SPECIAL RISK :ISSUE Deep Foundations Risk Analysis Is Fundamentally Changing the Landscape of Dam Safety in the U.S. during the past 30 years has resulted in the dam safety community (1) openly recognizing, in a formal manner, the many ways a dam can fail and the consequences of those failures, (2) using risk as a tool for prioritizing risk reduction actions, and (3) focusing monitoring programs and remediation efforts on the highest risk dams and their potential failure modes. One example of a substantial change in emphasis resulting from risk con- siderations is an increased recognition of internal erosion risks, relative to those from rare events such as large floods and earthquakes. The application of risk analysis has fundamentally changed the practice of dam safety engineering in the U.S., and will continue to do so in the future. Several considerations have influenced the trending shift to risk-informed thinking in the dam safety industry. There are more than 90,000 dams in the U.S., including approximately 16,000 high-hazard potential facilities (loss of life potential upon failure). Presently, the average age of dams in the U.S. is over 55 years. In addition, many older dams do not meet current design criteria, although, there are several cases of dams that met standards- based criteria that still experienced failure. The quantity of high-hazard potential dams in the U.S. that are considered NO INTERNAL EROSION NORMAL POOL YES deficient is more than 2,200 and increasing, and it is currently est imated that i t would cost approximately $50 billion to repair these deficient dams (ASDSO 2017). In addition, state dam safety inspectors have, on average, over 200 dams assigned to each of them. The limited financial and labor resources available to owners and regulatory agencies drive the need for strategic prioritization to address the most critical structures first. The increasing application of risk analysis and risk considerations in the U.S. FLAW? NO Risk Analysis Practice in the U.S. Prior to the application of risk analyses, dam safety engineering practice in the U.S. focused on evaluating dams through visual inspections and com- parison of analysis results with determin- istic criteria. INITIATION? YES NO CONTINUATION? YES Representative examples of such criteria include: NO • Comparing calculated factors of safety for stability to recommended or required minimum factors of safety • Comparing calculated stresses in structures to allowable or ultimate strengths of the corresponding materials Although there was general consensus, with some variability, regarding criteria for spillway capacity, factors of safety for stability and structural stresses, there was significant divergence concerning criteria for seepage and internal erosion. As a result, conclusions concerning seepage and internal erosion were based principally on qualitative professional judgments. PROGRESSION? YES • Comparing spillway capacity with a specific inflow design flood (e.g., a probable maximum flood or a 100-year flood for high- or low- hazard dams, respectively) Potential Failure Mode Analyses Beginning around the mid-1980s, the U.S. Department of the Interior, Bureau of Reclamation (Reclamation) led the way in introducing the potential failure modes analysis (PFMA) methodology to the U.S. dam safety practice. PFMA changed the basic thought process in dam safety engineering from one of evaluating dams as described above to one of critically assessing the ways a dam could fail, along with the r e l a t i v e likelihoods NO UNSUCCESSFUL INTERVENTION? YES NO BREACH? Example event tree for an internal erosion failure mode (Reclamation/USACE 2015) AUTHORS Jennifer L. Williams, P.E., and John W. France, P.E., D.GE, D.WRE, AECOM DEEP FOUNDATIONS • JAN/FEB 2018 • 61 YES FEATURE ARTICLE
