Visualized liquefaction behavior of sandy soil deposited in water under undrained cyclic shearing

Water films probably form beneath less-permeable interlayers in liquefiable deposits during an earthquake, further resulting in devastating disasters. However, the development of such water film and its effect on the liquefaction behavior of stratified deposits are still unclear. Herein, a water sedimentation method was developed in laboratory to reproduce the layered structure of sandy soil deposited in water during land reclamation. Undrained cyclic torsional shear tests were conducted to visualize the evolution of localized liquefaction behavior incorporating an image-based measurement technique. The test results indicate that the liquefaction resistance of layered specimens with less-permeable fine layers was similar to that of specimens without fine layer, which was possibly due to the lower liquefaction resistance of the sand layer compared to the fine layer. Large local strains periodically observed at the bottom of fine layer during undrained cyclic shearing implied that pore water concentrated into thin zones, which was also directly proven by the differential pore pressures in the sand and fine layers within the layered specimen. Moreover, large local strains also manifested a relatively lower liquefaction resistance of specimen compared to that evaluated by global strains. As specimen was closely liquefied, a hydraulic gradient significantly higher than its critical value to drive upward particle migration was formed at the fine layer, which overcomes the resultant force of gravity and buoyancy acting on soil particle. This hydraulic gradient and the less-permeable fine layer provided the formation conditions of a subjacent water film.