Influence of salinity on the unconfined compressive strength of sands at different freezing temperatures

Abstract

Salinity modifies the freezing behaviour of soils and can strongly affect the strength of frozen ground, yet the coupled relationships between phase behaviour, unfrozen water and mechanical response remain insufficiently documented. This study investigates the influence of NaCl on the freezing behaviour and unconfined compressive strength of fully saturated, narrow-graded sand. Freezing curves were recorded and interpreted together with nuclear magnetic resonance measurements of unfrozen water content and unconfined compression tests at -10 °C and -25 °C, representing conditions above and below the eutectic temperature of the NaCl-H2O system.

Increasing NaCl concentration lowered both the supercooling temperature and the freezing temperature. At -10 °C, the unfrozen water content increased almost linearly with salinity, and even a small NaCl addition caused a disproportionate loss of strength: 0.5% NaCl reduced peak strength to about one quarter of the salt-free reference value. At -25 °C, the freezing curves exhibited a second plateau in saline specimens, consistent with eutectic solidification of residual brine. Under these conditions, the strength response differed qualitatively from that at -10 °C, showing a local plateau or partial recovery at low salinity before decreasing again at higher NaCl contents. Comparison between measured unfrozen water contents and a corrected lever-rule estimate showed good agreement up to about 5.0% NaCl for -10 °C.

The results provide a controlled thermo-mechanical dataset linking phase behaviour, unfrozen water and strength in saline frozen sand. The results show that salinity must be treated as a primary design variable in frozen ground. Above the eutectic, even low NaCl contents can severely reduce strength, whereas below the eutectic the phase assemblage and mechanical response change fundamentally, producing a non-monotonic strength trend that requires further microstructural investigation.