在堤基管涌发展机制的研究中，需考虑管涌通道四周破坏扰动松散层的影响。目前针对松散层渗透特性变化的定量试验数据较少。基于此，通过一维垂直向上渗流作用下均匀砂的渗透试验来研究临空面表层土体的松散过程和渗透特性变化情况。砂样垂向渗透变形发展过程中伴随着临空面表层土体松散破坏和松散层逐渐向深部扩展的过程。通过分析砂样沿程水头分布的调整变化情况得知，松散表层土体的渗透系数分布呈突变形式，即临空面松散薄层的渗透系数远大于其下松散砂层的渗透系数。由此类推，管涌通道边壁四周松散薄层的渗透系数远大于砂样整体渗透系数，松散层的存在使得管涌通道尖端坡降并不会达到很大值。这一点认识对管涌发展机制的研究至关重要。

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.