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Mammoet aids Seattle tunnelling machine salvage

Mammoet has been contracted to help repair a damaged tunnel boring machine (TBM) that is delaying work on the Alaskan Way Viaduct Replacement Tunnel project.

The TBM, known as Bertha, stopped 1,000ft into the underground operation, which involves construction of a road tunnel in Seattle, US. 

Design and build contractor Seattle Tunnel Partners (STP) announced last year that a 120ft-deep recovery shaft would be created to access and repair the machine.

As part of the operation, Mammoet has designed a modular lift tower capable of skidding over the 120ft-deep shaft and repair area.

This gantry system sits in saturated soil that exerts four times atmospheric pressure, as if 105ft undersea. The soil on site will not withstand the machine’s heavy payload, so the lift tower will be constructed on two concrete foundation beams that are installed over the existing concrete piles.

The piles were originally designed to protect adjacent structures and utilities during groundworks. To allow the piles to support the gantry’s heavy payload, a hydraulic skid base, with 48 hydraulic cylinders, has been designed to equalise the loads on the concrete foundation beams.

The system will lift Bertha’s entire 2,200t front end as a single hoist, rotate it a quarter-turn in the air, and set it on pre-constructed repair supports. The gantry will then be used to disassemble the drive unit.

According to Mammoet, the 16 electric motors will be removed first, and then the seals and main bearing will be removed and replaced. Finally, the front end will be reinstalled and placed back in the shaft with a reverse single hoist.

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