., ▁part ▁of ▁Ireland ' s ▁W alls ▁Group . ▁Sub cont ract ors ▁then ▁handled ▁many ▁of ▁the ▁key ▁tasks ▁of ▁construction . ▁ ▁Const ruction ▁Const ruction ▁involved ▁the ▁exc av ation ▁of ▁a ▁large ▁casting ▁bas in ▁where ▁the ▁tunnel ▁elements ▁or ▁pieces ▁were ▁constructed . ▁After ▁construction ▁of ▁elements ▁was ▁complete , ▁the ▁casting ▁bas in ▁was ▁filled ▁with ▁water ▁and ▁joined ▁to ▁the ▁adjacent ▁River ▁Lee , ▁each ▁element ▁was ▁flo ated ▁out ▁and ▁s unk ▁into ▁position ▁into ▁a ▁carefully ▁d red ged ▁river ▁bed . ▁The ▁road ▁surface ▁was ▁laid ▁and ▁the ▁tunnel ▁opened ▁for ▁traffic ▁in ▁ 1 9 9 9 . ▁ ▁Const ruction ▁techniques ▁The ▁chosen ▁method ▁of ▁construction ▁was ▁the ▁imm ers ed ▁t ube ▁technique . ▁ ▁In ▁this ▁method , ▁a ▁tren ch ▁is ▁d red ged ▁in ▁the ▁bed ▁of ▁the ▁water ▁channel . ▁ ▁T unnel ▁sections ▁are ▁constructed ▁in ▁the ▁dry , ▁for ▁example ▁in ▁a ▁casting ▁bas in , ▁a ▁fabric ation ▁yard , ▁on ▁a ▁ship - l ift ▁platform ▁or ▁in ▁a ▁factory ▁unit . ▁ ▁The ▁ends ▁of ▁the ▁section ▁are ▁then ▁tempor arily ▁se aled ▁with ▁bulk head s . ▁Each ▁tunnel ▁section ▁is ▁transport ed ▁to ▁the ▁tunnel ▁site ▁– ▁usually ▁floating , ▁occasionally ▁on ▁a ▁bar ge , ▁or ▁assist ed ▁by ▁c ran es . ▁ ▁In ▁the ▁Jack ▁Lyn ch ▁T unnel , ▁the ▁ 6 1 0 - met re - long ▁rein for ced ▁concrete ▁imm