Mesoscopic interpretation of fines clogging-induced permeability changes in completely decomposed granite

Due to poor particle gradation, fine particles in completely decomposed granite (CDG) can migrate during seepage, thereby altering soil permeability. Previous experimental studies have demonstrated from a macroscopic perspective that permeability reduction in CDG is induced by fines clogging under seepage flow. However, the underlying mesoscopic interpretation remains unclear. Hence, the mesoscopic characteristics of CDG were investigated in this study by analyzing pore structure evolution and local permeability variations during seepage. The influence of pore structure on permeability was elucidated through comparative analyses of intact and recompacted CDG specimens. Furthermore, the progressive process of fines migration and clogging was systematically examined in recompacted specimens. The results demonstrate that intact CDG exhibits a more non-uniform pore structure than recompacted CDG, characterized by a seepage tortuosity 3.7 times that of recompacted CDG and a permeability coefficient one order of magnitude lower. Regarding local characteristics, fines migrate from the upstream zone and subsequently clog the downstream zone, thereby isolating the pores in the downstream zone into small-volume pores. As a result, the pores in the upstream zone mainly become long columnar, while those in the downstream zone primarily turn to be ellipsoidal. This process increased the void ratio by 35% in upstream zones while reducing it by 40% downstream, leading to a gradual decrease in local permeability coefficients along the seepage path. These findings provide a mesoscopic interpretation that enhances the characterization and understanding of permeability changes induced by fines clogging.