Pore water pressure evolution below a freezing front under saturated conditions: Large-scale laboratory experiment and numerical investigation

Subpermafrost aquifer hydrodynamics is generally poorly known due to monitoring technology issues. The few available data show that this aquifer is confined below continuous permafrost due to ice expansion. We conducted a 2 m x 1 m x 1 m sand box experiment under controlled conditions in a cold room to (1) evaluate the confinement of the unfrozen part of a saturated porous medium below a propagating freezing/thawing front, (2) assess the associated uplift of the soil surface, and therefore (3) quantify how the ice expansion translates into frost heave and excess pore-water pressure in the unfrozen part below the freezing soil. Pore water pressure, soil temperature and soil heave were monitored inside the sand box during a 70-day freeze-thaw cycle. A transient fully coupled heat transport and water flow model (called Ginette) was developed to reproduce the freeze-thaw experiment numerically. It takes into account excess pore-water pressure related to pore water phase changes and uses a simple hydro-mechanical term based on the storage coefficient to estimate soil heave. Fairly good agreement was obtained between measured and simulated pressure heads in the unfrozen part below the freezing front over time. Both experimental and numerical approaches show that the ice expansion is translated into excess pore-water pressure (maximum pore water pressure: 5.5 m) and frost heave (2.2 cm).