Seismic

Learn more about the failure of Lower San Fernando Dam during a seismic event at DamFailures.org

“The scope of the seismic hazard study at a site depends on the seismicity of a region or site-specific considerations, the types of structures involved, and the consequences of failure. The design and evaluation of dams for earthquake loading should be based on a comparable level of study and analysis for each phase of the study (seismotectonic, geological, site, geotechnical, and structural investigations) and that level of study should reflect both the criticality of the structure and the complexity of the analysis procedures".^[1]

"Risk management and risk analysis can be used in making seismic evaluation of dams. These methods may be applied to help accomplish the following: prioritize safety evaluations when considering a large number of dams; evaluate the benefits of alternative remedial measures; select load levels; and evaluate the structural response. Risk management and risk analysis tools may also be applied as a framework for the overall seismic evaluation leading to the final decisions. The application of risk assessment procedures in a comprehensive and quantitative manner is used as a tool by various agencies to improve the quality and consistency of decisions on the seismic safety of structures. It is recognized that risk is considered in the application of engineering judgement even when more formal risk analysis procedures are not employed". ^[1]

Design Criteria

A Seismic Hazard Analysis (SHA) is an assessment of naturally occurring earthquakes using faults or past earthquakes to predict the hazard. The purpose of conducting a seismic hazard analysis is to develop seismic design criteria for use in evaluating the seismic response of a given structure or facility. Presently, there are three ways by which the design requirements can be ascertained: use of local building codes; conducting a Deterministic Seismic Hazard Analysis (DSHA) Deterministic Seismic Analysis; or performing a Probabilistic Seismic Hazard Analysis (PSHA) Probabilistic Seismic Analysis.

Determining which of the three above mentioned analyses that need to be completed depends on the owner or regulatory commission in charge of the structure that is being designed. It is important to recognize that different agencies, local cities, counties, states or countries could have separate design guidelines that may need to be reviewed to ensure that all guidelines are being followed.

Types of Seismic Evaluations

After the applicable codes and regulatory guidelines are determined, the next step is to understand what results are expected from a seismic hazard analysis. It is often helpful to discuss how the results will be used with the lead structural engineer, geologist or geotechnical engineer. Certain projects may need to have inputs for a liquefaction analysis; slope stability analysis or perhaps reservoir triggered seismicity is a concern. It is always important to understand the studies that may need to be completed before jumping into a seismic hazard analysis.

In the feasibility level analysis pseudo-static coefficients and simplified methods of deformation are sometimes used to estimate deformation. The preliminary design normally includes more complicated analyses, which sometimes requires a site specific spectrum or include development of time histories. Finally, detailed design in many instances includes a finite element method (FEM) analyses which requires time histories. In addition, foundation studies may require the use of SHAKE to deconvolute the time histories to a level of shaking experienced beneath the foundation.

A critical aspect of any engineering analysis is communication. There are a variety of seismic modeling approaches and methodologies, and it is important to owners, consultants, and regulators that clear communication is integrated in the process. General guidance and recommendations regarding both pre- and post-modeling communication are provided on this page: Modeling Communication.

Hazard Analysis

Seismic Source Characterization

Ground Motion Attenuation Relationships

Ground Motion Characterization

Dam-specific Considerations

Performance Evaluation

Design and Mitigation

Monitoring and Instrumentation

The most common seismic-specific monitoring/instrumentation is to record acceleration at a dam site. The data from these recordings can be used to evaluate the dynamic response of a dam and typically measures velocity and acceleration in three mutually-perpendicular directions. The location of instrumentation is incredibly important to determine accelerations on the structure, at the foundation, and the abutments. Understanding site-response at the dam is vital to interpreting recorded accelerations. To prevent accumulation of unwanted data, most instruments will be triggered to measure under relatively small accelerations.

Following any significant seismic event, the dam structure should be inspected. An action-level threshold should be pre-determined depending on the vulnerability of the dam, downstream hazard, and level of shaking. Post-earthquake inspections focus on the earthquake-induced damage, signs of distress, and any changes to the foundation or dam that could indicate a problem. Embankments are typically evaluated for cracking, slumping, and other signs of movement. Concrete dams are checked for signs of structural distress such as cracking or spalling and movement. Spillways and outlets are checked for signs of instability and to ensure they are clear of debris and fully functional. Seepage is evaluated for changes in quantity, quality, and new locations of seepage. It is common for instrumentation to change suddenly after an earthquake and enhanced monitoring is often necessary following the event.