ABSTRACT Suffusion is a typical form of internal erosion and a primary cause of damage to hydraulic or underground structures. The initiation and progression of suffusion vary significantly with seepage direction. In this study, a novel permeameter featuring an adjustable seepage direction and a polydimethylsiloxane (PDMS) membrane lining is developed. This design eliminates interface leakage, ensuring high reproducibility and enabling clear visualisation of direction‐dependent particle migration. Suffusion tests are conducted in gap‐graded sand under varying seepage directions using the newly developed apparatus. The results demonstrate the apparatus's feasibility and consistency with published literature. The findings reveal that the suffusion process can be divided into three distinct phases: self‐filtration, initial erosion, and extensive erosion. The identification of the self‐filtration stage is crucial for understanding the initial water–soil interactions involved in suffusion. Furthermore, multi‐point monitoring and image analysis reveal significant spatial heterogeneity induced by inclined seepage. An ‘upper‐channelling and lower‐clogging’ pattern emerges, where gravitational deposition densifies the lower region while preferential channels form above. Quantitatively, critical hydraulic gradients increase monotonically with the angle between seepage and gravity directions, indicating enhanced resistance to suffusion due to altered force equilibrium and localised densification. The proposed apparatus provides a reliable platform for unravelling the coupling mechanisms between particle migration and hydraulic response under complex hydraulic conditions.
Chi et al. (Wed,) studied this question.