Abstract: Elucidating the development characteristics of the preferential flow within waste rock slopes is crucial for optimizing the reclamation plans and enhancing long-term stability. This study was conducted on the Hengtai Iron Mine in Fuxin City, Liaoning Province. By combining laboratory staining and tracer tests, the effects of gravel contents (65%, 75%, 85%) and infiltration volumes (90, 120, 150 mm) on preferential flow development were analyzed under the conditions of gravel particle size ranging from 2 to 7 cm. Using image processing technology, the preferential flow in gravel soil was evaluated through multiple indicators. The results showed that: (1) The gravel content had a dual regulatory effect on preferential flow development. Under the gravel content of 65%, preferential flow development was inhibited, characterized by the highest coefficient of variation in staining morphology but the lowest preferential flow ratio, length index, and total staining area ratio. As the gravel content increased, preferential flow development significantly intensified. The optimal preferential flow development characteristics were observed when gravel content reached 85%. (2) Infiltration volume was the key driving force for the development of preferential flow. Under different gravel contents, when the infiltration volume increased from 90 mm to 150 mm, both the preferential flow ratio and length index significantly increased, but the coefficient of variation of staining morphology significantly decreased (P<0.01). (3) The gravel content of 75% was a critical threshold for preferential flow development. When it was ≤75%, preferential flow development was mainly controlled by hydraulic drive, and when it was >75%, matrix action was dominant. (4) The preferential flow ratio, length index, and total dye coverage all exhibited a negative relationship with the coefficient of variation of staining morphology (P<0.01), collectively forming the comprehensive characteristics of preferential flow development. (5) Fourth-order polynomial function could accurately describe the nonlinear attenuation characteristics of preferential flows. In conclusion, a dual threshold regulation mechanism of gravel content and infiltration volume is formed during the development of preferential flow. Under the combined action of gravel content of 85% and infiltration volume of 150 mm, optimal water transport can be achieved, while 75% gravel content is the balance point for regulating hydraulic drive and matrix action.

Key words: engineering deposit, gravel soil, preferential flow, soil infiltration, staining tracer