Experimental investigation on erosion characteristics in double-layer soil-rock mixtures considering seepage direction and layer-specific fine particle contents

Soil-rock mixtures (SRMs) widely distributed in Southwest China are prone to internal erosion under rainfall and groundwater fluctuations, a process often accompanied by pronounced heterogeneity due to stratification and complex seepage directions. This study investigates the erosion behavior of double-layer SRMs under the coupled influence of seepage direction and layer-specific fines content (FC). A series of variable-direction seepage tests was conducted on gap-graded specimens with systematically varied FC combinations in the upper and lower layers. Erosion rate, final particle loss, and failure modes were analyzed. Results demonstrate that the final erosion amount is governed by a complex interaction between seepage direction and the FC in both layers. The spatial distribution of fines critically determines vulnerability: a high FC in the lower layer consistently promotes greater erosion due to shortened particle migration paths, whereas a high FC in the upper layer reduces overall permeability, thereby diminishing the driving seepage force. The dominant factor controlling erosion shifts dynamically with the specific skeleton structure and the relative magnitudes of the influencing variables. Response Surface Methodology analysis quantitatively revealed these synergistic interactions. The findings elucidate the erosion mechanisms in stratified SRMs, providing a predictive framework for assessing erosion risk in SRM slopes.