Considerations for Subsurface Investigations for Seismic Analysis of Embankment Dams and Levees

Seismic analyses embankment dams and levees depend on many important factors in order to reliably predict expected performance. The main factors for reliable forward seismic analyses include (1) seismic hazards evaluation, (2) characterization of subsurface conditions, (3) foundation and embankment characterization and materials parametrization to represent dynamic strengths and stiffnesses, (4) suitable characterization of any structural project elements, (5) appropriate applications of concepts, relationships, and constitutive models, and (6) (6) engineering evaluation of seismic analysis results, considering current modeling capabilities and constraints. Significant progress in seismology currently allows practitioners to evaluate and predict seismic hazard levels at project sites with greater confidence, and to better characterize and develop suitable seismic “input” motions. Similarly, recent advancements in numerical modeling and the engineering use of appropriate relationships, constitutive models, protocols, and analytical platforms have led to advances in our ability to perform project-specific seismic analyses for dams and other critical buildings and infrastructure potentially susceptible to seismic soil liquefaction hazard and/or cyclic softening and strain softening of soils. When it comes to performing geotechnical and geophysical site investigations for seismic analyses, sometimes the importance of selection of appropriate methods, and key details of execution of these methods, are underestimated. This can lead to improper execution due to a lack of understanding of method-specific limitations for some sets of site-specific conditions. Most of these method-specific considerations are addressed in ASTM procedures. However, due to lack of awareness regarding intricacies or details of many geotechnical and geophysical investigation methods, incomplete or misleading data can be produced. In some cases, the resulting data can lead to incorrect seismic analysis results. An evaluation of current state of practice protocols has been performed based on experience from a number of USACE projects. These project-specific and site-specific experiences provided insights regarding details involved in performing and interpreting different investigative methods such as the Standard Penetration Test, Cone Penetration Test, Becker Penetration Test, Dynamic Penetration Test, borehole and surface geophysical tests, soil sampling methods, and laboratory tests. A set of guidelines have been developed that may be useful in selection and performance of appropriate methods, considering (1) project-specific objectives of seismic analyses, (2) project site conditions and the selection of suitable field investigation methods and details of their execution in field, and (3) suitable data interpretation and application to analytical model parameterization. If properly implemented, these guidelines may help to prevent misleading interpretation and characterization of in-situ conditions, and may increase reliability in performing forward seismic analyses to evaluate expected seismic performance.