Melding Pathways and Mechanisms for Internal Erosion Failure Modes

摘要

Potential failure mode analysis (PFMA) and risk analysis procedures have evolved over the past two decades, particularly for internal erosion failure modes. Initially, internal erosion failure modes were commonly described based on typical pathways - internal erosion through the embankment, internal erosion through the foundation, internal erosion along the outlet conduit, etc. - and often the term "piping" was used interchangeably with internal erosion. More recently, industry guidance documents have tried to re-focus the understanding of internal erosion failure modes according to their processes and mechanisms - backward erosion piping, concentrated leak erosion, contact erosion, and suffusion/suffusion - with "piping" being a subset of internal erosion. Concurrently with this shift in focus to the different internal erosion mechanisms, over the past few years, the FERC and other organizations have required increasing detail in the descriptions of failure modes. This increased detail is needed to fully document the loading condition and mode of failure, from initiation to breach, to confidently categorize the failure mode for a PFMA or estimate nodal probabilities for risk analysis. It has been the authors' experience, from a workshop at the 2016 ASDSO conference and from recent PFMA sessions, that there is confusion among practitioners on how to address pathways and mechanisms in describing internal erosion failure modes. In our opinion, the description needs to include both pathway and mechanism, as the two combined will define how the failure mode initiates and progresses, what considerations should be used to judge its likelihood, and where and how signs of the failure mode may be exhibited at the dam. This paper explains the mechanisms and pathways and how they are combined to fully and accurately describe an internal erosion failure mode.