The Office of Geotechnical Design North (OGDN) provides expert Soils, Rock, and Foundations recommendations for structures and Roadways throughout the Northern and Central Portions of California. The Service Area for OGDN is comprised of Districts 1, 2, 3, 5, 6, 9, and 10.

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SAMPLE CURRENT PROJECTS

Feather River Bridge at Pulga
03-BUT-70

A foundation study was conducted from October to December 1999 by personnel from Geotechnical Services for the proposed strengthening and retrofitting of the Highway 70 North Fork Feather River Bridge at Pulga. The field study involved drilling borings at the existing pier and bent locations. The existing bridge is an arch bridge with the piers and bents notched into the cliff sidewalls on either side of the river. The bridge is in a scenic and environmentally-sensitive setting, so all investigations were carefully conducted to minimize potential impacts.

At the bent locations, drill crews were able to drill through the existing bridge deck using conventional drill rigs and set casing to the ground below. However, at the pier locations, a portable CS100 drill rig was needed. Parts for the portable drill rig were lowered approximately 100 feet from the bridge deck to the top of each pier and assembled. Climbing gear was required for the project geologist, Tim Alderman, and drill crews to rappel each day to the top of the piers where the borings were performed. Hazards encountered by Caltrans crews included falling rock along with hazardous climbing conditions during rainstorms. However, the most annoying hazard turned out to be a constant deluge of storm water from one of the deck drains which unfortunately happened to be located 100 feet above one of the piers where drilling was performed. Despite the challenges, the geotechnical exploration was completed quickly and the retrofit is expected soon.

Buckhorn Summit Improvements
02-SHA-299, PM 0-7.4

The Buckhorn Summit Improvement Project is located on State Route 299 just west of Redding in Shasta County, California. This section of highway has a design speed of just 25 mph and 53 curves with associated radii of curvature as small as 49 meters (160 feet). A realignment is planned to combat higher than normal accident rates, high maintenance costs, and an uncommonly high rate of closures resulting from accidents and inclement weather. For this project, District 2 has had 27 alternatives under consideration which crisscross the landscape over the approximately 7.5-mile design length of highway.

The Office of Geotechnical Design - North was contacted to assist in preliminary design and alternative selection for this project and to provide input to the Buckhorn Erosion Control subcommittee. This project presents geotechnical challenges due to the highly erodible nature of a majority of the site soils/bedrock, the rugged terrain with steep natural slopes, the lack of good overland access to proposed alignments, and the need for large structures. Unique aspects of this project may include a combination of large bridges, MSE or soil nail walls, RSS slopes, tunnels, and large cuts and fills. Geotechnical investigation for the structure portions of this project will likely involve the use of helicopter placement of drill rigs and support personnel.

Highway 12 Passing Lanes and Shoulder Widening
10-SJ-12 KP 0.2/16.3

Route 12 in San Joaquin County connects I-80 and I-5, and is the principal connector between Fairfield, Rio Vista and the Central Valley. The existing 2-lanes of roadway that composes the majority of Route 12 are projected to be inadequate for the traffic loads it will experience in the near future. The long-term goal for Route 12 is conversion to a 4-lane expressway. For this project, passing lanes and a median will be added to a section of Route 12. The project section of highway traverses Bouldin Island between the South Fork of the Mokelumne River and Potato Slough. At the project location, Highway 12 is built on fill with a maximum height of approximately 3.5 meters above the adjacent ground.

Section E’s role in this project was performing a geotechnical investigation in support of widening the existing fill embankment to provide passing lanes for both eastbound and westbound traffic as well as widening the shoulders. The project covers a distance of approximately 6.6 kilometers.

The site is underlain by a significant amount of compressible soil. Layers of peat and soft clay underlay the site to a typical thickness of 12 meters. The existing embankment has historically experienced significant settlement damage, requiring significant efforts by maintenance to keep the highway in acceptable condition. Construction of the new fill is expected to result in significant settlements. Section E’s recommendations to mitigate long-term settlement damage resulting from the new fill placement includes wick drains, surcharge loading, lightweight fill, staged construction and an instrumentation program.

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Last Updated June 5, 2007

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