Next to the bridge of Highway 302 in Nagoya’s Ama District Marutai Doboku CO. LTD. is building a second bridge. Due to the poor soil conditions a special method was chosen to build and anchor the bridge pillars.

Thirty casings with a diameter of 1000mm and a thickness of 14mm were placed in a circle with a total diameter of around 12 meters. Marutai welded a special profile on two sides of each casing (see illustration). These tube shaped profiles interlock perfectly when the casings are driven forming a cofferdam. After these interlocks were filled with grout a watertight separation was formed. Water was pumped out and the soil removed until the desired depth was reached and the construction of the foundation of the bridge pillar could be started. The casings that were used for the cofferdam remained in the soil as integral part of the foundation. After the bridge pillar was finished the casings were cut just above the foundation level, dismantling the cofferdam but leaving the rest in the soil as a permanent part of the foundation.

The soft seabed on site and the required capacity of the foundation has ensured that the tubes had to be no less than 30 meters long. Three pipes, each with a  length of 15 meters, were welded on top of each other on site after the previous casing was vibrated with the PVE 55M into the soil. When the hard layer was reached a IHC S-200 Hydrohammer was used to drive the casing a bit further.