AVALANCHE IMPACT PERFORMANCE OF A LIGHT-WEIGHT DIVERSION STRUCTURE

ABSTRACT: The 138kV Snettisham electrical transmission line is the main power source for Juneau, Alaska. The line was constructed in 1973 and traverses steep remote terrain exposed to numerous avalanches. Tower 4/6 is particularly exposed and has a history to prove it. On April 7, 1976, less than three years after construction, it was impacted and damaged by an avalanche. In 1982, it was moved to reduce its exposure. On April 16, 2008 a large avalanche destroyed tower 4/6. Several protection options were evaluated, including tower reinforcement, active control, a submarine cable, and a massive reinforced concrete splitting wedge. Based on cost and risk factors, the power company selected an unconventional defense structure consisting of a guyed and braced steel wedge with gaps between wing members. The structure is approximately 11 meters high with an apex angle of 37 degrees. Construction was completed in October, 2009. The steel wedge had its first full scale test on March 8, 2012, when it was impacted by a natural avalanche with an estimated size of R3 and D3.5. This paper describes the observations and performance of the structure and its interaction with the avalanche debris. Weather records were analyzed, avalanche deposits were measured, and flow directions were estimated by field crews. Avalanche impact pressures were calculated based on snowpack data, crown geometry, and runout distances. The impact pressures and avalanche heights were compared to the design values. The information acquired and the lessons learned will be applied to future similar structures planned for the Snettisham transmission line