Today, typical compressed air work, or hyperbaric intervention as it is perhaps more accurately termed, is concerned mainly with changing TBMs cutters and other maintenance tasks on the cutterhead or in the TBM pressure chamber. Working pressures are usually kept below 3.5bar, which is the maximum allowed by normal compressed air regulations in most countries. This is usually achieved by the planning of the tunnel alignment in appropriate strata, groundwater lowering, or other exclusion techniques such as grouting or ground freezing.
Other common requirements for a hyperbaric working atmosphere include the rescue of TBMs stuck in ground under a water table, repair of failed tunnel linings in similar circumstances or, sometimes, shaft sinking through an aquifer. In addition to the health and safety considerations of the pressurised atmosphere, related enhanced hazards such as fire potential and heavy handling in small spaces must also be properly considered.
The deeper tunnels go, the more difficult it becomes to avoid higher groundwater pressures when face work becomes necessary. Also, as tunnelling always has a chance of nasty surprises, there need to be methods and equipment available to tackle circumstances such as trapped or stuck TBMs, and failures of the tunnel support lining or waterproofing.
In the limited, but seemingly increasing, circumstances requiring intervention at pressures above 3.5 bar, saturation diving techniques come into play. The techniques involving not only higher pressures but also non-air breathing mixtures, long periods immersed under pressure, and intense medical and fitness supervision, have been developed mainly through deep-sea diving.
Advanced thinking
As with many other contingency procedures to do with tunnelling, contractors and maybe clients have a natural tendency to want to save money when something might never happen. Whereas cutter changing interventions can be planned in advance on long drives, they may be a surprise on shorter tunnels with higher than expected wear rates. The expected and actual groundwater pressures and flows are critical.
“We would like to get across the message,” says Tony Ridley, managing director of Tony Ridley Hyperbaric Associates, “that the possible need for hyperbaric intervention in a tunnelling project should be assessed at the tender stage. The tender documents should explain what may need to be put into place so that the successful contractor is aware of this and the potential consequences of avoiding or not doing it.”
There are a number of centres of expertise in hyperbaric work for tunneling around the world including tunnelling and medical consultants, equipment manufacturers, equipment rental companies and the provision of specialist operatives and on site lock and chamber attendants, so lack of qualified support for a project should not be a problem so long as the geotechnical criteria for the operations are known as well as possible.
One of Specialist Plant’s particular specialities is the supply and installation of a wide variety of airlocks, chambers and other hyperbaric equipment throughout UK tunnelling projects, including Morgan Est’s recent work in repairing the Lovat TBMs on the new Belfast sewers where shaft ’soft eye’ construction was also carried out under compressed air, plus projects in France, Singapore and Taiwan. Specialist Plant director Ian Matthews explained the company’s assurance service. “More often than not hyperbaric equipment is ordered as insurance by a contractor in case access is required into a TBM head under pressure to carry out repairs. Sometimes the equipment is delivered to site and stored there awaiting possible use. In other cases the equipment is prepared and held at the depot for the duration of the contract, reserved for call off by the contractor.”
Who does what
There is a recurring debate about who should actually carry out the necessary work under hyperbaric conditions. Ideally the tunnel operative should be trained to the fullest in both tunnelling work and hyperbaric techniques. This is not always possible, and considerations must also include adequate fitness as well as training. In saturation work the extra demands of the procedures indicate that hyperbaric training has to come first, although skills to meet the tasks such as cutting and welding will also be required. Clear instructions on what is required of the diver will need to be given, aided by good communications including CCTV if possible.
At pressures below 3.5 bar there are good arguments for both ‘miners’ and ‘divers’. Former HM Inspector of Health & Safety, Donald Lamont of Hyperbaric & Tunnel Safety favours tunnelling miners to be trained in compressed air work. “They have the skills and expertise required to carry out work required,” he says. “You can’t expect a diver to be able to excavate a failed lining safely and replace the support, even if it is under compressed air.”
Ridley agrees, “I believe that, in most circumstances, where possible existing miners/tunnellers should be trained in the appropriate high pressure working techniques rather than bringing in divers. Divers do not, normally, have an intimate knowledge of the work tasks and safety aspects in tunnelling/TBM applications generally, and particularly not those associated with cutterheads and exposed faces. However, in some scenarios, diving personnel may be beneficial as part of an intervention team when they have previous experience of similar techniques.”
Providers of hyperbaric intervention teams and equipment understandably think otherwise. Andrew Hickox of Bekk Solutions (BSL) headquartered in Hong Kong and with bases in India, Sabah, Singapore and the UAE, believes fitness rather than skills is the key, and that means regular hyperbaric operatives. “We feel that divers are best equipped to carry out this type of work. They are generally more physically and mentally prepared to work in the arduous conditions,” he says.
Equipment and regulations
While there is enough equipment available, such as hyperbaric chambers, airlocks and pressure management systems available around the world, especially if diving industry suppliers are taken into consideration, ensuring that the equipment meets the particular requirements of tunnelling and local regulations is not always so easy.
“We sometimes get called in by TBM suppliers or tunnel contractors to ensure that the airlocks provided within the TBM meet the relevant standards,” says Ridley. “This can also involve good ergonomic, safety and common sense necessitating modifications to the hyperbaric equipment proposed or supplied.”
“Unfortunately we can also be called in too late, after a problem occurs or because it is discovered that what provision is necessary is not in place. This may be because the main equipment, or TBM, supplier has been ‘economical’ in compliance with the relevant standards and good hyperbaric practice. Also, relevant equipment and airlocks are often brought in after a requirement has been identified instead of in advance. This, together with installation, mobilisation, training and medical requirements, causes project delays unless planned beforehand.”
Even the regulations are fallible as they often take considerable time to keep up with advancing technology or are not, for various reasons, adopted by the relevant authorities. As far as the US is concerned, Gerry East of Global Diving & Salvage points out that that country’s OSHA compressed air regulations (including decompression tables) are antiquated in that they were made before the advent of modern TBMs and hyperbarics, but they still apply. Contractors must get a ‘variance’ to not follow any of the OSHA standards. He says, “Many states have adopted their own occupational safety and health plan, but they must adopt standards identical to, or at least as effective as federal standards.
Eastern make-up
BSL is a leading provider of hyperbaric services and other subcontracted specialist tunnel construction services. The Government of Hong Kong approves both the ‘Blackpool’ Tables and French tables for operating pressures, work and decompression times, and Bekk says it can work with both.
The nature of the ground that has to be tackled in Hong Kong, with a high proportion of made ground or fill, means that tunnelling can be even more unpredictable. Consequently Bekk’s work has included a higher than usual proportion of TBM ‘rescues’ under compressed air. These have included recovery work for China Harbour & Engineering Company on the DSD Drainage contract in Wo Che, Hong Kong. Hickox says, “The rescue work we have carried out has generally been in front of the shield to remove steel obstructions. This can be quite dangerous in HK as most of the ground is reclaimed. This makes it very difficult to maintain a constant pressure, and there is always the constant threat of total ground failure.”
Other work includes more standard interventions for cutter changes. Currently the company is working on compressed air intervention for Chun Wo Construction on the Wan Chai Project, and was recently awarded a contract to support hyperbaric tunnelling operations for Penta-Ocean Construction and Kane Tunnelling on the MTRC XRL Contract 825 for the 2.19-km twin tunnels between Mai Po and Ngau Tam Mei for the high-speed rail link to Shanghai. Bekk’s work includes interventions on the two Herrenknecht TBMs being supplied for the project.
Bekk Solutions has not needed to engage in saturation techniques, although teams have worked with surface-fed breathing systems with mixed gas supplies for working at depth. It’s equipment, such as chambers, compressors and gas storage is supplied by Unique Systems of Dubai of the Unique Maritime Group, who recently acquired Hydra Marine.
Westerschelde
One of the most significant interventions of recent times was a series necessary for one of the two Herrenknecht TBMs forming the Westerschelde twin highway bores under the River Scheldt southern Netherlands. A paper describes what was involved including experts from JCLP Hyperbarie, Paris, Leiden University (Netherlands) and Arbo Hypercon (Netherlands). The very high pressure at the face (up to 6.9 bar) was largely unexpected and caused difficulties with the shield itself as well as necessary cutter changes, made worse by the need to work in the bentonite slurry.
Nordseetaucher performed the saturation interventions necessary. As a result of the project demands, this was the first time that saturation technology and mixed gas breathing were used in tunnelling operations. The divers had to be transferred by a 4 bar pressurised mobile chamber using breathing air from their accommodation, where the atmosphere was also at 4 bar pressure using Trimix gas (helium, nitrogen and oxygen).
Trimix was also used when working at the 6.9 bar face pressure. The diving team on each intervention numbered three, with some 37 excursions made outside the habitat chamber, giving a total working diving time of 400 hours.
Brightwater
Necessary repairs to the Herrenknecht slurry-shield cutterheads on the Brightwater Conveyance System sewerage project near Seattle also required hyperbaric intervention, although, at first the necessary pressure was reduced by pumping to lower the groundwater head to atmospheric pressure up to one bar instead of five bar. Hyperbaric facilities had been planned from the start however.
The project is featured in the lecture by Edmund Kay, ‘Deep Hyperbaric Tunnelling – Experience with 600 Interventions’ as Kay is project physician for the four hyperbaric tunnels operating at up to 8 bar. The lecture also describes the DART, or diver attendant recompression transportable module, the use of oxygen self-rescuer units, the new Biopak 240 Revolution lightweight self-contained rebreathing apparatus and the medical conditions that may be associated with hyperbaric work. Kay is medical director of the Divers Institute of Technology in Seattle and director of hyperbaric medicine at HealthForce Partners, also of Seattle, engaged in workplace medicine.
Lake Mead
An example of the increasing number of saturation projects due to deeper tunnelling, interventions at the Lake Mead Number Three Intake, Nevada, US, are being carried out by Ballard Diving & Salvage, another of the cluster of diving specialist headquartered in Washington state/ Vancouver, Canada.
Inside a 1.8m-diameter decompression airlock equipped for oxygen breathing (Photo: Specialist Plant) The DART (Diver Attendant Recompression Transportable) chamber arrives at the TBM back-up on the Brightwater project (Photo: Edmond Kay)
