Malcolm Drilling Co. Inc. was recently awarded the support of excavation work to rescue Bertha, the incapacitated 57.5-ft diameter tunnel boring machine digging the SR 99 tunnel in Seattle. The TBM, the world’s largest, has stopped all forward progress after just more than 1,000 ft of mining. Bertha’s stoppage from mechanical difficulties places her precariously 80 ft below the surface between the aged Alaskan Way Viaduct and the southern terminus of the dilapidated Seattle Seawall.
After vetting out all possible repair/rescue methods, Seattle Tunnel Partners JV (STP) decided that accessing Bertha with a vertical shaft provided the best approach to access all facets needed for repairs. Due to the limited surface area above the tunnel alignment and physical constraints bound by existing infrastructure, the dimensions of the otherwise symmetrical shaft are heavily impacted thereby requiring special attention to design, layout and water control processes. Consequently, the shaft design requires a combination of 3-, 2.5-, 2.2-, 1.5- and 1-m diameter unreinforced drilled secant piles coupled with various jet grouting techniques and dewatering. The shaft will allow STP to remove Bertha’s entire cutter face and inner workings to facilitate a complete repair.
“Rescuing a one-of-a-kind TBM requires a one-of-a-kind access shaft.” said Clint McFarlane, project superintendent, Malcolm Drilling. “We’re happy to be working with STP and WSDOT to help get Bertha back on track and moving forward.” Malcolm’s crews and equipment have mobilized from their regional office in Kent, Washington, and will work around the clock to expedite completion of this time critical project.