Compressed air environments are used in shaft sinking and tunnelling as a means of controlling water ingress and hence ground stability. A working chamber is formed by installing an airdeck or bulkhead in the shaft or tunnel after which compressed air is injected to pressurise the space between the airdeck and the shaft invert, or between the bulkhead and tunnel face. Access through the airdeck or bulkhead is by means of airlocks. In contemporary tunnelling the bulkhead is most likely to be part of the structure of a TBM cutterhead.

The main health and safety risks are decompression illness following the return to normal atmospheric pressure, and fire due to the enhanced mass concentration of oxygen in the pressurised atmosphere.

Air pressures of up to around 3.5 bar, equivalent to 35m of water head, are permitted by health and safety legislation in most countries. Pressure limits have not changed over many decades, and probably reflect the capability of early compressors to produce reliable supplies of compressed air that were relatively free of contaminants and the requirements and state of knowledge of the tunnelling industry at the time.

Advances in medical technology in other areas of hyperbaric medicine are being introduced to tunnelling to reduce health risks associated with decompression.

The current UK legislation regulating compressed air work is the Work in Compressed Air Regulations 1996 supplemented by Health and Safety Executive guidance document L96 – ‘A guide to the Work in Compressed Air Regulations 1996’ along with the addendum to L96 covering oxygen decompression and the use of non-air breathing mixtures. L96 is complemented by guidance in BS 6164:2011 – ‘Code of practice for health and safety in tunnelling in the construction industry’, which has a clause dealing specifically with the interaction between the pressurised workings and the surrounding ground. Within Europe, EN 12110 – ‘Tunnelling machinery – safety – Airlocks’ is the relevant standard for the manufacture of bulkheads and airlocks.

Shaft sinking
In the past, shafts were sunk using underpinning techniques or as caissons. Compressed air was sometimes required to control water ingress to enable the underpinning to continue below the water table. In caisson sinking, compressed air allowed men in the working chamber below the water table to remove spoil from the base of the caisson which in turn allowed the caisson to sink in a controlled fashion. The most recent large caisson to be sunk in the UK utilizing compressed air was that at Ramsden Dock in Barrow-in-Furness in 1991. The 25m by 25m concrete monolith formed the basic structure of a new dock entrance, which allowed the passage of nuclear submarines from the nearby shipyard where they were built to the open sea.

Also in shaft work compressed air is sometimes required to counter water ingress when forming the tunnel eye during the launch of a TBM. A recent example of this application was in Belfast where it was used on a tunnel sewer contract.

Underpinned shafts are now more likely to be constructed using sprayed concrete lining (SCL) lining techniques than precast concrete segments. With the availability of slurry wall techniques having the capability to form shaft walls to depths of around 100m in water bearing ground, the use of compressed air for caisson sinking has all but ceased.

Tunnelling
Historically, compressed air was used to facilitate the excavation by hand and/or open shield of tunnels in soft ground below the water table. Bulkheads to form the airlocks were usually installed in the tunnel lining, close to the shaft bottom and the whole tunnel pressurised with air. As the whole tunnel was ‘under air’, there were large numbers of exposures to pressure.

Over recent decades the development of sophisticated TBMs for soft ground tunnelling – slurry TBMs, EPBMs and their variants – has done away with the need for pressurising the whole tunnel. Some exposures to compressed air are still required for cutterhead inspections, tool replacement and other interventions in the cutterhead of the TBM. Typically however, the use of TBMs has reduced the number of exposures by two or three orders of magnitude. On very large TBMs it is now possible to change cutters at atmospheric pressure from within the spokes of the cutterhead itself.

Although the majority of compressed air working is now associated with TBM drives it should be remembered that compressed air still has a role in responding to the unforeseen circumstances which make tunnelling the challenge that many in the industry enjoy. Ground stabilisation to enable the replacement of a broken pipe in an undersea pipejack or insitu repairs to a damaged TBM cutterhead are only two examples of where compressed air can still be required.

Exposure to pressure in tunnelling has led to numerous cases of decompression illness (DCI). Over the past 60 years for which UK records exist, around 0.6 per cent of all exposures have resulted in DCI. This is an average figure which does not reflect the true nature of the problem. Not surprisingly, detailed analysis of the DCI figures has shown that those most likely to experience DCI are the miners. It was not unusual for between a quarter and half of the miners on a contract to experience at least one DCI ‘hit’.

The predominant manifestation of DCI in tunnelling is Type One decompression sickness or ‘the Bends’, as it is often referred to. This occurs in over 90 per cent of tunnelling DCI cases. Although often considered to be a ‘hazard of the job’, and of little consequence, decompression sickness resulted in occasional fatalities throughout the 20th century until the introduction of the Blackpool tables of exposure limits and decompression times in the mid-1960s. Fortunately Type Two DCS, which results in severe neurological symptoms, is relatively rare from tunneling exposures.

By the mid-1960s chronic DCI (dysbaric, also known as aseptic, osteonecrosis) had been identified. This is a degenerative and ultimately crippling disease affecting particularly the hip joints. There is no cure for osteonecrosis and the only treatment is surgical replacement of the bone ends or joints. To aid early diagnosis of osteonecrosis, everyone working in compressed air at pressures above one bar, had to undergo regular long-bone X-rays. Research in the mid 1990s showed that the strongest correlation for factors resulting in osteonecrosis was a previous Type One DCS event. This finding significantly altered the perspective on the seriousness of Type One DCS.

Not only has the reduction in the number of exposures led to significant reductions in the number of cases of DCI, and hence Type One DCS, but the use of oxygen breathing during the latter stages of decompression, introduced in the UK in 2001, is now a routine procedure intended to further reduce the incidence of DCI.

Legislation and standards
At present, there are no planned changes to the UK Work in Compressed Air Regulations. The HSE guidance has been revised and awaits publication. However the perception that official publications are a burden on business has led the current government to impose a moratorium on publishing guidance, including L96.

The revised text of L96, once published, will incorporate a number of changes from the current version. The most significant change in working practice to be accommodated is that compressed air working now tends to be intermittent in nature. Head inspections and tool changes are carried out at intervals of days or weeks. It is not cost effective to maintain lock attendants and medical lock attendants permanently on site. Likewise compressors can be hired in when needed. This has led to the introduction of the role of hyperbaric supervisor – a person in day to day control of the compressed air operation with responsibility to bring together the personnel and equipment required to undertake the compressed air work as and when it is required. A further change, although not imposed by the regulator, has been to merge the roles of lock attendant and medical lock attendant. This leads to greater flexibility by having a small pool of lock attendants on site who can take on either role as required.

Another major change set out in the revised L96 is the use of magnetic resonance imaging (MRI) in preference to x-ray as the means of detecting bone necrosis. This change results from medical safety legislation prohibiting the unnecessary use of X-ray when other means of scanning are available. Unfortunately unlike with X-ray, there is currently no recognised correlation between abnormalities detected by MRI and dysbaric osteonecrosis. MRI is considered to be a more sensitive technique than X-ray so a number of false positive results can be expected. It is likely that X-ray will be required to confirm, or otherwise, the results of the MRI.

The revision of L96 has also given the opportunity to regularise the use of Doppler monitoring as a means of assessing in real time, the effectiveness of the decompression regime being used. Doppler techniques are routinely used in a number of medical applications but have only recently been introduced in tunnelling. Although a group of persons after decompression may not exhibit any overt symptoms of DCI, the quantity of inert gas bubbles in their blood can still be unacceptably high. Doppler monitoring is able to detect this and whilst it cannot be relied upon to identify individuals requiring prophylactic recompression, the Doppler results can allow the contract medical adviser to act proactively to prohibit exposures resulting in an unacceptably high risk of DCI.

BS 6164 ‘Code of Practice for health and safety in tunnelling in the construction industry’ has recently been revised and the most recent edition was published in July this year. Clause 11 of BS 6164 still deals with compressed air working. It provides extensive guidance on the interaction between the pressurised structure and the ground in which it is being built and, in this respect, it very much complements the HSE guidance. Otherwise BS 6164 covers a wide range of topics but not in the level of detail in L96.

EN 12110 is also being revised. The majority of the work has been completed and the revised text should be published in 2012. In addition to a general editorial revision of the text, a small number of technical changes have been made. These include consolidation of the requirements for fire fighting, clarification of the requirements for electrical power supply, an increase in minimum lock diameter from 1.5 to 1.6m and more extensive requirements for the oxygen breathing system.

Unlike L96, ENs are not UK government departmental guidance. However, as they are European Standards, the British Standards Institution (BSI) is obliged to introduce an EN into the UK as a dual-numbered BS EN within six months of the EN being listed in the EU ‘Official Journal’.

HPCA
Perhaps the most important development in compressed air working practice currently is so-called ‘high pressure compressed air (HPCA) work’. This involves the use of higher exposure pressures than currently permitted by the legislation in most countries. At such pressures whilst pressurisation of the working chamber and manlocks is by compressed air, it is not desirable to breathe that air due to the adverse response by the body to high pressure nitrogen and oxygen. Consequently a major difference between HPCA work and conventional compressed air work is that HPCA requires the use of non-air breathing mixtures, and in some circumstances the use of ‘saturation’ techniques. Typical breathing mixtures are oxygen and helium blends (heliox) or oxygen, nitrogen and helium mixtures usually known as ‘trimix’. Breathing mixtures are supplied by line-fed mask. Exposure to excessively high oxygen pressures leads to lung degeneration and other adverse health effects. High pressure nitrogen is both more difficult to breathe due to its density and is narcotic. Helium, whilst expensive, acts as an inert and low density diluent.

Saturation techniques are commonplace in offshore diving and those concerned live for periods of up to 28 days under pressure with a single decompression at the end of that period. The decision whether saturation is required or not in HPCA work is based on the exposure pressure and the amount of work to be done. For pressures of up to around six bar, non-saturation exposures can be the more cost effective. At these pressures, exposure periods of between 30 and 45 minutes are possible within the recommended limits on decompression time and total exposure time. Where the work to be done under pressure requires longer exposures or higher pressures then saturation techniques will be required.

Saturation techniques require significant resources in equipment and personnel. A living complex on the surface is required along with the means to transfer the compressed air workers under pressure between the living complex and the TBM. In addition to the normal lock attendants in the tunnel, life support personnel are required to look after those in the living complex. Fortunately some equipment, personnel and knowhow are available from the offshore diving industry. There is likely to be continuing debate about whether divers or tunnel workers should be chosen for HPCA work but probably a team formed from a combination of both is the optimum available solution.

Not surprisingly HPCA work involves considerable health and safety risks. DCI, fire and the prevention of sudden decompression as a result of a blowout are some of the more obvious.

A number of tunnel projects around the world are being constructed in situations where high water pressures can be anticipated. Amongst these is the Lee Tunnel in London, which will be constructed at around 60 to 70m below ground. This means that the theoretical maximum air pressures could be as high as six or seven bar, and should entry under compressed air be required at these pressures, HPCA techniques will be required.


A medical lock attendant managing airlock entry Looking through a Bessac TBM being prepared for work on the Toulouse Metro to a technician in the airlock View from the front of a Brightwater Project TBM to the pressure chamber