The TBM, Bertha, is being used by Seattle Tunnel Partners - a joint venture of Dragados USA and Tutor Perini – to build a 3.2km tunnel to replace the SR 99 Alaskan Way Viaduct in Seattle, USA. Bertha has tweeted that she is working fine but has encountered an obstruction. The machine – which has a record-breaking diameter of 17.5m - had completed her first 1,000ft (305m) of tunnelling a day earlier.
Washington State Department of Transportation (WSDOT) program administrator Todd Trepanier said that Seattle Tunnel Partners decided on 7 December to stop tunnelling after experiencing unanticipated and increasing resistance, possibly due to an obstruction. This significantly decreased the advancement rate of the machine and the volume of tunnel muck on the machine’s conveyor belt.
“Seattle Tunnel Partners is currently evaluating various strategies to determine what is slowing the machine’s progress,” he said. “The contractor is considering the shallow depth of the tunnel boring machine, the high groundwater table and the challenging nature of the historic fill soils at this location as they assess the strategies. Together, WSDOT and Seattle Tunnel Partners are taking the necessary steps to ensure that the chosen strategy can be implemented safely.
“Tunneling is challenging work. This section of the tunnel drive is very difficult due to the location of the machine and the nature of the historic fill soils in the area. Seattle Tunnel Partners has completed more than 1,000 feet of the 1.7 mile tunnel. WSDOT and Seattle Tunnel Partners have taken a number of precautions to minimize risk along the route, but have understood that challenges would be encountered along the way.
“Seeing some reports that I’m stuck,” tweeted Bertha on Monday. “I’m working fine, but have encountered an obstruction. I’ll keep you posted.”
Bertha currently still belongs to Hitachi but is due to be handed over to Seattle Tunnelling Partners shortly, once the 200th concrete ring is installed. So far, 305m have been tunnelled and 147 rings installed. She left her launch pit in August.