Mt. Pisgah Dam

Challenges

Mt. Pisgah Dam is an 80-foot-tall, significant-hazard dam located in Teller County, Colorado.  The original outlet works consisted of a concrete intake structure, lined inlet tunnel, parallel 30-inch-diameter steel pipes, gate valves for flow control, and an unlined rock discharge tunnel.  The steel pipes were significantly corroded and the gate valves had become difficult to maintain and could only be opened partially.  The outlet works lacked an upstream guard gate required by Colorado Office of the State Engineer (SEO) dam safety regulations.  The dam also had two previously abandoned 16-inch-diameter outlet pipes and it was unclear if these pipes required remediation.

The owner required construction to be completed without draining the reservoir.

Solution

The reservoir was initially dewatered, and RJH performed a detailed inspection and assessment of the outlet works.  A prior attempt to perform the inspection underwater was unsuccessful because of water clarity issues.  A video inspection of the two abandoned 16-inch-diameter outlet pipes was also performed as part of the initial assessment.  Based on the inspection and assessment, RJH developed a series of alternatives to rehabilitate the outlet works.

RJH performed conceptual through final designs, and provided construction phase services for the project.  The selected alternative included installing a new inlet structure with a hydraulically-actuated slide gate, slip-lining the existing 30-inch-diameter steel pipes, and installing new downstream gate valves.  Other site work included full-length grouting of the abandoned outlet works pipes and installation of a filter diaphragm at the downstream end.

RJH utilized early contractor involvement for underwater construction to evaluate construction details and challenges of underwater construction.  All construction was subsequently performed without draining the reservoir.