According to Donald Lamont high-pressure compressed air (HPCA) work is: "work in compressed air at pressures above historical statutory limits, which in most countries is between three and four bar (gauge), and which involves the use of breathing mixtures other than compressed natural air and can involve the use of saturation techniques". The differences between HPCA and traditional compressed air work includes the use of line-fed full-face masks for routine breathing of gases such as trimix in the excavation chamber, saturation exposures above six to seven bar, a hyperbaric living habitat possibly on the surface and transfer under pressure in the tunnel. Despite this the hyperbaric aspects of HPCA were not new, but they were new to tunnelling. HPCA work should be seen as an extension of existing compressed air working methods.

Recommendations of use
As with traditional compressed air work, the pressurising medium was compressed air. The number, pressure and duration of exposures should always be minimized whilst maintaining the overall safety of the work. The ITA/BTS guidelines do not recommend the use of HPCA, but only makes recommendations once a commercial decision to use HPCA had been made.

HPCA is being used or considered for contracts from Hong Kong and China, through Singapore and London to Vancouver and Seattle involving pressures from five to 10 bar and future contracts with theoretical maximum pressures of 10 to 15 bar are also being considered.

The ITA/BTS document is needed because HPCA is a relatively new and high risk technique which involves the transfer to the tunnelling industry of hyperbaric procedures more familiar in diving. There is currently no guidance on the technique.

The strength of the joint ITA/BTS publication lay in the background of the respective organisations. The ITA has some 65 member nations and is a recognised source of authoritative guidance in many countries around the world. The BTS Compressed Air Working Group (CAWG) is a multi-disciplinary group of hyperbaric specialists whose recognised expertise gives considerable technical credibility to the guidelines.

Guidelines
The guidelines are aimed at all parties involved in HPCA work including regulators, clients, designers, contractors and insurers. They are not a standalone document but draw heavily on other guidance sources including HSE L96/BTS Guidance, EN 12110, BS 6164:2011, diving industry guidance and other national standards.

Terms such as ‘saturation’ and ‘non-saturation’ exposures along with ‘transfer under pressure’ are given. Non saturation exposure is a short duration exposure comprising a compression phase, a working period under pressure, followed by a decompression phase.

It does not involve any storage time in a hyperbaric living habitat. Whereas saturation is long duration exposure during which the person exposed lives under pressure and may make transfers into the working chamber.

Possible breathing mixtures such as nitrox, heliox and trimix are outlined. The physiological properties and problems of breathing oxygen, nitrogen and helium are described. The importance of partial pressure when considering exposure limits is stressed.

The recommended exposure limits for the various gases are: oxygen, partial pressure of 0.2 to 1.6 bar; nitrogen – partial pressure not exceeding 3.6 bar. Because total oxygen dose is also important, a maximum oxygen dose based on units of pulmonary toxic dose (UPTD) of 400 UPTD per day and 2,000 UPTD over seven days has been set. It is emphasised that these limits represented good practice but that not all contractors currently adhere to them.

Because of the discomfort of wearing masks and the use of small decompression locks, non-saturation exposures should be limited to a maximum of two hours decompression time, or a maximum of three hours under pressure.

Saturation exposures should be limited to 28 days for experienced saturation workers and 14 days for others.

The choice of non-saturation or saturation exposure is a commercial one based on the amount of work to be done and the working pressure.

Non-saturation exposures typically allowed 45 minutes working time at six bar. Saturation exposures should be considered from five bar upwards, and should be used from seven bar upwards.

The main safety issues include preventing sudden loss of pressure, selection and sourcing of gas mixtures, saturation and living under pressure along with worker selection and training.

In most jurisdictions, HPCA work would probably need some form of exemption, approval, dispensation and/or variance to allow it to be done legally, hence the guidance for regulators in the document. Clients had to recognise the costs of HPCA. Contractor selection should be on competence and capability with local diving contractors, not necessarily the best option. The ability to work safely in a tunnelling environment was paramount.

Types of intervention
Werner Burger began his presentation by describing types of interventions including inspection of face conditions and mechanical equipment in the excavation chamber, maintenance including replacement of cutting tools and wear protection elements, maintenance or cleaning of other equipment in the chamber, removal of natural or man-made obstacles and repair of structural damage. The intervention technique used depends on the pressure and duration of the exposure. For pressures up to 3.6 bar, short duration exposures in compressed air are normally used with decompression in the TBM airlock. In exceptional circumstances air could be used up to 4.5 bar. For short duration exposures up to 6 bar, mixed gas should be used for breathing with decompression in the TBM airlock. For long term interventions above 4.5 bar the use of saturation techniques with an above ground hyperbaric habitat is recommended. Personnel should be transferred in a pressurised shuttle between the habitat and the TBM airlock.

The TBM designer can opt for parallel or in-line compression chambers for workers to pass through the bulkhead. A major and ongoing problem that TBM designers have to overcome was lack of space to locate the chamber. For all countries however, the fundamental requirement at all times, excepting when the lock was actually in use, was to have one lock compartment open to free air whilst the other was open to the workings.

Exposure minimisation
Continuing the theme of minimising exposure, Werner Burger gives the design features to reduce the need for compressed air interventions. These include the increased durability of components in the excavation chamber; a better understanding of soil behavior in the excavation chamber; more and reliable information about excavation chamber and cutting tool conditions, engineered solutions for maintenance not requiring exposure to compressed air and the use of safe havens, blocks of treated ground in which to carry out maintenance. Other design features aimed at reducing exposure include remote visual inspection techniques such as using an endoscope from within the air bubble chamber or from behind the bulkhead; fixed cameras in the excavation chamber; a range of wear detection systems covering both tools and the cutterhead structure. Systems which allowed maintenance to be undertaken in free air includes, on mixshields, a gate to isolate the crusher and suction intake from the slurry in the air bubble chamber. On large cutterheads it was possible to fit the tools within housings into which they could be withdrawn for maintenance. The housing could then be sealed against the outside pressure and thus permit tools to be changed from within the spokes of the cutterhead in free air. This system was first used on the Fourth Elbe Tunnel in 1996.

Lee Tunnel HPCA
Thames Water’s Lee Tunnel is the first UK project to potentially require HPCA techniques.

For much of its length the Lee Tunnel will be driven in chalk, however there is a short length close to the end of the drive where the Thanet Sands are predicted to dip into the tunnel horizon. Contingency planning to be able to undertake maintenance interventions in the cutterhead was a contractual requirement applicable over the full length of the drive. Within the chalk, it is planned that by a combination of pumping and/or ground treatment, routine interventions could be conducted in free air or in compressed air at pressures not exceeding 3.45 bar.

However, where an intervention is required within the Thanet Sands, there is a potential for water pressures up to 6.9 bar to be encountered. It was a contingency planning exercise for this scenario, which identified the potential use of HPCA on the Lee Tunnel Project. HPCA options that have been considered includes the use of nitrox for saturation exposures, however the useful range of nitrox was too limited to make this a viable option. This left the possibility of non-saturation exposures or saturation exposures using heliox or trimix.

Compressed air could be used above 3.45 bar, but at that pressure it became increasingly unsafe because of the risk of nitrogen narcosis and the high gas density which was why it was not recommended in the ITA/BTS guidelines.

Saturation options would require high establishment costs which had to be balanced against a very low possibility of use and were thus eliminated.

Non-saturation options had a relatively low establishment cost and with the limited possibility of their adoption became a cost effective option. Trimix – a mixture of oxygen, nitrogen and helium is the preferred breathing mixture. Ridley’s company has been developing this technique for some time prior to Lee Tunnel for contingency interventions on projects in other countries.

The main advantages of trimix is the control of nitrogen narcosis by limiting the proportion of nitrogen in the mixture, a lower gas density by incorporating helium in the mixture to reduce the work of breathing and control of oxygen partial pressure by adjusting the proportion of oxygen in the mixture by mix adjustment. In addition the higher thermal conductivity of helium could be of benefit in reducing heat build up in the working chamber and only minimal voice distortion was expected.

There are no dedicated trimix decompression tables so heliox tables will be used. This gives a further benefit as an improved decompression outcome has been predicted from research, against equivalent heliox exposure. This is due to asymmetry in the take up and wash out of helium in the body.

The key procedural aims for the Lee Tunnel were to adapt existing validated procedures for trimix use; to provide realistic working time for non-saturation exposures; to minimise technical and logistical complications; to minimize oxygen dosage and to maximise safety along with worker comfort. The make up of the trimix would be determined by limiting values for nitrogen and oxygen with the balance made up with helium. Although the mixture can be varied with pressure, a single mixture for use over the full working range is preferable.

Procedures
The Work in Compressed Air Regulations 1996, limit exposure to a maximum of 3.45 bar. Because of this and in order to be allowed to undertake HPCA work, applications have had to be made to the HSE for an exemption under Regulation 21 to exceed the maximum pressure of 3.45 bar permitted under Regulation 11(2) and for approval under Regulation 11(1) of a regime for decompression following exposures using trimix at up to 6.9 bar.

That process has been somewhat protracted and onerous as it involved exemption from existing legislation. Additionally it is the first time such an exemption and approval has been requested. Until publication of the ITA/BTS guidelines, no appropriate guidance had been available. Because of all this, the Health and Safety Executive (HSE) requested extensive justification and validation of the procedures and their development, along with access to supporting research and information on the mitigation measures that would be employed to avoid or reduce the use of HPCA. The process remains ongoing.

Diving experience
Claus Mayer has had extensive international experience in HPCA work. His early experience had been as a saturation and experimental trials diver at pressures to 60 bar. He also has extensive experience of compressed air tunnelling. Amongst notable tunnel projects on which he had worked were the Fourth Elbe tunnel, where both compressed air and diving work in bentonite had been done in the cutterhead.

Mayer also worked on Westerschelde which was the first tunnel project to use saturation techniques. In his experience, the main reasons for interventions were maintenance of the cutterhead.

Lessons from Westerschelde
Saturation working at Westerschelde had necessitated the use of transfer under pressure techniques between the surface habitat and the TBM. Other work in the cutterhead could include inspection and assessment of tool damage. Occasionally the removal of obstructions ahead of the face was required. Cutting and welding as a high risk activity in the cutterhead. Lack of inspection and routine maintenance could result in major repairs being necessary. Sometimes this required the construction of a small heading across the front of the cutterhead from which the work could be carried out. Mayer warns of some of the hazards of HPCA work including fire and blow out. Water mist is the most effective fire fighting medium for airlocks, according to Mayer. Detailed safety rules for HPCA were required. Total oxygen dose should be limited with air breaks provided every 30 minutes. Only decompression tables incorporating air breaks should be considered. Air locks should be kept clean and free from rubbish.

Oxygen concentration in the locks should be monitored and kept within recognised limits. A further essential precaution was the provision of a recompression chamber on site.