Development of a novel high-pressure triaxial apparatus and investigation of the undrained behavior of sand in ultra-deep sea environment

Deep-sea mining is typically conducted on the seabed with water-depths exceeding several thousand meters, necessitating accurate understanding of soil mechanical behavior under ultra-high back pressure (bp) while preserving conventional effective stress states. However, current high-pressure triaxial systems fail to achieve sufficient measurement accuracy for effective stress, and investigations on undrained behavior of sand under ultra-high back pressure remain scarce. To overcome this fundamental constraint, we propose a testing method that incorporates an auxiliary high-precision differential pressure sensor into high-pressure devices, enabling direct measurement of effective stress and substantially improving measurement accuracy. Moreover, the principles of calibration procedures and additional techniques to further enhance accuracy are demonstrated. To isolate the role of bp, undrained monotonic and cyclic tests are performed on quartz sand under bp ranging from 300kPa to 29MPa utilizing this enhanced high-pressure triaxial apparatus. The results demonstrate that bp variations neither alter the slope of the critical state line in p'-q space nor significantly affect the evolution of effective stress ratios during shear, confirming the validity of the effective stress-based mechanical principle under ultra-high hydrostatic pressure. Furthermore, an energy-based analytical framework reveals a unique relationship between normalized energy dissipation and pore pressure evolution during cyclic loading regardless of bp.