TBMs are circular. Everyone knows that. Tunnel boring machines always have been, and in the opinion of some, perhaps always will be circular, because it is the easiest and simplest shape to bore tunnels.
Many tunnels that people want, on the other hand, are rectangular. An arched roof may be acceptable, but people generally want flat vertical sides and, in particular, a flat, smooth bottom surface that you can drive vehicles along.
The traditional way around this has been to excavate a circular tunnel and then infill the invert with concrete – insitu or precast. It seems a shame, not to say also wasteful and inefficient, to carefully dig a hole then straightaway fill 30% of it back. A TBM that bores a rectangular tunnel would therefore be a big advantage.
Robbins has just created one. It is in operation at Mexico’s oldest silver mine at Fresnillo, and it has completed 2,000m of rectangular-section tunnel, through rock hard enough to defeat road-header machines. The geology of the mine is andesite and shale with quartz intrusions. The tunnel the machine creates is 5m wide by 4.5m high with a smooth flat surface that the mine’s rubber-tyred trucks and other large equipment can immediately drive on – to the satisfaction and pleasure of the mine owner.
Called the MDM5000 (Mine Development Machine five metre width), the rectangular TBM uses cutters, just as on a conventional TBM, but instead of being set in a rotating disc they are in rows on a plate or bar which is mounted on arms at the front of the machine in roughly the same way as a bulldozer blade. The plate swings up and down, through a little over 180°, making the cutters traverse the face of the tunnel from bottom to top and then back again. The result is a rectangular cut.
MOVING FORWARD
Propulsion is very similar to a conventional TBM: it uses grippers and propulsion cylinders, as normal. “The main difference is that the cutters are pressed against the rock using extension cylinders rather than propulsion cylinders,” explains Robbins owner Lok Home.
“We’re duplicating what we do in TBMs. Steering is through the grippers and cylinders as on a TBM. We use disc cutters; we use them on the same spacing as we would on a TBM. The difference between round and rectangular machines is that we have a swinging motion. There’s a big plate in front of the machine containing all the cutters, and they are set at around 75mm spacing across the face, and we just load them up with a thrust from the machine like a TBM. Then we swing this plate up and down, and it gives a sort of arc-like cut. It means that the cutters stay in their same grooves all the time as in a round TBM, and that is important for efficiency. So that is how we cut rock and break it out.”
A scraper blade along the bottom gathers the spoil into a vertical belt conveyor, from which a horizontal conveyor carries it to the portal. “When you need to change the cutter blades, it is easier than in a round TBM because the plate that carries them can be lowered to put them in easy reach.”
Conventional TBMs fit lining segments behind them. The flat roof at Fresnillo is lined with wire mesh. “So as you progress and the machine moves ahead, you roll the screen out and rock-bolt it in place; so everyone stays safe in a very safe environment.”
So, a finished tunnel is delivered – flat floor, safe roof, smooth vertical walls, ready for installing ventilation and services – as excavation proceeds, and that is something that is really valuable to the mine owners. Drill and blast, which is the alternative, leaves rough walls which need lots of further attention. And the MDM disturbs the overburden much less: it causes minimal damage to the surrounding rock so it needs much less in the way of ground support.
MADE FOR MINING
“The MDM is designed not for excavating ore bodies, but for rapid development of the ore bodies, for reaching them quickly and giving access to them. So that is the application and that is what it is designed to do. Mechanical excavation with the MDM is significantly faster, as well as safer, than drill and blast. One reason it is faster is that it is continuous. You don’t have to evacuate the tunnel then wait for the blast to clear as you do with drill and blast; and as I said our cutters stay in the same groove, which is the secret of high advance rates for TBMs.”
It should be remembered that TBMs which bore near-rectangular tunnels are not a new innovation, as Home explains: “The Chinese have TBMs that have four or six small circular cutter heads arranged next to each other in a rectangle configuration, and the Japanese have done quite a bit of work too, having built maybe twenty civil projects that way. But those only work in softer ground. The gauge is picked up by the outer edge of the box structure of the machine and by just forcing it through the ground. That cutting principle and that way of doing it would never work in ground like this. Our goal was to have a machine that could mechanically cut through even the hardest rock formations.”
So, how did the contract with Fresnillo come about? “We talked to them; we have a very good dialogue with them. They told us what they wanted; and they placed the order. We have a certain payment on performance, but they stepped up and effectively bought it on faith. They committed to put it in their mine, and believed that Robbins would be able to do as we said, and so from their standpoint it has been an interesting project.
“From Robbins’ point of view, we took it on as a real project, as if it were an order for a conventional TBM. The Robbins team has the know-how and did the designing – I helped out a little with the application. We designed it, set all the specifications, sent those out to our standard suppliers of sub-components. On our standard machines, we do first-time assembly on site, but this was a completely new design so we shop-assembled the whole thing in our workshops in Ohio to check that everything fitted together and we did an acceptance test there. But you cannot load test in a factory. We shipped all the components to Mexico and put them together in a cavern at the end of a ramp. We actually built special crawlers for it to move the big components down.
“We had this operating within two years of taking the order. The whole machine, the rock-support equipment, all the backup gear, the ventilation, cutters for the first ten thousand metres, spare parts, and the conveyor system to go with the machine, which we supplied, came to about US$20 million. That is in line with a circular TBM of similar horsepower in the same range for cutting the same cross section. Our machine is a little over 1,000KVA.
“Operator numbers are similar to those of a round TBM – we can bring that down in future with automation. Since this was a first in concept and design, we kept the operating systems simple.”
A LEAP OF FAITH
So has Fresnillo’s leap of faith paid off? “It is making a beautiful tunnel in the mine and that’s a starting point. One good thing about it is that we are getting close to a couple of thousand metres of tunnel, which I guess is proof that the concept works. Fresnillo are headed to be able to drive into a tunnel where the profile is regular and known and all set up for automation, with no bumps and dips like you get with drill land blast. They like the safety aspect of it and they like going into a tunnel where you have put up the ground support and the ventilation and installed all the utilities straight away as the TBM progresses. That is a concept they are not used to, and it is a product and a process that they really like.
“We are making certain modifications to it, but that was expected, by the mine and by us. It is a first unit, and you don’t get it all right when you step out of your comfort zone. In digital software language, it is an alpha machine, and our next ones will be the beta version. But substantially, two thousand metres shows that we got quite a bit of it right.”
So, will Robbins make more of them? “We are going global, and I will be happy to sell two or three of these a year. It is a design for mines rather than for civil applications like road and rail tunnels. There are reasons for that: I have to say that the MDM is not as fast as a circular TBM, nor does it have the same cutting efficiency or advance rates. The reason is that although the cutters stay in the same groove, and we reach some of the same peripheral speeds as on a round machine, we start the cutter plate and arm at the base of the tunnel, at the bottom of its swing, and we accelerate it to speed as it rises. Then we slow it down towards the top, where it reverses direction and starts to accelerate downwards again. There is a big loss of efficiency and energy as you accelerate and decelerate that a continuously-rotating circular cutter does not have.
“That said, the MDM is much better (more efficient) than say a drum cutter, which has a rotating drum with cutters on its periphery. On that, the cutting head enters the rock at say zero depth and only once, when it is halfway through its drum circle, does it see its full load depth; then, for the next half of the circle, as it is on the half of the drum that is in clear air, it is at zero depth. So the MDM is much more efficient than that, and it is faster than a roadheader; but it is not as efficient a cutting machine as a round TBM. But for mining applications, the speed and the tunnel profile and smooth finish can outweigh that.”
Mines are generally slow to adopt mechanical excavation because of two big conceptual hurdles for them. First, a TBM is a big bit of kit to get underground. Unless you have a ramp or are well equipped to lower large pieces, and that can cause reluctance.
“And the second thing is that even if you see the advantages and figure out how to utilise TBMs in your mine, then you have to re-design your mine and your mining practice to use them. Currently, when we try to get acceptance for the idea, the mine operators will say ‘OK, we want a machine that will burrow in 50 metres from here, then two days later back up out to be over there for the next strip, and so on’…” But that’s just not how you can use the advantage of current mechanical excavation. So going forward, we as manufacturers have to change our mindset on how to make our machines more agile and portable, and they as operators will have to change their mindset to make the mines more adaptable to exploit the technology.
“We got the project at Fresnillo because they realised they needed this machine-built drift to reach the lower ore bodies and they already had a ramp that they could take the machine down to start it off. So that’s why we ended up doing it at this particular mine; and hopefully it will bring greater acceptance of the MDM.
“That is one goal for the future: we would like to see if we can position ourselves to be the leaders in this technology. For the Fresnillo people, and for ourselves, I think we both know that we are onto a winner.”
