Membrane penetration reduction technique for undrained cyclic torsional shear tests on gravelly soils

This study investigates the liquefaction behavior of gravelly soils using undrained torsional shear tests, with a focus on mitigating membrane penetration effects. A novel approach is proposed involving a 4-mm-thick sand coating (the membrane-penetration-reduction or MPR layer) applied to both inner and outer surfaces of hollow cylindrical specimens. All specimens were prepared at a target relative density (Dr) of 50%. Comparative tests showed that specimens without the MPR layer substantially overestimated liquefaction resistance due to membrane penetration. The MPR layer effectively reduced this bias, providing more reliable measurements of excess pore water pressure generation and deformation. Furthermore, the effect of gravel content (GC = 0–70%) on liquefaction resistance was examined using specimens equipped with the MPR layer. While an increase in GC enhanced liquefaction resistance, an energy-based analysis revealed that the cumulative dissipated energy required for significant pore pressure generation was largely independent of GC. However, for a given amount of dissipated energy, shear strain accumulation was markedly lower at higher GCs.