The use of compressed air to support the excavation face is not uncommon in modern tunnelling. Almost all EPB and slurry TBMs use compressed air to support the face (inside the excavation chamber or as an air cushion behind the submerged wall). Airlocks are required to allow workers to access the pressurised working chamber to perform inspections, replace the face tools, or carry out other maintenance work.
Working underground in hyperbaric conditions is typically a high-risk activity that can lead to major health and safety concerns if the appropriate risk-mitigation measures are not applied. A typical application relates to maintenance work in the TBM cutterhead (e.g replacing cutterhead tools) where compressed air is required to stabilise the face and control groundwater ingress.
Workers in compressed air must be physically fit and mentally conditioned to deal with the challenges, including the hostile environment, high temperatures, high PPE levels, heavy physical work in a confined space, work at height and, in some cases, complex breathing requirements.
Hyperbaric workers must have training in health and safety, as well as for working in the confined space of the cutterhead.
They also need back-up from experienced hyperbaric and medical specialists. Generally, time is critical because of the requirement to work within recognised limits on working periods which are linked to the exposure pressure and the time required for decompression.
High-pressure hyperbaric operations can be safer and more cost effective when saturation techniques are employed, thus eliminating the need for daily decompression and its associated risks. Saturation workers spend a number of days continuously under pressure followed by a single, long decompression. This is followed by the same number of days at atmospheric pressure away from the jobsite. While in saturation, workers rest and sleep in a surface habitat in which ‘hotel services’, including food, drink and laundry, are provided. Travel to and from the TBM is in a specially-designed pressurised transfer shuttle – effectively a mobile personnel lock .
Well-planned compression/decompression procedures are critical to avoid the health risks associated with compressed air working. The most common issue is decompression illness (DCI) – often referred to as ‘the bends’. DCI results from the inert gas absorbed by the tissues during compression and the working period being released too quickly, thereby forming bubbles in various parts of the body.
Normally in tunnelling, these bubbles form in limb joints and cause pain. Less common manifestations include neurological decompression sickness leading to bubbles in the spinal cord or brain, causing numbness or paralysis, and barotrauma, where a change in surrounding pressure causes direct damage to the body’s air-containing cavities directly connected with the surrounding atmosphere, principally ears and sinuses. Other manifestations of DCI result in lung damage which can occur in divers but is extremely rare in tunnelling.
Work in compressed air at pressures above statutory limits – between 3-4 bar in most countries – is termed a high pressure compressed air (HPCA) operation and requires the use of breathing mixtures other than compressed natural air. There are different breathing mixtures available for working in HPCA, but all use helium as the diluent to reduce the proportion of nitrogen, and for higher pressures, the proportion of oxygen in the breathing mixture. Helium has the double benefits of reducing the density of the mixture to make breathing easier and reduces the narcotic effect from exposure to high-pressure nitrogen. Mixtures are referred to as ‘heliox’ (helium/oxygen), or ‘trimix’ (helium/oxygen/nitrogen). Because of the lower helium content, trimix is cheaper and is becoming the mixture of choice for exposures above 3.5 bar in tunnelling.
Modern tunnelling machine designs include provisions for undertaking hyperbaric operations. Usually, these may include two or more personnel lock compartments to allow for escape and rescue in an emergency. Many TBMs have separate materials locks for a more efficient transfer of tools and material through the bulkhead.
Some modern machines use an articulated robot that can access the excavation chamber under hyperbaric conditions to change worn or damaged cutter discs, reducing to a large extent the need for human intervention.
