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| A diesel locomotive coming out of the tunnel. |
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Des Walters: Under Pressure Underground
Sydney's Airport rail link, the Transfield Bouygues Joint Venture project, needed diving expertise for their hyperbaric training contract. Though project intervention workers don't actually go under water, being under pressure requires a knowledge of diving theory and practice. Physiology, physics, decompression tables, the ability to equalise ears and operate a decompression chamber are all essential to their safety.
It was very like a scene from a science fiction movie. A dull yellow glow illuminated the tunnel as I approached the machine 30 metres below the earths surface; the tunnel behind me was black. There was a constant humming created by the hydraulic motors on the machine, and although I felt the air movement of the mechanical ventilation system, I was already beginning to sweat with the heat.
The light got brighter as I stepped onto the back gantry of the Tunnel Boring Machine (TBM), the noise increased - and so did the heat. On my first visit I'd expected the dirty environment of a traditional tunnel: jack hammering and drills and trolleys of dirt. But this high-tech machine is way past that. The tunnel is a clean smooth concrete tube, completed and sealed as the TBM progresses, and not a trolley of dirt or any excavation can be seen. After many visits I'm still amazed at the technology and the sheer size of this extraordinarily complex world-class project by the Transfield Bouygues Joint Venture, The Airport Rail Link Project, joining the internal and domestic airports to Sydney central.
The tunnel boring machine is huge. Over 75 metres long, it's bigger than a jumbo jet. It's rotating cutter head cuts a tunnel 10.75 metres in diameter, the largest ever used in Australia and the fourth biggest diameter tunnel in the world. The huge rotating cutting wheel is mounted on a control drive shaft, and is incorporated within a shield which also houses the pressurised cutting chamber, the hyperbaric air lock system, the thrust rams that advance the machine into the ground, a battery of hydraulic motors and the tunnel lining segment erector. The cutter head and shield assembly is connected to six supporting gantries towed behind like huge carriages which house the main hydraulic, compressed air and electrical services, the control system, mortar storage and pumping, slurry pumps and electrical extension systems.
 | | The TBM crossing a station box. The Bentonite delivery and return pipes are in the foreground. |  |
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This TBM is state of the art and is trialling, for the first time in the world, a sophisticated computerized collapse detection system. It also has a laser guidance system and is pioneering the use of a slurry system in Australian tunneling. This tunnel is being cut into soft ground, which only a few short years ago would not have been possible. Water ingress through the soft ground into the tunnel is prevented by air pressure pumped into the scaled cutter head chamber in the front of the shield. The unstable ground is then sealed and supported by pumping in Betonite, a special clay substance mixed with water to form a slurry. The spoil, (excavated material mixed with Betonite) is pumped out to a surface refinery where the Betonite is separated from the spoil and recycled back to the TBM. In the tunnel the only evidence of excavation taking place are the two large pipes along the side of the tunnel, one delivering the Betonite to the machine, and one taking the spoil back to the refinery.
As the tunnel moves forward it is lined with concrete rings 1.6 metres wide and 250 millimetres thick and consists of seven segments, each weighing about seven tons. These segments are manufactured at the Tempe construction site and delivered to the machine via a small diesel locomotive. The TBM erector grips each section by suction, spins it into position and the crew on the machine bolt the sections into place. The machine automatically injects a sealing gout behind the segments, then the giant hydraulic rams pushes off the newly-erected ring to force the cutter head into the ground to cut another 1.6 metre ring length. When cutting at full production the machine can cut and erect about 20 rings every 24 hours.
As the machine cuts into the ground, the cutting discs and picks on the rotating cutter wheel wear at a rate dependent on the type of ground being excavated, so it's necessary to inspect the wheel and replace the cutters as they wear out. This requires a manned intervention crew to enter the pressurised environment in the excavation chambers. This is possible via the hyperbaric man locks - which is my reason for being on the TBM.
My company, Descend Underwater Training Centre, won the contract for training the intervention workers and the lock operators in October 1996, and since that time we've trained approximately 130 M.I workers and about 40 lock operators. Now, after two years of training. I have the chance to see how they perform by going under pressure with a crew whose job will be to inspect the cutter wheel.
 | | Rotating Cutter | |
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The sheer magnitude of the rotating cutter is apparent when compared to a mere human! The tunnel boring machine behind this is huge. Over 75 metres long, it's bigger than a jumbo jet. This rotating cutter head is the largest ever used in Australia.
The hyperbaric centre consists of two 60" walk-through twin lock recompression chambers and a materials lock giving access to the cutter head. They are very much like a normal diving chamber except the main locks are fitted with four seats instead of bunks, and the gas monitoring equipment is much more sophisticated. The lock operators can digitally monitor the oxygen, carbon dioxide, carbon monoxide and explosive gases in the system from the lock operator station. The system is also fitted with six communication systems, the normal divers' radio, a powerless genephone, a normal telecom phone, a press-button intercom, closed circuit television and, if that doesn't attract attention, there's an air horn to indicate communication is required with the lock operator. The lock has three separate air supplies, making a total of 100,000 litres of breathing air available.
We enter the man lock through the auxiliary lock and the four man intervention crew gets seated. Unlike a diving chamber, these guys were dressed in their normal working gear - overalls, safety harness, hard hats and safety boots. The lock door is then shut - the inside crew controlling their own descent at a rate that suits the ear-clearing ability of all the team to the working depth of 27 metres. The maximum depth on the project so far has been 40 metres. The heat is oppressive but there's more to come! When the gauges between the main lock and excavation chamber read the same pressure, the crew opens the balance valve between the lock and the excavation chamber to ensure the pressures are equal, then opens the door and enters the work area. The ground heat is stifling and the humidity close to 100%. Sweat immediately pours off all the crew.
 | | The left hand man lock on the TBM. | |
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This area of the machine is served by a compressed air injection and exhaust system, known as a Sampson system. It is a safety system used to inject air to hold the pressure constant as air is lost out through the Bentonite cake into the unstable ground. As I entered, the Sampson was gently making up the pressure, but the noise is like hitting a steel plate with a sledge hammer.
My crew are old hands and familiar with the routine. They blow down the betonite level, open the hatch to the cutter wheel and set up the high pressure water blaster. The noise of the water blaster adds to the clang of the Sampson system as the crew blasts the clay off the cutter wheel so we can inspect the cutting discs. The blow-back from the blaster covers the workers with mud and water.
One worker is dedicated to the freshwater hose; his job is to hose the blaster operator's face and safety goggles so that he can see what he is doing. He also periodically hoses the rest of the crew to help keep them cool. By now we are all totally drenched.
In a small niche carved into the rock a statue of Saint Barbara, the patron saint of tunnellers watches over the men under pressure.
The work continues for just on two hours. When we hear the communication horn blow I slip the communication headset on and hear the lock operator giving us the 15 minute warning; our time was nearly up. The hoses are retrieved and the hatches shut. A final wash-down with the hose and we changed into dry overalls before reentry to the lock to face another 2 hours 25 minutes of decompression before leaving.
All the decompression stops from 9 metres and shallower are completed on 100% oxygen. From what we have learnt, this is the first time 100% oxygen has been used for decompression as a project of this type and the results have been excellent.
After some 2000 manned interventions to a maximum depth of 40 metres, most needing heavy decompression, the hyperbaric problems have been minimal and well below levels experienced on similar projects world wide. This project has become a bench mark for future tunneling and hyperbaric training.
As we exited the excavation chamber, a second crew was being locked in via the other hyperbaric system. They would continue the job and start to change some damaged picks. Our period under pressure was nearly five hours and as our decompression time was in excess of two hours, our hyperbaric procedures required us to spend the next three hours resting in the vicinity of the surface recompression chamber. This is called the 'bends watch' period; really a way of ensuring the crew don't do any heavy work. So its time to ride out on the man rider behind the loco, shower, change and get a cup of coffee.
As I walked up Bourke Road towards my hotel I watched the traffic passing by. The drivers and passengers were oblivious to the twenty man crew running the $50,000,000 machine making tunneling history 30 metres below their wheels.
Article URL: http://www.descend.com.au/commercial/projects/2003413191.htm
Descend Underwater Training Centre
Shop 1 826 David Street
Albury NSW 2640
Australia
Tel: +61 2 6041 1405
Fax: +61 2 6021 6732
ABN: 83 251 221 741
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