Discharging Compacted Sediment by Underwater Blasting in Conjunction with a Desilting Tunnel in the Tseng-Wen Reservoir of Taiwan

Abstract

Sediments transported by heavy rainfall result from frequent typhoons in the Tseng-Wen reservoir, which now comprise one-third of the capacity of the reservoir. Yearly increases of 6.56 million m3 deposits are planned for ejection via a newly operational desilting tunnel to extend the serving capacity of this reservoir, which is the most important reservoir in Taiwan. The first worldwide experimentation of cooperative underwater blasting to loosen compacted sediment within a reservoir, then dispelled by a desilting tunnel, is carried out for demonstrating the feasibility and efficiency. Precautionary measurements are very strict for managing underwater explosions in an operating reservoir without jeopardizing its mission and structure safety. When choosing the detonating locations, the sediment thickness is determined by a shot wave spectrum ratio method with a small amount dynamite test shot for avoiding the bed rock fracture affected by the blasting. Vibrations, turbidity, underwater life, and water content affected by the explosion have all been assessed. When applying counter measurements before the detonation, affecting the underwater life could be reduced; the influence of water quality by a water gel dynamite explosion was very minimum and vanished a few hours after the blasting. A distance of 50 m away from the dam is set as a safety perimeter for an underwater explosion, for which the specified amount of dynamite is determined with a consideration of distance and the applied method of blasting. The free drop method can use 10 times the amount of dynamite than in-hole blasting at the same location and where the affected deposit volume is around 10 m3 per kg via free drop blasting. In-hole blasting disturbs more compacted sediment than free drop methods, and the effect of an applied in-hole blasting in combination with a traditional mechanical dredge is shown to lead to a 250% improvement. To achieve the best desilting tunnel efficiency, a deep underwater groove via blasting is recommended. With proper design, half of the existing compact deposits in the Tseng-Wen reservoir could be removed, thus increasing the capacity of the largest reservoir in Taiwan as much as 15%, which is equivalent to 75 million m3. Recompacting the underwater blasting disturbed sediment still could be expelled via desilting tunnel water discharging six months after the blasting operation shown by this experiment. This finding supports the feasibility of operating the blasting work at the dry season and waiting for the rainfall the loosen sediment away via desilting tunnel discharging.