Abstract: Fine particles in an internally unstable sand and gravel soil may be washed out under seepage, thus the bearing capacity of a dam foundation with those sand and gravel soil layers reduces. The determination of critical piping permeability gradient of gravel soil on dam foundation is an important step in dam design and seepage prevention safety analysis. In this paper, the prediction of the critical vertical upward hydraulic gradient of piping in sand and gravel soils were developed based on the force balance theory of a single particle or a group of particle under seepage. Based on 64 groups of vertical upward piping tests of gravel soils in literatures, Kenney-Lau stability criterion method and Kezdi particle size ratio method were used to analyze the internal stability of gravel soils. The results show that the proposed method is an effective method to access the internal stability of gravel soils using particle gradation curve. By comparing the predictions of critical permeability gradient with the experimental results of 41 groups of sand-gravel soils, it is a reasonable assumption that the permeability of particles in sand-gravel soils under seepage is proportional to the surface area of particles .

Key words: piping, internal stability, sand and gravel soil, critical gradient