Sponsored by ASCE's Geo-Institute and ASCE Continuing Education.

"This was an excellent webinar, very excellently presented."

Up until recently, the methods for determining the seismic coefficient that have been used to design and build retaining structures have neglected important criteria.  This webinar discusses a performance based methods that will help determine much more accurate seismic coefficients that, in turn, will lead to much more economical slope and retaining structures design.

Pseudo-static analysis remains the most common method used in engineering practice today for the seismic design of slopes and earth retaining structures. While the seismic coefficient is one of the most important parameters in these types of analyses, relatively little guidance is available on the appropriate value to use in design. Rational evaluation of the seismic coefficient should consider the characteristics of the earthquake ground motions, the response characteristics of the slope or earth structure, the ductility of the slope or earth structure, and the ability of the slope or earth structure to sustain permanent ground displacement. Most guidance on evaluating the seismic coefficient was based upon work done over 30 years ago that failed to take into account most of these factors.

Recently, several performance based approaches for establishing the seismic coefficient used in pseudo-static analyses of slope stability and retaining walls have been developed. In both of these approaches, the seismic coefficient depends upon the acceptable permanent seismic displacement as well as upon factors representing the earthquake ground motions and the response of the slope or earth structure to the ground motions. Analyses using these new methods demonstrate that a seismic coefficient equal to no more than 50% of the free-field peak ground acceleration (PGA) at the site, and in some cases less than 25% of the free-field PGA, is appropriate if the soil and structure behave in a ductile manner (i.e. no or small post-peak strength decrease) and 1-2 inches of permanent seismic displacement can be accommodated. Smaller values of the seismic coefficient are appropriate if greater seismic displacement can be accommodated.

Use of these performance based approaches to evaluating the seismic coefficient can lead to substantial economy in seismic design in many cases compared to designs based upon other commonly used approaches to estimating the seismic coefficient (e.g. a seismic coefficient equal to the PGA).

Primary Discussion Topics

• The relationship between the peak ground acceleration and the peak acceleration of the failure mass
• The relationship between the peak acceleration of the failure mass, the seismic coefficient, and permanent seismic displacement
• Selection of the appropriate shear strength for use in limit equilibrium analysis
• Appropriate combinations of seismic coefficient and factor of safety for use in design

Learning Outcomes

• Learn about a relatively new performance based approaches to establishing the seismic coefficient for pseudo-static analysis of slopes and earth retention structures.
• Apply this method to practical engineering problems.

Webinar Benefits

• Learn the basis for the seismic coefficient method
• Understand the factors influencing selection of the seismic coefficient
• Learn a state of the art approach to evaluating the seismic coefficient
• See how the allowable permanent seismic displacement affects the value fo the seismic coefficient
• See the influence of the seismic environment on seismic coefficient values
• Learn how to apply performance based criteria to establish an appropriate value for the seismic coefficient
• Achieve economies in seismic design while providing adequate margins for life safety and protection of property

Intended Audience

This webinar is intended for geotechnical engineers who are involved in seismic analysis of slopes and earth retaining structures. It may also be useful for structural engineers involved in retaining wall design. A basic understanding of limit equilibrium analysis, shear strength of soils, and earthquake ground motion characterization is assumed for all participants.

Webinar Outline

The Seismic Coefficient Method of Design of Slopes and Earth Retention Structures

• What is the Seismic Coefficient?
• Historical Background
• NCHRP 12-70 Method / FHWA Method
• Design Examples