av ated ▁by ▁the ▁back ho e ▁d red ger ▁" Z enne ". ▁Two ▁b arg es ▁were ▁used ▁to ▁transport ▁the ▁d red ged ▁material ▁ 1 9   km ▁( 1 2   mi ) ▁down river ▁and ▁from ▁there ▁to ▁a ▁dispos al ▁site ▁four ▁miles ▁off sh ore . ▁The ▁cut ter ▁d red ger ▁" V la ander en ▁XIX " ▁removed ▁the ▁underlying ▁flu vi og la cial ▁material . ▁A ▁second ▁cut ter ▁d red ger , ▁" V la ander en ▁XV ", ▁was ▁deployed ▁to ▁bre ach ▁the ▁casting ▁bas in ▁per imeter , ▁or ▁" bund ". ▁ ▁A ▁rock ▁layer ▁was ▁encountered ▁on ▁part ▁of ▁the ▁tren ch ▁line , ▁which ▁was ▁de alt ▁with ▁by ▁the ▁jack - up ▁platform ▁" Ze eb ou wer ". ▁The ▁" Big ▁B oss " ▁vessel ▁was ▁employed ▁to ▁remove ▁the ▁rock . ▁This ▁back ho e ▁d red ger ▁was ▁equ ipped ▁with ▁a ▁" Back ho over " ▁( in ▁effect , ▁a ▁" min i " ▁precision ▁d red ger ). ▁This ▁system ▁proved ▁extremely ▁effective ▁in ▁removing ▁very ▁thin ▁layers ▁of ▁recently ▁depos ited ▁material ▁immediately ▁prior ▁to ▁imm ersion ▁of ▁the ▁tunnel ▁elements . ▁ ▁Con struct ing ▁the ▁imm ers ed ▁t ube ▁Building ▁the ▁tunnel ▁structure ▁required ▁the ▁pre - cast ing ▁of ▁ 5 ▁tunnel ▁segments ▁and ▁an ▁open ▁boat ▁unit . ▁These ▁were ▁constructed ▁in ▁a ▁casting ▁bas in ▁located ▁partially ▁on ▁the ▁line