The contractor for the project, SELI, needed a TBM solution that would protect workers in broken ground and provide a rapid rate of advance, while simultaneously lining the tunnel. To fit their need, Robbins invented the Double Shield TBM.
Before 1956, TBMs had never been used to excavate rock. The crystalline limestone on the project was too hard for the drag bits being used on machines at the time. So, Robbins invented the rolling disc cutter and the contractor succeeded in boring the first rock tunnel ever.
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