Permeability Evolution and Fine-Particle Transport in Gap-Graded Sand under a Cyclic Hydraulic Gradient due to Storm Surge and Waves

Abstract Internal erosion has been widely observed in geomaterials, resulting in many instability problems. However, there is little experimental research concerning mechanisms of internal erosion under oscillating hydraulic gradient resulting from storm surges and waves, which can be induced by extreme events and climate change. A series of laboratory seepage experiments on gap-graded sand specimens was conducted under a compound seepage mode including monotonic and sinusoidal cyclic hydraulic gradients, which were characterized by real-time permeability measurements and particle image velocimetry (PIV). The results show that a cyclic hydraulic gradient can initiate unclogging in previously clogged specimens and erosion in unstable specimens without reaching the critical hydraulic gradient. Increasing the inflow loading frequency from 0.005 to 0.02 Hz resulted in an increase in the final permeability, with all changing patterns affected by cyclic pore-throat narrowing and pore-throat reopening. The permeability evolution, fine-particle PIV, and fine-particle distribution showed good agreement, indicating that the effects of unclogging and erosion decrease with the progression of inflow loading cycles. Recent developments in laboratory testing and imaging analysis were combined to offer a comprehensive understanding of permeability evolution in gap-graded sand, which is relevant to climate change mitigation and adaptation of geotechnical structures.