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Tunnelling: Bertha’s rebirth

Breakdown of a tunnel boring machine middrive is every project’s worst nightmare. GE takes a look at the recovery work for the machine on the Alaskan Highway Tunnel in Seattle, US. 

Plans to put the US city of Seattle’s Alaskan Highway Viaduct, which carried SR99, underground will not only replace an ageing structure but also reclaim the ground level for residents. The concept of removing a barrier between the city and the sea is something that promoters of the Hammersmith Flyunder have used to support their plans.

Cutterhead recovery

The cutterhead was lifted out of the shaft in April and STP hopes to restart tunnelling with the upgraded TBM in July

While the initiative may seem the perfect solution, the realities on site have been far more difficult with breakdown of the tunnel boring machine (TBM), named Bertha, at the end of 2013 (see news for details). Work to lift the cutterhead to the surface has just been completed and design and build contractor Seattle Tunnel Partners (STP) – formed from a joint venture of Dragados and Tutor Perini – expects to restart tunnelling in July.

The client for the US$1.14bn (£737M) project is Washington State Department of Transport (WSDOT) and has been planned for a long time (see box). Tunnelling started in June 2013 but was halted after less than six months and 300m of progress following overheating problems with the Hitachi Zosen TBM. Initial interventions discovered damage to the machine’s seal system and contamination within the main bearing but repairs could not be undertaken insitu. The result was the need for a rescue shaft to bring the cutterhead to the surface for repairs.

Bertha breakthrough

The TBM was named Bertha after Bertha Knights Landes who was elected mayor of Seattle in 1926 and was the first woman to lead a major US city

In May last year preparations started with jet grouting to stabilise the ground around the site and utility diversions before installing a grout wall behind the TBM’s shield. Secant piling was then used to form a 24m diameter, 36m deep shaft to access the TBM which was 18m below ground level. Groundwater was lowered before the shaft was excavated and Bertha restarted to tunnel into the shaft and onto a concrete cradle in February this year.

Mammoet was contracted to crane the 1,700t cutter head out of the shaft and place it on preconstructed repair supports. The lift was successfully completed in April.

With the cutterhead now on the surface, STP is working with Hitachi Zosen to replace the damage with a more robust system, install enhanced monitoring systems and add new steel sections to strengthen the TBM and accommodate the new seal system.

Shaft construction

STP constructed a 24m wide, 36m deep shaft to recover the stricken TBM using a Mammoet crane

Current forecasts from STP suggest that, once restarted, the tunnelling work could be completed within 12 months and the fit-out work will take a further two years. At the start of work in 2013, STP had originally planned to complete the tunnelling work by the end of 2015 which was ahead of WSDOT’s expectations of November 2016. So as long as the rest of the work goes without a hitch the end date of delivery may be later than STP hoped but still within WSDOT’s timeframe.

Who pays for the cost of the additional work still needs to be resolved. STP’s request for US$125M (£81M) in compensation from WSDOT was rejected on the grounds of having no contractual merit, but STP may still decide to take the issue to court – a decision that could take longer to resolve than the completion of tunnelling.

NEED FOR CHANGE

Old viaduct

Deterioration of the existing 1950s Alaskan Highway Viaduct (pictured right) had already been recorded in the late 1990s, but the 2001 Nisqually earthquake further weakened the structure.

The viaduct stands on fi ll material placed behind a seawall that has also been weakened by age, which led to concerns over failure of both structures in the event of liquefaction during another seismic event.

Weight restrictions and automatic closure systems in the event of another earthquake were placed on the viaduct in 2001 while a replacement plan and funding was sought. The solution is the tunnelled option and central and local government funding will be supplemented by tolls levied on the new tunnel.

Part of the viaduct was demolished in 2011 and replaced with a bypass to reduce the risk and enable the tunnelling work for the 3.2km long twin-deck tunnel to be planned. 

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