Abstract
Benten-yama tunnel, about 1.3km long, is located near by Kozu, South-West of the Oiso hill land mainly consisted of diluvium-tertiary formations. Along the tunnel site, the hill is chiefly constituted by alternation of sand gravel beds and tuffaceous mudstone beds.
Because of the existence of the unconsolidated sand gravel beds with much poor water enclosed by impermeable mudstone beds, pilot tunnel is often damaged by collapse with see page water. The maximum total tunnel water abount to 8400l/min. Relations between the geological conditions and the amount of the tunnel water are discussed in this paper.
By the methods of the roundabout pilot tunnel driving at the collapse points, the pilot tunnel finished in Dec. 1961, and the tunnel completed in June.1962.
