Dam Seepage Sensing and Modelling

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Earthen embankment dams are widely used to contain hydroelectric reservoirs, water supplies and wastes such as those produced by mining and oil sands operations. Concentrated water seepage through such dams must be avoided to prevent internal erosion of fine-grained materials and the development of piping - a major cause of dam failure. Although standards for dams in Canada are amongst the strongest in the world, best practices have progressed over time, and the need for improved seepage monitoring methods that can be applied to aging structures is recognized internationally. UNB and NB Power will design and evaluate, through field trials and modelling, three minimally invasive geophysical approaches for the early detection and quantification of concentrated water seepage. The project will be based at NB Power's Mactaquac Generating Station on the Saint John River near Fredericton, NB, which includes a 500 m long zoned embankment dam, completed in 1968, that rises 32 m above its toe. The methods will rely largely on monitoring of seasonal temperature variations within the dam, as measured directly using distributed temperature sensing (DTS), and indirectly by electrical resistivity imaging (ERI). Regions of higher seepage are expected to exhibit greater seasonal variations in temperature and hence resistivity as water from the dam reservoir moves through it. Zones of particular interest include the dam's clay till core and the left abutment where the dam adjoins a concrete diversion spillway structure that is undergoing differential expansion as a consequence of Alkali Aggregate Reactivity in the concrete. The former will be investigated using time-lapse 2D resistivity monitoring along the dam crest which has yielded encouraging results on similar dams in Sweden. The abutment region will be investigated in more detail using borehole DTS complemented by time-lapse 3D resistivity monitoring with electrodes on the back of the dam. The research will build on valuable insights obtained from prior monitoring research in the abutment region using DTS and self-potential (SP) methods. It will be applicable worldwide but particularly in regions such as Canada where seasonal temperature variations are large.