Malcolm was awarded the support of excavation contract to drill the rescue shaft for Bertha, the world’s largest Tunnel Boring Machine (TBM). Bertha stopped all forward progress after just 1,000-ft of mining the new SR99 transportation tunnel below downtown Seattle, Washington. The TBM was stalled 80-ft below the surface. After vetting all possible repair/rescue methods, Seattle Tunnel Partners decided that accessing Bertha with a vertical shaft provides the best approach to gain access to the cutter-head and facilitate 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 were heavily impacted, requiring special considerations for design, layout, and water control processes.
The final design for the rescue shaft required the drilling a series of secant piles with varying diameters, ranging from 3-ft to 10-ft. Shaft sizes and depth were developed together with the shoring designer, Brierley Associates. The secant piles overlapped two 1,000-ft long walls, constructed out of 5-ft dia. non-accessible shafts that Malcolm originally installed to support the initial TBM launch. The existing drilled shafts, with depths ranging from 75- to 138-ft, were installed to protect the existing sea-wall and control the below sea-level ground conditions. Jet grouting and dewatering wells were also installed to control the ground water inflow during the rescue operation.