Replace Low Level Outlet Works

Replace Low Level Outlet Works

Location: Highland, Ill.

Client: City of Highland, Illinois

Completion Date: 2007

The City of Highland, Ill., has retained Burns & McDonnell to design the replacement of the low level outlet works at Silver Lake Reservoir. Silver Lake is a 600-acre lake with a watershed of approximately 50 square miles that serves as the primary water supply for the city.

The project, 4 miles north of Highland, consists of the following elements:

  • The outlet tower, a vertical concrete structure that is the emergency spillway, will be completely demolished and replaced. Replacement is necessary because of structural damage to the existing tower. The existing dewatering conduit will remain in service. The water gates in the new tower will be replaced with new vertical slide gates of identical size and sill elevation as the existing gates.
  • A temporary sheet pile cofferdam will be built around the tower site (on three sides) and dewatered to allow construction to take place in the dry. When construction is complete, the sheet piles will be cut off at the mud line, rather than being pulled out, to minimize disturbance to the dam. Following customary practice, the detailed design of the cofferdam and dewatering system will be the responsibility of the construction contractor.
  • A pad, or “working platform,” will be built out into the lake to support the construction cranes. The platform will be built by dumping rock fill on the northeast slope of the dam. It will remain in place after construction. The working platform is sized to accommodate two cranes: a 150-t crane to drive sheet piles and erect the tower, and a 20-t crane to unload trucks and assemble the 150-t crane.

The project has an estimated construction cost of $1,600,000 and was constructed in 2007.