One of Illinois’ most heavily stocked lakes for fishing, Lake Vermilion is a 1,000-acre reservoir that serves as the primary water supply for the Danville area. It’s impounded by a high-hazard dam owned and operated by Aqua Illinois. The utility enlisted Schnabel to perform routine safety inspections as well as other services that ultimately led to recommended improvements to the dam.

Built in 1926, Lake Vermilion Dam consists of an earth embankment with a concrete-core wall, a concrete spillway section with 10 radial gates and an upper-level trash gate, and a non-overflow concrete gravity section. In 1986, the dam was modified to accommodate a 5-foot increase in normal pool. Thirty years later, engineering evaluations revealed naturally aging infrastructure in need of rehabilitation in order to help ensure continued reliability for years to come.

We provided annual independent dam safety inspections starting in 2016. In 2017, we performed new hydrology and hydraulic modeling and breach analysis, and updated the Emergency Action Plan, to understand impacts of having multiple gates out of service during construction. In a hands-on, detailed inspection of the gates, we used rope access techniques to evaluate possible reuse of portions of the gates or anchorage, followed by structural analysis of the gates. We also evaluated spillway capacity and global stability.

Based on the results of our analysis and inspections, we recommended replacement of the gates and bridge, the installation of post-tensioned anchors, and spillway capacity upgrades. Our detailed gate inspection and hydraulic analysis were used to develop a prioritized replacement sequence. We developed an alternatives analysis for gate and bridge replacement as well as to improve spillway capacity and global stability of the dam. A site-specific probable maximum precipitation (PMP) study was performed and resulted in reduced design inflows and a significant cost savings for potential spillway capacity upgrades.

The dam’s rehabilitation was tackled through a phased approach. Phase 1 (2018) included interim repairs to the gates based on the hands-on inspection, a subsurface exploration, and early contractor involvement during rehabilitation design for constructability reviews and cost-estimating. Phase 2 (2019–2020) included replacing the radial gates, trash gate, and bridge and the installation of temporary ballast blocks to compensate for a reduction of stabilizing weight during construction. Phase 3 (2020) included the installation of 26 post-tensioned anchors to improve the stability of the concrete dam and the removal of ballast blocks.

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