Abstract:
The Mosquito Road Bridge Project consists of the construction of a 1,200 foot long cast-in-place balanced cantilever bridge spanning up to 400 feet above the South Fork American River gorge at a remote site near Placerville, CA. The bridge has two main hollow column piers that are over 230 feet tall. The piers are founded on 9 foot thick pile caps approx. 1,100 cyds each in volume that sit on 36-inch CIDH piles and require the foundations to be embedded in the mountain side requiring over 10,000 square feet of soil nail and ground anchor retaining wall. Access to the remote pier locations on either side of the river is via steep, winding county roads, with no prior construction access between the two sides of the canyon. The owner of the project, El Dorado County, decided on a bid process where the bidders would be required to design and implement construction access. The Shimmick/Geoengineers team decided to approach access via design-build forest roads that lead to two design-build access trestles totaling over 12,000 square feet of working space, with one trestle over 60 foot tall at its highest point. The access trestles are supported on foundations consisting of 36-inch diameter CIDH piles with embedded steel pipe piles drilled into varying conditions of residual soil composed of sand, over decomposed to weathered bedrock. The CIDH piles required excavation of challenging cut slope and fill benches and detailed sequencing on the steep terrain, some of which was 40 degrees or steeper.  The presentation will provide an overview of the access trestle and pile design from the original siting; load rating and pile design; to the challenges encountered and overcome during construction.