Change in mechanical behaviour of gap-graded soil subjected to internal erosion observed in triaxial compression and torsional shear

Abstract

Internal erosion can be a cause of failure in hydraulic structures such as levees, dikes and embankment dams. Suffusion is a type of internal erosion in which the fine particles migrate through the voids between the coarse particles under seepage flow, leaving behind the coarse skeleton. In this study the impacts of suffusion on the mechanical properties of soil in triaxial compression and torsional shear were investigated using a gap-graded soil mix of Silica sand No.5 and non-plastic silt. It was found that the erosion process varied depending on the initial density of the soil and higher density specimens showed relatively lower rates of erosion, degree of erosion, and compressive straining during erosion as compared with loose specimens. Effects on the mechanical behaviour​ of the soil were also dependent on the initial density of the specimen, with changes in the small strain stiffness, secant stiffness degradation, yield loci, small strain peak strength and medium strain strength in both drained and undrained conditions. In triaxial loading, force chains of coarse particles’ skeleton reinforced by fines developed in the suffusion process in the direction of major principal stress, then the stiffness at small strain increased. On the other hand, in torsional shear testing, the reinforced soil skeleton established by the erosion did not resist against shear loading. Increase in stiffness at small strain range was not observed.