Swinging Bridge Dam

Swinging Bridge dam, constructed in the 1920s, is a hydraulic fill earthen structure, 965 feet long and 135 feet high, that retains a reservoir some 1000 acres in surface area. The dam is traversed by a penstock/tunnel that feeds water to one of the two powerhouses located at the base of the dam. When a 50-foot long, 30-foot wide, 20-foot deep sinkhole depression appeared in the crest of the dam, a fast-track investigative program was initiated to determine the underlying cause of the sinkhole, assess the overall condition of the dam, and determine the remedial measures necessary to meet all current dam safety standards. The investigation attributed the sinkhole formation to longterm settlement of the penstock/tunnel, causing it to crack. Water seepage through the cracks and joints created piping paths that resulted in soil erosion and the creation of subsurface voids. As the voids increased in size, the overlying arching soils became unstable and raveled into the void, forming the characteristic surface depression.

A complete remediation program was implemented under the jurisdiction of the Federal Regulation Energy Commission (FERC). In the first phase, designated for emergency activities, Moretrench repaired the main dam crest sinkhole and other smaller sinkholes along the crest and slopes of the dam face, and performed ancillary work. Second phase work focused on the installation of a conduit filter and toe drain system to limit the potential for the development of piping paths, further sinkholes, and uncontrolled seepages. Wave protection was also installed.

Given the emergency nature of the work, the sinkhole repair was a continuous, 24-hour per day, 7-days per week operation. The unstable materials were removed to stable subgrade, the excavation was then backfilled with engineered materials and compacted, and the dam was restored to its original condition. Thirteen sinkholes were repaired during the course of the work. Ancillary work included construction of an access road along the crest of the dam, and removal of a steel footbridge and its concrete foundation and complete replacement.

The conduit filter and toe drain system was installed on the downstream side of the dam. Components of the work included installation of two braced, steel sheet cofferdams; design and installation of a deep well system to maintain excavation stability; excavation of in-place soils; placement and compaction of engineered backfill aggregates including fine and coarse sand, gravel bedding, imported fill, and light and medium stone rip rap; and installation of horizontal drain piping. Approximately 40,000 tons of materials were imported and placed for the project.

Raising the effective height of the dam to provide wave protection was accomplished by the placement of alternating layers of controlled foundation materials and medium stone rip rap. Ancillary work included installation of permanent inclinometers, settlement monuments and piezometers, installation of access roads, and restoration of the site to its original condition.