Small-strain shear modulus of fine-grained gassysoil under different gas content and initial pore water pressure

• Initial pore water pressure has different effects on ESP, shear strength and moduli • Gas content reduces mobilized strength in shearing, instead of peak strength • Point ε=5% marks max pore water pressure, min modulus, precedes strength peaks • Unique power law links small-strain modulus to secant shear modulus under shear The fine-grained gassy soils are prevalent in coastal regions worldwide. The inadequate knownledge of their mechanics, leads to engineering geological issues such as seabed landslides, amid marine and offshore advancements. Through a series of triaxial tests combined with bender elements, this study investigated the stress-strain behavior of fine-grained gassy soil with varying initial gas contents and pore water pressures, along with the variations in the small-strain shear modulus during shearing, thereby facilitating a better understanding of gassy soils mechanics in situ exploration. Our findings show the gassy soils at initial pore water pressure of 150 kPa resemble saturated soils in the stress-strain behaviors, but differ in small-strain moduli. A distinctive inflection point at 5% strain signifies peak pore pressure and valley shear modulus, precedes strength and strain peaks. Additionally, there is an unique power relationship between the small-strain modulus and the secant shear modulus during shearing.