Onset of suffusion in gap-graded soils under upward seepage

Suffusion is an important mechanism that leads to the failure of hydraulic structures such as dams and levees. To evaluate the onset conditions, i.e., the critical hydraulic gradient of the suffusion, a set of laboratory experiments is conducted to explore the internal erosion behaviors under upward seepage conditions. During these experiments, the particle erosion in soils with different particle size distributions and dry densities is investigated. Based on the results of the experiments, a low critical hydraulic gradient (LCHG) and a high critical hydraulic gradient (HCHG) are defined corresponding to the local moving and the global loss of fine particles, respectively. The results show that these two critical hydraulic gradients are significantly influenced by the particle size distribution and the dry density of the soil. Moreover, the experiments reveal that the characteristics of the soil, especially the particle size distribution, have a significant impact on the internal instability of the soil. The effects of the particle size distribution and the dry density on the critical hydraulic gradients are then explained using a capillary model, which is built upon the characteristics of the voids in the soil, including the porosity and the void size distribution.