ABSTRACT
A common duct for underground installation of cables has been under construction beneath National Highway Route 20 (the Koshu Kaido Ave.). A multi-stage shield machine that allows smooth switch from a large to small machine was adopted for excavating the tunnel of two different diameters at reduced costs. The shield machine cutter head was designed to suit with the boulder-containing Tachikawa Gravel Bed, and appropriate additives were used for tunneling. For branching of power and communication lines, shafts were excavated at three locations along National Highway Route 20 (one of them being a destination shaft). Fiber reinforced foamed urethane (FFU) walls were used as sheathing walls when starting the shield machine to ensure safe and smooth start of tunneling. The plan and the results of the tunneling work are described in this report.
