Studying the behavior of unsaturated soils using pore-scale numerical modeling with the multiphase lattice Boltzmann method




Hosseini, Reihaneh

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Advancements in unsaturated soil mechanics have historically lagged behind saturated soil mechanics, due to the complexities associated with the behavior of unsaturated soils. The presence of two fluid phases in the pore space of unsaturated soils gives rise to varying phase distributions and interfacial interactions, which play an important role in the mechanical and hydraulic behavior of the soil. Fundamental understanding of the behavior of unsaturated soils requires an understanding of the phase distributions at the pore scale. This dissertation presents early work on using pore-scale numerical modeling for studying the behavior of unsaturated soil. A code based on the multiphase lattice Boltzmann method (LBM) is developed for this purpose. Using this tool, two fundamental investigations are carried out. The first investigation is on the underlying source of soil-water characteristic curve (SWCC) hysteresis. Primary drainage and imbibition simulations are performed for a granular soil specimen. It is found that even in the absence of well-known contributors to SWCC hysteresis, such as contact angle hysteresis and air entrapment, SWCC is still hysteretic. The underlying source of this hysteresis is found by comparing phase distributions during drainage and imbibition. It is revealed that during imbibition narrow throats are filled by capillary condensation, while during drainage these narrow throats have to be emptied by high-curvature menisci pushing through them, therefore, requiring a higher suction at the same degree of saturation. The second investigation is on the hysteresis of the effective stress parameter as a function of degree of saturation. The effective stress parameter is measured at the base of a granular soil column during primary drainage and secondary imbibition. The independent contributions of suction and surface tension forces to the effective stress parameter are monitored throughout the simulation. It is found that the contribution of suction forces to the effective stress parameter is slightly lower during imbibition compared to drainage, however, the contribution of surface tension forces is much higher, resulting in a larger effective stress parameter during imbibition. The reason for this behavior is linked back to the differences between the phase distributions during imbibition and drainage.


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