Dam Completion Facts

Features to be constructed in this contract include placement of zoned earthfill dam embankment, manufacturing of highly engineered sand and gravel filter and drain materials, special dam foundation treatment, concrete lining the outlet works tunnel, construction of an intake structure, installation of control and release valves and piping, construction of a control house at the outlet end of the tunnel, and improvements to Basin Creek downstream of the dam.

Excavation for Ridges Basin Dam foundation, outlet works, and tunnel were completed, or are nearing completion, under four separate previously awarded contracts. This contract will complete the construction started by these early contracts with actual construction of the dam and outlet works facilities.

Prior to placement of the embankment it is necessary to perform special foundation treatment to the rock to ensure the foundation does not permeate reservoir water. This will involve an extensive foundation grouting program. When completed the foundation grouting operation will require approximately 1300 drill holes, 51,400 feet of drilling and 28,400 bags of cement.

Ridges Basin Dam is a zoned earthfill dam composed of six distinct earth material types totaling approximately 5 ¼ million cubic yards of material. The impervious core of the dam is constructed using clay material obtained from the future reservoir area. Immediately downstream and adjacent to the clay core is an engineered processed sand filter backed by a processed gravel drain. These materials are held in place on both sides using a massive shell of material consisting of a mixture of sand, gravel and cobbles obtained from a source located downstream of the dam. The upstream surface will be protected using a rip rap (large rock boulders) to protect the dam from reservoir wave action erosion. The downstream face of the dam is protected from precipitation erosion with cobbles and small boulders.

Previously awarded contracts provided for the excavation of the dam's outlet works and tunnel. These excavations provide diversion capability for routing Basin Creek around the dam foot print foundation excavation which is some 70' below the elevation of the existing Basin Creek stream channel. These same excavations will receive the dam's permanent outlet works facilities to be constructed with this contract. The permanent outlet works consists of a 55.5 foot high intake structure, 72.5 foot long by 7.5 foot diameter concrete conduit section between the intake structure and tunnel, 659 feet of 7.5 foot diameter upstream tunnel concrete lining, a 25-foot diameter, 41.5-foot high concrete lined gate chamber with a 4-foot by 6-foot guard gate, 682 feet of 11-foot diameter downstream tunnel concrete lining, 752-feet of 66-inch steel discharge pipe located in the downstream half of the tunnel, and a 45-foot by 54-foot by 32-foot high concrete control building housing a 60-inch jet flow gate for emergency releases and a 48-inch sleeve valve for making releases down Basin Creek. Downstream of the control building the outlet works consists of a 95-foot long, 20-foot wide 16.5-high concrete lined discharge channel.

This contract will also complete Basin Creek Channel Improvement work which was begun with previously awarded contracts. An additional nine structures will be constructed in the existing Basin Creek Channel and approximately 2.6 miles of rock lined channel constructed. These improvements will allow water released from Ridges Basin Dam to reach the Animas River without increasing sediment loads above those historically contributed by Basin Creek.

The dam is scheduled for completion in 2008.