PROJECT PROFILES
Dumbarton and Antioch Bridges
Seismic Evaluation
San Francisco Bay, California
In November of 2006, Gregg Drilling was contracted to provide site investigation services for the Dumbarton and Antioch bridges located in the San Francisco Bay Area. The Dumbarton Bridge connects Newark, Alameda County to East Palo Alto, San Mateo County. It is the southern most highway bridge to span the San Francisco Bay and is also the shortest at 1.63 miles. The Antioch Bridge crosses the San Joaquin River, linking Antioch to Sacramento County and is part of California State Route 160. It was the first toll bridge built across a San Francisco Bay tributary and was originally opened in 1926. New regulations require that all bridges be re-evaluated under current seismic design criteria. CALTRANS turned to the Earth Mechanics Inc., a local California consultant for the expertise to perform this evaluation. In turn, Earth Mechanics subcontracted the major portion of the site investigation to Gregg Drilling.
For the Dumbarton Bridge, boreholes and Cone Penetration Tests (CPT’s) were completed on land where possible using standard truck and track mounted mud rotary drill rigs and 25 ton CPT rigs. In addition to Standard Penetration Test (SPT) and California modified samples, vane shear testing and PS logging were also completed at select locations. The vane shear tests were conducted in the bay mud clays to determine in-situ values of the undrained shear strength. On-land CPT’s were supplemented by seismic testing using the Seismic Cone Penetration Test (SCPT) to 100ft below the ground surface. Obtaining shear wave velocity from both PS logging and SCPT allows computation of a site specific seismisity analysis.
To complete the over-water work, Gregg utilized its custom designed drill ship, the Quin Delta. With a flat bottom and shallow draft of only three feet, the Quin Delta was able to sit on the mud at low tide and complete drilling and CPT operations. In total, 21 CPT’s were completed to 100 ft or refusal and 10 boreholes were drilled down to 250 ft.
At the Antioch bridge location, boreholes and CPT’s were completed on-land totaling 21 CPT’s with seismic testing to 100 feet. PS logging was also carried out in the boreholes to provide shear wave velocities in conjunction with those collected from the SCPT. Over-water work included 14 CPT’s to 100 feet and a number of boreholes to 200 feet complete with PS logging.
The thorough and complete evaluation of these two bridge sites illustrated the wide range of site investigation tools and equipment available. From in situ tests such as the CPT, SCPT, vane shear, and PS logging to more invasive testing such as drilling and sampling, geotechnical engineers have a large selection of site investigation tools available. It was shown that by supplementing the in situ tests with a number of boreholes and samples, the site investigation program was completed in a short amount of time and at a reduced cost compared to more traditional methods.
Dumbarton Bridge
Quin Delta drill ship conducting site investigation at low tide.