Poroelastic modeling of soap hole formation

Sarah Reid, Rachel Lauer

Soap holes were first identified >50 years ago as areas of localized surface weakness characterized by a thin and fragile crust covering sand, silt, clay, and water. It was hypothesized that they form where groundwater is moving upward to the ground surface through unconsolidated sediment. Soap holes are ubiquitous across the prairies and manifest as either mounds or flat exteriors underlain by liquefied mud. They range in diameter from less than 1-m to several meters and can reach up to several meters in depth. Due to their thin and fragile crust, they pose a risk to farming equipment and livestock, with several farmers reporting loss of cattle and extensive portions of land that are no longer farmable. Previous work has provided hydrological and geochemical constraints to create a conceptual model for soap hole formation. In this conceptual model, pressurized water from a confined aquifer travels upward through preferential flow paths in glacial till to a lacustrine deposit at the ground surface. There, the combined effects of increased fluid pressure and clay dispersion cause the soil to liquefy and form a soap hole. This study tests the conceptual model for soap hole formation by determining which parameters and processes impact the extent and volume of liquefaction in a 3-dimensional model using a steady state solution in COMSOL Multiphysics. In COMSOL, we employ Darcy’s Law, solid mechanics, and poroelasticity to successfully approximate a simplified version of the observed field data. Variations in hydraulic, elastic, and geometric parameters were explored to determine their impact on the volume of liquefaction in the model. The results provide insight into the conditions required for soap hole formation, and serve to verify the conceptual model developed through field studies.