Hard rock TBMs are highly capable, specialized machines designed for any number of project parameters, from stressed high cover conditions to long, single bore tunnels in variable ground. But what makes a good TBM great? From setting world records for speed to boring 25km, the answer is anything but straightforward.

Optimizing the rebuild
Many Robbins TBMs have operated on numerous projects, totaling up to 50km of combined tunneling. There aremultiple factors contributing to longer machine life, including proper rebuilds and refurbishments, TBM maintenance, TBM design and geology of the machine’s previous projects.

In general, as long as the TBM is well maintained, there will be jobs it can bore economically. If this was not done on a previous project, or the project conditions are very different, then refurbishment is the best option. Optimal TBM refurbishment on a used machine requires a broad knowledge of the project conditions, and there are some limitations:
• Machine diameter can be decreased within the limits set by free movement of the grippers and side/roof supports.
• Machine diameter can be increased subject to the structural.
integrity of the machine and the power/thrust capabilities.
• Propel force can be increased only to the level supported by the grippers’ thrust reaction force.
• Cutterhead power must be adequate to sustain the propel force in the given rock, but cannot be increased beyond the capacity of the final drive ring gear and pinions.
• Cutterhead speed increases must not exceed the centrifugal limits of muck handling or the maximum rotational speed of the gage cutters.

Increasing the power of the TBM is one way to make the design more robust for a longer equipment life. Strong designs have been developed in recent years, including Robbins High Performance (HP) TBMs, used on projects such as the Karahnjukar Hydroelectric Project in Iceland. “The HP TBM is much more powerful and has a longer life, in part because of the greater strength of core structure and final drive components. They are stronger and can be used over a much wider range of diameters, whereas older machines from the 1970s are typically limited to a range of less than 1m plus or minus their original size,” explains Joe Roby, Robbins vice president production and logistics.

HP TBMs have the capability of operating over a broad range. For example, a 4.9m TBM can be refurbished between 4.3m and 7.2m diameters—a range of 2.9m. Main bearing designs have allowed for greater flexibility, evolving from a tworow tapered roller bearing to the three-axis, three-row cylindrical roller bearing used today. This configuration gives a much higher axial thrust capacity for the same bearing diameter and far greater life in terms of operating hours or revolutions.

There are other factors influencing machine and equipment life, including geology. In general, homogenous rock creates a steady load for even wear of machine components. Mixed face conditions result in impact loading of individual cutters as they pass from broken to solid rock, which causes uneven wear on the main beam, thrust and gripping elements. The uneven wear reduces the life of those elements even when the failure is not immediate.

“Overall, what determines how long a TBM will last is a function of the fundamental design, such as the thrust and gripper load path through the machine and the robustness of the core structure. On old TBMs, you can increase the strength of the ring gear and pinions, and you can add larger motors. With sufficient core structure strength, it’s also possible to increase the thrust capacity. The limitation is the capacity of the gripper cylinder to handle the increased power and thrust. Once replacement of the gripper cylinder and carrier are required, TBM modification costs are generally considered uneconomic,” says Roby. He adds one more note about building the perfect TBM: “Of course, the main limiting factor for further increasing TBM design life is economics. For a new machine, we can overbuild thrust capacity and power, but this also increases the TBM cost and the tunnel project cost—something adverse to most contractors.”

Calculating advance
Once the machine has been supplied, it is important that contractors know what advance rates to expect. A recent research project has shed some light on how this can be achieved with algorithms calculated to predict TBM advance. The factors on each project are numerous, and depend largely upon the expected penetration rate. To estimate advance, the researchers included:
• Expected penetration rate in the geology (calculated by analyzing the machine, rock type and rock mass).
• Cutterhead rotation speed.
• Estimated downtime (correlating to a percentage of system availability).
• Time available for tunneling, minus restricted hours and maintenance shift.
• Abrasiveness of the ground (correlating to expected cutter changes).
• Quantity and type of ground support.
• Logistical and operational conditions during tunneling.

The factors are input into a TBM Simulator model, using data from numerous actual projects to determine an estimated rate of advance. It is these field proven projects, however, with all of their obstacles and successes, which may be the most accurate predictor of actual TBM performance and machine wear.

Anatomy of a record-breaker
A number of Robbins TBMs have achieved landmark feats, including world records that have stood the test of time over many years. Still other records are broken every few years as technology advances and optimal project conditions present themselves. Two of these ‘Super TBMs’ are operating or have recently finished on projects in China and in Maryland, USA.

Bi-County Water Tunnel TBM, Montgomery County, Maryland, USA
One of the longest-running machines on record is a now 3.0m diameter Robbins Main Beam TBM originally owned by Affholder. The machine has been used on at least 10 different projects between 1973 and 2011, in excess of 48km total length. Back in 2000, when the machine was boring its fourth job at the Plateau Creek Tunnel, it achieved a world record in its size class for a best 10-hour shift of 67m. Bob Stier, then of Affholder, was involved in the project. “This was an older machine that we had refurbished for harder rock conditions. We increased the machine thrust and increased cutterhead power from 300 to 600kW. We had new gear boxes, motors, and larger thrust rams added. It was originally the first 2.7m diameter machine Robbins had built, and it was remade to cut very hard rock on the job, up to 175MPa, very efficiently.”

The 4.0km tunnel in Palisade, Colorado was part of a water transfer project for the UTE Water Conservancy District, and was completed in March 2001. “Overall maintenance of the machine by the crew helped to keep everything up and running, and advance rates high, for the whole project—not just our record-breaking week,” says Stier.

The machine’s latest iteration is at the Washington Suburban Sanitary Commission (WSSC’s) Bi-County Water Tunnel in Montgomery County, Maryland. The 8.5km long transfer tunnel will improve capacity in Montgomery and Prince George’s Counties by carrying 380 million liters 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 27 and 84m below the surface. The 10.6m diameter, 50m deep Connecticut Avenue shaft is the only place from which to launch and remove the TBM.

Work is being carried out by a JV of Oscar Renda, Southland and SAK, with Black & Veatch as the primary designer. “We did consider a new TBM, but we knew the history of this machine and its performance in the past. We decided that the rebuild would be better for us economically,” says Tim Winn, principal in charge for Oscar Renda.

The 3.0m diameter machine, with its original name of ‘Miss Colleen’, was launched in August 2010 from the main shaft. Work was done prior to the launch to boost the machine power again, from 600 to 900kW, in order to accommodate hard granite rock of 140MPa UCS. “We also tried to simplify the operator’s cab, and increased the bearing size to 2.5m. The rock was very hard, and we were concerned about the length of the tunnel drive with no access. We knew that the larger bearing would provide a higher likelihood of success,” says Winn.

He cites several reasons for the machine’s long history of success: “The most important factor is probably the main structure of the TBM, which was built very rigid. It was definitely built for severe rock conditions. We also maintain the machine on a regular basis, checking the lube system, changing belts and hoses, and monitoring cutter wear.”

The machine holed through its first 1.2 km of tunnel into a shaft in the Stoneybrook area in December 2010. Advance rates averaged 12m per 10-hour shift, with some rock bolts, steel straps, and wire mesh being added in less competent ground. As of January 2011 it was being backed down the bored tunnel for re-launching at the Connecticut Avenue shaft on a 7.2 km long drive in the opposite direction. All excavation is expected to be complete in January 2012.

Pinglu Tunnel TBM, Shanxi Province, China
In November 2010, a veteran Robbins Double Shield completed one of the world’s longest single-drive TBM tunnels, at 25.4km. The 4.8m diameter machine bored the Pinglu Tunnel, the most recent leg of China’s Yellow River Water Diversion Project, for the Sino-Austria Hydraulic Engineering Co. Ltd (SAHEC) JV, led by Alpine Bau.

The Pinglu Tunnel connects the North Main Line to the General Main tunnel the Yellow River Water Diversion Project. Due to go online in October 2011, the remote tunnel will transfer water to drought-prone Pinglu, Shuozhou, and Datong areas. Over 100km of the South Main Line were excavated by five TBMs, including four Robbins Double Shields, between 1999 and 2001. The entire scheme will transfer water from the Yellow River to dry regions of Shanxi Province, an area that receives just 400mm of rainfall per year on average.

The Robbins machine that excavated the Pinglu Tunnel was the same used to bore the 12km long Lot 5 tunnel on the Yellow River South Main Line in 2000. During the course of this original excavation, the machine set two long-standing world records in its size class of 4 to 5m: best month (1,855m) and monthly average (1,352m). The 12km long tunnel wascompleted in less than 10 months.

Fortunately, the ground conditions at the previous Yellow River tunnels were similar to the geology at the Pinglu Tunnel, requiring minimal modifications. One major overhaul was to the design of the back-up system—a modification that allowed for sufficient muck removal and materials delivery capacity for two TBM strokes.

Muck was removed by trains of rota dump muck cars in two tracks using California switches. The back-up system was equipped with floor chain movers to shunt the muck cars as they filled. Even with these measures, average transit times of 70 minutes were required for trains traveling from the tunnel entrance to the machine, and vice versa.

Lining for the Pinglu Tunnel, consisting of unique hexagonal segments, was produced in a segment factory near the remote jobsite. “The hexagonal segment concept uses dowels instead of bolts, so the rings are quick to build. We also had back-filling going on simultaneously with ring building, so this step did not have to be done afterwards,” says Meik Mueller, technical director for Alpine’s Asia division.

During tunneling, segments are placed in rings of four elements, in a honeycomb configuration staggered longitudinally. The non-bolted lining requires dowels to assist in accurate placement, as well as packers to negotiate curves and manage alignment corrections. The annulus is then backfilled with pea gravel and subsequently consolidated with a thin backfill grout.

Crews successfully mined through difficult and variable geology on the project, including 12m thick coal seams and abrasive sandstone that required intensive monitoring of tunnel air for particulates. Up to 70 per cent quartzite content made the rock very abrasive. “The combination of 70 per cent quartzite and 6 per cent corundum made the rock seven times more abrasive than quartzite—this is the stuff that grinding wheels are made of. On some sections this was eating our cutterhead, requiring that we change the bucket lips,” says Mueller. Despite the obstacles, advance rates topped out at 50 ring sets, or about 70m, per day.

Crews kept the machine in good condition throughout the drive with daily four hour maintenance shifts. “The most important thing is the key people that you have in each area,” continued Mueller. “Joseph Glantschnig, our Equipment Supervisor, has 120km of TBM experience. He was on the site from the first day, and was often the first one there in the morning and the last one to leave. By the end he could hear if the machine was working properly—he knew every bolt on it.”

TBM performance, whether it involves high penetration rates or long equipment life, is a function of several variables. Optimal geology, planning, and proper equipment selection all contribute to the mix, as well as daily TBM maintenance.

Optimizations for fast ground support and muck removal are also factors in the overall performance. Paul Bargmann, head of the machinery department for Alpine’s tunneling division, credits another important reason for his project’s success in China: “The key to this successful breakthrough was the crew. We had the right mix, and that is what made it work—including both experienced people and young people hungry to learn.”

Alpine Bau crew members celebrate the breakthrough of one of the world’s longest single drive tunnels. One of the longest running TBMs on record is a 3.0m diameter machine boring Maryland, USA’s Bi-County Water Tunnel. The TBM was lowered partially assembled into the 50m deep Connecticut Avenue shaft to commence boring of Maryland’s Bi-County Water Tunnel The Oscar Renda/Southland/SAK JV refurbished the TBM before lowering it down the shaft for launch, by increasing the cutterhead power and bearing size. Unique hexagonal segments, produced onsite, were used to line China’s Pinglu tunnel for faster ring builds China’s 25.4km long Pinglu Tunnel, using a refurbished 4.8 m Robbins Double Shield, was excavated in abrasive sandstone with quartzite and corundum