Muck removal: It’s one aspect of TBM tunnelling that is often overlooked, yet crucial for efficient advance. “Muck removal is always critical for any TBM to advance at high rates, whether it is in soft ground or hard rock. You may have the highest powered machine out there, but if you cannot remove the muck it goes nowhere,” says Dan Martz, project manager for the contractor JV of Shea/Traylor.

The process, however, can be quite complex when the tunnel is located in an urban area, when space is limited and when conveyed materials vary widely. Continuous conveyors have made it possible to achieve higher rates even in these difficult setups, using advancements such as variable frequency drives, spacesaving vertical belts and specially designed systems for soft ground.

Case studies, beginning at a massive 1970s tunnelling project in Chicago, highlight how conveyor systems have improved over the years. Recent and ongoing tunnelling projects in California and New York detail the use of today’s conveyor systems in complex ground conditions and urban environments.

Better, faster, safer
The use of continuous conveyors in difficult conditions has produced great successes on modern projects. Conveyors were used on a number of record-setting tunnels at Chicago’s Tunnel and Reservoir Plan (TARP) in Illinois, USA—a project that began in the 1970s and involved more than 20 TBMs over two decades. “I was one of the Project Engineers at the Chicago TARP Tunnels in the late 1990s,” says Richard Redmond, now operating Redmond Construction Engineering, a construction management and cost estimating company. “The project utilized a horizontal conveyor, through 14 curves of 300m radius, dumping onto a specially designed vertical conveyor, and then running onto radial stacker conveyors to spread the material on site. Challenges included the installation of booster drives every 760m, and training the belt through so many curves.”

Martz, then working for Affholder, was also part of tunnelling at TARP. “I worked in Chicago for many years seeing the start of conveyor systems and watching their progress. The Little Calumet tunnel was set up for high production, with a highpowered TBM, 19-inch disc cutters, and high-capacity muck removal. When you mine 300m per week, and 1,600m in a month, something is working pretty well and it is not a result you can achieve with muck cars,” he says.

During tunnelling, muck was removed using a side-mounted belt conveyor that emptied onto a vertical belt with a 700t/h capacity. The Little Calumet project, excavated in limestone with curves down to 305m radius, still holds world records for best day (99.1m) and best week (562m) in the 5m to 6m diameter range.

Earlier successes such as those at TARP have inspired a host of improvements for the use of conveyors in difficult project schemes. “A good conveyor requires good maintenance, including maintaining the belt alignment, greasing, and belt splicing. We can manage our belts much better today using data logging, real-time monitoring and cameras,” Martz says.

Variable frequency, greater efficiency
Variable frequency drives (VFDs) have many advantages over hydraulic couplings, an earlier form of drive motor still used on short tunnels. VFDs actively set the speed and torque that a drive contributes to the conveyor in both single and multiple-drive setups. In this way, a controlled start-up is possible and belt tension is kept low to reduce stress, allowing belting to better negotiate tunnel curves.

The drive systems are also lower in cost because they use AC three-phase motors, which require less maintenance compared to earlier DC motors. Newer systems are also digitised, so a separate frequency motor is not needed to control the torque supplied by each drive. “One of the most important factors in maximising conveyor availability is maintaining the drive motors. This is much easier to do with variable frequency drives,” Redmond says.

Since their power output is controlled, VFDs are also more energy efficient than their hydraulic coupling counterparts. “Old scoop tube hydraulic couplers required a balancing act through adding or deleting oil. Later fluid couplers came with this parameter preset, but they still didn’t allow us to utilize torque and speed at different RPMs during startup,” Martz says.

Because they are digitised, VFDs also allow monitoring and maintenance using Programmable Logic Controllers (PLCs) from the operator’s cabin or a location on the surface. PLCs have dramatically decreased downtime and increased safety of long, complex conveyor systems. When combined with Emergency (E-stop) pull cord systems, the crew can identify problems in real time, as well as the exact section of conveyor that has tripped a switch. E-stop pull cords automatically halt a conveyor belt, while magnetic impulse belt-slip and whisker-type belt run-off switches provide additional protection at conveyor discharge points.

Curved drives
Curved tunnel paths are one of the most common challenges for continuous conveyors. Self-adjusting curve idlers, patented by Robbins, help to solve the problem by pivoting to accommodate changing load tensions around curves. The pivoting action is also favourable because it does not unnecessarily alter the carrying capacity of the conveyor or the belt tension. Recent decreases in the weight of the carrying idler assembly have also improved worker safety by making the curve idlers easier to lift, while reducing installation time.

New materials
New belt materials are also making curved systems safer and more efficient. A new type of fabric, known as straight warp belting, is thinner and lighter without reducing strength or durability. The belting is able to move through small radiuses at a variety of working tensions. In addition, the fabric is resistant to string formation, a process that occurs as the edges wear and begin to fray. Minimisation of edge wear tempers the risk of strings catching on pull cords, and reduces the chance of worker injury.

Going vertical
New advancements for limited space projects, including J-Type vertical belt conveyors that are loaded using a vertical receiving hopper and feature a small vertical drive section, are capable of reducing the footprint both in the shaft and at the surface compared with conventional S-type designs. Vertical belt cassettes further reduce the space normally taken up by horizontal belt cassettes by as much as 170 per cent.

Moving rock under New York City
New York City’s East Side Access Project has been in the works for over 30 years and is needed to relieve heavy traffic congestion between the densely populated boroughs of Queens and Manhattan. The project was revisited in earnest in 2006, when contracts for two parallel TBM tunnels below Manhattan were awarded. A total of eight TBM headings, drilled by two TBMs, connect the boroughs via an existing submersed tube below the East River. The setup utilizes continuous conveyors drawing from multiple headings, hauling muck up a shaft and across a six-lane highway.

Excavation with a 6.7m diameter Robbins TBM began in December 2007 and was completed in July 2010, while a Double Shield TBM of the same diameter completed tunnelling more recently. Geology has consisted of competent Manhattan schist up to 200 MPa UCS.

Current operations at the project, as of April 2011, included drilling and blasting of large station caverns below Manhattan’s Grand Central Station.

“This job is really logistically unique. Not only is it a massive undertaking—we also have up to 11 different headings happening at one time,” says Don Hickey, Project Manager for the Dragados/Judlau JV (project contractor). “There are two TBMs, two road headers, drill and blast operations, concrete operations, and shotcrete final lining, all happening out of one major shaft.”

The muck removal system for the project is equally complex, consisting of every type of commonly used belt conveyor. During TBM tunnelling, the conveyor system was in its longest iteration, stretching over 4.8km in length. Two extensible straight warp fabric belt conveyors (914mm in width) travelled behind each TBM operating in the tunnels, and dumped via a crownmounted cross conveyor onto a single 1,863m fixed-length conveyor mounted inside the submersed tube.

From the tunnel, muck is transported up the 23m deep Queens shaft using a fixedlength, steel cable vertical conveyor. Once the muck has reached the top of the shaft, it is transferred to a storage area using three overland conveyors and a radial stacker. The second overland conveyor, 37m in length, is a completely enclosed box truss that crosses Northern Boulevard, a major thoroughfare in Manhattan. Once the muck reaches the storage area, it is transferred to a radial stacker, which can rotate through 60 degrees to deposit muck in kidney-shaped piles.

Though the conveyors are no longer handling muck from behind the TBMs, they are still in heavy use and being loaded at multiple points. “The conveyors have worked well; we still have a lot of material for them to remove,” Hickey says. “Theyhave been running since mid-2007 and will be running to the end of 2012—that’s about five years of life required on this one job,” Throughout the project’s excavation, the conveyor system has averaged over 90 per cent availability.

“We have removed 2,300 to 3,000 cu m of muck a day with this conveyor system when the TBMs were running—it’s the only way we could keep this project on schedule,” Hickey explains.

“This belt has seen a lot of different types of material, from TBMs, road headers, and crushed drill and blast material. It has handled all of it well, though dealing with fines has been a challenge. We are using cleaning apparatus to make sure running material is removed and not returned back into the system.”

Sacramento’s soft ground
While continuous conveyors have been tried and tested in hard rock, their entrance on the soft ground scene has been much more recent. The design of such systems must take into account the differences in material between soft ground and hard rock, including belt cleaning, sealed transfer points and other elements.

“Probably the most challenging job I’ve worked on was the Lower Northwest Interceptor, a short river crossing job in Sacramento,” Martz says. “The EPB TBM was launched at six per cent down grade, mined 300m through a vertical curve, and then up at six per cent for another 300m to hole through. The tunnel went under the Sacramento River, through several different types of soil, clays, and sand. The conveyor was a big part of what allowed us to do the job safely and efficiently.”

That project—one of the first to use a continuous conveyor for soft ground TBM tunnelling was completed in 2005. The Robbins conveyors, even with their steep inclines and declines, achieved system availability above 90 per cent for the entire project duration.

A more recent adaptation of this setup, also in Sacramento, took place at the Upper Northwest Interceptor (UNWI) project in 2009. The 5.8km long tunnel was excavated using a 4.25m diameter Robbins TBM and continuous conveyor through running sand and silt. The contractor, the Traylor/Shea JV, opted for a continuous conveyor system spanning the length of the tunnel, with custom adaptations for varying ground conditions and water inflows.

Design features included sealed transfer points and receiving hoppers. Urethane rubber was used to seal the points and minimise spillage. Additives mixed with the wet ground, such as foam and bentonite, ensured a smooth consistency of muck flow on the conveyor and eliminated clumping, which can contribute to muck fallback and spillage. Cutterhead design aided in injection of the additives, with four independent foam injection points to mitigate the risk of clogged lines.

During TBM operation, an extensible fabric-belt conveyor was constructed behind the TBM and over the top of the back-up system in the crown of the tunnel. The muck was discharged from the screw conveyor onto the belt conveyor at the front of the back-up system. From there, the conveyor system was elevated over top of the back-up system and into the crown of the tunnel. As the conveyor travelled through radiuses down to 400m, Robbins self-adjusting curve idlers transferred the load and enabled the system to run through curves. A tripper assembly, located at the open cut in the tunnel, redirected the conveyor up through the open cut at a 12 degree incline for discharging onto a stacker conveyor. The system was powered by a 150kW variable frequency main drive, as well as two 150kW booster drives.

By machine breakthrough, the conveyor had operated at well over 90 per cent availability for the duration of the drive. The 210m weekly advances, as well as daily advance rates of up to 50.3m, are believed to be records for EPBMs in the 4m to 5m diameter range.

Continuous evolution
Conveyance has become more efficient within the last decades, making continuous conveyors ideal for urban tunnels and variable ground. Both Martz and Redmond see a growing acceptance of conveyors in a wide spectrum of geology. “I think conveyors are being used more today than ever—when we used the belt in Sacramento just a few years ago to remove soft ground, a lot of people thought we were crazy. But, that belt made the job a success,” Martz says.

Redmond sees several improvements that could be made to conveyors in general, with regards to better control of muck spillage and handling of large pieces of material in blocky ground. Martz also hopes improvements will be made with conveyor operation: “It is important to remember that this is not a magic piece of equipment that moves huge amounts of material at the flip of a switch. I think the biggest challenge is in finding experienced people to set up, operate and maintain the system, so that it can achieve what it is designed to do.”


Belt cassettes, like those on the Robbins conveyor system at East Side Access, are designed with electric rather than hydraulic winches for constant torque and PLC control California’s UNWI tunnels used a 5.8km long Robbins conveyor system, resulting in both TBM and conveyor system availability of above 90 per cent A Robbins fabric belt conveyor with sealed transfer points conveys clay and sand mixed with anadditive in Sacramento, California Hazardous belt strings can be minimised with new materials such as straight warp belting, which is string formation resistant and aids conveyors on curved tunnel drives Robbins self-adjusting curve idlers allow for curved conveyor drives by pivoting to accommodate changing load tensions