Crater Lake Inspection
Tapping the bottoms of two glacial-fed mountain lakes, the Snettisham Hydroelectric Project provides reliable and low-cost power to residents and businesses in Juneau, Alaska. Constructed by the Alaska District of the U.S. Army Corps of Engineers between 1968 and 1973, its facilities were developed in two stages. Schnabel was retained to perform inspections of the two power tunnels four times between 1996 and 2015.
The project was originally authorized in 1962 by Congress to provide additional power to the growing population in Juneau. The first stage, Long Lake, consists of a 8,400-ft long, 13.5-ft diameter power tunnel, an 8.5-ft diameter, 900-ft long steel penstock, and an underground powerhouse housing two Francis-type turbines with an installed capacity of 47 megawatts. The second stage, Crater Lake, includes an underwater lake tap into a high mountain lake, over 8,000 feet of unlined power tunnel, a 900-ft deep surge shaft, an underground machine shop, and several large underground chambers, ranging in excavated volume from 37,500 to 180,000 cubic feet.
In 1996 and again in 2006, we inspected the Long Lake power tunnel. In each case, our team performed a visual inspection of the length of the tunnel and its installed support elements. While much of the power tunnel is unsupported hard rock, some areas required additional support systems such as rock bolts, shotcrete, mine straps, or full-perimeter concrete lining. We prepared a final inspection report documenting our findings as well as recommended actions and procedures for correcting any problems found.
In 2007 and again in 2015, we inspected the Crater Lake power tunnel. In each case, we conducted a walk-through of the tunnel while it was drained for maintenance. We performed a visual inspection of the tunnel facilities for any deterioration or damage and prepared a final inspection report documenting the results of our inspection as well as recommending actions and procedures to correct any problems identified.