Application of Time Domain Reflectometry to Triaxial Shear Tests on Hydrate-Bearing Sediments

Abstract

Dissociation of natural gas hydrates due to natural and artificial perturbations can induce or trigger submarine landslides, posing catastrophic risks to offshore facilities and coastal cities. The risk assessment of these geological hazards basically requires to fully understand mechanical properties of hydrate-bearing sediments which are widely quantified by performing triaxial shear tests. However, the key factor, hydrate saturation, has not been well measured. This study aims to show how a time domain reflectometry (TDR) method nondestructively measures hydrate saturation for triaxial shear tests on methane-hydrate-bearing sands. Hydrate saturation is determined based on the difference of volumetric water contents measured by using a pair of flexible TDR probes after calibration. The results suggest that apparent dielectric constant of hydrate-bearing sands increases with increasing volumetric water content, and this evolution can be depicted by polynomial and weighted-average equations. Values of hydrate saturation acquired by the TDR method are accurate, and the flexible TDR probes can avoid any reinforcement effects on triaxial shearing properties of test specimens. Both the peak strength and the secant modulus of hydrate-bearing sands increase with the presence of methane hydrate and increasing effective confining pressure. Strain-softening becomes more obvious during triaxial shearing when hydrate saturation and effective confining pressure are high. The flexible TDR probes developed in this study have a great potential to dominate the real-time measurement of hydrate saturation within remodeled unconsolidated sediments in the future.

Keywords: Gas hydrate, hydrate saturation, volumetric water content, mechanical property, Geological hazard