Nine Ways to Extend the Life of a Tunnel Boring Machine
Long distance TBM design makes tunnelling over 15 km a reality. Today’s Tunnel Boring Machines are often required to bore longer tunnels in harder rock at a faster pace – a trio of challenges that can be daunting for any contractor.
With the proper design, operation, and maintenance modern TBMs are very capable of reaching, and even exceeding their 10,000-hour design life. TBMs in the industry today have already accomplished the feats of boring upwards of 50 km on multiple tunnels over decades, and of completing single TBM drives totalling 27 km. With new capabilities, even greater feats may be possible.
From abrasive rock to fault zones to water inflows, geologic challenges become more common as tunnel lengths increase. In rock tunnels over 15 km long, a host of challenges may meet a TBM, requiring a versatile design. General wear and tear is an issue on machines boring long stretches of tunnel, and thus minimization of downtime is a key.
To counteract these challenges, a number of design features can be added during the manufacturing process, and these, combined with regular maintenance and well-designed logistics during tunnelling, can result in TBMs lasting for the tunnel length and possibly over multiple projects.
In order to design machines for such conditions, consideration must be given to the harsh aspects of tunnelling in hard rock over long distances. This can be broken down into what can be done at the TBM design stage, and what can be done during TBM operation and maintenance. Overall, oversizing components like the hydraulics and lube systems is a good idea, and overbuilding of steel structures is key.
Solution #1: Proper Cutterhead Design
Much of what counts in maximizing TBM life involves the proper design of areas directly in contact with the rock face, namely, the cutterhead and cutters. High strength materials, wear protection on the cutterhead, and cutter spacing all affect cutterhead wear in dramatic ways.
The cutterhead should be designed with regular cutter inspections and changes in mind. It must also be built to last: this can be difficult when a back-loading cutterhead is designed or required in order to change the cutters from inside the machine. The structure of a back-loading head is not unlike Swiss cheese, and needs to be made strong enough to withstand many boring hours.
In order to do that, much of the strengthening occurs during the manufacturing process when corners are welded and stress testing is conducted to test any fractures that may occur in the cutterhead steel. During operation, deflector plates on the cutterhead can bulldoze rock away from the leading edge of disc cutters, minimizing the impact of loose rock on cutters that might otherwise cause chipping or spalling.
Adequately-sized muck buckets are also a crucial component to allow for a smooth flow of muck, along with the right quantity and location on the cutterhead. Durable, replaceable bucket lips are also of key importance in these high-wear areas.
Solution #2: Opt for Rugged, Larger Diameter Disc Cutters
In terms of rolling disc cutter design, larger diameters are manufactured with larger bearings capable of withstanding heavier loads while also offering more wear volume. 19-inch or 20-inch cutters are preferable to smaller cutter diameters such as 17-inch.
The materials of the disc rings themselves are also crucial – exceptionally clean steel ensures high resistance to fatigue, for instance. Metal-to-metal face seals are also the industry standard in eliminating ingress of foreign materials and maintaining elasticity at high temperatures. These temperatures can often rise quickly when excavating very hard rock, and cutter designs must be able to withstand repeated heating and cooling.
Solution #3: Use a 3-Axis Main Bearing and Durable Seals
Large diameter 3-axis main bearings, with the largest possible bearing to tunnel diameter ratio, have larger dynamic capacity and therefore are capable of withstanding more load impacts and give longer bearing life. The bearing and ring gear are in a difficult-to-access spot on the TBM, and must be designed for longevity, with a super robust structure and high safety factor.
Robust seal design is also essential. Robbins TBMs utilize a proven seal design including hardened wear bands. Many other manufacturers don’t use wear bands, and so as the TBM operates, it wears a groove into the seal lip contact zone. Incorporating a sacrificial wear band into the design, that can be switched out or replaced, ultimately makes repairs easier.
The abrasion-resistant wear bands, made of Stelite™, can be changed in the tunnel in the unlikely event of excessive wear, or can be relocated on the bearing to ensure that damage is not done to the TBM structure itself on long drives. Other manufacturers utilize a ring of low-carbon alloy steel instead, which is non-replaceable.
In addition to the seal design, other elements of the main bearing such as the internal fasteners must be designed as durable and of high reliability, as these fasteners are difficult to access and are not easily replaceable. The studs connecting the cutterhead to the main bearing seal assembly must also be closely analysed for strength, deflection, and adequate fastening and protection must be provided for the fasteners against abrasive muck.
Solution #4: Dry Sump Lubrication
Dry sump lubrication is a critical way of keeping the main bearing cavity clean by filtering and recycling the oil at a constant rate. Any contamination is cleaned from the cavity, prolonging bearing life.
The system also has an added benefit in that the oil can be monitored and analysed for any indications of distress in the main bearing or gears. This monitoring has the potential to allow for correction or intervening maintenance of critical components and structures before a failure occurs.
Solution #5: Opt for Efficient Variable Frequency Drives
The right drive system is also important in long-distance TBM design. Variable Frequency Drives (VFDs) and planetary gear reducers allow for infinitely adjustable torque and speed control based on the encountered ground, which optimizes the TBM advance rate and reduces damage to machine components.
This is in comparison to older style drives, which were often single speed or 2-speed. If an older TBM bored into a fault zone, for example, there would be no way to slow down the cutterhead. Such drives would often result in undue wear to the TBM, or even damage to structural components.
Drive motors must also be designed to withstand high vibration as a result of excavating through hard rock conditions. Cantilevered motors must be able to withstand the high g-forces applied to them by the violent machine vibration that is induced by rock cutting action.
Solution #6: A Uniform Load Path is better
A uniform load path, from cutterhead to main bearing to cutterhead support, is always desirable. However for long distance tunnelling, the load path can be crucial as high stresses occur wherever the load path shifts.
A cutterhead with a cone-shaped rear section can help with this problem by evenly distributing the load across the circumference of the main bearing. All of these components must be designed in a more robust fashion, and the loads generated by the cutterhead must also translate into a heavier overall structure of the machine, making the load path all the more critical.
Solution #7: Don’t Forget about the Muck
The path of muck, from the muck bucket to the chute to the machine belt conveyor, must also be as smooth as possible. Smooth muck flow not only increases efficiency, but also prevents the problem of re-grind on the cutterhead.
It can also reduce wear on both the external and internal surfaces of the cutterhead. One way of achieving smooth flow is to oversize the TBM belt conveyor, allowing for increased capacity while reducing the overall belt speed, thereby reducing wear on the conveyor over a long period of time.
Solution #8: Minimize Downtime with Continuous Conveyors
A smooth muck flow path all the way out of the tunnel is also critically important. Use of continuous conveyors limits downtime when compared to the downtime experienced when a locomotive and muck cars are used. As a tunnel gets longer, the time to transport muck cars in and out of the tunnel becomes less and less efficient.
Continuous conveyors for long distances must be designed with robust transfer points to allow for a soft landing of the muck in order to reduce wear and tear. A good maintenance system including muck scrapers to keep the belt clean and flashings to prevent spillage is also of critical importance.
Solution #9: Daily Maintenance
Once the machine has been built, regularly scheduled maintenance based on tunnel length and geological conditions is key. While there are no universal guidelines for long-distance tunnels, contractors must be especially diligent and conduct more detailed inspections the longer a TBM is in operation.
Planned cutter inspections are a regular part of maintenance, which is recommended daily. Checking of oil levels, and all fluids, greases and hydraulics is also of primary importance. Daily logs are recommended for monitoring of all major systems on the TBM.
Depending on the tunnel length, some maintenance may be done beyond what is considered normal. Gear boxes, for example, may be designed for long tunnels but if it is known that the tunnel length will exceed the life of the gear boxes then planned refurbishment should occur during tunnelling.
This procedure has been done on several tunnels including India’s AMR tunnel. It will be the longest tunnel without intermediate access at 43.5 km once complete.
Proper Operations Secure Long Life
While the limits of TBMs are continually being pushed on long tunnel projects, much about what makes them last is already known.
Proper TBM design and maintenance are keys, as are training of the crew if needed and cooperation by all teams involved. In an intricate, long tunnelling operation, smooth communication can be the best pathway to success.