Sewer Tunnel Starts With Assembling a Kit of Parts
The forward shell and sections of the cutterhead were first to arrive. Later came motors, hydraulic cylinders and other parts that were scavenged from other tunneling machines around the world. Some of the components were shipped from as far as Italy, with stops in Baltimore, while others came from Ohio and West Virginia. As the disparate components arrived on site, crews began the task of piecing together a 200-ft-long Robbins double-shield tunnel boring machine they later deployed to mine the 165-ft-deep, 5,500-ft Black River Wastewater Tunnel in Lorain, Ohio. The job marks the first application of onsite, first-time assembly of a TBM in the U.S., according to Michael Cugini, utility tunneling program manager with tunnel machine maker Robbins Co., Solon, Ohio.
"Logistics can pose challenges," says Cugini. "It can be difficult to ship parts where they're needed, when they're needed. There can be issues of oversized or overweight parts that require special transport permits. It can take time. It's not necessarily for every job."
Now that tunnel excavation has concluded and crews have begun disassembling the tunnel boring machine, project engineers say the work most certainly put the contractor and crews through their paces.
"You're not only recruiting labor and cranes to assemble the machine, but bringing in electricians and other trades to weld in electric trays, install transformers and transfer breakers, and erect catwalks and railings," says Gregg Rehak, vice president of tunneling and project supervisor with Menomonee Falls, Wis.-based Super Excavators, Black River's tunneling contractor. "We also were charged with commissioning portions of the assembly as we went along and, when necessary, troubleshooting problems. Some of that required more time than we originally anticipated."
Yet time was the deciding factor as project team members, including the city of Lorain, weighed the merits of remanufacturing the TBM on site or ordering a factory-built, factory-tested unit for the $55.4-million project. Although site assembly carried a slight premium for labor and construction equipment, it shaved three to four months off the schedule, once Lorain accelerated the timetable.
The savings will help ensure the city meets consent-order milestones issued by Ohio's Environmental Protection Agency, which has ordered city officials to eliminate inflow-and-infiltration-induced sanitary sewerage overflows, or SSOs, into the Black River and, from there, Lake Erie during heavy rains.
Timing wasn't always so urgent an issue for Black River, a project that has been off and on the boards for more than a decade. Original plans called for construction of a 12-million-gallon surface equalization storage tank before planned waterfront development nudged the site—and the solution—out of contention. The city and project engineer ARCADIS, Columbus, Ohio, finally opted for a nominal 19-ft-dia deep storage and conveyance tunnel with a capacity of 11 million gallons.
As built, the alignment originates near a wharf and extends alongside the Black River before terminating near the Lorain Black River Wastewater Treatment Plant, the site of a new screening facility, where water diversion will occur. When completed next year, a pump station at the launch shaft will direct water to a sewer interceptor once a water event concludes.
Prior to assembling the TBM, crews dug a 175-ft-deep, 36-ft-dia launch shaft, then a 200-ft-long, concrete-lined starter tunnel at its base to accommodate below-grade assembly of components. As work on the starter tunnel proceeded, the contractor began receiving, sorting and, in some cases, pre-assembling parts, including the drill's cutterhead and shields, on a one-acre plot above grade.
"Operations like this are easier to negotiate above grade," says Rehak. "Whenever we could pre-assemble above grade, we would."
Achieving earlier arrival of the TBM cutterhead and other components slated for above-grade work required extensive coordination between the contractor and TBM supplier, the latter charged with customizing select components, assembling them for testing, then breaking them back down for shipment to Lorain. Among other modifications, the cutterhead and other outer segments were enlarged to accommodate a 23-ft-dia path, 3 ft wider than originally fabricated for the used TBM's previous work, says Cugini.
The forward shell arrived on site in four pieces, as did grippers. Outer telescoping assemblies arrived in two pieces; the cutterhead and gantry in five. "If components arrived in quarters, we'd assemble them in halves before lowering them to the starter tunnel," says Rehak. Robbins dispatched a chief assembly, hydraulics and electrical technician; a pair of mechanical technicians; and welders to oversee operations and instruct local labor in parts assembly.
For its part, Super Excavators assembled a team of 15 to 20 workers per day per shift, including crane operators, welders and electricians. "We had two or three of our welders working with two or three of theirs," recalls Rehak. "The lead technician would point and tell us what to do and we'd do it."
Super Erectors engaged a Manitowoc 555 and Kobelco crawler crane to assist in operations, the former to engage in lifts of 50,000 to 100,000 lb and the latter to maneuver smaller components from one location to another.
In all, assembly required 12 weeks, from Aug. 12 to Nov. 18, 2013, which was the launch date for mining operations. Of the TBM's myriad parts, only its 232,000-lb cutterhead was completely built above grade before a 550-ton crane supplied by Capital City Group lowered it down the launch shaft in late August.
As a result of final adjustments to both borer and the continuous conveyor system, mining began slowly, advancing only 200 ft during the first few weeks. In one instance, Super Excavators found itself contending with steering snafus, the result of mismatched hoses. Once final adjustments were made, excavation required six months, with soft Cleveland shale facilitating rapid progress—about 72 ft to 80 ft per day.
"That was pretty much our target," says Scot Pearson, ARCADIS senior engineer. "There really were no surprises. Conditions matched those of test borings."
At times, work proceeded at a rate that tested the continuous conveyor's ability to keep up, particularly its vertical segment, says Rehak.
There also were instances when geologic conditions weren't as favorable for lining operations as for mining. Up to 25% of the soft shale geology consisted of layered and laminated rock that broke from tunnel crowns as concrete ring beams were expanded, circumstances that required additional chipping and rock relief.
"That slowed us down a bit, to about 60 feet per day," says Pearson, "but, again, it was expected. In fact, we encountered less than expected."
"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.
