Experimental investigation of the influence of seepage direction on internal erosion characteristics in gap-graded soils

This study investigates the influence of seepage direction on internal erosion in gap-graded soils through a series of hydraulic tests conducted using a newly developed permeameter capable of adjusting the seepage direction. The erosion process is qualitatively captured through visual observations during testing and post-test inspections, while quantitative analysis is performed using data from multiple pore pressure transducers. The results reveal a nonlinear relationship between the critical hydraulic gradient and seepage direction. As the flow shifts from horizontal to vertically upward, the critical hydraulic gradient initially increases and then decreases. Meanwhile, the erosion pattern evolves from a non-uniform distribution to a more uniform one across the soil cross-section. The localized critical hydraulic gradient, indicative of early-stage instability, is found to be 40%-60% lower than the global critical hydraulic gradient. Theoretical predictions further validate the observed nonlinear trend. These findings provide new insights into the mechanisms of seepage-induced failure and offer a refined approach for identifying vulnerable zones in geotechnical structures, contributing to improved design and risk mitigation under varying seepage conditions.