The Enasan Tunnel is a vehicular tunnel with two lanes and a length of 8, 489 m from Iida city to Nakatsugawa city.
The geological survey of the Tunnel was started in 1957. On the basis of preliminary survey, many faults and sheared zones running from sounth to northward were recognized. Misaka fault have wide sheared zone about 700 m in width.
The pilot tunnel was first driven for the whole length of the tunnel. Its purpose was to explore the strata, fully and to simplify access, ventilation and drainage in the construction of the main tunnel. Geological conditions were much worse than anticipated. Tunnelers hat to bore through the mountain of granite and rhyolite crossed by over 130 faults and saturated with ground water. Maximum pressure ofgroundwater reached about 50 kg/cm².