Chorro Creek Bridge - Twin Bridges Replacement, Morro Bay, California
The timber-supported Twin Bridges were deemed structurally deficient and functionally obsolete due to the deterioration of the abutments and pile caps, and the frequency and duration of closures due to flooding of Chorro Creek.

The Twin Bridges were replaced with a single 300-foot-long, 7-span, concrete slab structure supported on concrete piles. Fugro provided geotechnical services, which included field investigations consisting of four exploratory drill holes and six cone penetration test (CPT) soundings. To minimize disturbance to environmentally sensitive areas, CPT tests were performed using an all-terrain vehicle to access heavily vegetated riparian areas without clearing access roads.

To reduce the potential for liquefaction to impact the pile-supported structure, ground densification was recommended using stone columns. Approximately 600 columns were installed prior to construction of the bridge. Post-installation testing indicates that the required densification was achieved. To reduce the potential for excessive settlement of soft soils between the bridge and the approaches, portions of the approach embankments were designed using lightweight concrete fill materials. The study evaluated expanded polystyrene, shredded rubber, cellular concrete, and lightweight concrete aggregate materials, with cellular concrete fill selected for the project.

Design challenges presented by the project included soft and liquefiable soils, high groundwater, and construction in an environmentally sensitive estuarine environment. Also, because the cost of closing the bridge was estimated at $32,000 per day, maintaining a fast-track schedule was critical.