MAMMOET USA TO RESCUE TRAPPED TUNNEL BORING MACHINE

Mammoet USA has been contracted to recover the largest tunnel boring machine in the world, known as ‘Bertha’.

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Bertha was built specifically for the Alaskan Way Viaduct Replacement Tunnel project, which entails the construction of a road tunnel in the city of Seattle in the U.S. state of Washington. The machine stopped 1,000 feet into its underground operation after the discovery of damage to the seals, located at the front of the machine. In order to get the front portion of the machine to the surface for repair, a 120 feet deep recovery shaft must be constructed.

Seattle Tunnel Partners (STP) is building a 120 feet deep, 90 feet wide ring‐shaped shaft, where the lift tower can safely retrieve Bertha’s front end in open air. For this rescue operation, Mammoet designed a unique modular lift tower capable of skidding (sliding) over the 120 feet deep shaft and repair area. This lifting tower, also called a ‘gantry system’, was custom‐engineered to fit the limited space next to the Alaskan Way Viaduct, and to steer its payload within millimeters of a target.

The machine sits in saturated soil that exerts four times atmospheric pressure, as if 105 feet undersea. The soil is too soft to withstand the gantry’s heavy payload, so the gantry will be constructed on two concrete foundation beams which are

installed on top of the existing concrete piles.These piles were originally designed to protect adjacent structures and utilities while digging, and will now also serve to support the gantry’s heavy payload. In order to allow the piles to support the gantry’s payload a hydraulic skid base with 48 hydraulic cylinders has been designed to equalize the loads on the concrete foundation beams. It is the first gantry in the industry which is fully supported by hydraulics.

The lift tower will lift Bertha’s entire 2,200‐ton 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. First to be removed are the 16 electric motors. Next, the seals and main bearing will be removed and replaced. The front end will then be reinstalled and ultimately placed back in the shaft with a reverse single hoist.

An international team of Mammoet engineers designed the system within 4 weeks. Mammoet has been gathering and erecting steel parts since September from Malaysia, Europe and Canada to prepare the lifting tower. By putting together an international team of specialists and combining in-house expertise, Mammoet designed this unique system within 4 weeks to get Bertha moving again as soon as possible.