Enasan Tunnel, Road tunnel in Achi and Nakatsugawa, Japan.
The Enasan Tunnel is a road passage in Achi and Nakatsugawa extending 8489 meters through the Kiso Mountains. Separate tubes carry uphill and downhill traffic within the expressway system.
Geological surveys began in 1957 and identified more than 130 faults in the rock. Construction crews managed high groundwater pressure reaching 50 kilograms per square centimeter during the work.
The tunnel connects traditional post road routes between two prefectures and offers modern travelers easier access to remote valleys. Commuters and freight vehicles pass daily through this underground passage beneath the mountains.
Vehicles move at 80 kilometers per hour (50 miles per hour) through two lanes in each tube. Driving through reduces travel time between Shimoina District and Nakatsugawa City considerably.
Construction teams first dug a pilot tunnel along the entire length to study rock formations. This advance tunnel helped engineers set up a ventilation system for the main tubes.
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