Influence of salinity on the behaviour of frozen soils

Salinity strongly affects the freezing and mechanical behaviour of soils, but systematic studies combining thermodynamic interpretation with strength testing are limited. This study examines the influence of NaCl on the freezing process and unconfined compressive strength of saturated sands. Freezing curves were recorded at –10 °C and –25 °C, and unfrozen water content was measured using nuclear magnetic resonance (NMR). Results show that increasing salinity lowers both the supercooling temperature and the equilibrium freezing point in accordance with the NaCl–H₂O phase diagram. At –25 °C, a second plateau in the freezing curves was observed, reflecting eutectic solidification of residual brine. Even small salt additions caused disproportionate strength reductions: at –10 °C, 0.5% NaCl reduced strength to about one-quarter of the salt-free value. At eutectic conditions, strength partially recovered as brine solidified, though the ice–soil skeleton incorporated salt crystals, producing a response distinct from fresh water soils. NMR tests confirmed a near-linear increase of unfrozen water with salinity, while the strength–liquid relation was strongly nonlinear. The novelty of this work lies in combining freezing curve analysis with direct strength testing of saline sands.