Seismic assessments of various assets given the large risk and complexity of the problem involved can be overly conservative using conventional methods. Many asset owners face the problem of responding to regulators regarding their asset seismic stability. To respond appropriately and cost-effectively, it is essential to manage their risk by applying solid engineering practice and using the “Best Applicable Practices” (BAP) approach.
Best Applicable Practices in seismic assessments can be defined as the application of the best available yet practical, technologies and methods to determine potential earthquake loading, define representative soil parameters, and apply site-specific analyses, all with minimum use of assumptions.
Site Review
First and foremost, Best Applicable Practices start with the diligent review of site-specific information from available documents. This enables an engineer to determine the critical subsurface materials, i.e., delineation of any loose or strain softening deposits or delineation of competent rock, where supplemental information may be necessary to collect.
Subsurface Characterization
In any geotechnical assessment, subsurface characterization is the most crucial portion of the assessment. One example of critical soils for seismic assessments are soils that tend to reduce their volume, i.e., contract under rapid loading, like earthquakes. These contractive soils, if saturated, can dramatically lose strength during an earthquake loading. The field and laboratory testing programs need to be designed to target characterization of these critical soils.
Field Investigation
- Seismic Cone Penetration Tests (sCPTu) can provide lots of invaluable data quickly. Using CPT data to guide and plan for sampling can improve field data quality.
- Good quality drilling data with mud-rotary borings and high-quality soil samples using Osterberg samples are key for field investigation for seismic assessments.
- Instrumentation that includes piezometers, inclinometers, and settlement plates is quite powerful. Measured pore pressure data can have a direct effect on stability results because the actual conditions are rarely hydro-static.
Laboratory Investigation
- X-ray photographs of soil samples help in selecting quality samples.
- Performing a suite of index tests helps to classify each layer and plan for advanced testing.
- Conducting advanced tests help to characterize the contractive materials and their representative shear strengths.
- Performing advanced cyclic and dynamic tests on critical, contractive materials helps detail the subsurface characterization for dynamic models.
Stability Assessment
Site response to seismic load
Site amplification analysis, either 2D or 3D, is essential for evaluating a slope’s response to seismic loading, a critical factor in assessing seismic stability. Although 3D analysis provides a detailed response, it is often prohibitively expensive, especially for asset owners managing multiple or historical sites. In contrast, 2D site amplification offers a practical and cost-effective approach to estimate a slope’s response to seismic forces, capturing the important 2D effects that influence slope stability. A 1D analysis typically falls short, as it lacks these considerations. By using 2D analysis, engineers can better understand how an earthquake impacts the slope’s sliding mass and overall stability.
Stability analysis with seismic load
Inherent to the seismic assessment fundamentals, engineers need to consider some level of movement of the slope, often called “tolerable displacement”. However, regulators generally speak the language of “factor of safety,” where no movements are allowed, leading to very cautious assessments with large, simplified coefficients representing the seismic load.
A Best Applicable Practice in seismic slope stability is to determine a seismic load coefficient that aligns with a tolerable level of displacement. This displacement-compatible seismic load coefficient is found by performing a series of sliding block analyses, which estimate how much a sliding mass would move under different seismic loads. These analyses create a relationship between displacement and seismic load, allowing engineers to select a seismic load coefficient that matches their acceptable displacement level. This coefficient is then used in a limit equilibrium analysis to calculate a factor of safety. This method enables engineers to present a stability assessment based on displacement in terms that are readily understandable for regulatory review, specifically through the language of factor of safety.
Seismic assessments of slopes by no means are simple assessments, however, with Best Applicable Practices followed, comprehensive yet practical analysis methods can be performed and can be communicated to the asset owner and their regulators.
To learn more about our experience in seismic assessments, contact us at info@geocomp.com.
Post by: Seda Gokyer Erbis, PE, PhD. Seda is a Geotechnical Engineer and Senior Project Manager for Geocomp’s Consulting Group. She has been with Geocomp for nine years, holding a doctoral degree in Geotechnical Engineering. She has been leading the project management and technical efforts for one of Geocomp’s largest consulting projects on seismic assessments. With more than 13 years of experience, Seda specializes in geotechnical and earthquake engineering, focusing on research, development, and consulting. She has led numerous research and consulting projects utilizing advanced geotechnical and geophysical field methods, as well as sophisticated laboratory testing programs. She has also authored and co-authored several publications in peer reviewed ASCE and ASTM journals and conference proceedings.
Cover Image: The Asahi Shimbun/Getty Images
