Risk Indexing Tool to Assist in Prioritizing Improvements to Embankment Dam Inventories

A risk indexing tool is proposed to assist in the prioritization of maintenance, repair, and evaluation tasks on embankment dams that are generally less than 33 m (100 ft) high and for which there is little or no instrumentation, limited or no information concerning as-built conditions, and little or no information on the performance history. Under such circumstances, there may not be enough information available to perform anything other than an “indexing” type of analysis to assist in prioritization. The risk indexing tool is based upon identifying potential deficiencies in the physical condition of the dam and rating the overall importance of these deficiencies to the safety of the structure. It is meant as an index of risk (indication) and not as a direct measure of risk. Checklists are presented for onsite inspections to determine current physical condition. Condition is defined in terms of a condition function that is based upon a condition indexing scale. Four potential failure modes are considered: (1) overtopping; (2) external erosion; (3) piping; and (4) mass movement (slope instability). Absolute probabilities are specifically not factored into the analysis. Conditional probabilities are estimated for each failure mode using a Bayesian updating procedure based on dam attributes. A simple failure criticality analysis is performed wherein specific changes in physical condition of the dam are considered to contribute to the probability of failure for each mode. From this failure criticality analysis and the conditional probabilities of failure, the relative importance of the various changes in physical condition is determined. The physical condition and the relative importance are then combined for each observable deficiency to form a risk index. These risk indices are used to prioritize expenditures for improvements on the premise that actions to address the most significant physical deficiencies are preferred.