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Wednesday, January 18, 2012

The Geology of Seismic Site Class and the Effect of Site Class on Structural Design
A Joint CAGE/SEAC Event

Presenter: David Butler, Zonge International, Inc.


At least seven methods are used to measure Vs30. The gold standard of these measurements is crosshole seismic testing followed by in order of decreasing quality (and cost) downhole seismic, P-S suspension logging, cone penetrometer, s-wave refraction, multichannel analysis of surface waves (MASW) and refraction microtremor testing (Remi). Suspension logging, seismic cone, and s-wave refraction are seldom used due to difficult equipment, depth limitations, and interpretational barriers. Crosshole seismic has an ASTM standard (6429) but is costly. The other two methods depend on the correlation of surface wave velocities and s-wave velocities in layered rocks. The dispersion of surface waves makes them effective in noisy, i.e. most urban, situations.


Building codes and regulations attempt to sort natural materials into categories, specifically site classifications by Vs30 values. Mother Nature frowns on this engineering technique and has placed most building sites in the Denver at the boundary of site class C (Vs30>1200 ft/s) and D (Vs30<1200ft/s). A statistical compilation of more than 30 sites near Denver will be presented.
Depth to bedrock variations and precision of the measurements are the two main contributors to a potential error of up to 8-12%. The sounding model used for calculation in the surface techniques is one-dimensional; the measurement is two (or three) dimensional using a line up to 300 feet long. Engineering judgment is required when the Vs30 comes out as 1199 ft/s or when severe bedrock topography is measured or suspected. The exposure of the site to potential seismic events is an additional factor usually underestimated in the Denver area.

 

 

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CAGE is a proud partner with QBS Colorado. For more information, go to http://www.qbscolorado.org.


The Colorado Association of Geotechnical Engineers (CAGE) was established in 1995 by a small group of owners of geotechnical engineering firms in Colorado. This visionary group of founding members saw the need for a much greater exchange of technical and professional information between practicing geotechnical engineers, in an effort to advance the profession and to promote the public welfare.

Since CAGE was formed it has experienced steady and significant growth, with more than 120 current members from more than 60 different companies and organizations. Beyond the growth in the size of our membership, the organization has made great strides in promoting and improving the practice of geotechnical engineering in Colorado.
CAGE’s contributions to the profession have included the hosting of regular luncheon meetings with guest speakers, sponsoring local geotechnical seminars, establishing committees to evaluate and respond to various geotechnical issues, issuing several publications, and developing Geotechnical Study Guidelines For Light Commercial and Residential Buildings in Colorado. In addition, CAGE has promoted collegial interactions between members and practitioners in related fields of geology, as well as civil and
structural engineering.

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2/8/12 CAGE Luncheon - Lakewood Country Club
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