Temperature-dependent undrained behaviour of sand in an ultra-deep marine environment

Deep-sea mining operations now exceed water depths of 3000 m, necessitating a thorough understanding of the role of low temperature of the seabed (approximately 5°C) in sand behaviour under ultra-high hydrostatic pressure. This study presents the first systematic experimental investigation into the effects of temperature on the undrained behaviour of granular materials in an ultra-deep marine environment, employing a high-pressure triaxial apparatus designed for precise effective stress control. Strain-controlled monotonic and cyclic undrained triaxial tests were conducted on Fujian sand under a temperature (T) range of 5–55°C and a back-pressure (bp) of 29 MPa. Comparative experiments were conducted under conventional back-pressure (bp = 300 kPa) to isolate the temperature effects from those observed in terrestrial environments. Results revealed that specimens exposed to lower temperatures exhibit significantly larger and more rapid positive excess pore pressure (u) during both undrained monotonic and cyclic shearing. Notably, this temperature effect is nearly independent of bp, although specimens subjected to ultra-high bp demonstrate a higher u and reduced undrained strength. Furthermore, the critical state line in the p′–q plane remains unique, independent of temperature and back-pressure. In addition, a framework for analysing the state-dependent mechanical properties of sand was established considering varying T and bp conditions.