The invention of the tunnel boring machine has had a profound impact on the tunneling industry around the world. The speed, safety and efficiency of the machines have allowed tunnels to be built throughout the world in varying geologies for a variety of civil purposes, as well as mining, military, research and other uses.
But the TBM itself is only part of the equation. The best TBM design is limited by the ability to move spoil from the cutting face out of the tunnel. Early TBMs relied solely on muck cars to move excavated material from the face. When conveyor belts were introduced, however, the speed and efficiency of the machine greatly increased.
For this issue, we talked with Dean Workman, vice president of the conveyor division at The Robbins Company, about the past, present and future of belt conveyor systems for TBMs. Workman began his career designing underground equipment for the Long-Airdox Company, and has spent the last 20 years with Robbins’ conveyor division.
What are the advantages of belt conveyor systems?
As TBMs were being developed, manufacturers did a lot of work so that the machines could mine faster and more efficiently. However, they were restricted by the ability to get material out of the tunnel, especially as the tunnels became longer and longer. With rail haulage, you end up having to wait for locomotives, you have to set up switches, and there can be issues with locomotives getting derailed. Additionally, you need to upsize your ventilation if you are dealing with diesel locomotives. With the belt conveyor system, the only time tunneling stops is to add belt. Once the belt conveyor systems were introduced, we started to see TBMs around the world breaking speed records.
How have belt conveyor systems evolved over the years?
One of the most significant changes has been the introduction of belt conveyor systems for Earth Pressure Balance TBMs. The use of belt conveyors with EPBs opened up a new market and has increased the number of conveyor systems being used around the world. The use of conveyors with EPBs, however, requires a little more effort in the planning and design stages. You need to consider what type of material you will be conveying because there is a significant difference between muck coming from a rock TBM — which is generally reasonably dry chips that just flow through the transfer belt’s points — and that of an EPB. With EPB TBMs you can get material that ranges from sticky clay to wet, abrasive material. To deal with the different ground conditions you need to treat the material with additives in the cutting chamber of the TBM to turn it into a consistency that can be conveyed. If you wait until the material comes out of the cutterhead, it is too late. So, it is important to work with the contractor to ensure you have the proper setup ahead of time.
What innovations have you seen in belt conveyor design?
At Robbins, we have developed a patented load-sensitive idler that allows us to negotiate horizontal and vertical curves – it super-elevates automatically depending on the tension and load on the belt. When the tension is high and there is not much load, the idlers will rock up to their highest super-elevation angle. Then as the material starts to come, that weight on the rollers will force the rollers to come down, so the belt is not going against the guide rolls and damaging the edges of the belt as it goes through the curves.
Robbins is supplying its 100th conveyor system for the 30-ft diameter Akron CSO project. What considerations are there when designing a conveyor system for large-diameter tunnels?
Large-diameter tunnels are actually easier to work with than small-diameter tunnels because you have much more room in the tunnel. In small-diameter tunnels everything is compacted and it is hard to find real estate for the conveyor in addition to the ventilation and utility lines. In the larger tunnels there is not only more room in the profile but also in the trailing gear.
A point of emphasis about the Akron conveyor is that it is a mix of equipment that we have redesigned and rebuilt. The main drive is a customer-supplied component from Long-Airdox; the other parts came from our previous systems at the Mexico City Metro and San Francisco Central Subway. We always try to work with the contractor to use the components they have.
What unique projects is Robbins working on currently?
One project that is particularly challenging is the Westside Purple Line Extension in Los Angeles. In that case, there are twin tunnels being bored in opposite directions from a single jobsite. All the equipment – the vertical conveyors, the splice stand, the storage unit – had to be designed so that the system could be flipped to mine in the opposite direction without having to move any of the components. And, being in an urban environment, you are also confined by space at the jobsite where even a few inches can make a difference. It is interesting to see that one of the contractors on that job – Traylor Bros. – is still using some pieces and parts for the conveyor system that we originally supplied at Long-Airdox at least 25 years ago. As with the Akron conveyor, we often use refurbished components in combination with newer components.
What is the rule of thumb for deciding between a belt conveyor and muck cars?
It is always the contractor’s decision, but generally if the tunnel is a mile or more in length, conveyors are used. If it is shorter, then muck cars will probably be used. But that is changing. Over the past decade or so belt conveyor systems have been built for more efficient setup and can allow the contractor to start mining more quickly vs. a muck car arrangement. This is because the components themselves are modular, making for easier shipment and installation.
What is the next step for belt conveyors?
With our small boring units (SBUs), there is an opportunity to develop smaller belt conveyor systems to increase the efficiency. Typically, you do not see belt conveyors smaller than 24 in. wide, but we envision belt conveyors 18 or even 12 in. wide that can be used with SBUs since we are starting to see the drives lengths increase with those machines.
How do you get the most out of your belt conveyor system?
Like many other pieces of equipment, maintenance is the key. It is particularly challenging with a belt conveyor system because it can stretch for miles and you can’t see the entire system at one time. But if you have one bad idler or one weak point in the system, the entire belt is going to cross it. There have been improvements with electronic monitoring to help the operator see what is going on with the system. Planning is also key, in the early stages of the project, there is a lot of belt strength and power in the system, but as the belt extends farther and farther, the load on the system increases.