Mechanical behavior of unsaturated frozen soil-rock mixtures: Insights from cryogenic triaxial tests

Frozen soil-rock mixtures (FSRMs), characterized by a mixture of soil matrix and rock aggregates in a frozen state, are widely distributed in cold high mountain and plateau regions. Understanding their mechanical behavior is vital for assessing the stability of geotechnical structures and geological hazards in these areas. This paper presents a comprehensive experimental investigation into the mechanical properties of unsaturated FSRMs through a series of drained cryogenic triaxial tests. Compacted specimens with varying rock block contents (10%, 30%, 50%, and 70%) were tested at different freezing temperatures (–2 °C, –5 °C, –10 °C, and –15 °C). The analysis focuses on stress-strain relationships, volumetric characteristics, and shear strength envelopes. The effects of freezing temperature and rock block content on the evolution of strength indexes and grain size distribution were elucidated. A comparative analysis between ‘peak’ and ‘large deformation’ strength was provided, alongside an examination of three characteristic stress ratios (end-of-testing, peak, and dilatancy stress ratios). Furthermore, aspects crucial for constitutive modeling, such as the attainment of critical states and stress-dilatancy relationships, were discussed. The results highlight that lower temperatures and lower rock block contents enhance dilatancy and strain-softening under low confinements, whereas high confining pressure and high rock content promote particle breakage and ductile behavior.