Internal instability in soils: a critical review of the fundamentals and ramifications

Seepage-induced fine-particle migration that leads to a change in the conductivity of a soil matrix is referred to as internal instability. This could jeopardize the structural integrity of the soil matrix by initiating suffusion (or suffosion), a form of internal erosion. Susceptibility to suffusion has been studied mostly under extreme laboratory conditions to develop empirical design criteria, which are typically based on the particle size distribution. The physics governing the process have not been comprehensively uncovered in the classical studies because of experimental limitations. Mainstream evaluation methods often over-idealize the suffusion process, holding a probabilistic perspective for estimating constriction sizes and fines migration. Prospective studies on constitutive modeling techniques and modern computational techniques have allowed a more representative evaluation and deeper insight into the problem. Recent advances in sensing technologies, visualization, and tracking techniques have equally enriched the quality of the data on suffusion. This paper sets out to present the long-standing knowledge on the internal instability phenomenon in soils. An attempt is made to pinpoint ambiguities and underscore research gaps. The classical empirical studies and modern visualizing techniques are integrated with particle-based numerical simulations to strengthen the theoretical understanding of the phenomenon.