INSIGHT | H&S - COMPRESSED AIR WORK
Above left: Personnel lock Above right: Gas distribution panel where mixed gas is distributed to the various compartments of the habitat
Section 8 provides guidance on hyperbaric
procedures – which is probably the most challenging aspect of HPCA work. It sets out extensive information on gas properties, the numerous exposure limits arising from these gas properties and on breathing mixture selection. Compression and decompression procedures are covered along with pressure and time limits on periods spent in the TUP shuttle and working on the TBM. It also sets out recommendations for working patterns and shift structuring along with time limits on exposure. Section 9 deals with record keeping and is an
extension of BTS Guide requirements. Section 10 covers emergency procedures and fire.
One side effect of using low oxygen breathing mixtures is the lower flammability of materials as the oxygen concentration in the atmosphere is reduced.
Flammability reduces with reducing oxygen
concentration till there is total inflammability at around 6% oxygen by volume. This is typically achieved in storage at pressures over 6 bar(g). However, on the negative side, for fire safety reasons
the oxygen concentration should be kept below ~23% by volume which limits the oxygen concentration in the habitat atmosphere during the later stages of decompression. Emergency procedures should also address
accelerated decompression – removing a critically injured/ill casualty for urgent hospital care, from saturation as rapidly as possible without triggering a life threatening DCI (decompression sickness) event. HPCA work will never become a mainstream
tunnelling technique, but it will enable deeper more complex tunnels to be built.
REFERENCES ● Lamont, D., Colvin, A., Heili, A., Ridley, T., Slocombe, R. & Wendling, J. (2024) ‘Advanced high
pressure hyperbaric techniques in tunnelling.’ Undersea Hyperb Med. 2024 Second Quarter; 51(2):159-171
● BTS CAWG (2021) ‘Guidance on good practice for work in compressed air.’ Revised guidance, published in 2021, available at:
https://britishtunnelling.com/pages/work-in-compressed-air
28 | July 2025
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