CHALLENGE: Century-old dam rehabilitation project

CLIENT: Aquarion Water Company of Connecticut

PROJECT: Means Brook Reservoir Gatehouse and Dam Rehabilitation, Shelton, CT

THE CHALLENGE: The Means Brook Reservoir Dam, a concrete gravity dam constructed on rock in 1916, required repairs in order to improve the safety and reliability of the structure, replacement of the deteriorated upstream and downstream faces of the dam and the crest, and rehabilitation and modernization of the gate chambers. Hydraulic, hydrologic and structural studies over the last several years have shown that the dam would be overtopped by about 3.9 feet by the Probable Maximum Flood (PMF), and that the dam does not have adequate stability factors of safety for this extreme loading condition. The upstream and downstream faces and the crest of the dam were severely spalled and delaminated, and the dam reportedly overtopped by about four inches during the 1955 flood. In 1977, the spillway was extended to its current length of 100 feet and a 2.1-foot high concrete parapet wall was constructed along the upstream crest of the dam.

THE SOLUTION: Concrete repairs were made to the crest and both faces of the dam. The deteriorated and delaminated shotcrete and concrete were removed to sound concrete, anchor dowels and reinforcing installed, and new concrete surfaces poured. The upstream face was resurfaced to a depth of about six feet below spillway, and the downstream face to two feet below grade. Depth of concrete removal was between three and six inches. The new concrete surface was designed with a nominal thickness of 12 inches on the downstream face and six inches on the upstream face.

Rock anchors were installed and post-tensioned in holes drilled through the dam into the rock foundation. The anchors will improve the stability of the dam and provide adequate factors of safety for overtopping by the PMF.

One section of the spillway training wall was removed and replaced. The deteriorated concrete on the remaining sections was removed and the wall resurfaced.

The repairs and improvements undertaken to the gatehouse and chambers included reshingling the gatehouse roof, providing a new steel plate exterior door for additional security, and blocking up the existing windows. The antiquated electrical system was replaced. Both of the inoperable sluice gates and their appurtenances in the upstream gate chamber were replaced. The former vertical brass ladders in the intake gate chambers were replaced with new fiberglass ladders. A base for a fall protection and retrieval system was installed.

Wooden stairs in the lower gatehouse were replaced with a new aluminum ships ladder. The pneumatically operated butterfly valve on the 30-inch service main in the basement of the lower gatehouse was replaced with an electrically operated butterfly valve that can be remotely controlled. An extension stem and electric operator were provided. The operator was installed in the operating room above the valve, not in the basement. The 8-inch chamber drain and 10-inch auxiliary outlet gate valves were also replaced. Floor openings within the gatehouse previously covered with wooden hatches were replaced with aluminum grating with cover plates.

Chain link fence was installed along portions of the spillway training wall for safety. Chain link fence and gates were installed at both ends of the dam for security. Galvanized steel pipe railings have been installed along the upstream and downstream crest of the dam for safety. Steel pipe rails were chosen for the dam crest instead of chain link fence because the dam crest is subject to overtopping.

PROGRESS: The project was completed in August of 2015.