To facilitate the post-process of excavated debris.

The excavated debris passes in a primary force feed pipe 16a, is put into the hopper section of a primary relay force feed pump P1, and is extruded to a secondary force feed pipe 16b via a discharge port in the prescribed pressure state from the hopper section by the primary relay force feed pump P1. The water sent from a water injection pump 26 is fed to the excavated debris via a water injection ring 18 to reduce the frictional resistance with the inner face of the secondary force feed pipe 16b. The force feed pressure value p1 of the primary relay force feed pump P1 is detected, and the revolving speed of the water injection pump 26 is set according to the detected force feed pressure value p1 to adjust the water injection quantity.