"It's not the type of problem you encounter if you used concrete segments," adds Rehak.

Along with ring beams, a lining composed of lagging and wire mesh will minimize joints once crews solidify the assembly with a monolithic pour, says Greg Osthues, director of technical knowledge and innovation with ARCADIS. The issue of leaks is critical, he says, as the Black River structure also is designed to function as a conveyance tunnel, should the city proceed with plans to relocate its wastewater treatment plant.

Ring beams were erected from within the TBM's tail shield, with crews setting 12 to 14 per day in less stable sections and 18 to 20 per day in more stable sections.

"At our peak, we hit 22 per day," says Rehak.

Ribs were spaced 4 ft on center, with lagging spacing varying from continuous to 18 in., edge to edge. Each fully assembled rib weighs about 2,500 lb and is designed to withstand rock loads of 2,200 lb per sq ft.

As mining progressed, tunneling workers installed Trolex gas monitors at 500 ft intervals to contend with the potential buildup from the underground's gassy shale. With computers charting its course, and lasers measuring its progress, the TBM broke through to Black River's reception shaft in late April, within millimeters of where plans specified.

"Due to the soft shale, we changed only seven of the borer's cutters during the entire operation,' Rehak says. "Many of them were changed simply as a precaution."

Crews are set to begin pouring the concrete lining for the Black River tunnel later this summer, once the TBM is completely disassembled.