INSIGHT | H&S - COMPRESSED AIR WORK
HPCA – BREATHING MIXTURE GASES
The ‘big 3’ breathing mixture gases are oxygen, nitrogen and helium. Oxygen is a relatively dense, non-narcotic, metabolically
active gas which causes a potentially fatal response if breathed at too low or too high partial pressures. Nitrogen is a relatively dense, inert gas which is narcotic and ultimately fatal at high partial pressures and which, following exposure to it at pressure, requires controlled decompression to mitigate the risk of DCI. Helium is a low density, inert, non-narcotic but expensive gas which is used as a diluent to reduce the gas mix density and the partial pressures of oxygen and nitrogen to counter the harmful effects of exposure to high oxygen and/or nitrogen partial pressures. Carbon dioxide is frequently ignored but is a relatively dense, narcotic gas that is not inhaled but which is produced
in the body as the end product of human metabolism. It is prone to being retained in the lungs thus preventing the inhalation of oxygen. All gases increase in density with pressure hence, as a
consequence, increase the work of breathing. Contrary to common perception it is not the difficulties in
decompression but the increased gas density and associated work of breathing along with the narcotic potential which trigger the need to introduce helium into the breathing mixture. Generally accepted upper limits on gas density and narcotic potential are reached in air at pressures of around 3.5 bar(g).
(*) Partial pressure of a gas (in a gas mixture): the pressure it would have if it alone occupied the space filled by the gas mixture
Saturation - a physiological state
Understanding saturation and hyperbaric mixed gas physiology is one key to understanding HPCA work. Another is recognising that, in HPCA work, immersion is in a gas and not in water as in diving. This results in a lack of buoyancy and also in a pressure profile for compressed air that can be assumed to be constant with depth; in water, pressure increases linearly with depth. Breathing apparatus is only required in HPCA work
where the cost of pressurisation with mixed gas makes this option prohibitive. Saturation is the natural default condition of the
body in which the partial pressures of the inert gases in the tissues equal the partial pressures of the inert gases being breathed. The partial pressure of oxygen in the tissues is always slightly lower than that inhaled due to human metabolism. Once saturation has been established, and
provided there is no further change in pressure or breathing mixture, there is no change in tissue gas no matter how long the exposure lasts. This is the key benefit of saturation exposure techniques. Saturation is a physiological state and is not restricted to diving, hence terms such as ‘saturation diving in tunnelling’ show a complete lack of awareness of what saturation really means. Remember – we all live in saturation in a ~20/80 oxygen/nitrogen mixture called ‘air’ at atmospheric pressure and very few of us are divers!
ITA/BTS CAWG REPORTS Report 10 is divided into a number of sections, which are described in some detail below. One important aspect of HPCA work, highlighted in the definition of such work,
26 | July 2025
is that it is undertaken at pressures above national statutory limits and hence by definition is illegal without some form of exemption, approval or variance from the national regulator. Section 3 of the report provides guidance on the content of applications to regulators in terms of data supporting the application, trial runs, mathematical modelling, and physiological monitoring. Over the past decade or so, HPCA has gone from
being a radically new high-risk technique in tunnelling to a novel technique with some history of successful application. Report 10 now recommends that regulators become more restrained in their demands for modelling and trials but continue to require data on recent use. The development of mixed gas and saturation
exposure techniques in diving was largely driven by the need to explore and exploit oil and gas reserves in water depths of 50m – ~150m and beyond which is far beyond the air diving limits of ~ 50m. Until relatively recently there has been little demand in soft ground tunnelling to go beyond national limits of typically 3 bar(g) – 4 bar(g) (gauge pressure, measured in excess of atmospheric pressure) but as the underground space gets more crowded and the demand for undersea tunnels increases, deeper tunnels requiring higher pressures and the use of more sophisticated hyperbaric techniques are required. Section 4 covers organisational aspects of HPCA work
identifying the persons/roles additional to those set out in the BTS Guide (2021), which are required for HPCA work. Report 10 recommends that a person is appointed to manage all surface activity along with a second person to manage the deployment of the hyperbaric personnel from the habitat to the TBM and back to the habitat. Additionally a person should be appointed to be responsible for all aspects of the mixed gas supply.
GUIDELINES FOR GOOD WORKING PRACTICE IN HIGH PRESSURECOMPRESSED AIR
ITA Working Group 5 ‘Health & Safety in Works’ In Association with the British Tunnelling Society Compressed Air Working Group
N° ISBN: 978-2-9701670-6-8
ITA REPORT N°10 V3 / MARCH 2024
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45
