Risk of Dam Failure Due to Sinkhole Formation

A sinkhole can lead to different failure modes depending on the size, depth and location of the sinkhole. A large sinkhole under a downstream embankment slope is likely to result in slope stability failure, overtopping and breach of the embankment. A small sinkhole at the toe of an embankment may not result in slope stability failure, but if undetected it can increase exit gradients causing backward erosion, piping and eventual breaching of the embankment.
This paper describes the risk evaluation performed for a large reservoir in Florida to determine the risk of a significant uncontrolled release of water from the reservoir as a direct or indirect result of sinkhole formation. Three aspects of the risk evaluation are emphasized: (1) the method for estimating the annual probability of sinkhole occurrence within the footprint of the dam, (2) the potential failure modes analyses that address the various different failure modes that can develop depending on the size and location of the sinkhole, and (3) the methodology for estimating the conditional probability of a sinkhole-induced failure.
The annual probability of a sinkhole occurring under the dam footprint in any one year was estimated by comparing the dam footprint area with the area of the representative study area, and considering the number of historical sinkholes that occurred in the study area, their size, and the time period over which they occurred. Geological expertise in local karst conditions was essential to determine the boundaries of the study area and the sinkhole size that need to be considered. This probability of sinkhole occurrence was then modified by two factors: (i) the extent and results of geotechnical explorations and (ii) the presence of the reservoir pool and the potential for increased downward hydraulic gradient in the subsurface below the reservoir.
Within each general failure mode (overtopping, piping or slope failure) there was further differentiation with regards to the size and location of the sinkhole; 25- and 100-foot diameter sinkholes were considered, and it was assumed that a sinkhole could form under the upstream slope, center, or downstream slope of the embankment.