Strain rate effects on intact deep-sea sediments under different confining pressures and overconsolidation ratios

Abstract

Underwater construction has gone to greater depths below the sea level for the extraction of resources such as oil and minerals. During different construction stages of deep-sea structures, different loading rates may be applied to deep-sea sediments. Generally, the strength of soil usually increases with the loading rate, and the sensitivity to strain rate effects may be more pronounced in deep-sea sediments. Previously, the strain rate effects were mainly studied on shallow marine deposits, but rarely on deep-sea sediments due to the difficulty of sampling. In this study, the strain rate effects on intact deep-sea sediments were investigated by a series of undrained triaxial tests with different strain rates of 0.5%/h, 2.6%/h and 13.2%/h under normally and over-consolidated conditions. In addition, scanning electron microscopy (SEM), nitrogen adsorption (NA) and mercury intrusion porosimetry (MIP) tests were conducted to illustrate the unique microstructure and pore size distributions of the deep-sea sediments, which showed a flocculent structure with abundant mesopores and macropores. It is found that higher strain rate leads to higher undrained shear strength due to the delay of increase of excess pore pressure as a result of low permeability. Strain rate effects are more obvious with the increase of confining pressure and then become less significant when the confining pressure is increased further. This change of trend is attributed to the increase of particle contact and more particle rearrangements when the confining pressure is first increased, but the particle rearrangement is suppressed under higher confining pressure. The sensitivity to strain rate effects increases with the overconsolidation ratio as more particle rearrangement is enabled under less confinement.