New gravity dam protects contaminated sediments at Lake Conestee

A new roller-compacted concrete dam has been completed at Lake Conestee near Greenville, South Carolina, replacing the critical containment role of a structure dating from 1892 and removing a long-standing risk associated with contaminated sediments trapped within the reservoir.

For more than a century, runoff and industrial activity had contributed to the accumulation of contaminated material behind the original dam. Much of that material now lies beneath approximately 5 to 8 ft of cleaner sediment. The condition of the ageing masonry dam had therefore become a major concern, since a failure could have released contaminated sediment downstream and created a significant environmental impact.

The adopted solution avoided disturbing the reservoir deposits. Rather than demolishing or extensively modifying the existing structure, a new roller-compacted concrete gravity dam was constructed approximately 50 ft downstream, allowing the original dam to remain in place.

Creating a dry and controlled construction area within the active river required temporary river management works. Crews used 721 large sandbags together with a cofferdam system and diversion culvert to redirect flow away from the new dam footprint. This enabled foundation excavation and preparation to proceed while limiting interaction with the contaminated material upstream.

Foundation treatment became one of the most important parts of the works. Around 16,875 sq ft of bedrock was exposed and cleaned, with high-pressure water and compressed air used to remove loose material and prepare the surface. Grouting and dental concrete were then applied where required to create a sound and continuous bearing surface before placement of the RCC structure.

An onsite batch plant supplied the roller-compacted concrete, allowing material to be produced close to the dam and placed in rapid successive lifts. RCC is particularly suited to gravity dams because it combines the mass and durability of conventional concrete with a placement method that resembles earthworks, enabling large volumes to be constructed efficiently.

The completed dam has a crest approximately 637 ft long and includes a central overflow spillway and energy dissipation directly onto bedrock. The design therefore combines the stability of a massive gravity structure with a controlled route for flood flows and protection against downstream erosion.

The project also illustrates how environmental constraints can fundamentally shape construction methodology. Disturbing the contaminated sediment would have introduced additional risk, so the new structure was designed and built around the requirement to leave the historic reservoir deposits largely undisturbed.

Construction reached substantial completion around 12 months ahead of the amended contractual schedule. The result is not simply a replacement dam, but a new permanent barrier that secures a reservoir containing more than a century of accumulated industrial legacy material while allowing the original 1890s structure to remain in place.