On the Assessment of Internal Erosion of Dam Cores of Glacial Till

Sammanfattning: The objective of this study is twofold; on the one hand it investigates the susceptibility to suffusion of glacial till, and, on the other, it presents approaches to evaluate internal erosion susceptibility of dams composed of this type of core soil. Concentrated leak erosion, backward erosion, contact erosion, and suffusion erosion are the mechanisms associated with the initiation of internal erosion. How these interplay in the continuation phase is still a relatively unresolved issue. In engineering practice, these mechanisms have all been seen in dams with glacial till cores; backward erosion activity as seen in Juktan dam, Juklavatn dam, Grundsjöndam, Wreck Cove dam, and in Brodhead dam; possible suffusion as seen in Lövön dam and in Stenkullafors dam; and, furthermore, signs indicating concentrated leaks, such as in Songa dam. A laboratory program of tests was performed on four natural, non-plastic glacial till soils from the core or its borrow area of existing earth dams in Sweden, and on eight mixturesbased on these tills, making in total 12 tests. The longest running for 77 days, and the highest applied gradient was 9.5, in bounds of what can occur in dam cores. The tests revealed that gradations of low fines content (<§15–20%), low sand fraction (<§20%), and high gravel fraction (>§60%) are susceptible to suffusion. These are grading characteristics present in the coarse end of the envelope of glacial till cores in many existing dams. Available state-of-the-art assessment criteria for internal stability were evaluated in reference to these experimental data, yielding a modification to the Kenney-Lau adaptation by Li-Fannin. Furthermore, a screening tool for the assessment of internal erosion of dams is proposed, i.e., the unified-plot approach, which combines attributes of the filter gradation and the probable occurrence of internal erosion in a dam. Given the variety of potential mechanism interplaying in the internal erosion of dams with cores of glacial till, it lend confidence to an assessment approach which addresses, susceptibility to several mechanisms.