In most works, unsaturated soil is considered a three-phase medium composed of air, water, and solid, neglecting effects caused by the presence of residual water and/or entrapped (immobile) air.
We present a set of governing equations for unsaturated soils with residual water and entrapped air considering a mass transfer mechanism between the free air and the entrapped air. The performance of the model is examined in simulations of water retention curves, a rapid drawdown experiment as well as in the dynamic analysis of a centrifuge test on a dam subjected to earthquake loading.
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