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Tuesday, 24 December 2013 11:24

Groundwater flow delays the unblocking of Bertha

Groundwater flow delays the unblocking of Bertha Credits: KING5.com

Attempts have already been made to identify this object of mystery that forced Bertha, the world's largest TBM in Seattle, to halt suddenly on December 6. High groundwater pressure though is preventing crews from performing a close inspection and as Chris Dixon of Seattle Tunnel Partners said last week, almost two more weeks will be needed for workers to discover the unknown object.

The obstruction is thought to be an unstable boulder, or a giant "glacial erratic" rock. Six wells have been dug and four more wells are expected to be drilled to pump the water away from the cutting head in order to lower the pressure and allow for a safe access for workers. If the de-watering wells fail to reduce the water pressure, compressed air will be forced into the cutting-face area to keep water and mud away, allowing access to specially trained divers. It's too early for engineers to estimate any change on project schedule or the increase of cost but non-tunneling work has already begun in other areas in order to mitigate the delay.

See also: ‘Mysterious object obstructs the world's largest TBM in Seattle

Sources: The Seattle Times, KING5.com

Read 2273 times Last modified on Tuesday, 24 December 2013 11:34

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