January 23 - Mammoet USA has been contracted to recover the largest tunnel boring machine in the world, Bertha, from the ground in Seattle, Washington.

Bertha, which was shipped from Japan to the USA back in 2013 onboard Jumbo's Fairpartner, was built specifically for the Alaskan Way Viaduct Replacement Tunnel project, comprising the construction of a road tunnel in the city of Seattle.

The machine stopped 1,000 ft (304.8 m) into its underground operation after damage was discovered to the seals, which are located at the front of the machine.

In order to lift the front portion of the machine to the surface for repair, a 120 ft (36.5 m) deep and 90 ft (27.4 m) wide recovery shaft is being constructed by Seattle Tunnel Partners (STP).

For the lifting operation, Mammoet has designed a bespoke modular lift tower capable of skidding over the shaft and repair area. The gantry system was specifically engineering to fit the limited space next to the Alaskan Way Viaduct. 

Since the machine will be sitting on soil that is too soft to withstand the gantry's heavy payload, it will be constructed on two concrete foundation beams, which are installed on top of the existing concrete piles, originally designed to protect adjacent structures and utilities while digging.

In order to allow the piles to support the gantry's payload, a hydraulic skid base with 48 hydraulic cylinders has been designed to equalise the loads on the concrete foundation beams.

The tower will lift Bertha's entire 2,200?tonne front end in a single hoist, before rotating it by a quarter?turn in the air, and setting it on pre?constructed repair supports.

The gantry will then be used to disassemble the drive unit, first removing the 16 electric motors, and then removing and replacing the seals and main bearing. The front end of Bertha will then be reinstalled and placed back in the shaft with a reverse single hoist.