Borehole Velocity Logging For The Caltrans Bridge Retrofit Program

The influence of local soil conditions on ground shaking and the potential for resulting<br>structural damage has long been recognized. Two of the primary soil properties affecting the<br>intensity of ground motion are shear- and compressional-wave velocity. As part of a multidisciplinary<br>approach to the seismic retrofit of toll bridges, the California Department of<br>Transportation uses a suspended borehole probe to measure in situ compressional- and shearwave<br>velocity, both for onshore and offshore environments. These data are used as input for<br>ground-motion and foundation response analysis. The probe contains a dipole source that<br>directly produces compressional waves. Shear-wave velocity is measured via the low-frequency<br>component of the borehole flexural mode. A new empirical relationship between P-wave<br>velocity and density improves the fit to observed data. Experiments with logging through freehanging<br>plastic casing produced mixed results. Although resolution is low compared to<br>conventional sonic logs, the velocity data provide good delineation of gross stratigraphy (bed<br>thickness > 1 meter).