At JCSE, we continue to receive questions whether the 2007 California Building Code (CBC) requires seismic loads induced by retained soils be accounted for in the design and construction of retaining walls. The short answer is yes.

There is little evidence that indicated retaining walls had failed due to seismically induced loads. Thus, the CBC didn’t prescribe a seismic force be accounted for. How the CBC has been modified is to simply not be silent on the topic. The CBC (ASCE 7-05 11.8.3) requires a Geotechnical Engineer consider what the seismically induced loading on a retaining wall is. The Geotechnical Engineer could of course, determine that force to be zero if the conditions and soil type of the site are such.

The simplified Mononobe-Okabe formulation (initially postulated in the 1920s) shown below captures the seismically induced loading on a retaining wall reasonably well:

Pae = (3/8)kjh^2

Where j is the weight of the soil and H is the height of the retained soil against the wall. k is where the crux lies. It can range from 0 all the way up to the site’s peak ground acceleration as determined by Sds/2.5. This can lead to values in excess of 1g depending on the site. Good judgment in the selection of kh must be used. Simply using the site’s peak ground acceleration will lead to an unduly conservative retaining wall design with a corresponding construction cost premium.