Assessing the risks associated with internal erosion phenomena in aging embankment dams: a New Zealand perspective

Crawford-Flett, K. and Haskell, J. (2016) Assessing the risks associated with internal erosion phenomena in aging embankment dams: a New Zealand perspective. In: ICSE 2016 (8th International Conference on Scour and Erosion), 12-15 September 2016, Oxford, UK.

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Abstract

Earth embankment dams form a vital part of New Zealand’s hydropower, agricultural, and water supply infrastructure. The challenges faced in the management of aging embankment dams are compounded by factors specific to New Zealand, including large variability in soil types and the highly tectonic environment in which the dams are located. Internal erosion, triggered by both seismic and non-seismic events, is considered one of the primary risks to New Zealand embankment dams. Spurred by the recent Canterbury Earthquake Sequence, hydropower asset owners in New Zealand have expressed a need for improved guidance for the evaluation of embankments (1) following significant earthquake ground motions, and (2) from a whole-life perspective. This study considers the applicability of existing empirical methods to assess the potential for internal erosion in the New Zealand context. Two distinct mechanisms of internal erosion are considered: (1) internal instability, and (2) filter incompatibility. Four existing empirical geometric methods were used to assess the potential for internal instability in 19 widely-graded New Zealand soils. One existing method was found to be mathematically ineffectual with respect to the widely-graded soils considered in this study and all methods lack reliable verification using volcanic soils. Existing screening methods suggest that a number of glacial, alluvial, and volcanic materials used in construction of New Zealand’s large earth dams may be susceptible to some degree of internal instability phenomena, irrespective of seismic hazard. Secondly, a case-study concerning a common type of widely-graded base-filter soil interface demonstrates ambiguous analysis results arising from overlap in No Erosion and Excessive Erosion thresholds. Uncertainties in interpretation could be resolved by the future development of statistical guidelines for filter assessment. With regard to both internal instability and filter incompatibility mechanisms, the applicability of existing empirical analysis techniques to New Zealand soils appears limited due to a lack verification for the diverse geological range of fill soils encountered. In addition, existing stability thresholds have not been verified for long-term or seismic loading conditions inherent in the New Zealand context. This study highlights significant shortcomings in the applicability of existing screening methods used to assess the potential for internal erosion in New Zealand soils.

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