Abstract: The loosened zone around a pipe must be considered when the mechanism of backward erosion is investigated. However, few quantitative analyses were found in the literature regarding the permeability characteristics of the loosened zone around a pipe. The present paper focuses on the process of grain loosening and permeability changes of surface soil by sand column experiments subjected to upward vertical flow. The loosening top soil and downward progression of the loosened zone were observed in sand column experiments, indicating that the seepage failure modes of the top soil and the mode of whole sand column are different. Analysis of the hydraulic head distribution along the sand samples showed that the permeability of loosening top soil occurred as an abrupt change. The permeability coefficient of the thin loosened zone at the surface is larger than that of the following loose sand. Similarly, the permeability coefficient of part of the loosened zone around the pipe is larger than that of the original sand, resulting in insufficient gradient near the pipe tip. The finding is essential for investigating the erosion mechanism near the pipe tip.

Key words: backward erosion piping, loosened zone, permeability, laboratory experiments