Qingfeng Feng, Visiting PhD Researcher, Tsinghua University
Internal erosion, which is known as the detachment of grains out of soil fabrics, constitutes a major threat to the dam operation and foundation. Gap-graded soil gradation is a decisive factor of internal instability, which furthermore, leads to internal erosion. In this study, a numerical model was developed to study the internal erosion process in its entirety and clarify the effect of gap-graded soil gradation on the internal instability. The proposed model was validated with experimental data from other research. It has been found that there exist two critical factors that contribute to the soil particle movement: 1) when the imposed shear stress on soil particles is larger than the critical value initiating the soil particle movement; and 2) when there exist sufficient space areas within the soil fabric for particles to move through. The model showed good accuracy in simulating the process of erosion and results disclosed that the soil with gap-graded size gradation is more likely to reach the above thresholds, causing internal erosion. This result can be compared against soil with a continuous size gradation. Furthermore, a universal quantitative evaluation method was also proposed to determine the internal instability of soils based on their initial size gradation and other relevant factors such as the permeable conductivity, confining pressure and porosity. Moreover, some new experiments have been launched to ensure that this model could be applied to more sophisticated engineering situations, for instance, investigating how a cut-off wall prevents soils from being internal eroded.