  The water retention curve (WRC) defines the relationship between matric
suction and saturation and is a key function for determining the
hydro-mechanical behavior of unsaturated soils. We investigate possible
microscopic origins of the water retention behavior of granular soils using
both Computed Tomography (CT) experiment and multiphase lattice Boltzmann
Method (LBM). We conduct a CT experiment on Hamburg sand to obtain its WRC and
then run LBM simulations based on the CT grain skeleton. The multiphase LBM
simulations capture the hysteresis and pore-scale behaviors of WRC observed in
the CT experiment. Using LBM, we observe that the spatial distribution and
morphology of gas clusters varies between drainage and imbibition paths and is
the underlying source of the hysteresis. During drainage, gas clusters
congregate at the grain surface; the local suction increases when gas clusters
enter through small pore openings and decreases when gas clusters enter through
large pore openings. Whereas, during imbibition, gas clusters disperse in the
liquid; the local suction decreases uniformly. Large pores empty first during
drainage and small pores fill first during imbibition. The pore-based WRC shows
that an increase in pore size causes a decrease in suction during drainage and
imbibition, and an increase in hysteresis.
