The second Herrenknecht machine (BT2), excavating the eastbound drive to the North Creek shaft and pump station, has been shut down since mid-December for extensive wear repairs to the cutterhead. With work being done under air pressure of up to 4.4 bar, compressed air maintenance crews work for 45 minutes only, followed by three hours of decompression. The machine is about 6,500ft (2,000m) into its 11,600ft (3.5km) drive.
http://www.tunneltalk.com/Brightwater-Mar09-Brightwater-sinkhole.php
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