The development of closed-face, earth pressure balance (EPB) and slurry shields has largely negated the need to pressurise the whole workings. These machines have allowed increasingly larger tunnels to be constructed in ever-more difficult ground conditions and at significantly higher pressures.

However, these machines require picks and cutters to be changed at frequent intervals. To facilitate this task, it is generally necessary to make manned interventions into the plenum chamber under compressed air. Many of the issues discussed in the previous article remain relevant to such interventions. In this second article, a number of projects that illustrate the problems and solutions to making such interventions will be discussed.

GENERAL

At pressures up to about 3.5bars, such interventions are normally possible in compressed air in single stages, although allowable durations reduce – and decompression times increase – significantly with increasing compressed air pressures. Specially trained divers and operatives can work at pressures up to about 5bars, but above this pressure, 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. These can either be Heliox (helium and oxygen) or Trimix (helium, nitrogen and oxygen). The proper composition of these gases depends upon the type of operation and the exposure pressure.

As shown in Figure 1, the use of these gases can extend single-stage working interventions up to pressures of about 8bars, however working periods remain short. Exceptionally, single-stage interventions up to about 10bars are possible. More practically, at pressures above say, 4.5 bars, it is necessary to resort to saturation diving techniques where the gas levels in the workers’ blood become fully saturated due to the length of the exposure. When mixed gases and saturation diving techniques are used in the tunnelling industry for TBM cutterhead interventions, the operatives work inside the TBM excavation chamber under compressed air but breathe mixed gases via an umbilical hose. In saturation dives, after the operatives have worked in the chamber for a few hours, they are transported back to a pressurised ‘living habitat’ on the surface via a transfer under pressure (TUP) shuttle. These saturated workers can stay at work continuously in a hyperbaric environment for up to a month before they need to decompress back to atmospheric pressure.

Saturation diving techniques are common in the diving industry where maintenance of off-shore oil and gas drilling rigs and equipment is required. The maximum depth reached in demonstrations is 534m (53.4bars), and in actual commercial operations the maximum depth reached is 350m (35bars). In the tunnelling industry, intervention pressures remain considerably below these limits.

STOREBÆLT PROJECT, DENMARK

The Storebælt project generally operated on the then current French compressed air tables. Practical periods for interventions and decompression limited maintenance working interventions to a maximum pressure of 3bars. However, short interventions for inspection purposes could be made at pressures up to 5bars. These inspections were made by specially-trained operatives accompanied by an experienced diver.

In the author’s experience of the Storebælt Project, it became necessary to maintain picks and cutters on TBM ‘Dania’ in a face of glacial sand tills with up to 5bars of hydrostatic pressure. To complicate matters, when excavating through sand, the EPB screw conveyor system was unable to hold the hydrostatic pressure and as a consequence a piping connection had developed through over 30m of overlying till, establishing a direct flow-path connection with the seabed. As a result, significant ground losses occurred, as shown in Figure 2, with the resultant depressions in the sea bed measured at up to 2,200m3.

In order to limit these ground losses and minimise wear on the cutter head, a programme of deep wells was instigated. These were installed from drilling barges in the Storebælt into the marl formation underlying the tills. The programme was christened Project Moses (Method of Safety by Emptying Storebælt). The wells – designed to lower pressure in the sand tills by under-drainage from the marl unit – provided some local relief but were largely ineffective if a piping connection had been established from the cutterhead to the seabed. Recharge from the seabed greatly exceeded the effect of the under-drainage.

When operating in the sand tills, TBM Dania developed such a piping connection to the seabed, and this had followed the machine for some time. Inspection interventions into the cutterhead indicated that all discs and cutters showed considerable wear and further operation risked considerable damage to the cutterhead. The solution developed is shown in Figure 3

Alternate shield rams were removed and freeze pipes were installed through the shield skin to intersect and cut off the piping connection to the seabed. Four deep wells were installed from the surface by drilling barge. Forming part of Project MOSES, these wells were drilled some 20m into the underlying chalk marl and were screened through the sand body – effectively creating a low-pressure atmospheric sink within the sand body. Flow net modelling indicated that with this configuration of wells, stable face conditions i.e. a net outflow of air, could be established by the application of compressed air as low as 1.7bars within the plenum. Complete refurbishment of the cutterhead was achieved at this pressure.

The above method serves to illustrate the effectiveness of combined compressed air and well-point systems. However, it should be noted that establishing the freeze hood and installing the deep wells took some six months.

WESTERSCHELDE TUNNEL, NETHERLANDS

Completed in March 2003, the 6.6km-long WesterscheldeTunnel in the Netherlands carries highway N62 under the estuary of the Western Scheldt River. It consists of two tubes, each of 11.3m diameter, and each having two car lanes with no hard shoulder.

At the lowest point of the alignment, 65m below sea level, both TBMs required tool changes, and one required heavy repairs to the stone breaker. Knowing that such operations would require both mixed gases and long working times, a full saturation habitat was installed on the site, two transfer shuttles were built and a hyperbaric transfer train was set up. The Westerschelde Tunnel was the first tunnel where saturation diving techniques were employed for pick and cutter changes. This aspect of the work is described in detail by Le Péchon et.al. (2008)

Due to the sandy nature of the ground and the thin cover (less than two tunnel diameters), it was considered dangerous to evacuate the bentonite from the cutter head chamber with compressed air. It was therefore decided that manual intervention had to be carried out by divers in immersion in the bentonite. This resulted in working with no visibility or light at a pressure equivalent to a 69m head (6.9bars).

Interventions were carried out using a Trimix of gases (helium, nitrogen and oxygen). The composition of gases was varied to suit the different situations encountered in the working chamber, transfer shuttle and habitat. Figure 5 shows how the relative percentage of each gas was varied throughout the intervention cycle. The high-pressure deep mix in the working chamber provided the same partial pressure of nitrogen as that used in the habitat, but with increased pressure of helium and oxygen.

Interventions were performed by a team of three divers working for a maximum period of four hours in the working chamber. The change of tools on each TBM required two six-day saturations and 25 excursions outside the habitat. Repairing the stonebreaker was completed in one saturation of 13 days and required 12 excursions. A total of 111 decompressions after excursions were carried out. No incidence of decompression sickness and no circulating bubbles were detected after returning to the habitat.

In conclusion of this work, Le Péchon noted:

? Personnel saturated or breathing mixed gases under pressure need special training;

? Work in bentonite should only be necessary on rare occasions when compressed air cannot be maintained in the working chamber; and

? Standards and procedures that have been devised for diving operations can be used as a reference for tunnelling works, however it is recommended that diving decompression tables should not be used directly for ‘in-the-dry’ operations. Many physiological factors associated with the corresponding working and decompression conditions will change the outcome of decompression safety.

TUEN MUN-CHEK LAP KOK LINK SUB-SEA TUNNEL, HONG KONG

A more recent experience in the use of saturation diving techniques is the Tun Mun-Chek Lap Kok Link Sub Sea Tunnel project completed in 2020. The project comprises 9km long, dual two-lane road tunnels between Tuen Mun in the New Territories and North Lantau adjacent to Hong Kong International Airport.

Construction of the tunnels was carried out by two 14m-diameter TBMs. The tunnel alignment is up to 55m below sea level with the deepest seabed level at -21mPD. In order to maintain cutting face stability during each intervention, the required compressed air pressure could be up to 6bars. Saturation diving techniques were used over the deepest part of the alignment, but conventional compressed air and single-stage Trimix ‘bounce’ interventions were also carried out where lower pressures permitted. Figure 6 shows the different modes of intervention used throughout the alignment.

Conventional compressed air working: This was permitted at pressures up to 4.2bars. Decompression of hyperbaric workers is required after a period of working time which varies according to the pressure. At 4.2bars, the allowable working time was up to a maximum of 100 minutes followed by a 147-minute period of decompression.

Single-stage Trimix Bounce Mode: This was used at pressures from 3.45bars to 6bars. The Trimix gas comprised a mixture of oxygen, nitrogen and helium in the ratio 20/47/33 (O2/N2/He). The allowable working time and decompression period varies with the pressure; as an example, at a pressure of 5.5bars, the allowable working time was up to a maximum of 90 minutes, followed by a decompression time of 305 minutes.

Under saturation mode: A team of four hyperbaric workers were employed in each intervention. The workers were permitted to stay and live under a Trimix gas environment for periods of up to 28 days. During this time, they were allowed to carry out TBM cutterhead interventions for a maximum period of six hours for the first 25 days, with the last three days of the cycle for decompression. After completing the 28-day cycle, the workers were rested for a further period of 28 days before commencing another saturation cycle. In addition, saturation works were limited to a total of 180 days within a year. Chan (2021) reports that a total of 18 saturation cycles were conducted with more than 1,200 disc cutters changed for both TBMs. There were zero cases of decompression illness reported.

The use of saturation diving techniques for TBM cutterhead interventions had not previously been used in Hong Kong, hence considerable effort was required to gain statutory approval from the various statutory authorities. However, there was considerable experience in Hong Kong for compressed-air working gained from the construction of the MTR system. However, the CAP59M Factories and Industrial Undertakings (Work in Compressed Air) required that no person shall be employed in compressed air at a pressure exceeding 3.45bars without permission from the Commissioner. In order to gain exemption from these regulations, the contractor was required to submit a detailed proposal – based upon renowned international guidelines – for the authority’s review and consideration.

The considerable establishment in both staff and equipment required to support saturation diving techniques should also be appreciated. The saturation diving team comprised over 50 staff members, including 30 saturation workers, six hyperbaric doctors, three nurses, three hyperbaric operation supervisors and 12 life-support supervisors and technicians. In addition, a local trauma surgeon and backup trauma surgeon were also appointed by the contractor.

The living habitat was constructed onsite near the tunnel portal. It comprised a compound housing the hyperbaric chambers including a living chamber where the saturation workers lived under pressure in a trimix gas environment; a medical chamber where medical treatment or surgical operations could be conducted under pressure; a wet pod equipped with shower and toilet facilities for use under pressure by the saturation workers; a decompression chamber where saturation workers could stay for the three-day decompression; and two TUP shuttles which were used to transport the saturation workers under pressure to the TBM for cutterhead interventions.

The experience gained in this project has been incorporated into the updated version of the International Tunnelling Association’s ITA Working Group No. 5, version 3, issued in March 2018.

SYDNEY METRO CITY & SOUTHWEST, AUSTRALIA

This project comprises a new 30km metro line linking with Metro Northwest at Chatswood, continuing under Sydney Harbour, through the CBD, and south west to Bankstown.

Site investigation for the Sydney Harbour crossing indicated that, on the preferred alignment, the tunnelling machines will generally be excavating within the Hawkesbury Sandstone Formation, but will encounter an approximately 150m section of soft ground at mid-channel. Hydrostatic pressure at this point was approximately 4.5bars to the invert of the tunnel. (See Figure 7).

Construction of metro tunnels through the Hawksbury Sandstone is normally with open-face double-shield hard rock boring machines. However, the use of this type of equipment for the Sydney Harbour crossing was precluded, as any deep-level alignment that remained within the Hawksbury and passed below the soft ground would have required track gradients that exceeded the traction capability of the rolling stock. As a further consideration, the energy requirements for operating the rolling stock at, or close to the limiting gradients would have added considerable operational costs over the lifetime of the project.

It was therefore desirable to establish the shallowest alignment that could safely be excavated through the soft ground section without the risk of a blow-out, in the event that an intervention should became necessary in this area. Stability calculations as described in Part 1 of this article were performed in this regard.

CONCLUSIONS

The history of compressed air working for tunnel construction was described in Part 1 of this article (Tunnels & Tunnelling, March 2022). In modern practice, the need to pressurise the whole of the workings is limited to short sections of alignment where the use of modern closed-face tunnel boring machines is uneconomic. Yet these machines have allowed increasingly larger tunnels to be constructed in evermore difficult ground conditions and at significantly higher pressures.

The need to maintain these machines and make pick and cutter changes at such high pressures is a developing challenge for which, as yet, there is limited experience and regulation. The author is aware of at least one project in the US where the possibility of intervention pressures up to 17bars is being studied. The viability of these projects rely not only upon the medical aspects and physiological factors of interventions at such high pressures, but also upon the basic engineering considerations of limiting air losses and controlling compressed air blow-outs.