Compressed air has been used for more than 150 years as a cost-effective ground support technique for tunnelling and shaft sinking in soft ground below the water table. Major tunnelling projects in the UK, France, Germany, Singapore, Cairo, Hong Kong, Australia and the US have all been driven successfully with the assistance of compressed air. For much of that time there was little change in working practices. Major UK road tunnels such as the Clyde Tunnels in Glasgow and the Dartford Tunnel under the Thames were driven using hand excavation techniques with large numbers of miners working in compressed air. On these contracts, tens of thousands of exposures were recorded. Tunnels of similar size are driven today using machines such as EPBMs or slurry machines, with compressed air being applied only in the head of the TBM to allow access for inspection or maintenance purposes. Consequently, the number of exposures to compressed air has been reduced by between 90% and 95%. In most countries the legal maximum working pressure is around 3 bar.
Principles of Compressed Air
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The physical principle behind the use of compressed air is extremely simple and is the reason for its effectiveness. The tunnel or shaft is sealed by a bulkhead and is pressurised with air. The air pressure is increased sufficiently to balance the ground water pressure in the face. When the air pressure equals or exceeds the ground water pressure, the flow of water into the tunnel ceases. By controlling the inflow of water, face stability can be controlled.
Ground water pressure varies linearly with depth across a tunnel face, whereas the applied air pressure is approximately constant. There is therefore only one level at which the ground water pressure can be exactly balanced by the air pressure. Above the balance point the air pressure exceeds the water pressure, while below the balance point the water pressure is greater and inflow continues. Over-pressure will result in a net outflow of air which, if allowed to become excessive, can result in a blow-out and subsequent face collapse.
Compressed air is effective irrespective of ground conditions, provided air losses can be controlled. It can therefore be used in all types of soft ground and has been used in shallow tunnels in fissured rock to control ground water inflow.
Plant and Equipment
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By GlobalDataA typical compressed air installation consists of a number of compressors or blowers feeding air through coolers to remove the heat generated by the compressors, and filters to remove traces of oil. Back-up compressors and power supplies are also required as a contingency. For convenience and to make the equipment quieter and thus more environmentally acceptable, plant suppliers have tended to containerise the equipment. Airlocks are required to permit access through the bulkheads and may be used for personnel, materials or both. Airlocks in the tunnel can be classed as "horizontal" while those in shafts are "vertical" airlocks.
In the past it was normal to pressurise most of the tunnel length, with excavation and lining erection taking place "in air". Typical air lock systems would consist of a number of bulkheads built into the tunnel lining to form separate compartments of the air lock or a multi-compartment steel pressure vessel built into a single bulkhead in the tunnel or shaft.
Compressed air is normally fed directly to the tunnel, which acts as an air receiver and reservoir from which air is drawn off to supply the air locks. There should also be the facility for a direct supply of air from the compressors to the air locks for use in emergencies such as in the event of a fire in the tunnel. With tunnels now being driven mainly by TBM, the air locks are much smaller and form an integral part of the machine. TBM locks are normally fed directly from compressors sited on the surface. Current practice requires that any air lock should have at least two compartments. One is normally open to the tunnel to allow people in the tunnel to escape to a place of safety in an emergency, while the other is open to free air to allow quick access by an emergency team should the need arise. An on-site medical lock, for the treatment of acute decompression illness, is normally required for some or all compressed air work. The pressure threshold for it’s requirement varies according to national legislation, but is normally 0.7 bar – 1.0 bar.
Personnel
Key personnel to operate compressed air systems should include a member of the site management team as the person in charge of the works, lock attendants to control the routine compression and decompression of personnel and materials in the tunnel and compressor attendants to look after the plant and equipment. They should be on duty whenever people are at work in compressed air. In most circumstances, medical lock attendants are required to be available on site for the treating any worker who might suffer decompression illness.
Hazards of Compressed Air Working
The most significant hazards associated with work in compressed air are decompression illness and fire.
During compression, the body tissues take up oxygen and nitrogen until they become saturated for the pressure to which they are exposed. On decompression the oxygen is rapidly given up or metabolised. However the nitrogen takes longer to be released. Although many of the primary organs give up nitrogen quite rapidly, the release of nitrogen from fat tissue is extremely slow. Too rapid a return to atmospheric pressure results in bubbles of inert nitrogen being formed in the blood stream. This can lead to tissue damage and decompression sickness.
Decompression illness is the commonly accepted term for all medical conditions arising following decompression from work in compressed air. The acute conditions include pain-only or Type 1 decompression sickness "the bends" which manifests itself as pain, normally in the shoulders, arms or legs; serious or Type 2 decompression sickness, which can affect the spinal cord or brain and result in paralysis or in extreme circumstances death and barotraumas, which is an injury caused by pressure. The chronic condition is dysbaric or aseptic osteonecrosis which is a degenerative bone disease normally affecting the hips but which can also affect the shoulders. In both cases it causes serious disability.
Over the years, progressively more rigorous decompression procedures have been introduced where compressed air working is carried out. In the UK, the pressure from which staged decompression is required has been progressively lowered from ~1.5 bar (22 psi) in 1936 to 0.7 bar (~10 psi) in 2001. Decanting – a procedure whereby compressed air workers were rapidly decompressed in the tunnel or shaft lock before being transferred to another "decant" lock in which they underwent compression back to working pressure followed by a normal staged decompression, has been prohibited. In the most recent development in decompression practice the tunnelling industry has adopted a technique from the diving industry where decompression illness was also a problem. The technique involes the use of oxygen as a breathing gas during decompression. Oxygen breathing during decompression significantly reduces the risk of decompression illness.
Fire is another major hazard. Because of the increased amount of oxygen in compressed air, fires ignite more easily, burn more vigorously and are more difficult to extinguish. Again over the years, the fire precautions required by the regulatory authorities have been increased. Comprehensive fixed fire extinguishing systems are now an integral part of all tunnelling equipment associated with compressed air working. In the UK, the Health and Safety Executive has undertaken an extensive programme of research to quantify the effects of pressure on fire and is shortly to examine the problems of extinguishing such fires.
Other techniques from the diving industry which have been trialled by the tunnelling industry include the use of non-air breathing mixtures such as Trimix, a mixture of oxygen, nitrogen and helium, and saturation techniques where people live at pressure for a period of time, only undergoing decompression after a number of working shifts. This has been used on a tunnelling project now under way in Holland. As ever more challenging tunnels are driven, a greater transfer of technology from the offshore diving industry to the tunnelling industry can be expected.
Standards and legislation
Many countries have national legislation regulating work in compressed air. In the UK, the Work in Compressed Air Regulations 1996 and accompanying guidance document (L96) apply. These regulations place most of the duties on the "compressed air contractor", including the need to have safe systems of work and key competent personnel to carry out compressed air work. An additional source of guidance on good practice is BS 6164:2001 "Code of practice for safety in tunnelling in the construction industry". Apart from national legislation and guidance, a draft CEN standard, prEN 12110 "Airlocks – safety requirements" has been prepared. It sets out safety requirements in the design and manufacture of airlocks. The final text of the standard should be published in 2002. Early drafts of the European Community directive on "Physical Agents" indicated it would include pressure. However this has been dropped from its scope.
Nevertheless there have been calls within Europe to develop common standards for working in compressed air including decompression techniques.
