Construction of the SR-99 TBM Recovery Shaft

"The world’s largest tunnel boring machine (TBM), at the time of her launch in 2012, the TBM locally referred to as “Bertha,” is incapacitated due to mechanical failure. Bertha’s stoppage places her 25m below the surface between the aged Alaskan Way Viaduct and the failing Seattle Seawall. Accessing Bertha with a vertical shaft provides the best approach to access all facets needed for repairs and will allow the full removal of Bertha’s cutter head and related inner workings. Due to the limited area above the tunnel alignment and physical constraints created by existing infrastructure, the dimensions of the otherwise symmetrical shaft are heavily impacted, thereby requiring special attention to design and layout. Consequently, the shaft design required a combination of 3m, 2.5m, 2.2m, 1.5m and 1m diameter unreinforced drilled secant piles coupled with various ground improvement techniques and dewatering, to fully satisfy the shoring design requirements. This paper will describe the shaft installation procedures, equipment used and pitfalls encountered while undertaking access shaft construction of this magnitude. INTRODUCTION Mining on Seattle’s SR99 tunnel began in July of 2013. Initial mining progressed slowly through a 4.5 m thick fiber-reinforced concrete headwall before immediately entering the first of three planned safe havens. Once through the headwall and safe haven the machine would move through roughly 122 lineal meters of jet grout improved soil at a rate of slightly over 1.6 m per day. It was anticipated that once mining has passed through the improved area, “Bertha” – now the world’s 2nd largest tunnel boring machine (TBM), would be moving at a rate of almost 11 m per day. By the end of October 2013, Bertha had mined roughly 131 m, averaging 1.4 m per day, and had begun mining within native soils. By the start of December 2013, tunneling had reached the 244 m mark and Bertha was now averaging 3.6 m per day, achieving as much as 12.8 m of progress on given days. Unfortunately, on December 6th, 2013, Bertha began to stumble and all progress quickly came to a halt. Initial reports stated an obstruction had stopped Bertha in her tracks. The preliminary investigation involved installation of 10 deep dewatering wells to reduce the hydrostatic pressure within the ground around the TBM, to allow the contractor’s personnel to inspect the cutter head from within and also to investigate for the presence of obstructions at the face of the cutter head. During inspection, the contractor’s crews found fragments of steel pipe entangled in the cutter head. The steel was apparently from an abandoned 254 mm diameter monitoring well casing, previously installed by WSDOT to monitor ground water conditions for the design and planning of this tunnel project. After the steel was removed from the cutter head and cleared from Bertha’s path, she attempted to move forward but was only able to generate a slow crawl."