Specialist heavy lifting contractor Mammoet has released images of the 16-hour operation to lift a 1,700t tunnel boring machine cutter to the surface in Seattle.
After a year of planning, Seattle-based Mammoet has lifted out the Cutter Drive Unit (CDU) from tunnel boring machine ‘Bertha’. The unit was lifted to the surface, rotated to lie face down and then skidded onto supports ready to be repaired.
In December 2013 Bertha stalled 36.5m underground, while boring from South Main Street to the Alaskan Way Viaduct in Seattle.
The repairs could not be completed, so a deep, concrete-lined shaft was created down to the cutter.
The lifting of the heavy CDU was complex due to its shape and asymmetrical weight distribution. Weighing 1,700t and with a diameter of 17.5m, the cutter was located close to a highway with complex soil stability.
A special gantry mounted on a skid system was designed to lift the tunnel cutter. The gantry was supported on twin rows of concrete pillars, originally designed for stabilising the surrounding soil during tunnelling. Using the pillars as foundations the gantry was constructed across shaft.
Engineers developed the design in four weeks. Using steel beams a specialised modular tower was constructed to safely support the weight of the load.
The concrete foundations were not designed to support a load at height, so engineers devised a hydraulic equalisation system to place under the tower. Made up of 48 hydraulic cylinders, the system was designed to help the gantry withstand wind and tail forces when skidding.
Once the repairs are completed, the CDU will be manoeuvred back down the shaft and reinstalled in the machine ready to complete the Highway 99 tunnel.