Ultrahigh static pore pressure effects on the undrained shear behavior of deep-sea cohesive soil

Due to the unique ultrahigh water stress environment, the physical and mechanical behavior of deep-sea sediments is significantly different from those of offshore and terrestrial soils. However, the effect of water depth-induced ultrahigh static pore pressure on the mechanical properties of deep-sea soils has rarely been considered. Here, a series of consolidated undrained (CU) shear tests on cohesive soil from the deep South China Sea were conducted to investigate the ultrahigh static pore pressure effect on undrained shear behavior. The test results confirmed that there is a slight deviation in Terzaghi's expression of effective stress under ultrahigh static pore pressure stress conditions. The soil strength will be underestimated without considering the real static pore pressure condition in the deep-sea environment. A distinction is made between the external pressure-induced and pore pressure-induced contact stress between soil particles, providing a good understanding of the skeleton stress that determines soil strength. Then, the mechanism of ultrahigh static pore pressure on the shear strength of cohesive soil is discussed. Additionally, to estimate the effect of the water depth-induced pore pressure, a strength-related parameter η is introduced to the modified strength expression. Based on the Mohr–Coulomb criterion and test results, a novel method to determine the value of η is proposed. The research results herein could help provide deeper insight into the mechanical properties of deep-sea cohesive soil.