August 2025 Webinar: Annual Conference Virtual Presentations

Best Practices for Event Tree Construction and Portrayal in Dam and Levee Safety Risk Analysis:

Event tree analysis is the most common approach for estimating dam and levee safety risks. Complex potential failure modes can be decomposed into a more understandable sequence of events, and event trees assist the understanding, analysis, and communication of dam and levee safety risks. Creating event trees for potential failure modes and communicating the results of risk analysis can be difficult. This paper will provide best practices for decomposing complex potential failure modes into event trees that can be used in a risk analysis, an overview of the RMC Typical Event Tree Database to aid the development of project-specific event trees, and how to communicate key events from the event tree that drive the risk estimate to decision makers.

Does LifeSim Work with Tailings Hazards? Estimating Life Loss using LifeSim for the Feijão B-1 Tailings Dam Failure 25 Jan 2019:

Interest in the capability to estimate potential life loss for Tailings Storage Facilities (TSF) has become more prevalent within the dam safety industry. USACE’s LifeSim has been developed, and applied, to estimate consequences caused by ‘clean’ water flooding. Few studies have attempted to apply LifeSim to the TSF consequences estimation problem. New advancements in Non-Newtonian hydraulic modeling provide an opportunity to use LifeSim to estimate life loss for tailings events.On January 25, 2019, the Feijão Dam B-1 at Minas Córrego do Feijão in Minas Gerais, Brazil failed. A devastating mudflow inflicted extreme downstream damage and caused 270 fatalities. The Feijão Dam B-1 event provides a unique opportunity to demonstrate how LifeSim can be used to analyze TSF failure events, provide insights into how life loss can be estimated for these events, and to show how this approach could be used to inform consequences estimation and risk assessments for TSFs.In 2022, USACE initiated a study of the Corrego do Feijão B-1 Tailings Dam Failure with multiple goals.-Determine the applicability of the LifeSim method to tailings hazards.-Identify potential modifications that may be necessary to effectively estimate potential loss of life from tailings events.-Identify opportunities for improvement in the existing LifeSim method, including short-term solutions and longer-term research efforts.-Demonstrate how LifeSim can be used to inform TSF consequences classification systems. The study constructed multiple iterations of the LifeSim model and improved the resolution through research and data findings. The result shows how LifeSim can be used to estimate potential life loss and support risk assessments for TSFs. It uses 59 unique alternatives to explore multiple scenarios, various warning and evacuation outcomes and different Population at Risk (PAR) distributions. The study demonstrates the dimensions of the LifeSim simulator and identifies potential improvements for estimating consequences for non-water hazards.

Weak Rock: The Effect of Water Content on Strength and Deformation Properties:

Weak rocks, with the subcategory of intermediate geomaterials (IGMs), have engineering properties between soil and rock. Sampling and testing these types of deposits can be difficult, with sample preservation techniques more similar to soil than rock. High reactivity with water leading to slaking, and a strong dependency of the strength and deformation properties on the water content and saturation history make these materials particularly challenging. A geologic exploration and laboratory testing program was performed in 2023 and 2024 to characterize the foundation materials of a dam to support a major modification. The foundation is Eocene Spencer formation sandstone, comprising poorly indurated thick bedded to massive, friable, fine- to medium-grained sandstone with occasional siltstone and claystone interbeds. The Spencer formation is known to be susceptible to slaking and deterioration when exposed to air, but in place appears massive, typically with a high RQD. The drilling program included HQ wire-line coring of the bedrock, with select samples waxed in the field to preserve the in-situ water content.The laboratory program included testing air-dry and waxed (to preserve the in-situ water content) sandstone for susceptibility to degradation when exposed to water, and static uniaxial and triaxial compressive strength. The testing found that the strength of the air-dried sandstone was nearly three times that of the in-situ water content specimens, with the in-situ water content specimens having about half the Young’s modulus of the air-dried specimens. This sampling and testing program illustrates the need for proper care to be taken to preserve the in-situ characteristics of weak rock to ensure that laboratory testing improves the understanding of a site, rather than contribute to uncertainty.

The Screening (and meaning) of Highly Unlikely Potential Failure Modes:

In a typical dam safety risk analysis, it is inevitable that some potential failure modes will receive greater scrutiny than others. By necessity, this is a result of the fact that there are usually other facilities within the inventory to consider and that both technical resources and time are inherently limited. In the past, the criterion for deeming some potential failure modes “highly unlikely” has been largely intuitive, and the reasons for such a designation have not always been clearly documented. This practice was criticized in the wake of the 2017 Oroville Dam spillway incident, with both the independent forensic team and subsequent reviewers calling for more systematic guidance. This paper describes a screening process that can be used not only to distinguish highly unlikely potential failure modes from those to be retained for risk quantification, but to identify a set of potential failure modes that fall into neither category. In doing so, it also proposes a set of risk analysis axioms that can be called upon to explain how the risk analysis process used by Reclamation differs from that of other dam owners, and why the form that it has taken on is considered appropriate for the needs of the organization.

Limitations of Commonly Used Simplified Seismic Analyses Methods for Embankment Dams and Levees and A Framework for Improvement:

Simplified seismic analyses of embankment dams are sometimes used for seismic deformation analyses and post-seismic stability assessments, generally for low-consequence dams and levees, or during the screening phase of risk assessments, or for planning for more detailed studies of high consequence dams and levees. However, there are serious limitations to some commonly used simplified seismic analysis procedures. These often include an inability to correctly predict the most critical potential failure mechanisms in zoned embankments, incomplete analytical protocols to compare with case histories, limitations in applicability to embankment dams such as water retention dams, and an inability to capture progressive reductions in soil strength and stiffness due to soil liquefaction, or strain softening of non-liquefiable soils. Sometimes, the use of these simplified seismic analyses may result in incorrect or misleading conclusions during the preliminary stages of projects regarding seismic potential failure modes or PFMs. For high-consequence dams, these incorrect or misleading conclusions during the initial risk assessment or planning stage may result in exclusion of seismic potential failure modes (PFMs) for further investigations, analyses, and potential retrofit. In this paper, a framework and set of protocols for simplified seismic analyses, using common engineering tools and software, are presented. The purpose of this framework for simplified seismic analyses is to address the limitations of current practices, while documenting their limitations in a more transparent manner. The proposed simplified seismic analysis protocol should enhance the reliability of screening level analyses during risk assessment or planning studies.

Screening Level Finite Element Seismic Analysis of a Spillway Pier

A screening-level analysis method to assess seismic failure risk for reinforced concrete structures is being presented. The approach demonstrates a range of applicability for reinforced concrete structures, though it is developed within a Dam Safety context for the seismic analysis of spillway piers. The approach is based on an adaptation of the pseudo-acceleration response spectrum concept for use with limited-scope nonlinear finite element models of reinforced concrete structures. This seismic analysis approach is meant to be efficient and effective, while providing a defensible first iteration estimation of the load bearing capacity of a reinforced concrete pier when subjected to earthquake loads. The methods assumes that a site-specific Uniform Hazard Response Spectra is available, along with nonlinear material properties for concrete and reinforcement steel. Results show that the structural response of the spillway can be estimated for a given acceleration spectrum, this information can then be used for screening-level Dam Safety risk estimation purposes.This nonlinear finite element modeling approach aims to expand the tool set available for structural seismic analysis by offering a defensible first iteration structural response estimate without developing a time-consuming and expensive full-scope 3D finite element model.