ast ing ▁techniques ▁and ▁t unn elling ▁shield s , ▁which ▁were ▁placed ▁at ▁three ▁locations ▁and ▁driven ▁towards ▁each ▁other . ▁The ▁shield s ▁proceeded ▁west ▁from ▁Manh attan , ▁east ▁and ▁west ▁from ▁We eh aw ken , ▁and ▁east ▁from ▁the ▁Ber gen ▁port als . ▁▁ ▁Under ▁the ▁river ▁itself , ▁the ▁t unn els ▁started ▁in ▁rock , ▁using ▁dr ill ▁and ▁bl ast , ▁but ▁the ▁str ata ▁under ▁the ▁river ▁was ▁pure ▁mud ▁for ▁a ▁considerable ▁depth . ▁As ▁a ▁result , ▁this ▁part ▁was ▁driven ▁under ▁com pressed ▁air , ▁using ▁ 1 9 4 - ton ▁shield s ▁that ▁met ▁about ▁ ▁from ▁the ▁We eh aw ken ▁and ▁Manh attan ▁port als . ▁ ▁The ▁mud ▁was ▁such ▁that ▁the ▁shield ▁was ▁sh oved ▁forward ▁without ▁taking ▁any ▁ground ; ▁however , ▁it ▁was ▁found ▁that ▁the ▁shield ▁was ▁easier ▁to ▁ste er ▁if ▁some ▁mud ▁was ▁taken ▁in ▁through ▁holes ▁at ▁the ▁front , ▁since ▁the ▁mud ▁had ▁the ▁consist ency ▁of ▁to oth paste . ▁After ▁the ▁tub es ▁had ▁been ▁exc av ated , ▁they ▁were ▁l ined ▁with ▁ ▁segment al ▁cast - ir on ▁rings , ▁each ▁we igh ing ▁ 2 2 ▁tons . ▁The ▁segments ▁were ▁bol ted ▁together ▁and ▁l ined ▁with ▁ ▁of ▁concrete . ▁The ▁two ▁ends ▁of ▁the ▁northern ▁t ube ▁under ▁the ▁river ▁met ▁in ▁September ▁ 1 9 0 6 ; ▁at ▁that ▁time ▁it ▁was ▁the ▁longest ▁under water