Length-effect can be an important consideration when estimating risk for long earthen structures. Length-effect is difficult to define because it is dependent on the length of the structure, failure modes, and variable geologic & engineering characteristics along the length of a structure. Length-effect was considered for the 7.5 mile-long Moose Creek Dam for a risk assessment performed by the U.S. Army Corps of Engineers in 2016. Three risk driving failure modes were considered; backward erosion piping with a vertical exit, backward erosion piping with a horizontal exit, and contact erosion. Total project risk was assessed first by using a best practices approach for risk assessment of dams. The best practices assessment divided the project into three characteristic reaches primarily based on geomorphology, with consideration also given to the construction, hydraulic loads, performance history, and consequence centers. Risk was estimated using expected worst-case failure probabilities using most-critical cross section for each characteristic reach, and aggregated using DeMorgan's Rule to compute total project risk. The total project risk was then reevaluated to consider length effect. Characteristic lengths for this second evaluation were defined using ranking factors for the risk-driving failure modes at each 500-foot reach over the full 7.5 mile length of the dam. A natural division of the project into nine characteristic lengths was observed by the risk cadre using ranking factors and by autocorrelation of the ranking factors for each of the 500-foot reaches. The total project risk increased by about one order of magnitude above the original best practices estimate. This case history demonstrates the value of using simplified risk- based weighting factors to define characteristic lengths where length effect is an important consideration in long earthen structures.
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