Mechanical Characteristics of Methane Hydrate-Bearing Clayey Silt Containing Carbonate Sand and the Modified Duncan–Chang Model

The mechanical characteristics of hydrate-bearing reservoirs are essential for understanding reservoir stability and the security of geological engineering. To mimic the methane hydrate-bearing sediments in the South China Sea (SCS), a given weight of carbonate sand (CS) was saturated in airless water and then mixed with quartz silt and kaolin. The excess-gas method was used to form methane hydrate-bearing specimens (MHBS). A series of consolidated drained triaxial tests were conducted to investigate the effects of effective confining stress (σ3’), hydrate saturation (Sh), and clay content (CC) on the stress–strain behavior of MHBS. The results showed that the elastic modulus (Ei), peak strength (qf), and axial strain corresponding to peak strength (εam) increased with the increase of σ3’, while the strain-softening phenomenon decreased. This is attributed to the increase of the interlocking effect and friction among soil particles, hindering soil particles’ movement and rearrangement. In addition, with the cementation effect of hydrate, increasing Sh led to increased Ei and qf, while εam decreased and the strain-softening phenomenon intensified since the brittle hydrate broke, resulting in unstable movement and rearrangement of soil particles. However, with increased CC, Ei and qf decreased, εam increased, and the strain-softening phenomenon diminished, which is attributed to its plasticity and lubricating effect on interlocking and friction between particles. Furthermore, a modified Duncan–Chang model considering strain-softening was proposed and verified, in which the effect of CC on hydrate cementation was taken into consideration.