Abstract:  During flood season, piping risk in the downstream of embankment is the most dangerous hidden trouble. The discrimination of piping in the past was generally based on seepage gradient which is not only limited theoretically by Darcy ' s flow but also inconvenient in practice. In view of this, a method of identifying piping below embankment, which can be called critical velocity or unit seepage discharge observation method, is suggested in this paper. It is based on the experimental curve of drag coefficients for single immersed spherical body moving steadily in water as well as the seepage experimental results for porosity relationship of grained soil mass. Furthermore, the empirical formulas for different flow patterns (Darcy and non-Darcy ' s flow) of the experimental curve are given to calculate the critical velocity. A table for the critical velocity and seepage discharge of different kinds of soil mass is also given. In practice, it only needs to measure the seepage discharge per unit by surrounding a suitable ring on the permeable surface to determine whether piping will happen or not.

Key words:  piping  ,   embankment  ,   critical velocity ,   critical seepage discharge  ,  non-Darcy ' s flow , flood prevention during flood season