The role of fines on internal instability and its impact on undrained mechanical response of gap-graded soils

This study presents an experimental investigation of the contribution of non-plastic fines to the development of seepage-induced internal instability and its impact on the undrained mechanical response of gap-graded sands. The purpose of the laboratory tests is to observe the internal instability of two key microstructures: (1) an underfilled microstructure in which the coarser particles control the stress transfer and (2) an overfilled microstructure in which the finer particles play a primary role in the stress transfer. Tests on medium dense sands with seven different fines contents are conducted using a pressure-controlled triaxial erosion device. The device enables internal erosion tests with high back pressure under a pressure-controlled condition. The results indicate that the initial fines content significantly affects the initiation and progress of internal instability. The phenomena of self-filtering and suffusion, evident for underfilled soil, can occur at relatively small hydraulic gradients. Depending on the initial fines content, overfilled soil is vulnerable to suffosion, seepage-induced failure, or an internally stable state, which can occur at large hydraulic gradients. Undrained compression tests on eroded soils reveal that suffusion makes the soil looser and more contractive, while suffosion makes the soil more dilative at large strain levels. As suffusion may create an unstable structure in the soils, sudden drops in deviator stress and sharp increases in pore water pressure and radial strain with axial straining are detected in all the post-suffusion soils at small strain levels. Finally, the identificaton of internal instability is illustrated in terms of the void ratio and the fines content, for assessing the initiation and progress of instability phenomena, as well as the possible soil microstructures.