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Advanced high-pressure hyperbaric techniques in tunneling

Advanced high-pressure hyperbaric techniques in tunneling

HIGH-PRESSURE TECHNIQUES IN TUNNELING

 

ABSTRACT

Work in compressed air and diving are both occupational activities that have been around since the mid-19th century, and those undertaking their work under elevated pressure. Meeting the demand to go to “higher pressure for longer” in tunneling has lagged in diving, but both activities have found it necessary to adopt mixed gas breathing and saturation exposure techniques. This paper explains how work in hyperbaric conditions at high pressure is undertaken in tunneling and is illustrated by the hyperbaric activity likely to be involved in constructing a large-diameter road tunnel below a body of water such as an estuary. It also explores the practical differences between work in compressed air and diving.

Keywords – decompression; hyperbaric; mixed gas; saturation; transfer under pressure; tunneling;

Key points - the paper reviews work in compressed air in tunneling and describes the developing use of mixed gas and saturation exposure techniques in tunneling whilst highlighting important differences between the application of such techniques in tunneling and in diving.

INTRODUCTION/BACKGROUND

Work in compressed air is a construction technique developed in the mid-19th century in which compressed air is applied to a tunnel or shaft at a pressure equivalent to the surrounding groundwater pressure to control water ingress and thus stabilize the ground [1]. A comprehensive but slightly UK-centric summary of the development of work in compressed air, decompression practice, and regulation over almost two centuries is given by Lamont (2006) [2].

Compressed air work began with caisson sinking for foundation construction. In this technique, an open-bottomed structure is placed on the bed of a river or lake, and compressed air is pumped into the structure to displace the water so that the base can be excavated in the dry, allowing the caisson to sink. The pressure has to be progressively increased as the material is excavated, allowing the caisson to sink sufficiently to act as a foundation for a bridge pier or similar. By the late mid-19th century, there was considerable demand from railway entrepreneurs to build bridges over rivers and estuaries. Caisson sinking was effectively an adaptation of early bell-diving techniques. Caisson sinking with compressed air is still used, but other foundation engineering techniques are now available. The successful use of compressed air for caisson sinking soon led to its adoption as a face stabilization technique in tunneling, and that became the dominant application.

Bell-diving techniques have moved on and developed. Wet bell diving has been undertaken for many years, and now closed bells are the norm for saturation diving. These have a bottom opening to allow divers to leave and enter the bell. When open, water is kept out by the gas pressure inside the bell.

For much of the 20th century, large tunnels were excavated by miners hand excavating in open-face compartmentalized shields with a cast-iron or pre-cast concrete segmental lining being erected in the tailskin of the shield. When hand excavated, the whole tunnel was pressurized, and decompression was on air. However, since the 1980s, mechanization using tunnel boring machines (TBMs) has increasingly replaced hand excavation and allowed deeper tunnels to be built. In the 1980s and 1990s often, the whole tunnel, including the TBM, was pressurized.

TBM technology has advanced and now typically utilizes a closed-face machine, which has a rotating cutterhead located in front of an air-tight bulkhead separating the excavation chamber from the rest of the machine with an airlock built into the bulkhead for access (Fig 1). Now, with closed-face TBMs, only the excavation chamber is pressurized. Modern closed-face machines allow larger tunnels to be built at greater depth and in more challenging ground conditions. However, this normally means higher pressure also.

Figure 1 – Cutaway illustration of TBM

In the UK, exposure pressures never exceeded 3.5 bar(g) (1 bar = 105 Pa), which was reconfirmed as the UK statutory limit in 1996 [3]. However, work was mainly in the range of 0.7 bar(g) to 2.5 bar(g).  

A typical application for compressed air could be facilitating the construction of a road tunnel in the soft ground below a body of water with open-cut construction or adits leading down from ground level on both sides to the tunnel horizon, with a near horizontal length of the tunnel below the water itself. The pressure profile normally reflects the vertical alignment.   

The cutterhead is dressed with scrapers and disk cutters to suit the type of ground to be excavated (Fig 2). To maintain face stability when access is required for maintenance, the spoil in the excavation chamber is drawn down and replaced by compressed air. All cutting tools can be subject to wear or damage; hence, access for inspection and replacement is a routine part of the tunneling cycle. Tool changing on a cutterhead is a physically demanding task, as disc cutters can weigh up to 250 kg.

Figure 2 – Typical cutterhead on large diameter TBM

The paper is based on the authors' experience in the UK, Europe, and internationally.

Pressure categories

In 2012, the International Tunnelling Association (ITA) defined high-pressure compressed air (HPCA) work as “work in compressed air at pressures above historical statutory limits, which in most countries are between 3 and 4 bar (gauge), and which involves the use of breathing mixtures other than compressed natural air and can involve the use of saturation techniques” [4].

Prior to formal categorization by ITA in 2012, “high” pressure in compressed air work normally meant a pressure exposure that required stage decompression. This pressure was usually between ~1 bar(g) and ~3.0 – 3.5 bar(g), with exceptionally a maximum of 5 bar(g), depending on national legislation [5].  What was previously “high” pressure is now categorized by the ITA as “intermediate” pressure [4], and work is still done in non-saturation mode with air as the breathing and pressurizing medium but with oxygen-assisted stage decompression.

The first formal, but rudimentary, guidance on mixed gas breathing in UK tunnelling was published in 2001 [6]. An account of early contracts using mixed gas at pressures over 3.5 bar(g) is given by Le Péchon and Gourdon [7]. Much of the work on Westerschelde was not HPCA work but high-risk saturation diving in the bentonite-filled excavation chamber [8].

            The number of tunnels constructed using HPCA remains small but has grown particularly during the second decade of the 21st century. Perhaps the most complex high-pressure project to date has been the Tuen Mun – Chek Lap Kok Link tunnel in Hong Kong, which opened in late 2020 [9] and reached a pressure of just under 6 bar, supported by two teams of operatives in saturation, simultaneously servicing two tunnel drives at different pressures, all from a single habitat. Probably the highest pressure used to date was on the Eurasia Tunnel in Istanbul at around 10.5 bar(g) [10].

Exposure patterns – low and intermediate pressures

Historically, shift durations of up to 12 hours were worked for exposure pressures of under 1 bar. Following the introduction of the Blackpool tables [2] in 1963, the exposure period plus decompression time for intermediate pressure exposures was limited to 10 hours. Current practice favors a reduction to 8-hour shifts covering working time and subsequent decompression, which should be oxygen-assisted [11].

 

Exposure numbers

The adoption of mechanization and advances in TBM technology have dramatically cut the number of exposures. For example, in the late mid-20th century, hand excavation in compressed air was undertaken on several UK sub-estuarine road tunnels, such as the Clyde tunnel opened in 1963 [12], and the 2nd Dartford tunnel opened in 1980 [13].   

The hand-dug 2nd Dartford tunnel generated ~122,000 exposures by ~1200 compressed air workers [13]. By comparison, the nearby twin tunnels on the HS1 line at Swanscombe opened in 2007, of roughly the same diameter, length, and ground conditions, required ~120 exposures to excavate by TBM with only 20 - 30 men exposed [2]. 

The choice – work in compressed air or diving

Normally there is no question about which technique should be used – work in compressed air when the work is underground in an air pressurized environment and diving when the work is underwater. However, in a very small number of tunnels with very low ground cover and hence poor face stability, exceptionally, the excavation chamber has been flooded with water or bentonite, and diving techniques have been used to facilitate maintenance of the cutterhead [8].

Such practice and using personnel from a diving background led to phrases such as “bounce dive” and “saturation diving in tunneling.” The authors of this paper believe that for HPCA work these terms are inappropriate and misleading and show a lack of understanding of the many significant differences between the two techniques. The authors support the use of the terms “non-saturation” and “saturation” for describing exposure techniques in tunnelling as in ITA Report 10 [4]. They also feel it is important to explain these differences to the wider hyperbaric community by identifying hyperbaric issues and working practices common to both or sometimes widely differing between tunneling and diving.

Immersion

Two fundamental differences between work in compressed air and diving are those of the use of breathing apparatus and immersion.

In theory compressed air workers do not require to use breathing apparatus. While pressurizing the TBM excavation chamber with mixed gas can be technically feasible, the costs would be prohibitive. Hence HPCA work is undertaken in an air pressurized environment but with a breathing mixture supplied through umbilical-fed breathing apparatus.

Being immersed in air means that compressed air workers do not experience buoyancy or some physiological effects from water immersion [14]. Compressed air workers tend to work in a more upright position and expend much more physical effort in their lower limbs, supporting their body weight along with equipment such as helmets fitted with CCTV and lighting. The important physiological effects of immersion are set out by Wingelaar [14].

These differences mean longer decompression times are required following (non-saturation) tunneling exposures, and despite the different stage pressure at which oxygen administration is commenced, the differences can be seen in Table 1.

Table 1 – Comparison between decompression times for diving and tunneling exposures [15], [16]

(JORF, 2019) (Bulletin Officiel, 2013).

Bail-out bottles, as used in diving for emergency breathing supply, are not used in tunneling. Instead, emphasis is placed on redundancy, diversity in the gas supply, and proximity to the relative safety of the intermediate chamber or personnel lock.  

 

Temperatures

The excavation chamber usually presents a warm – 20 – 40 0C – and humid working environment, and a period to allow the cutterhead to cool can be necessary before entry. Hot water suits are not required, and the problem of hypothermia seldom occurs in tunneling. It is important to avoid extreme temperature variations between the TBM excavation chamber and decompression lock and monitor and give fluids to counter dehydration from sweating.

Risk

The safety risks differ with the technique. Fire is a risk in air-pressurized chambers, but it is not a risk in chambers with <6% oxygen by volume. Good practice requires that to minimize fire risk, no productive or maintenance work is undertaken on the TBM when HPCA work is being carried out. Hot work in the excavation chamber should be subject to a rigorous permit-to-work regime. Against that, drowning is not a significant risk in tunneling.

The significantly greater risk of decompression sickness in tunneling compared with diving persists, although that risk has been reduced in non-saturation exposures through the adoption of oxygen decompression. Although dysbaric osteonecrosis (DON) has effectively been eliminated from most commercial diving, it remains a recognized risk in tunneling, especially for exposures followed by routine air-only decompression. We have no proof yet that DON has been eliminated in modern tunneling; however, no cases have been reported in recent years.

 

Measuring pressure

Other basic differences between tunneling and diving exist over pressure. In tunneling, the basic metric is “pressure”, whereas in diving, “depth” is used. The internationally recognized pressure unit in tunneling is the bar (105 Pa), and tunnellers are still coming to terms with the difference between bar(g) and bar(a). Although pressure in tunneling is not normally measured by water depth, the general approximation of 10 m of (fresh or salt) water being equivalent to 1 bar of air pressure is familiar to tunnel engineers when assessing the required working pressure [17]. Historically in tunneling, “up” means an increase in pressure, and “down” means a decrease in pressure. “ascent” and “descent” should not be used in tunneling.

 

Terminology

Traditionally the term “intervention” was used for any work in the excavation chamber, but tunnellers are now becoming aware that with saturation techniques, “interventions” refer to a work period away from the habitat at storage pressure and “excursions” to a work period away from the habitat at a pressure different to storage pressure [18]. ITA recommends that for clarity, excursions are categorized as being to “a pressure greater than/less than storage.”

Regulation

In some countries, differences exist between the regulations covering tunneling and diving. Both activities have been heavily regulated since the early mid-20th century due to their hazardous natures. If anything, the regulation of work in compressed air has tended to be more prescriptive than the regulation of diving, particularly with respect to specifying decompression procedures and maximum exposure pressures but making no reference to breathing mixtures.

No country currently is thought to have legislation specifically regulating HPCA work utilizing mixed gas breathing or saturation techniques. At best such work might be permitted by specific approval, waiver, or exemption, as is the case in Hong Kong [9], Singapore, the UK [11], and the USA.

Tunneling industry guidance

In 2012, ITA recognized the need for industry guidance and published, in conjunction with the British Tunnelling Society Compressed Air Working Group (BTS CAWG), its Report 10 [4], and the latest version [18] is currently being revised. Report 10 covers only the requirements for HPCA work in addition to those for air mode compressed air work, which are set out in the BTS CAWG “Guidance on good practice for work in compressed air” [11]. Report 10 references diving industry guidance (e.g., IMCA) where appropriate and remains the only internationally recognized guidance on HPCA work.

A corresponding explanatory guide for tunnel project clients unfamiliar with HPCA work (Report 20) was published in 2019 [19].

Standards

The safety of tunneling machinery as manufactured for use in Europe is currently covered by two standards – prEN 16191:2023 [20] for tunnel boring machines and prEN 12110:2023 [21], [22] for personnel locks. The previous (2014) versions of both standards [20] and [21] addressed low and intermediate-pressure applications only, along with oxygen decompression. As part of the recent revision process, a second part [22] was drafted to address the machinery safety aspects of personnel locks and pressurized transfer shuttles for high-pressure exposures to a maximum pressure of 20 bar. No standard for habitats exists.

British Standard BS 6164:2019 [17] is widely used not only in the UK but also in many other countries and provides guidance on health and safety in tunnel construction. It contains normative references to the BTS guidance [11] and ITA Report 10 [18], thus formalizing their role in compressed air work in the UK.

Air/water interface and pressure profiles

Orientation of the air/water interface is another fundamental difference between work in compressed air in tunneling and diving. No matter how large the opening in the base of a diving bell or caisson is, the air/water interface is always horizontal, and pressure is constant across it. As long as the interface is at or slightly above the base opening, no air is lost at the interface, and pressure in the bell or caisson is maintained. The exposure pressure is easily determined from the interface depth. 

However, in tunneling, the air/water interface is vertical, and consequently, the water pressure profile across the face is the normal triangular hydrostatic distribution, with pressure increasing linearly with depth (Fig 3).  Although, in theory, the air pressure also increases with depth across the face, the much lower air density means that this very small pressure differential is ignored, and a rectangular pressure profile is assumed to exist.

Figure 3 – Simple illustration of pressure profiles [2]

Consequently, there is only one point on the tunnel face where water and air pressure balance.  Above this point, air pressure exceeds water pressure and can lead to an outflow of air, particularly in granular soils, with accompanying loss of ground stability. Below the balance point, water pressure exceeds air pressure which leads to an inflow of water and wet soil, which can also be accompanied by a loss of ground stability. The larger the tunnel face, the greater the potential pressure imbalance across the face. With typical road tunnels driven below estuaries and lakes now in the 12.5 – 17.5 m diameter range, a pressure differential of 1.25 – 1.75 bar can exist across the face. Thus, maintaining face stability can present a major challenge in compressed air work.

Balance point

Determining the air/water pressure balance point for any particular type of ground and, hence, determining the exposure pressure is highly safety critical. Air overpressure at the crown of the tunnel, along with the permeability of the ground, determines air loss. Tidal fluctuations should also be considered. Controlling the air pressure to maintain the balance point at the desired level in what can be a varying air loss situation across the face is a safety-critical operation for those in charge of the compressed air work, unlike in diving, where the exposure pressure is merely a function of water depth. This is another significant difference between tunneling and diving.

Exposure pressure

As a rule of thumb, the tunnel alignment is designed to give around one diameter of ground cover above the tunnel to ensure geotechnical stability and prevent flotation. For a 15 m diameter tunnel below a 10 – 30 m deep estuary, that means a depth to invert of 40 – 60 m. Whilst pressures of 4 – 6 bar(g) are high by historical compressed air experience they are relatively shallow in terms of offshore commercial diving practice.

As an order of magnitude check, maximum tunnel air pressure can normally be assumed to be 1 bar for every 10m of water [17]. However, a significantly reduced air pressure can suffice in relatively impermeable ground conditions where the water inflow, already reduced by low ground permeability, can safely be removed by other techniques such as pumping. 

Pressure profiles - advantages

Whilst the differences in air and water pressure profiles can cause problems with the control of face stability, the same difference provides one very important benefit for interventions and excursions. Once in the air-filled excavation chamber, the whole face is accessible at the same pressure, whereas, in a liquid, there would be an increase in pressure with depth.

Shallow saturation restrictions

Those familiar with offshore commercial diving practice will recognize that with “shallow” saturation, one of the issues that arises is the restrictive excursion limits that apply at such pressures and which have to be planned around.

Until there is tunneling industry guidance on excursion limits, reference should be made to diving sources such as NORSOK U100 [23] or JORF [15].  NORSOK U100 gives excursion limits in the 5- 6 bar(g) pressure range as +/- 8 msw, I.e., 0.8 bar (g), with a strongly worded warning against excursions to pressures less than storage pressure. In this case, working in compressed air gives more flexibility than diving, as the pressure is constant across the entire face.

Decision to adopt mixed gas techniques

Unlike diving, where regulations often mandate the use of heliox saturation at depths over 50 m (5 bar(g)), although diving contractors are free to use it at depths as low as 20 – 30 m (2 – 3 bar(g)), tunneling contractors are not mandated by regulation to use a particular technique above a specific pressure threshold unless included as a condition of approval or variance issued by local health and safety regulators. Even compliance with ITA Report 10 [18] is not necessarily mandatory, although it is encouraged and can be a contractual condition or a requirement of the regulatory authority.

Report 10 recommends using non-air breathing mixtures from 3.5 bar(g) upwards. This recommendation is based on considerations of nitrogen narcosis, the work of breathing, and CO2 retention, taking into account the high work rates of those working in the excavation chamber, which is important in such a safety-critical environment. A further restriction on gas density to limit the work of breathing is expected. Such a limit would also address narcotic risks along with the risks associated with CO2 retention.

There is widespread misunderstanding in the tunneling industry that decompression is the only factor to be considered when deciding to adopt mixed gas breathing. As a result, some less scrupulous contractors consider 5 bar(g) or higher to be the appropriate pressure to switch to breathing mixed gas.

Decision to adopt saturation techniques

prEN 12110-1 [21] sets a minimum lock diameter for a given maximum decompression time in the lock.  ITA Report 10 [18] also sets pressure and time limits on non-saturation high-pressure exposures linked to lock diameter. In the 4 – 6 bar(g) pressure range, both typically restrict exposure periods to ¾ – 1 ½ hours, depending on the decompression tables used.

These relatively short exposure periods limit the amount of productive work that can be done on each excavation chamber entry. The amount of work required is usually a function of cutterhead diameter because the bigger the head, the more tools on it – 150 – 200 discs on a ~15 m diameter TBM. Tools wear due to the abrasiveness of the ground or impact damage from the presence of hard boulders such as flints in the ground and need to be replaced periodically. Consequently, the limited productive work from non-saturation exposures can make them unattractive to contractors with large TBMs. Non-saturation exposures can be relatively easily accommodated using existing equipment with a few additional penetrations through the locks for gas supply lines etc.

The decision to use saturation is normally a commercial decision based on estimates of the likely exposure pressure, the amount of productive work required at that pressure, the proportion of the exposure taken up preparing the excavation chamber for work, downtime resulting from decompression in the TBM lock, gas costs, the cost of on-site surface installations and transfer under pressure shuttles.

The recommendation in ITA Report 10 is that saturation techniques are appropriate for exposures over 3.5 bar(g) when longer productive work periods and a reduced risk of DCI are desirable. Contractors are free to adopt a lower threshold if desired. In recognition of the more controlled environment in tunnelling, Report 10 sets a limit of 7 bar(g) over which all exposures should utilize saturation techniques but with an exception for smaller tunnels.

Not all tunnels are 15 m diameter road tunnels. Rail tunnels are around 8 m in diameter, and utility tunnels are often smaller than 5 meters, making a transfer under pressure virtually impossible. Therefore Report 10 permits a maximum exposure pressure of 8 bar(g) for non-saturation exposures where transfer under pressure becomes impossible in a small diameter tunnel, provided that decompression is undertaken in the relative safety of a personnel lock. Limits are given on the lock diameter, total time under pressure each day, and the number of such exposures that may be undertaken in a 35-day period.  

Transfer under pressure

Another major difference occurs with transfer under pressure (TUP). TUP in tunneling is essentially an untethered near-horizontal movement of a self-contained mobile personnel lock (referred to as a “pressurized transfer shuttle” in prEN 12110-2) (Fig 4) between an on-site habitat and a TBM personnel lock.

The vertical travel distance will typically be <100 meters. However, the horizontal distance between the surface habitat and the TBM can be several kilometers as tunnel construction progresses. The time for transfer could typically be between one and two hours or more. During this time, the personnel in the shuttle are essentially at rest.

As part of the TUP operation, the shuttle must be taken through the TBM backup, for which a clear “shuttle path” is required. The space required for the path dictates the minimum diameter of the tunnel below which saturation exposures can only be undertaken with partial or total disassembly of the TBM backup equipment. 

Tunnel workers transfer into the TBM personnel lock into an intermediate chamber and then through the bulkhead into the excavation chamber. The intermediate chamber provides an air-pressurized dirty working place of relative safety between a clean TBM lock and a dirty excavation chamber from which entry to the excavation chamber is made [18]. 

The shuttle and TBM lock would normally be pressurized with mixed gas, with the intermediate chamber and excavation chamber being air pressurized with personnel breathing mixed gas by umbilical and mask.

The shuttle is built into a protective frame to provide impact protection and handling/lifting capability. The number of access doors and compartments in the shuttle depends on the regulatory authorities in the country of use -  the UK requires two of both. Fire suppression is permanently mounted within the protective frame.

Figure 4 - Pressurized transfer shuttle

Normally for transfer, the shuttle and frame would be mounted on a support structure accommodating power, gas, environmental control equipment, and working space for one or more shuttle attendants. The support structure would typically be transported by a double-ended tunnel service vehicle.

prEN 12110-2 requires an external services connection panel on the protective frame to which gas, power, communications, etc., can be connected when the shuttle is in the shuttle path or the docking position at either the TBM or the habitat. If the shuttle becomes stranded during transport, emergency service connections can be made via the panel to supplement the extensive standby capability already carried on the support structure. Food, drink, and basic toilet facilities can be locked in via the entry compartment or a small supplies lock for a two-compartment shuttle.  

The preferred transport procedure is by a tunnel service vehicle for the entire journey from the on-site habitat to TBM; however, lifting a shuttle down a shaft to a vehicle in the tunnel below has to be undertaken in some circumstances. Lifting by crane is a high-risk option that should be avoided if possible, and extensive mitigation measures are required.

TUP in diving is very different. A closed bell involved in TUP is tethered by an umbilical to a surface vessel or installation and descends vertically some 100 – 300 meters to working depth. Divers exiting the bell are tethered to it by an umbilical.

Breathing mixtures

Although some early high-pressure tunneling contracts used heliox, at present, trimix seems to be the breathing mixture of choice, particularly for non-sat exposures. Saving on helium, ease of achieving the desired mix in chambers starting with compression on air, and reduced risk of narcotic shock in a mask-off incident are among the claimed advantages. However, there is little published data to support any specific advantages. Helium reclaim technology has yet to be adopted in tunneling. At the pressures in the 3.5 - 6 bar(g) range, a trimix with 20% oxygen and around 30 – 40 % helium meets the current needs for non-saturation exposures, and a trimix with less than 20% oxygen and around 50 - 60% helium is appropriate for saturation exposures. However, limits on gas density may change this.

The currently recommended partial pressure limits for oxygen and nitrogen exposure in Report 10 largely reflect the limits in the diving industry. However, as experience of HPCA work increases, industry-specific limits are starting to emerge. Experience has already shown that a short-term partial pressure of nitrogen (PN2) limit of 3.6 bar is acceptable for non-saturation exposures, but for saturation exposures, a lower PN2 limit of 2 bar is necessary to avoid worker fatigue. The current revision of Report 10 is expected to specify overarching limits on gas density as a primary control measure on breathing mixture composition.

Decompression tables

A major disadvantage for tunnelling contractors using trimix is the current lack of proven decompression tables. One set of non-saturation trimix tables with some history of use in tunneling  [7] does exist, but it is commercially confidential and not widely available. Trimix at pressures of 3.5 – 6 bar (g) is not commonly used in non-saturation commercial or military diving hence there is only a very small number of diving tables available, appropriately modified for use in tunnelling if necessary.

Heliox diving tables are available for non-saturation exposures in the 3.5 – 6 bar(g) pressure range which specify longer decompression times than for the equivalent dive on air. A proposition is circulating in the tunnelling industry that breathing a 20/40/40 trimix is between breathing air (a 20/0/80 trimix) and heliox (a 20/80/0 trimix) and that breathing trimix but decompressing as if on heliox could be possible. This could be a topic for industry research.

Another issue to be addressed is the different decompression requirements for tunneling and diving. Should tables considered appropriate for tunneling use become available, then their effectiveness should be tested by applying the Doppler monitoring protocols in clause 11.8 of BTS CAWG guidance [11] for tables without a history of satisfactory use.

The most readily available saturation diving decompression tables are those of the US Navy, NORSOK U100 [23], and the “Bulletin Officiel" [15]. All are heliox tables. Their use should be considered for tunnelling as there are no differences between tunneling and diving in the habitat phase. The tables are referred to as “heliox” tables despite there possibly being a significant PN2 of potentially 1.6 bar or more depending on how the initial PO2 in the habitat was achieved.

 

Interventions and excursions

Obviously, the ideal pattern for saturation exposures is a tunnel at constant pressure where storage and work can be undertaken at approximately the same pressure. Unfortunately, this is not always possible. Road tunnels are more likely to conform to a down/under/up profile (Fig 5) in which the working pressure increases over weeks, remains roughly constant for a period, and then decreases over weeks. Careful planning of storage, intervention, and excursion pressures is required.

 

Figure 5 – Typical tunnel profile [24]

During the first period, where pressure increases, work in the excavation chamber can be done as interventions with storage and working pressure kept roughly equal but increasing incrementally with tunnel advance. Alternatively, storage pressure can be kept constant for a period with a series of excursions to an ever-increasing pressure before storage pressure is increased and the excursions restart from the higher storage pressure. For the period of roughly constant pressure, the preference should be for interventions to be undertaken. For the decreasing pressure stage, storage at a lower pressure with a series of ever-decreasing excursions to higher pressure could be undertaken as the working pressure decreases. This is a procedure where industry research into more effective methods of gradually reducing storage pressure in line with working pressure could prove useful. Excursions involving decompression stop should not be undertaken. Unfortunately, such research is unlikely to find funding anytime soon.

Servicing two TBMs being driven in echelon from one multi-compartment habitat adds further complexity in pressure selection as, for much of the drive, one TBM will be at a different working pressure than the other.

Caking the face with bentonite

Returning to the topic of air loss, any air movement through soil, particularly through granular soil, leads to drying out of the soil. This, in turn, leads to the opening of preferential flow paths in the ground, which exacerbates the air loss. One way of dealing with the problem is to temporarily flood the excavation chamber with bentonite slurry to form a bentonite cake over the face and thus seal the face. After the bentonite has been drained down from the excavation chamber, the face should be observed, and air loss monitored for a period to confirm that a satisfactory seal has been successfully established over the face. Criteria on air loss need to be set to confirm face stability and also to determine when the excavation chamber needs to be evacuated due to increasing air loss.

Any rotation of the cutterhead to position tools for changing can remove the surface seal, so careful planning of the work is necessary to minimize the removal of stable material from the face.

Working time

ITA Report 10 currently follows the diving practice in recommending a maximum working period but adapted to suit tunneling practice. That period is 6 hours in the excavation chamber with a ½ hour mask-off break mid-period. However, experience has shown that where excessive air loss requires frequent episodes of flooding the face with bentonite, personnel can enter and exit the excavation chamber three or four times during a working period. Although the personnel lock lacks the full welfare facilities of a surface habitat, it can be argued that the workers are effectively resting with access to food, etc, between work periods. Therefore, some relaxation of the 6-hour limit might be appropriate, provided the total working time did not exceed 6 hours in an eight or ten-hour stay on the TBM, particularly if followed by a 24-hour rest in the habitat.

Use of diving personnel

With low and medium-pressure exposures, the work in compressed air in the excavation chamber has traditionally been done using tunneling labor such as miners, fitters, and electricians. Before the mid-1990s, in the UK, lock attendants and medical lock attendants were largely sourced from within the tunneling industry. In the mid-1990s, lock attendants with a background in offshore diving or life support and seeking a lifestyle change entered the UK tunneling industry. This brought about a major advance in the professionalism by which first line supervision of compressed air work was undertaken.

The introduction of mixed gas techniques is bringing about a similar change with many aspects of the hyperbaric activity associated with excavation chamber entry being undertaken by personnel with a commercial diving background. Such personnel come ready trained with a high level of hyperbaric knowledge, and many can demonstrate practical experience of living in saturation. However, they lack the experience of working in the underground environment and are devoid of mining skills. Consequently, extensive training will be required for some time to ensure personnel are available with both the necessary hyperbaric skills and tunneling skills to work safely in HPCA [25].

On-site habitats

One area of commonality between tunneling and diving is the provision and management of on-site habitats. In principle, the same habitat and its management procedures can service either a tunneling or diving operation.

Medical emergency management

Work in hazardous environments under pressure has inherent risks of injury that are normally mitigated to an acceptably low level. Primary prevention for both tunneling and diving is project-specific and based on risk assessment.

Secondary prevention, however, is typically different. Tunneling operations are normally in relatively accessible locations where hyperbaric and specialist medical support is possible on-site within one to two hours or less. Support can range from first aid by trained rescuers from outside the chamber to a medical team being compressed into the airlock. ITA Report 10 requires at least two in the excavation chamber to be diving medical technician (DMT) qualified, with a further DMT-qualified person to be outside the chamber also. All external emergency response personnel should be trained and certified fit to enter the pressurized environment.

Commercial diving operations usually take place in remote locations where onsite medical support would not be realistic or only achievable in extreme emergencies with considerable delay. Advanced first aid competence Is crucial for all divers who get DMT training.

Research and development

The greatest advances in commercial diving practice arguably arose from the North Sea oil boom of the 20th century. Large sums of money have also been available from naval research and development budgets. While tunneling now benefits from that research, there never has nor will likely be any significant hyperbaric research and development programme in tunneling. Most hyperbaric medical research is also related to diving and not to tunneling.

CONCLUSIONS

The differences between work in compressed air and diving are numerous and varied. They stem from the different environments in which they are undertaken – immersion in air against immersion in water - and the working practices undertaken. It is important, particularly in tunneling, that those undertaking the work recognise the differences and make appropriate allowance for them. Other branches of the hyperbaric community should be aware of them.

REFERENCES

  1. Bert P, 1878. La Pression Barométrique, Translated from French by Hitchcok M. A. and Hitchcok F. A – Columbus College Book Co - 1943, Republished by the Undersea Medical Society Md.
  2. Lamont DR. Decompression illness and its regulation in contemporary UK tunnelling – an engineering perspective, PhD thesis, Aston University, Birmingham, 2006.
  3. HSE 1996, A guide to the Work in Compressed Air Regulations 1996 – Guidance on Regulations, L96, Sudbury, HSE Books, 1996. Out of print but accessible at https://www.cdc.gov/niosh/docket/archive/pdfs/NIOSH-254/compreg1996.pdf.
  4. ITA. Guidelines for good working practice in high pressure compressed air, International Tunnelling Association/British Tunnelling Society Compressed Air Working Group Report 10, 1219 Chatelaine, Switzerland, 2012. <https://about.ita-aites.org/publications/wg-publications/content/10-working-group-5-health-and-safety-in-works>.
  5. Anderson JM, Lamont DR. A comparison of international legislation concerned with tunnelling in compressed air” Tunnelling ’91, Institution of Mining and Metallurgy, Elsevier Applied Science, London, pp 17 – 28.
  6. Addendum to "A guide to the Work in Compressed Air Regulations 1996 Guidance on Regulations", Guidance on OXYGEN DECOMPRESSION and the use of Breathing Mixtures other than Compressed Natural Air in the Working Chamber. Construction Division Technology Unit, HSE, Bootle, UK.
  7. Le Pechon JCl, Gourdon G, Compressed air work is entering the field of high pressures. UHM 2010 Vol 37 No 4,
  8. Sterk W, Le Pechon JCl, Van Rees Vellinga TP, (2002), Dive techniques in the Westerschelde tunnelling project. In: Proceedings 2nd International Conference on Engineering and Health in Compressed Air Work. Oxford. eds Slocombe, Buchanan and Lamont, British Tunnelling Society, pp. 93-102.
  9. Schwob A, Cagnat E, Chen S, Chan AWY, Ng CCW. Tuen Mun–Chek Lap Kok Link: an outstanding subsea tunnel project in Hong Kong, Proceedings of Institution of Civil Engineers, London, Civil Engineering, Volume 173, Issue 5 (special issue on underground construction), May 2020, pp. 33-40.
  10. Mirasoglu B, Arslan A, Aktas S, Toklu AS. Eurasian Tunnel Project: the first saturation dives during compressed-air work in Turkey. Undersea Hyperb Med. 2018 Sep-Oct;45:489-494. PMID: 30428237.
  11. BTS. Guidance on good practice for Work in Compressed Air, British Tunnelling Society Compressed Air Working Group, 2021, <https://britishtunnellingtunnelling.com/pages/work-in-compressed-air> .
  12. Haxton AF. The Clyde Tunnel: Constructional Problems, Proceedings of the Institution of Civil Engineers, London, Vol 30, pp. 323 – 346, 1965.
  13. Campbell Golding F, Griffiths P, Hempleman HV, Paton WDM, Walder DN, Decompression Sickness during Construction of the Dartford Tunnel, British Journal of Industrial Medicine Vol. 17, No. 3 (Jul, 1960), pp. 167-180.
  14. Wingelaar TT, van Ooij P-J, van Hulst RA, Oxygen Toxicity and Special Operations Forces Diving: Hidden and Dangerous, Front. Psychol., 25 July 2017, Sec. Movement Science and Sport Psychology, Volume 8 - 2017 <https://doi.org/10.3389/fpsyg.2017.01263>.
  15. JORF. Ministère du travail, France, Arrêté du 14 mai 2019 relatif aux travaux hyperbares effectués en milieu subaquatique (mention A et mention B), Journal officiel de la République Française N° 120 du 24.05.2019 (for diving tables).
  16. Bulletin Officiel (2013), "Ministère du travail, de l'emploi, de la formation professionnelle et du dialogue sociale (2013), Annexes de l'Arrêté du 30 octobre 2012, relatifs aux travaux subaquatique effectués en milieu hyperbare", Bulletin officiel du Ministère du Travail, de l'emploi, de la formation professionnelle et du dialogue social 2013:1 du 30 janvier 2013 (tables for all types of diving, hyperbaric medical treatment and tunnelling).
  17. BSI. BS 6164:2019 – Health and safety in tunnelling in the construction industry - Code of practice, London, British Standards Institution.
  18. ITA. Guidelines for good working practice in high pressure compressed air”, International Tunnelling Association/British Tunnelling Society Compressed Air Working Group Report 010-V3, 2018, 1219 Chatelaine, Switzerland. <https://about.ita-aites.org/publications/wg-publications/content/10-working-group-5-health-and-safety-in-works>.
  19. ITA (2019), “Guide to ITA/BTS CAWG Report 10 for Clients and others not familiar with high pressure compressed air work”, ”, International Tunnelling Association/British Tunnelling Society Compressed Air Working Group Report 20, 1219 Chatelaine, Switzerland. <https://about.ita-aites.org/publications/wg-publications/content/10-working-group-5-health-and-safety-in-works>.
  20. prEN 16191, Tunnel boring machines – Safety requirements, Brussels, Comité Europeén de Normalisation.
  21. prEN 12110-1. Tunnel boring machines - Air locks - Part 1: requirements for air locks utilising compressed air as the pressurising or breathing medium along with requirements for oxygen breathing systems for decompression purposes, Brussels, Comité Europeén de Normalisation.
  22. prEN 12110-2. Tunnel boring machines - Air locks - Part 2: Safety requirements for the use of non-air breathing mixtures and saturation techniques in personnel locks and for pressurised transfer shuttles, Brussels, Comité Europeén de Normalisation.
  23. Norsok U-100. Manned underwater operations, (Edition 5, December 2015, corrected version 2016-05-09, amended 03/15/2022), Standards Norway, Oslo.

24]  Herrenknecht AG, internal company document.

  1. Le Péchon J Cl. (2002). Training for compressed air work. In: Proceedings 2nd International Conference on Engineering and Health in Compressed Air Work. Oxford. eds Slocombe, Buchanan and Lamont, British Tunnelling Society, pp. 425 -437.