Work in compressed air, also called compressed air work or hyperbaric work, is occupational activity performed in an enclosed environment where the ambient pressure is deliberately raised above normal atmospheric pressure. The practice is employed primarily in civil‑engineering projects to counteract groundwater ingress by balancing hydrostatic pressure with pressurised air inside sealed work areas such as caissons, shafts, or tunnels.
Applications
- Groundwater control: Pressurising the work chamber prevents water from entering excavations, enabling dry construction in otherwise flooded conditions.
- Tunnelling: Used in the construction of underground passages, especially when conventional dewatering methods are impractical. Notable examples include high‑pressure tunnelling in Hong Kong where ambient pressures of 5.5 bar were employed.
- Caisson construction: Provides a dry environment for building bridge foundations and other submerged structures.
Pressure categories
Compressed‑air work is commonly classified by the gauge pressure applied:
| Category | Typical gauge pressure | Decompression requirement |
|---|---|---|
| Low pressure | < 0.7 bar | No staged decompression required |
| Intermediate pressure | 0.7–3.5 bar | Staged decompression required |
| High pressure | > 3.5 bar (varies by jurisdiction) | Mandatory staged decompression |
Historically, operations were limited to 3–4 bar, but experience from offshore saturation diving has shown that higher pressures can be managed safely with appropriate procedures.
Physiological considerations
The principal physiological differences between compressed‑air work and underwater diving stem from the air environment rather than immersion in water:
- Reduced drowning risk; however, fire hazards may be greater due to the presence of oxygen‑rich atmospheres.
- Thermal management is more straightforward, with less risk of hypothermia or hyperthermia.
- Decompression is performed in an airlock separating the hyperbaric zone from ambient pressure. Pure‑oxygen breathing may be used to accelerate decompression.
- Personnel density in hyperbaric work areas is typically higher than in diving operations.
Saturation hyperbaric work
When ambient pressures are sufficiently high that daily decompression would be prohibitively long, workers may remain under pressure for extended periods (days to weeks) in a saturation regime. This approach reduces cumulative decompression risk and improves economic efficiency but requires dedicated hyperbaric living quarters and pressurised transport between accommodation and the work site.
Health risks and medical surveillance
Compressed‑air work is recognised as a potentially hazardous environment. Documented health concerns include:
- Decompression sickness (the “bends”)
- Barotrauma during compression and decompression phases
- Dysbaric osteonecrosis (bone tissue death associated with repeated hyperbaric exposure)
Consequently, many jurisdictions mandate medical screening and ongoing surveillance for workers, analogous to diving‑medicine requirements.
Regulation
Legislation governing compressed‑air work varies internationally. In the United Kingdom, the Work in Compressed Air Regulations 1996 (SI 1996/1656) set statutory pressure limits and prescribe safety procedures, medical fitness testing, and emergency recompression facilities. Other countries may integrate these provisions within broader construction or diving regulations.
References
- Compressed Air Working Group (2021). Guidance on good practice for Work in Compressed Air – Based on the Work in Compressed Air Regulations (SI 1996/1656). British Tunnelling Society.
- ITA Working Group 5 (2018). Guidelines for Good Working Practice in High Pressure Compressed Air. ITA Report n°010‑V3.
- Imbert, J.P.; Sidali, A. (2024). “Deep Tunnel Workers Go Trimix”. InDepth.
- Occupational Health and Safety Act Regulations (2009). Southern African Legal Information Institute.