April 23 - After a year of planning, Mammoet has lifted the tunnel boring machine (TBM) Bertha's 1,700-tonne cutter drive unit (CDU) from the ground in Seattle.
HLPFI reported on January 23 that Mammoet had been contracted to recover the TBM after damage to the machine was discovered and it was stopped underground.
To reach the machine and make the repairs it was necessary to create a deep, concrete-lined shaft down to the cutter. Mammoet then lifted the CDU out through the shaft and once raised to the surface, it was rotated and skidded onto a pre-constructed repair support system.
To carry out the tricky extraction, a special gantry mounted on a skid system, which was supported on twin rows of concrete pillars, was designed to lift the heavy tunnel cutter. Using the pillars as solid foundations the gantry was constructed across the wide span.
Using steel beams from Mammoet's fleet of heavy lifting equipment a specialised modular tower was constructed, which could safely support the forces exerted by the weight of the load. Comprising 48 hydraulic cylinders, the system was designed to help the gantry withstand wind and tail forces when skidding.
It took around 16 hours to lift the 1,700-tonne CDU, which has a diameter of 17.5 m. With Bertha now at the surface, the remaining repairs and lifting operations will take place and once completed, the CDU will be manoeuvred back down the shaft and reinstalled in the machine ready to complete the tunnel.