Investigation of Seepage Induced Internal Instability in Dense Gap-graded Soils under Complex States of Stress

As well as providing and storing water and regulating flows, hydroelectric dams function as a sustainable and clean source of energy and energy storage. However, many large embankment dams were constructed before engineers had a sound understanding of filter design. As a result, seepage induced internal erosion of fines through the coarser matrix is the biggest challenge to existing embankment dams, such that nearly 50% of embankment dam stability problems are due to internal instability. The initiation and later effects of internal erosion depend on a combination of both geometric and hydromechanical criteria, with the complex stress states that the soil is subject to being one of the determinant factors. The fact that internal erosion is stress state dependent has been recently recognised and reported. Therefore, this thesis aims to develop an understanding of the role that the critical stress condition plays in the erosion process. This research investigates the influence of varying different hydraulic gradient, fines content, stress path and finally, shearing rate. All erosion tests were conducted while changing the stress state (continuously shearing while eroding). To attain this objective, a triaxial permeameter was developed to study the effect of seepage flow through a dense gapgraded samples under controlled stress conditions. This enables the permeability to be examined, as well as the quantity of eroded soil. The triaxial permeameter is equipped with a pressurised water supply system, fine particles collection system and water collection system. The pore pressure differential across the sample, volume change, outflow rate of water and the mass of eroded particles were recorded during all tests. This allowed the hydro-mechanical parameters including hydraulic gradient, seepage velocity, permeability and the internal erosion rate to be determined. The tests reveal that shearing under seepage results in fines being released continuously from the soil, with the rate of release depending on hydraulic gradient, initial fines content, stress path and shear strain developed. However, shear strain rate did not affect the rate of fines release. In all tests, permeability decreased under downward seepage, irrespective of fines loss. It was also concluded that the intensity of fines erosion could affect the shear strength of the soil.