Geophysical Applications for Highway Infrastructure, a Retrospective-Paper

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The California Department of Transportation (Caltrans) has used geophysical methods for highway and bridge projects since the 1950s, initially on a limited, ad hoc basis. Their use expanded substantially after the 1989 Loma Prieta earthquake, when the state began a large seismic retrofit program. By 1996, Caltrans had brought geophysical logging in-house, and later established a dedicated geophysics unit. Guidance published in 2012 and 2017 formally incorporated geophysics into seismic design and geotechnical investigations.<br /><br />Caltrans combines geophysical surveys with drilling, sampling, and laboratory testing. Methods include seismic reflection and surface-wave surveys, borehole velocity and resistivity logging, acoustic televiewer imaging, and sonar. These techniques guide borehole placement, extend subsurface information between borings, and increase the value of data collected from each test hole. The goal is to reduce uncertainty during design and prevent construction delays and cost overruns caused by unexpected ground conditions.<br /><br />Two major projects demonstrate their value. For the San Francisco–Oakland Bay Bridge east-span replacement, extensive marine seismic surveys and borehole logs helped map bedrock, paleochannels, and sediment layers. The findings clarified foundation risks and supported alternative pile designs that improved constructability. For the proposed SR-710 tunnel in Los Angeles, seismic reflection and borehole investigations refined geological models, identified and repositioned faults, and assessed groundwater, rock conditions, and tunnel-construction challenges across several alignments.<br /><br />Caltrans has also broadened its work to include nondestructive evaluation of bridges, pavements, and tunnels, as well as underground utility locating. Its experience supports the Federal Highway Administration’s Advanced Geotechnical Methods in Exploration initiative and offers a model for other transportation agencies. The authors conclude that integrating geophysics into routine investigations can improve design decisions and lower overall project costs, despite modestly increasing early investigation expenses.

Keywords

Caltrans

geophysical methods

highway projects

bridge projects

seismic retrofit

borehole logging

seismic reflection

geotechnical investigations

Bay Bridge

tunnel construction