Challenge

Seismic site response analysis is an analysis performed to evaluate the behavior of soil deposits and/or earthfill facilities such as landfills and dams when subjected to strong ground shaking.  The most advanced version of this analysis is called effective-stress site response analysis because it includes seismically-induced pore water pressure generation and dissipation.  Effective-stress analysis is commonly used in engineering practice, yet practical guidance has historically been unavailable.

Solutions

In 2021, the National Academies of Sciences, Engineering, and Medicine (NAS) hired GLA to provide a practical guidance document for effective-stress analysis. Our team, led by Dr. Neven Matasovic, began an extensive review and evaluation of computer programs and constitutive models, including site-specific evaluation of design earthquake ground motions and element tests. Our team used design accelerograms, or seismological input into Site Response Analysis (SRA), and focused on effective stress SRA, i.e. SRA with porewater pressure generation and dissipation, which may be used for effective design purposes and to evaluate soil-foundation-structure interaction effects. Our team used these tests to develop guidance on the best uses for effective-stress SRA. We also performed various element and system tests to develop recommendations for element testing to include both the fitting of results of drained testing and results of undrained cyclic testing. In addition to computer programs, our team also studied in-situ sampling and testing of previously liquefied soil, advanced geotechnical laboratory and centrifuge testing, and back-analysis of seven well documented case histories.

Results

Our team successfully completed a Guidance Document in early 2024, and it was formally published by NAS in August 2024. In addition to practical guidance, the Document provides answers to several common questions, such as: (i) How accurately can a nonlinear effective-stress analysis program simulate actual phenomena? (ii) What is the proper way to use these programs in engineering practice? (iii) Which site-specific input parameters are required, at a minimum, for analyses (e.g., for site characterizations, constitutive models, seismic loading / input motions etc.)? (iv) What are the capabilities and limitations of commercially available programs and to what extent and for what problems/geosystems have they been validated? (v) What is the process of model setup? and (vi) How should the results of nonlinear effective-stress modeling be interpreted and used? The U.S. Department of Transportation subsequently included our Guidance Document as Chapter 5 in the Geotechnical Engineering Circular 3, commissioned in August 2025. The 10th edition of AASHTO LRFD Bridge Design Specifications will be based upon GEC-3.

We are particularly proud of our work in producing a much needed practical guidance in an area that has historically been lacking

— Neven Matasovic, PhD, PE, GE / Project Manager