Hazard Knowledge

This guidance focuses on tunnels under construction, along with the refurbishment of existing tunnels. The information may be relevant to disused or decommissioned tunnels.

Fire and rescue services should ensure that all reasonable arrangements are made to liaise with those constructing the tunnel system and should review Site-Specific Risk Information (SSRI) and response plans so that they reflect the current situation.

As construction nears completion, it will be necessary to re-evaluate information previously collated and to work with infrastructure managers, tunnel user representatives, regulators and other multi- agency partners to ensure that the final emergency plan is validated. Plan validation should take place before commissioning exercises and the official opening, through exercises using the access and systems that are in place.

Many aspects of the construction of a below ground structure fall outside the knowledge and skills of fire and rescue service personnel. It is recommended that fire and rescue services liaise with experts to ensure that proposals fulfil statutory duties, legal requirements and specific construction standards and that the services required to support a fire and rescue service incident are established.

Tunnels under construction can present challenging and unusual hazards such as:

• Limited access and egress
• Extreme travel distances
• On-site machinery
• Presence of hazardous materials and explosives
• Complex layouts
• Compressed air working

Compressed air working

Work in compressed air relates to any activities within any working chamber, airlock or decompression chamber that is used for the compression or decompression of people. This includes a medical lock used solely for treatment purposes, where the atmospheric pressure exceeds 0.15 bar. Access to a pressurised working will involve an airlock. or air washing lock, also referred to as a manlock. Work in compressed air is regulated under:

• The Work in Compressed Air Regulations
• Work in Compressed Air Regulations (Northern Ireland)

These regulations apply to all people employed in tunnelling, pipe jacking and shaft and caisson sinking operations carried out in compressed air, including the use of tunnel boring or shaft excavating machinery and similar operations, as part of construction work.

Atmospheric pressure at sea level is fractionally above 1 bar pressure, pressure gauges are scaled to read zero at this at normal atmospheric pressure. In a pressurised atmosphere a reading of 1 bar refers to double that of atmospheric pressure. At 1 bar the equivalent volume of air at atmospheric pressure is halved, therefore inhaling the same volume of gas contains double the concentration of air.

Fires involving compressed air workings will involve an accelerated combustion process, due to increased oxygen levels in the pressurised atmosphere.

During construction, tunnels may be pressurised to prevent water ingress, particularly boring under a river or in very wet layer. Regulations under the Health and Safety at Work etc. Act are in place in respect of people employed in pressurised workings.

Further information can be found in Tunnels and underground structures supplementary material - Work in compressed air (Pressurised atmospheres)

Health risks

When working in a pressurised atmosphere, the body's internal pressures balance to match the external pressure; therefore, the amount of air inhaled at 1 bar pressure will be double that at atmospheric pressure.

There are various types of health problem (decompression illness) which can be caused by working in compressed air. According to the Health and Safety Executive (HSE) information, About work in compressed air, the most common are:

• Decompression sickness, which predominantly occurs as a condition involving pain around the joints, or, more rarely, as a serious, potentially life-threatening condition that may affect the central nervous system
• Barotrauma, where a change in surrounding pressure causes direct damage to air-containing cavities in the body directly connected with the surrounding atmosphere, principally the ears and sinuses
• Dysbaric osteonecrosis, which is a long-term, chronic condition damaging the long bone joints, such as hips and shoulders

Breathing apparatus

The working duration of self-contained breathing apparatus (BA) is significantly reduced by increased pressure. The balance of the exhale valve and diaphragm controlling the demand valve are likely to be affected by the increase in the external pressure. In addition, the stress and exertion of working in these environments is likely to increase breathing rates.

Personnel need to consider the potentially limited intervention they can make at incidents in tunnels or other below ground structures under construction. The need to wait for additional core and specialist resources will inevitably add moral pressure to personnel and incident commanders to take life-saving action.