively predicted in the capillary and funicular saturation regimes (20 %

Pore-Scale Model for Estimating Saturated and Unsaturated Hydraulic Conductivity from Grain Size Distribution

AbstractAn approach is presented for predicting saturated hydraulic conductivity (ksat) and the unsaturated hydraulic conductivity function (HCF) of coarse-grained soils using pore-scale modeling of liquid configurations in idealized unit pores. Procedures are described for estimating ksat and the HCF from simple measurements of grain size distribution (GSD) obtained using mechanical sieve analysis. Measured GSD is converted into an equivalent population of spherical particles arranged to form subassemblies representing relatively loose and relatively dense particle configurations. Capillary theory and the geometry of unit pores formed within the particle subassemblies are used to quantify pore-scale liquid configurations as a function of matric suction. Corresponding hydraulic conductivity is calculated from pore-scale hydrodynamic considerations. Comparison between measured and predicted ksat for a suite of sand-sized soils demonstrates that the approach is an improvement over existing approaches, based...

Laboratory Fall Cone Testing of Unsaturated Sand

AbstractLaboratory fall cone penetration is measured as a function of saturation for four sands using equipment more commonly used to quantify plasticity and undrained strength of fine-grained soils. Relations between penetration depth and saturation display nonmonotonic behavior characteristic of the more general behavior of unsaturated sands. Penetration-saturation (P-S) relations are interpreted in three behavioral regimes: P is largest for dry sand, decreases sharply with increasing S up to residual saturation, continues to decrease slightly and approximately linearly between residual saturation and S of approximately 50 to 90%, and then increases at full saturation to a penetration value less than that under dry conditions. Observations are partially attributed to suction induced by a dilative soil rupture zone and are interpreted using an effective stress framework that links soil-water retention curves and suction-stress characteristic curves for the sands.