Comportement thermo-hydro-mécanique d’un sol exposé à un cycle de gel-dégel dans le contexte de la congélation artificielle des sols

Artificial Ground Freezing (AGF) is widely implemented to enhance soil strength and reduce permeability in deep excavations. Despite its proven benefits, managing frost heave and thaw-induced deformations remains a significant challenge. Field observations from deep excavation projects reveal that even under high overburden pressures, freeze-thaw (FT) deformations can exceed project tolerances. These unexpected outcomes raise critical questions: How does applied stress interact with the freezing process, and what are the implications for the soil’s mechanical properties once it thaws?To address these questions, a temperature-controlled oedometer was specifically developed to replicate AGF boundary conditions. This device imposes a thermal gradient across silty sandy soil specimens while applying a wide range of vertical stresses, from 10 kPa, characteristic of near-surface conditions, to 4000 kPa, typical of deep excavation environments. The study investigates two key soil states: normally consolidated (NC) and overconsolidated (OC) conditions. • The behavior of NC specimens:To isolate how applied stress influences FT deformations, experiments first investigated the role of thermal parameters (freezing rate and temperature gradient) and initial soil density under low-stress conditions. The results showed that reducing the freezing rate from 1 °C/s to 0.001 °C/s increased frost heave by 44% and increasing the temperature gradient from 0.4 °C/mm to 0.65 °C/mm amplified heave by 78%. As stress increases, these thermal effects diminish, and mechanical confinement begins to dominate ice lens formation, with frost heave reducing from 34% under 30 kPa to 9% under 1000 kPa. Nevertheless, this high stress reduces but does not eliminate frost heave, indicating that cryogenic suction-driven water migration, and thus ice lens formation, remains active even under high stress. Under low stress, initial density emerges as a major driver of FT response: looser soils undergo substantial densification upon thawing, whereas denser soils experience residual loosening. Moreover, water migration is suppressed when stress is applied, reducing heave and homogenizing the post-thaw response across different densities. The applied stress dictates post-thaw performance: under low stress, the soil softens and becomes more compressible, while high stress freezing imparts a pseudo-overconsolidation effect that stiffens the soil and raises its yield stress. • The behavior of OC specimens:Motivated by the question of whether “stress history” from prior consolidation can restrain FT-induced deformations as effectively as actively applied pressure, experiments on OC specimens yielded a nuanced picture. Moderate overconsolidation (OCR 8–33) effectively limits frost heave, but extreme preloading (OCR = 133) paradoxically triggers greater ice lens growth, suggesting that microstructural changes enhance cryo-suction. Notably, post-thaw tests on these overconsolidated specimens, even those with preloading histories up to 4000 kPa, revealed a rapid loss of the elastic domain and an immediate transition to plastic deformation following the FT cycle. Overall, these results demonstrate that high overburden pressure substantially mitigates frost heave yet does not fully neutralize ice lens formation. Soil density, thermal parameters, and stress history each play pivotal roles in shaping both the magnitude of FT deformations and the soil’s post-thaw structural integrity. By clarifying these coupled thermo-hydro-mechanical processes, this PhD work offers a stress-adaptive framework for designing and optimizing AGF operations, ensuring reliable ground control not only during freezing but also in the long term after thawing