Hydraulic Fracturing in Widely-Graded Dam Core Material: Fid-Bau Portal

Hydraulic Fracturing in Widely-Graded Dam Core Material

Waters, Ross D. / Crawford-Flett, Kaley / Stringer, Mark / Haskell, J. J. M.

Hydraulic fracturing in dam core can cause excessive seepage, internal erosion, and potentially lead to the total failure of earth dams. Previous hydraulic fracturing studies in soils have focused on fine-grained materials (clays and silts), and little attention has been given to investigating hydraulic fracturing in materials with a significant proportion of sands and gravels. This paper outlines a laboratory testing program that investigates hydraulic fracturing in a widely-graded till material. A new large-scale hydraulic fracture cell (a modified true-triaxial apparatus) was designed and built to test maximum particle sizes in the gravel range. Dam core sampled from an operational hydropower dam in New Zealand was screened into two batches of different maximum particle size and fines content (D_max = 2.36 mm, fines = 36%, and D_max = 9.5 mm, fines 26%), and tested in the fracture cell. It was found that the shape of the fluid injection pressure curves closely matched previous research on finer materials, suggesting the failure mechanism is still hydraulic fracturing in the widely-graded materials tested. As there are many variables that influence the onset of hydraulic fracturing, the influence of the wider grading on the magnitude of fracture initiation pressure was not sufficiently defined. However, this research demonstrates that hydraulic fracturing can occur in existing dams with a high proportion of coarse material in the dam core.