Concurrent Session 04A: Lessons Learned from a Complex Structural Problem - Design & Construction

Abstract Description: Indian Lake Dam is a cyclopean concrete and masonry dam constructed circa 1898. During rehabilitation to address stability, seepage, and general upgrades, a dive inspection into the gatehouse revealed extensive damage. Although minor repairs were always expected, the condition was much worse than anticipated and long-term performance of the structure was in question. The gatehouse is roughly 40 ft below water and the structure's issues complicated the repair solution. The challenges included 5-foot-diameter low level outlet conduits that had extensive bypass leakage making access dangerous/difficult, masonry construction with severe mortar loss that rendered the structural capacity extremely limited, and some portions of the support walls that were completely inaccessible making them very difficult to address. Design Lessons Learned: The design team developed plans to repair the structure using a three-sided reinforced concrete facing constructed with tremie concrete & divers. An early design modification enacted at the request of the contractor was to use precast concrete panels in lieu of conventional formwork which had several benefits. First, the thimbles for exterior sluice gates could be cast into the precast allowing the divers to more easily align the thimble with the existing opening. The exterior layer of reinforcing could be cast within the panel and formwork for the corners and vertical joints between the panel could be easily placed against the panels. The design team worked through the details which included room for lap lengths at vertical panel joints, room for standard hooks at the corners, and tolerance to allow for dimensional variations in the bedrock and existing dam. Construction Lessons Learned: Construction progressed through the harsh northern Adirondack, NY climate when the lake level was seasonably lowered. Even with the drawdown, the divers still had 30+- feet of water depth to contend with. How reinforcing detailing, tolerances, and interference challenges were overcome and other general lessons learned will be covered. What worked for specifics on tremie placement methods, placing reinforcement underwater, inspection, & verification of installation will be shared. Examples of how water management, diver safety, and other ancillary impacts will be included. This is a story of engineers, contractors, owners and regulators meeting the challenges presented by a complex problem.