Structure-controlled piping and flow-like erosion in granite residual soil: critical gradients and conceptual model

Granite residual soil (GRS) is highly disturbance-sensitive and prone to aggressive seepage, often triggering geohazards and engineering failures. However, previous studies have largely relied on simplified permeability tests that decouple hydraulic measurements from deformation processes, leaving seepage-induced failure modes, critical hydraulic thresholds, and structure-controlled mechanisms poorly understood. To address these gaps, we conducted laboratory infiltration–deformation tests on undisturbed (UD) and remoulded (RM) GRS using a modified permeameter that couples hydraulic monitoring with real-time surface observation. UD specimens exhibited piping failure, whereas RM specimens underwent flow-like erosion, revealing fundamentally different failure modes governed by soil structure. The lower critical hydraulic gradient (onset of sustained particle mobilisation, i_L ) and upper critical hydraulic gradient (bulk failure threshold, i_U ) were determined as i_L\approx 45 , i_U\approx 60 for UD, and i_L\approx 95 , i_U\approx 105 for RM, indicating that natural structure reduces critical gradients by approximately half. The hydraulic conductivity–hydraulic gradient (k–i) trajectory provides a robust, less subjective basis for identifying these thresholds, revealing a pre-failure dip (transient clogging), a rebound at i_L (sustained mobilisation), and a plateau beyond i_U (stable conduit formation). Integrating mineral–chemical evolution, particle-size distribution, seepage behaviour, and eroded-particle spectra, we propose a fabric-controlled conceptual model: key support grains (0.075–0.02 mm) pin critical throats, while kaolinite-rich fines (< 0.02 mm) and Fe-oxide cements bridge contacts. Under upward seepage, cement dispersion and key-grain mobilisation promote channelisation and piping in UD, whereas the homogenised fabric in RM delays channelisation and favours flow-like erosion. These findings advance the mechanistic understanding of structure-dependent seepage failures and offer practical guidance for GRS-bearing excavations and embankments, including limiting the hydraulic head difference relative to seepage path length ( \mathrm\Delta h/L ) below i_L and employing continuous hydraulic conductivity monitoring as an early-warning indicator for imminent piping.