The Rock Creek Dam is a concrete gravity structure 126 feet high with a crest length of 567 feet. Through dive inspections from another contractor, a depression in the rock backfill behind the left downstream training wall and void under the L-shaped training wall was discovered. If not mitigated, the undermining and void development could have potentially progressed into a dam safety issue.
Scope of Work
The repair occurred in a very remote location with challenging logistics for public road access that did not include access for equipment to the downstream toe. To facilitate construction, a temporary access scaffold was constructed on the downstream slope of the dam while all cofferdam and repair materials, along with the equipment, were lifted from a barge in the reservoir on the upstream side of the dam over the crest and lowered into the downstream toe.
Ballard teamed with a local engineering firm to review the provided concept cofferdam mitigation design and found it not feasible. The team hosted an engineering workshop to formulate cofferdam layout options and developed an innovative, viable option. This option, a two-sided sheet pile gravity cofferdam, eliminated the need for walers and struts initially found to impede construction and decreased the potential leakage over the originally proposed single wall system. Frames or guides, with adjustable support H-Pile spuds, were fabricated and set/leveled on the bottom. The spuds penetrated the sediment layer and were supported on the rock below, using a small vibratory hammer fitted with a sheet jaw. This frame was used for construction access and acted as a template for both sides of the sheet piling. The sheets were laced into the template and vibrated with the same small vibratory hammer to ensure they sat on the rock layer. Supersacks and membrane material were then placed at the base and sides of the sheet pile cofferdam to seal any openings. The inside of the system was then filled with rock and sand material. Once filled, the cofferdam became a gravity structure resisting sliding and overturning. This approach, when combined with a supersack cofferdam system, conformed to the irregular shapes and field conditions, as well as acted as an extension of the existing dam flume extension wall.
Once the cofferdam was dewatered, a post-inspection was performed, and it was determined that the undermining and void were far less than originally reported. An initial proper dive inspection could have potentially resulted in a far less scoped project or postponement of the project. Ballard repaired the existing damage and completed the project safely, on-schedule, and under budget.
Client: Pacific Gas & Electric
Location: Auburn, CA
Diver Depth: 25ft