Le Péchon  Jean Claude,          JCLP Hyperbarie, Paris, France


Sterk  Walter,                            Leiden University, The Netherlands


van Rees Vellinga T. P.,     Arbo Hypercon, The Netherlands


ABSTRACT : On the occasion of the boring of Westerschelde Tunnels, the need for manned interventions in bentonite at 6.9 bar(g) arose. For the first time saturation technology and mixed gas breathing have been used in a tunnelling operation. Pressures involved were : Habitat 4 b(g) breathing Trimix gas, transfer pressure 4 b(g) breathing air, working pressure 6.9 b(g) breathing Trimix. Total number of excursions outside the habitat is 37 with 3 divers each time. Total working time is 400 hours.




All pressure values are expressed in bar (105 Pa) when marked bar(g) or b(g) it refers to gauge pressure measurements. Bar or b when not marked "(g)" refers to gases partial pressures.

In the early 60's, after each deep dive, whether on air or on  exotic mixed breathing gases divers were facing long decompression times and thought about living under pressure all the time and decompressing only once the job is completed. This diving method is called saturation diving. First trials where carried out under the direction of the US Navy  (Sealab program) and Cousteau (Three "Conshelf" experiments) [4, 8]. Underwater habitat was the technical solution. Soon the offshore industry required the divers to remain under pressure for weeks and the habitats have been built on board the platforms or barges; transit from the habitat to the working site was performed via a pressurized diving bell. Sophisticated Diving Support Vessels are nowadays used throughout the oil and gas industry to perform all routine subsea work remaining under the control of divers. Maximum depth reached in demonstration is 534 meters -53,4 b(g)- and in actual commercial operations that maximum depth is 350 meters -35 b(g)-[1]

As soon as the depth exceeds about 50 metres of immersion -5 bar(g)-, the breathing gas density and the narcotic effects of nitrogen make air unsuitable as a breathing media. Compressed air has to be changed for helium containing breathing mixtures. It can be either Heliox (helium and oxygen) or Trimix (helium, nitrogen and oxygen) with the proper composition depending on the type of operation, and the pressure of exposure. In experimental dives hydrogen has also been used in Trimix [1].

Works in compressed air for tunnelling have been carried out since the middle of the 19th century [5, 9]. It is Behnke in the 60's [2] who proposed for the first time to operate under saturation conditions in a pressurized tunnel.

During the preparation of Transchannel tunnel construction, a study was carried out to evaluate the feasibility of manned interventions at 10 bar(g) in the tunnel boring machines (TBMs). The conclusions were that the cost and safety problems could be overcome by using ground freezing technique or soil injection, to operate at atmospheric pressure should repairs or inspections become necessary in locations where pressure would exist.

Since then, no tunnel ever reached the range of pressure at which mixed gases is a must (Elbe tunnel was in that range –4.2 b(g), but mixed gases were not used). However, caisson works in Japan have been performed with Trimix breathing gases [5]


The Westerschelde Tunnels, with a diameter of 11.3 metres and a total length of 6.6 km, have to be dug below the river Western Scheldt at a possible pressure as high as 6.5 bar(g), so synthetic breathing gases must be used to support operators at that pressure.

At the lower part of the project, 65 meters below sea level, both TBMs required tool changes and one of them heavy repairs on the stone breaker. Knowing that such operations would require mixed gases and long working time, a full saturation habitat has been installed on the site, two transfer shuttles were built, and an "hyperbaric train" set up. The "diving method" to be used had to be decided in accordance with the actual situation of interventions.

Conditions of the interventions which had to be performed were :

Working pressure : 6.9 bar(g),

Tools to be changed : Peripheral over cutting tools

Due to the thin ground cover (less than 2 diameters of the tunnels), it would have been dangerous to evacuate the bentonite from the cutter head chamber with compressed air. It was therefore decided that manned interventions had to be carried out by divers in immersion in the bentonite (which results in working with no visibility and no light, at a pressure equivalent to 69 meters of immersion –6.9 bar(g)-.





3.1   Breathing gases

The management of breathing gases required selecting the proper breathing media in different situations :

3.1.1   In the habitat


In the habitat, pressure is stable and should be selected as close as possible to the working pressure, to reduce the risk of decompression illness after transfer sessions. To simplify the procedure and to reduce the time spent breathing from masks, that pressure should also be such as to allow transfer into the TBM air lock while breathing compressed air atmosphere. To cope with those two constraints, the habitat and transfer pressure was chosen at 4 bar(g). At that pressure air could not be used as breathing media for long exposures due to oxygen toxicity. Reducing oxygen content would increase correspondingly nitrogen partial pressure and nitrogen narcosis/density to an unacceptable level. Final choice was a Trimix breathing gas with 0.4 bar of oxygen, 3.6 bar of nitrogen and the rest, 1 bar of helium. The atmosphere in the habitat is controlled 24 h a day by life support technicians and an environmental control unit located in each of the four compartments of the habitat.


3.1.2   In the shuttle


Going out from the habitat to the TBM could be conveniently carried out with almost the same breathing mixture as in the habitat. Only an increase of PO2 by 0.05 bar was done. The shuttle atmosphere is maintained by a closed circuit environmental control unit located on board the shuttle, and under the control of an attendant travelling with it on the train.


3.1.3   In the air lock


After mating to the TBM air lock, the operators can breathe compressed air at 4 bar(g), to get ready for further compression.


3.1.4   Compression and access to the air bubble


Breathing masks, fed with the deep mix from external supplies, are donned in the air lock. Compression procedure to 6.9 bar(g) is carried out in the air lock, and then the access door to the air bubble can be opened.  The deep mix is a Trimix providing almost the same nitrogen partial pressure as in the habitat, but with more helium and more oxygen (PN2 = 3.8 b, PO2 = 0.95 b, PHe = 3.15 b).


3.1.5   Diving


Two of the operators get dressed in diving gear (dry suits and KBM 17 Diving helmets). They exchange masks while breath holding and enter the bentonite to reach the door leading to the cutter head zone, then they swim to the external part of the wheel to perform the tool change. After the job is completed or 4 hours maximum, they come back to the air bubble, undress the diving suits and helmet, return to the air lock, close the door. Decompression back to 5 bar(g) is carried out under the control of the lock operator. 5 bar(g) is the pressure of the first decompression stop, which lasts 15 minutes before decompression to the next stop at 4.5 b(g).



Diagram 1;      Partial pressure of breathing gases



3.1.6   Returning to the habitat


The shuttle, still connected to the air lock has been flushed from the Trimix with compressed air at 4.5 b(g). Transfer takes place at that pressure, then the shuttle is returned on board the train, which moves back to the habitat.


That pressure corresponds to the second decompression stop lasting about 1 hour. When pressure is reduced back to 4 bar(g), final transfer back into the habitat is performed. A new team can get ready for the next excursion from the habitat to the TBM following the same procedure.


3.1.7   Final decompression


The breathing gases during final decompression are always changing (partial pressure of oxygen is maintained constant close to 0.5 b). Decompression rate is also changing by steps, to provide a long continuous bleed at reducing rate as atmospheric pressure is approaching. Total time for decompression is 4.5 days.


3.2      Equipment

3.2.1       Habitat

The habitat was built from parts saved from the GKSS previous diving research centre of Lubeck. It is a four compartment complex : Two main chambers which may accommodate up to 9 persons, two sanitary locks, one located between the main chambers is fitted with the mating flange for the shuttle.

       Photo 1.         Moving the shuttle from the habitat (on the right), towards the train

3.2.2       The Shuttle

The shuttle is equivalent to a diving bell, but it has been simplified since an external control panel is available for the attendant to monitor atmospheric conditions. The shuttle is connected to an umbilical for gas supply and power supply. The shuttle can work either in closed circuit mode with mixed gases or in open circuit mode with air ventilation. Three persons can be transferred in this way from the habitat to  the TBM.

         Photo 2          The shuttle being transported across the site

3.2.3       TBM  and air lock

From the rear part of the TBM. where the train stops, to the air lock, various cranes and rollers allows to move the shuttle across the TBM up to the air lock, where a mating flange is used for the transfer under pressure into the air lock. This lock is only pressurized with air. A lock attendant monitors the environment of the air lock.

Built in Breathing Systems (BIBS) with special air-refrigerated helmets connected to the diving supervisor's container located at the rear of the TBM are supplied with the deep breathing mixture to support further compression

3.2.4       Diving equipment

The diving equipment selected for the operations is classical KBM 17 and dry suits. Monitoring of the divers and the non diving member of the team is the responsibility of a diving supervisor. The diving supervisor is located in a special container located in the rear part of the TBM. He has the full control of gases, communications and timing of he dive. This container is also used to monitor short duration interventions which have been carried out in the dry at lower pressures breathing Trimix gas

4       The personnel

4.1   Divers and diving supervisors

All diving members of the team are mixed gas commercial divers with training at INPP (Institut National de Plongée Professionnelle) in Marseille. In addition a special 1 week course was organized for training to the specific tunnelling procedures. Furthermore, all of them had performed several non saturation interventions in compressed air and with mixed gases breathing for  repairs in the TBM at lower pressures, therefore they were very familiar with the environment and the tasks. Simulations of saturation at 1 bar(g) were run to control the whole system and make sure all team members had become experienced with the practical operation of the system.

4.2   Medical fitness and physiological controls

Assessment of medical fitness was carried out according to the standard for commercial saturation divers. Extra controls have been performed after the various phases involving decompression : after excursions and during final decompression to monitor the possible presence of circulating bubbles in the blood. The method of control is according to the Kisman-Masurel technique and grading [6]. This is a non invasive ultra sonic Doppler signal recording.




5.      RESULTS

5.1   Working performances:

The changes of the tools on each TBM required two 6 day saturations and 25 excursions outside the habitat. The repair of the stone breaker could be completed in one saturation of 13 days and 12 excursions. No major problems where encountered during the works.

5.2   Statistics

Total number of transfer is 37, involving each time 3 divers. The total time spent under pressure on the occasion of this series of operations is 38 days for 6 divers. Only one accident happened : a teapot with boiling water splashed on one of the divers in the habitat, resulting in second grade burns, which could be treated under pressure, although the diver was out of diving and therefore decompressed A replacement diver was introduced into the saturation team for replacement.

5.3   Physiology

Decompression after excursions (111 men exposures) has been uneventful and no circulating bubbles where detected after returning into the habitat.

Final decompression of the first saturation had to be stopped twice to cope with symptoms of decompression illness, which were successfully treated by recompression and oxygen breathing. The analysis of those cases showed that the after effects of the deep excursions and several difficulties in controlling the environmental parameters in the various chambers might have been the major factors in the genesis of the symptoms. Therefore the table was slowed down slightly and an air shift was introduced during the course of the next final decompressions. These changes have solved the problem, no more decompression illness symptoms were ever noticed during the second and third final decompressions.


This was the first occasion to carry out TBM repairs under very high pressure. Saturation technology was used for the first time in a tunnelling operation. Provided technical means are available, training of the personnel is adequate, safe procedures well adapted to the situation are followed, saturation in a tunnel is much easier and safer than in the offshore industry. In tunnels, all serious safety problems associated with deep immersion, weather, moorings vessel positioning, diving bell recovery are not existing.

For future projects, this operation demonstrates that the use of mixed gases is possible and should be considered necessary for pressures above 3.5 bar(g). For heavy repairs or long duration works, decision to implement saturation technology will have to be evaluated in the same range of pressure. However works in bentonite will only be necessary in rare occasions when compressed air cannot be maintained in the working chamber. Therefore the employment of commercial of divers for those interventions should remain exceptional.

 Personnel saturated or breathing mixed gases under pressure in the dry need a special training. Very few national regulations provide legal standard for that training. In France this training requirement is included in the law applicable to caisson workers, which does not refer to any maximum pressure nor restricts the use of appropriate mixed gases [7].

There are many combinations of breathing gases which can be used to cope with the various situations of pressure. Procedures which have been devised for diving operations can be transposed for almost any pressure which may be met in tunnel operations and be used in the dry. Standard for diving gases can be used as references for tunnelling works. However, it is highly recommended that diving decompression tables should not be used as such for in-the-dry operations since many physiological factors associated with the corresponding working and decompressing conditions will change the outcome of decompression safety.





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