Suffusion-induced change in spatial distribution of fine fraction in embankment subjected to steady and unsteady seepage flow

Abstract

Suffusion is the phenomenon that fine particles are detached from the solid matrix by seepage flow and are transported through the pores formed by coarse particles in the ground. This may cause deterioration of soil structures and, in the worst case, resulting in failure of the soil structures. In this paper, to examine the seepage-induced suffusion process in embankments, a series of physical model tests on seepage-induced suffusion on small-scale model embankments are presented. Binary mixtures consist of two Silica sands (Silica No.3 and No.8), having different dominant particle sizes, were used for the model embankment. The spatial extent of erosion-induced fines content variation is discussed through sieve analyses on subdivided areas of the model embankment after seepage testing. In particular, effects of seepage duration and unsteady seepage flow on soil erodibility and migrating of fines in embankments are discussed. The results of these experiments show that the temporal suffusion development under the steady seepage flow depends on balance between dislodgement of erodible fines from embankment and its deposition into the base in the early stage. After a certain elapsed time, suffusion may develop backward along the phreatic surface from downstream in the embankment. It was also observed that the erodible fines may not only move laterally by seepage flow but also move vertically due to the gravitational force and are deposited in the base. This deposition of the fines resulted in the expansion of the fine-rich region in the base and may have caused decrease in the nominal hydraulic conductivity of the embankment. When the unsteady seepage flow was adopted, changes in the spatial distribution of fine fraction were exaggerated and the downward movement of the fines became prominent.