TBM Recovery Shaft Construction- SR-99 Tunnel-Paper

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Bertha, the 17.4 m diameter tunnel boring machine on Seattle’s SR-99 project, stopped in December 2013 after its main bearing was severely damaged. Repair required removing the cutter head, so contractors rapidly designed and built a deep recovery shaft around the stalled machine.<br /><br />The site posed major constraints: limited working space between the Alaskan Way Viaduct and Puget Sound, shallow and variable groundwater, poor soils containing wood debris and abandoned structures, and existing settlement-mitigation piles. The shaft also had to provide excavation support and groundwater control without internal bracing that would obstruct removal of the cutter head.<br /><br />The solution was a circular secant-pile shaft, incorporating existing piles and using large-diameter drilled shafts. Construction included 17 one-meter, 14 1.5-meter, 16 2.5-meter, and 25 three-meter drilled shafts, each reaching depths of up to 40 meters. Jet grouting and cement grouting strengthened the ground and created cutoff walls. Sixteen dewatering wells—some extending 62.5 meters—were installed to control groundwater and prevent heave at the shaft bottom. Two horizontal drains relieved water pressure behind the shaft wall before the TBM entered.<br /><br />Work was complicated by tight sequencing, difficulty maintaining pile verticality, underground voids, sinkholes, and changing ground conditions caused by tunneling, dewatering, and site activities. Crews adapted methods in the field, including pre-drilling and alternative grouting where conventional jet grouting was impractical. Specialized casing rotators, oscillators, and drilling rigs enabled construction in the confined site.<br /><br />The recovery shaft required close coordination across disciplines and rapid responses to unforeseen conditions. Once completed, it allowed Bertha’s cutter head to be accessed and removed for repairs. The machine was subsequently repaired and resumed tunneling.

Keywords

Bertha

tunnel boring machine

Seattle SR-99

recovery shaft

main bearing damage

secant pile shaft

groundwater control

jet grouting

dewatering wells

cutter head removal