 There were geological anomalies along the way . Manhattan 's geology changes along the subway 's length , passing through rock and soft ground , consisting of sands , silts , and clays over Manhattan schist , and there are faults and shear zones as well as fractured rock . Hard @-@ rock Tunnel Boring Machines 6 @.@ 7 meters ( 22 ft ) in diameter tunneled during the first phase , progressing at a rate of about 20 meters ( 66 ft ) per day . 
