ATUNNEL project’s progress is only as fast as the ability to remove the spoil, which is why conveyor systems are increasingly becoming the mucking out method of choice. "Conveyor systems have proven themselves to be a very reliable means of muck removal," says Dean Workman, vice president, conveyor systems at Robbins. And, he adds, there are "multiple advantages" to using a conveyor belt system. "When the customer looks at the logistics of hauling the material with rail cars versus a conveyor system, they see that they can put in a lighter gauge rail, they can use smaller locomotives in the tunnel, and less ventilation is required because they’re not running as many locomotives." Terratec engineering manager and managing director Anton Jurasovic says the speed of muck removal provided by conveyors offers cost benefits, at least in long tunnels.
"Conveyors wouldn’t be used if the tunnel is short as the high installation cost means there’s not the opportunity to reduce the overall costs by speeding up the completion date. But on longer drives the main advantage is cost reduction of mucking out through reducing the time the full tunnel takes to be completed," he says.
Safety is also a big consideration. "The more locomotives or muck trains you have in the tunnel, the greater the risk or an accident or derailment," says Workman.
FIRST CHOICE
Now the use of conveyor systems has become accepted practice, he says. "Around the world contractors have an understanding of the systems and how they work so you don’t have the battle that we did 20 years ago. Then it was belt conveyors versus muck cars but it’s very rare now that we have someone ask what the advantages of the conveyor are over the muck car system. The mindset of the industry has changed," he says. "In my opinion, conveyors are now the favoured method of mucking out."
The introduction of a conveyor belt system behind a TBM has had a major impact on the tunnelling industry, speeding up the whole process and enabling world records to be set, says Workman, adding that the only time a conveyor needs to be shut down is when more belt is added.
Another perceived advantage of conveyor systems is their reliability and Workman says they are generally operational for 90 per cent of a project. "One of our customers in New York said they never gave a thought that the conveyor would be operating. They just had to push a button and it ran," he says.
Originally conveyors were used for tunnelling or mining in hard rock applications but a major development in the last five to 10 years has been their use with EPBs.
"Initially everyone was hesitant because of the always changing consistency of the material but we have proved that the belt conveyor can be used successfully in EPB applications," says Workman.
No matter what the method of removing the spoil, EPB-produced muck needs to be treated before it’s handled and there is now more work being done on the transfer point to minimise the amount of clean-up required around the belt conveyor.
"With EPB you have to take into consideration that you’ll have a different type of material than you’ll be conveying in hard rock applications so you have to seal up transfer points," says Workman. "Sometimes you have a lot of water on the belt and even though the water can be carried on the conveyor, when it goes to a transfer point there’s a big surge and splash and it can force the solid material out so you have to seal up the transfer points as much as possible."
Cool runnings
However, Jurasovic points out that manufacturers are addressing the potential problems posed by conveyors. "Manufacturers have ‘super elevated’ the belt to negotiate curves smoothly to minimise spills, and belts are now typically sized to ensure they are not ‘over filled’, which also minimises spillages," he says. "Also belt guides are added to control belt tracking."
According to Workman, the two 90-degree curves in the Deep Rock Tunnel Connector project in Indianapolis, have probably made it "one of the most difficult alignments that a belt conveyor has ever negotiated".
"We’ve not only proved that [the technology] can go to very long lengths but [it] can negotiate some difficult alignments," he says.
One of the biggest issues for such a sharp curve, and for conveyor systems in general, is safety.
"Going through a curve the belt can come off the idlers and go across the tunnel. We’re using a very high strength of belting so should it come off, the implications could be catastrophic so you have to make sure you design the system so that doesn’t happen," says Workman.
This is done by locating the boosters in the right places to control the tension, which not only enhances safety, but the life of the system as well.
"If the tension is too high it can cause premature failure through the curve."
But maintaining the tension is one of the biggest challenges of a conveyor belt system, coupled with locating the problem when something does go wrong.
"One of the biggest issues for the contractor is that you have to be able to determine quickly where the problem is if you have a fault or if the system goes down. If a major component is damaged all of the belt is going to cycle through that problem area in a relatively short period of time," says Workman.
"Some of these systems are very long – sometimes 36,000ft (11,000m) – and you can’t see all of it at one time." In projects around the world Robbins has seen incidents where the conveyor belt has nearly split in two, creating project delays and additional costs. "It takes a long time to replace a belt; you don’t just go in there with duct tape and put it back together," says Workman.
So minimising the chances of a problem or failure is key, and this is where monitoring systems and alerts come in.
Early detection
"Early detection is vital," says Workman, adding that electrical monitoring and alert systems have become increa
"We monitor not only the temperature of the electric motors and gear boxes, but also the vibration of these units so you can determine if there is something occurring that could cause premature failure. We monitor the torque of all the drives in the system so you know that if one drive starts to pull harder than the others that alerts you that there is a problem in that area, either pulling on the drive or with the running belt structure itself," says Workman.
Davis Derby designs and manufactures electronic control and monitoring equipment, software and data logging systems.
"Our systems monitor virtually all aspects of the belt," says contracts manager Dave Poole. "It monitors the belt for alignment and tearing, and temperatures on the gear boxes and motors."
Now the company is developing what it describes as a "total conveyor monitoring system". With WiPAN each pull key is fitted with a wireless connection system to create a wireless network along the length of the conveyor. The system identifies which pull key has been activated and feeds the information via co-ordinator and ethernet back to the surface.
"All that data is available to someone sitting at their desk but it’s like sitting alongside the conveyor," says Poole, adding that it allows managers to decide whether a problem needs to be addressed urgently or it can wait until the next maintenance slot.
"It gives you a lot more information and allows decisions to be made so you can maximise productivity and minimise downtime," he says.
"The last thing you want is a tunnelling machine that’s going at full bore then suddenly you can’t get the material out. That could have been foreseen and resolved during the maintenance schedule."
WiPAN is being trialled on two jobsites in the UK and is close to being fully launched. Future planned developments include wireless vibration and temperature sensing, says Poole. Robbins designs its systems so that, in the event of a problem, components can be changed as quickly and easily as possible. However, during his career Workman has noted that on tunnelling jobsites attention to maintenance is sometimes not as good as it could be.
"Companies mining a product to sell are putting in a belt conveyor system for long-term operation, normally 10 years. A tunnelling contractor is looking at the conveyor system being in operation for as long as they’re boring, which is usually a relatively short period of time," he says. "Mining companies really look at their conveyor and track and train their belts but in tunnelling that tracking and training is sometimes neglected and then you start to have problems with the system."
Having said that, conveyors are not a one-use-only system. Redesigned and modified, they can be used multiple times, as with the conveyor on the Indianapolis Deep Rock project which is now in at least its fourth incarnation.
At the same time, conveyor systems are constantly evolving in line with feedback from contractors and as components improve.
"They have become longer and faster – and consequently – higher powered," says Jurasovic. "This enables more types and lengths of tunnels to be use conveyor mucking, and faster belts can carry more muck for the size of the belt."
He believes the next technological developments will be lighter and stronger belts, faster belt speeds, and better head pulley lagging materials, with higher co-efficients of friction than are currently available.
Robbins holds regular design review meetings and aims to make improvements "with each new system we do", says Workman.
"Also the electrical control systems are improving as the industry improves the components that are used – from cameras that you can remotely control underground for monitoring, to variable frequency drives; there are so many things taking place right now."
