Shear Modulus of a Carbonate Sand-Silt Mixture with THF Hydrate

The maximum shear modulus (G(max)) is an important factor determining soil deformation, and it is closely related to engineering safety and seafloor stability. In this study, a series of bender element tests was carried out to investigate the G(max) of a hydrate-bearing carbonate sand (CS)-silt mixture. The soil mixture adopted a CS:silt ratio of 1:4 by weight to mimic the fine-grained deposit of the South China Sea (SCS). Tetrahydrofuran (THF) was used to form the hydrate. Special specimen preparation procedures were adopted to form THF hydrate inside the intraparticle voids of the CS. The test results indicate that hydrate contributed to a significant part of the skeletal stiffness of the hydrate-bearing CS-silt mixture, and its G(max) at 5% hydrate saturation (S-h) was 4-6 times that of the host soil mixture. Such stiffness enhancement at a low S-h may be related to the cementation hydrate morphology. However, the G(max) of the hydrate-bearing CS-silt mixture was also sensitive to the effective stress for an S-h ranging between 5% and 31%, implying that the frame-supporting hydrate morphology also plays a key role in the skeletal stiffness of the soil mixture. Neither the existing cementation models nor the theoretical frame-supporting (i.e., Biot-Gassmann theory by Lee (BGTL)), could alone provide a satisfactory prediction of the test results. Thus, further theoretical study involving a combination of cementation and frame-supporting models is essential to understand the effects of complicated hydrate morphologies on the stiffness of soil with a substantial amount of intraparticle voids.

Automated summary

The maximum shear modulus (G(max)) is an important factor determining soil deformation and seafloor stability. This study investigated the G(max) of a hydrate-bearing sand-silt mixture and found that hydrate significantly contributed to the skeletal stiffness of the soil mixture. The study also revealed the importance of hydrate morphology in determining the stiffness of the soil mixture, which was sensitive to the effective stress.