Small Tunnels, Big Challenges

Compact TBMs Tackling Longer Distances, Difficult Ground Across the United States

When we think of TBMs, the natural inclination is to picture 45-ft diameter behemoths moving millions of tons of rock and soil. The vast majority of tunneling machines, however, are of a more moderate size. Steady growth in the number of small diameter TBMs (less than about 12 ft in diameter) has U.S. contractors sizing up the compact TBM market, and reaping the benefits.

“The market for mid-range to smaller diameter TBMs is steady and growing. It might have to do with several factors, including the EPA mandates for CSO work, urban sprawl and aging water lines in the United States. We have been doing a lot of large water lines,” said Tim Winn, Director of Southland Contracting, a prolific U.S. company well versed in small diameter bores.
New projects from Maryland to Texas, and places in between, are pushing the limits of what is possible at small tunnel diameters. Whether the challenge is the logistics of space, difficult geology or very long tunnels, small TBMs are taking it on.

Making the Most of Limited Space

In Austin, Texas, contractor S.J. Louis utilized a Robbins 7.3-ft Double Shield TBM with great success despite confined tunneling conditions. The machine completed its 1.4-mile excavation in 2011 after a launch in April from a 55-ft deep, 20-ft diameter shaft. The project, for the Onion Creek Golf Course Wastewater Interceptor, was constructed as part of the City of Austin’s I-35 Water/Wastewater Improvements program.

The project’s geology in Austin chalk, a soft limestone of about 3,000 psi UCS, made for fast advance. “This was one of the smoothest jobs I’ve been on. There were really no major issues in the way of boring; excavating that Austin chalk is like cutting through butter,” said Robbins Project Manager Jeff Gioitta. Machine advance topped out at 150 ft in a single 10-hour shift.

The swift success did not come without extensive planning and some challenges, however. “The big challenge was that the tunnel was 6,000 ft long at a small diameter. We needed to mine productively at that diameter, so we planned for a two-push TBM setup and modified the back-up and muck cars to facilitate that,” said Gevan McCoy, Tunnel Division Manager for S.J. Louis. Another challenge associated with the size constraints of the tunnel was ensuring adequate ventilation. To meet the needs while still allowing train travel, a bag line system with multiple fans was utilized.

The need for fast excavation in a small space is a common challenge at this diameter, says Dan Liotti, President of Midwest Mole Inc., an Indiana contractor well-known for small diameter bores. The company used a 72-in. Double Shield machine to complete 2,014 ft of tunnel earlier this year for a project that faced similar challenges. “Installing small diameter tunnels at longer distances (greater than 750 ft) can sometimes be more challenging than large diameter tunneling. The limited space behind the TBM causes logistical issues with trailing gear, ventilation and getting adequate-sized muck cars.”

The Midwest Mole project, for Ohio’s Shayler Run Sewer Replacement Project, required relatively thin but long muck cars for increased capacity, as well as a high-pressure ventilation blower to ensure adequate air flow. The TBM design also aided in the logistics of space: “While the TBM itself was very small, the Robbins design of locating the Variable Frequency Drive controller on the surface instead of inside the tunnel tremendously increased the available working room,” said Liotti.

Long-Distance Tunneling

Also in Austin, the sprawling Jollyville Transmission Main will transfer up to 50 million gallons of treated water per day through 6.5 miles of tunnel. The pipeline will source water from Lake Travis and travel below the protected Balcones Canyonlands area.

Three TBMs were proposed to excavate the pipeline, with one contractor-owned machine having completed its 0.9-mile section in mid-2012. Robbins supplied a 10.7-ft Main Beam TBM, and refurbished a 9.8 ft Double Shield TBM for Southland Contracting. Both machines were launched in August 2012 from shafts up to 350 ft deep.
“We needed the Robbins High Performance Main Beam TBM for the longest bore, which is on the critical path for the project. It’s built to exceed our 230-day schedule, and for even tougher conditions than are foreseen here,” said Winn.

Conditions on the critical path are expected to consist of uniform limestone and dolomite rock. Although karst features are present throughout the formation, the depth of the tunnel should circumvent these features. Other obstacles are associated with the protected wildlife area — endangered cave-dwelling invertebrates including six species of arachnids and insects are present in and around the karsts. Because of this, no probe grouting can be performed due to the risk of seepage into the water features where the creatures make their home. “We don’t expect any features that will need significant support. Rock bolts will be the predominant form of support, and there may be areas that require wire mesh. Anywhere there is a water feature, we will install a liner that will be grouted in place to seal those zones,” said Winn.

As of late August 2012, the Main Beam machine had advanced about 300 ft within a week of launching. The fast advance is a result of extensive planning, as logistics are always a limiting factor in small tunnels: “We need to plan well in advance how ventilation and muck removal will work toward the end of tunneling. Ventilation is the biggest issue. We have multiple trains in the tunnel at once, so the requirements for ventilation are significant.

Our second biggest problem is getting people and materials in and out, which takes quite a bit of time,” said Winn. Southland has planned for two California switches and one shaft switch in the Main Beam tunnel alone. An oversized vent duct and additional fans will help keep ventilation up to par.

Stretching the Limits

Upcoming and ongoing projects are testing the boundaries of what is possible at small tunnel diameters. At Maryland’s Bi-County Water Tunnel, a 9.8-ft diameter Main Beam TBM, used on 10 projects since 1973, is nearly complete with 28,000 ft of tunnel in hard granite.

The water transfer tunnel, for the Washington Suburban Sanitary Commission (WSSC), will improve capacity in Montgomery and Prince George’s counties by carrying 100 million gallons of water per day between existing water mains. The tunnel route travels through urban and residential areas, resulting in limited access and a deep alignment between 90 and 275 ft below the surface. The 35-ft diameter, 164-ft deep Connecticut Avenue shaft is the only place from which to launch and remove the TBM.

Work is being carried out by a joint venture of Oscar Renda, Southland, and SAK, with Black and Veatch as the primary designer. “To have a machine go 28,000 ft with no intermediate shafts is a challenge that’s worked out well. We had some high water inflows from perched water, and we were going from hard rock to soft ground conditions intermittently, so we needed to be prepared for a lot. We put up ring beams and lagging in areas that contain significant voids or soft material, to stabilize the ground until grouting could be accomplished,” said Winn.

The diameter makes necessities such as adequate air movement and mucking a complex venture. To remove an appropriate volume of air, 35-in. diameter duct is run along the top of the tunnel. Scavenger fans also push air back from the boring face and toward the ducting that trails the machine. Air is removed by two 100-hp fans, then put through a silencer — a noise reduction device for work near residential areas.

Muck is being removed from the tunnel using single-track muck cars with California switches. A new switch is placed every 1.8 miles in the tunnel to allow multiple muck trains in a relatively small space. Full muck cars are then hauled out of the shaft for dumping on the surface. Proximity to the highways has resulted in restricted trucking and hauling hours, preventing muck haulage from 6 to 9 a.m. and from 3 to 6 p.m. daily. The excavated material is being stored at a firm nearby, to be reprocessed and used as fill on private property. With just 4,000 ft of tunnel left to bore, breakthrough is expected before the end of the year.

An upcoming project is notable for the difficult geologic conditions it is set to tackle. Southland’s Corbalis to Fox Mill Water Main in Fairfax, Virginia, will utilize an 88-in. Double Shield TBM in 54,000 psi UCS diorite. “It’s probably the most significant test for what small diameter Double Shield machines can do, because of the hardness of the rock,” said Tom Fuerst, Robbins Utility Tunneling Manager, who has been in the small diameter TBM market for over 15 years. “The TBM is being launched to complete the project and rescue a different machine that became stuck in the hard rock.” The machine is set to excavate 6,400 ft of water line using 17-in. disc cutters in the last quarter of 2012.

Looking Ahead

While these projects are just a few of the examples of small-diameter tunneling in the United States, the consensus is that the market is strong. Fuerst specifies that while interest in small-diameter TBMs is growing stateside, there is still a need for acceptance in many international markets.

“There are still a lot of companies around the world that rely on hand mining in soft ground and drill-and-blast techniques in rock,” Fuerst said. “Essentially, if speed of tunnel completion is important, small TBMs can be used and are a cost-effective solution. As tunnel lengths increase, the popularity of small TBMs will grow.”

Information for this article was supplied by The Robbins Co.

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