Abstract: The current internal stability criterion of existing soil fails to accurately describe the phenomenon of unsteady seepage in widely-graded gravel soil in both laboratory tests and engineering practice. This paper proposes a modified Istonima criterion by using porosity (n) based on the abrupt variations in sample flow velocity and cumulative erosion rate measured from experimental design of microscopic observation on fine particle movement. The results demonstrate the following: (1) The active state of fine particles increases with a higher uneven coefficient (C_u) and porosity (n), exhibiting three distinct stages of stability, transition, and instability based on their motion characteristics. (2) The flow velocity undergoes a nonlinear process as the hydraulic gradient decreases initially and then increases. The abrupt change point of seepage is identified as the critical point of seepage compaction and seepage erosion. The sensitivity of each factor to the cumulative erosion rate follows the descending order of hydraulic gradient, gradation unevenness, and porosity. The combination of C_u=20.47 and n=0.40 yields the highest cumulative erosion rate. (3) By comparing the result of internal stability obtained through the fine particles’ stability degree and the macroscopic parameter characteristics with those derived from various criteria, the Istonima criterion is found to offer relatively accurate results. In addition, modifications to the Istonima criterion are proposed to enhance the accuracy of determining the internal stability of widely-graded gravel soil.

Key words: widely-graded gravel soil, cylindrical infiltration test, fine particle movement, abrupt change point of seepage, internal stability