The Benefits of Deeper Subsurface Investigation at a Site with Unknown Bedrock Depth in Seismic Site Response Analyses: Fid-Bau Portal

The Benefits of Deeper Subsurface Investigation at a Site with Unknown Bedrock Depth in Seismic Site Response Analyses

Griffiths, S. C. / Frazier, J. D.

This paper discusses the data collection, ground motion scaling, development of the model parameters used and the results of a seismic site response analyses at a site where multiple surface wave investigations yielded different bedrock site classes. It is well known that input ground motion selection can greatly influence the predicted response from a site-specific analyses. Another important factor is the depth to bedrock in the soil model. These two factors are interdependent on one another. In fact, the stiffness (shear wave velocity) of the half space where input motions are applied, directly influences the target spectrum (i.e. hazard) used to scale the input ground motions. At a bridge site near Jackson, Wyoming, two different data collection, and hence shear wave velocity profiles have been used to predict surface spectral accelerations using site response analysis. At this site overly-conservative predicted spectral acceleration would have resulted in greater costs if deeper, better quality surface wave data had not been collected. The greater depths of investigation resulted in a stiffer half-space, and hence a more favorable target spectrum. This greatly reduced the predicted design response spectrum at the surface at all periods.