Geysers Road Bridge Replacement project over Big Sulphur Creek

 

Location

Los Lomas Drive, from Hall Road to Thomas Road

City or County Responsible for Project

Sonoma County

Category

Bridge: Efficient and Sustainable Bridge Maintenance, Construction and Reconstruction Projects

Description

The Geysers Road Bridge Replacement project over Big Sulphur Creek replaced a functionally obsolete, one-lane bridge with a new three-span concrete box girder bridge. The existing structure is a Historic County Landmark that remained in place, while the new bridge was placed downstream of the old facility.
The original single-lane bridge is one span of a three-span iron truss bridge with an interesting history. Originally a railroad bridge, it was reconstructed in 1909 and placed over the Russian River near Monte Rio. In 1937, the bridge was disassembled, and one of the spans was moved and reassembled at its present location over Big Sulphur Creek, approximately 18 miles north of SR128 and 9 miles east of Cloverdale. At that time, a 16-foot-long concrete span was added to the 130-foot-long truss span, for a total bridge length of 146-feet. This bridge is eligible for the National Register of Historic Places and is a Sonoma County Historic Landmark.
Deemed functionally obsolete in the Caltrans’ 2013 Bridge inspection report, Caltrans and the County determined that replacement was the most cost-effective alternative with low maintenance costs and sustainability for the future. Recent traffic counts indicated that the bridge carried a daily traffic volume of under 400 average daily traffic. The new two-lane bridge was designed to meet current standards including improvements to roadway approaches, travel lanes and road shoulders. The adjacent alignment of the new bridge curves across the stream to ease the existing abrupt, short radius turn at the southeast end of the bridge and minimize the amount of approach roadway required.
The new bridge is a 32-foot-wide, 250- foot-long, three-span, two-lane, cast-in-place/pre-stressed concrete box girder structure located within 40 feet downstream (southwest) of the original bridge, in the County right-of-way. The westbound and eastbound approaches extend approximately 300 feet and 400 feet, respectively, to reach the nearest attainable touchdown on either side. Located in the Big Sulphur Creek watershed, mitigations measures were established to protect fish and wildlife as well as the stakeholder properties nearby. Bypass culverts, a dam of clean river-run gravel, and K-rails were used during construction to temporarily divert the creek to ensure creek flow and velocities within the culverts were conductive for habitat life. Due to high scour potential on the creek, the new design uses 84″ diameter CIDH shafts at the piers and 24″ diameter CIDH shafts at the abutments along with “rock-slope protection” to protect the channel slopes from erosion. Traffic was maintained on the existing bridge during construction of the new bridge, as the available detour was excessively long.
The bridge was constructed in May 2025, and the former bridge was closed to vehicle traffic, including pedestrians and cyclists.
The project was funded under the HBP and LBRSA seismic retrofit program.