Blue Lake Hydroelectric Expansion Project-Tunnel Excavations

摘要

Conventional drill and blast methods were used to successfully excavate three new tunnels and two new shafts, employing typical rubber tired equipment and a mechanized raise climber, at the Blue Lake Expansion Project. The underground excavations were one phase of the ongoing $88 million Blue Lake Expansion Project owned by the City and Borough of Sitka, Alaska, a primarily fishing and tourism based community of approximately 10,000 residents located in southeast Alaska. Working in a remote island community accessible only by plane or boat presented many challenges. Underground excavations consisted of a 42.67 m (140 ft) long exploratory tunnel located 60.96 m (200 ft) from the base of the existing Blue Lake Dam, accessible only by crane; a 143.26 m (470 ft) long access adit developed to facilitate excavation of a 99.67 m (327 ft) surge shaft driven from the bottom up, using a Mechanized Raise Climber (MRC) breaking through in a remote surface location accessible by helicopter only; a 256.03 m (840 ft) long intake tunnel which portaled in 9.14 m (30 ft) above Blue Lake, Sitka's drinking water supply; and a 33.22 m (109 ft) gate shaft driven from the new intake tunnel, excavated with an MRC. This paper will provide a description of the logistical and engineering challenges involved with performing underground excavations in this isolated and demanding environment. Work areas accessible by crane and helicopter only, working directly above Sitka's drinking water supply, and excavating to within 9.14 m (30 ft) of the Blue Lake Dam and existing penstock tunnel are a few of the many factors which challenged this critical project.