TBM Hyperbaric Intervention: To Intervene or Not To Intervene

The application of hyperbaric intervention starts with the design phase of the TBM.

The application of hyperbaric intervention starts with the design phase of the TBM.

As TBMs are increasingly used in complicated geologies in urban areas, the use of face support technologies has also been increasing. Face support pressure is necessary to counteract pressures from groundwater levels, the substrate and the length of tunnel sections, and to prevent water ingress into the TBM and tunnel.
The use of face support pressure complicates the task of changing tooling and equipment at the face, oftentimes requiring the use of hyperbaric interventions. Hyperbaric intervention can use compressed air, mixed gases or saturation diving techniques, depending on pressure level and duration of expected intervention time. The type of TBM will determine which support medium is used to build the required face support pressure: compressed air, grout or bentonite slurry. Appropriate face support pressure allows for safe hyperbaric interventions to perform maintenance on the cutterhead and its tools.

Compressed air pressure within TBM cutterheads has been successfully used as a groundwater control technique. Resultant exposure of compressed air workers to the occupational health risk of decompression sickness and dysbaric osteonecrosis is well documented (Kindwall, 1997; Anderson, 2002). Kindwall (1997) further cautions that compressed air decompression schedules, including the OSHA tables, are inadequate and advocates oxygen decompression as the only viable method for safely decompressing tunnel workers. His experience showed that USN Treatment Tables 5 and 6 adequately treated decompression sickness symptoms, notwithstanding extremely long exposure times.

TBM Hyperbarics

Face support pressure
is necessary to counteract pressures from groundwater levels, the substrate and the length of tunnel sections, and to prevent water ingress into the TBM and tunnel.

However, the overall trend is that tunneling hyperbaric work continues to be relatively safe. For example, Andersen (2002) reported from Denmark’s Great Belt Tunnel work 13 cases of decompression sickness (DCS) in a population of 320 compressed air workers subjected to 9,018 pressure exposures ranging from 1.25-3.91 atm abs for working times between 2-339 minutes for an overall DCS incidence rate of 0.14 percent. How et al (1990) described clinical presentations of 164 cases of DCS of 1,737 compressed air workers who underwent 188,538 decompressions for an overall incidence of 0.087 percent during the Singapore Mass Rapid Transit Project. This low incidence was attributed to the adherence to appropriated decompression procedures, acclimatization of workers to pressures and strict medical selection criteria.

Prompt hyperbaric treatment of DCS symptoms further emphasizes the need for hyperbaric medical expertise and supervision and on-site treatment chambers. From the collective U.S. Navy and commercial diving experience, mixed gas saturation diving has clearly been shown to be the most efficient hyperbaric intervention methodology from a decompression safety and work efficiency standpoint when compared to bounce diving (Lang, 2013). Le Péchon and Gourdon (2010) stated that saturation in tunneling has been used on one work site only, i.e., the Western Scheldt Twin Tunnels construction in the Netherlands in 2001-02. Since then the Lake Mead Tunnel and Nanjing Yangtze Tunnel Crossing are two additional projects involving saturation diving with more anticipated given the trend toward deeper and longer tunnels.
The application of hyperbaric intervention starts with the design phase of the TBM. Business sense dictates this approach to save time and complications should hyperbaric intervention be needed during the course of excavations. Reliable application of a pressurized environment during excavation will allow compressed air workers to perform the maintenance, inspection and TBM cutterhead tool exchange. Manlock, mating flange integration, transfer-under-pressure shuttle and decompression chamber become integral components along with the hyperbaric medical expertise in decompression procedures and treatment of DCS incidents mentioned above.

TBM Hyperbaric Intervention

The use of hyperbaric interventions is often required to change tooling and equipment
at the face.

To not have to intervene means careful consideration of geological substrate assessment data, hydrogeology, the use of additives applied to excavated material, and TBM excavation alignment along the pre-established line and grade. Hyperbaric intervention avoidance techniques include water pumping and groundwater control techniques that alleviate the pressure exerted by surrounding water and prevent groundwater intrusion. Grouting techniques in highly permeable rock that use cementitious and bentonite grouts and a groundwater monitoring program were described by Terezopoulos and Singh (1987).

Timely response by a hyperbaric intervention partner must figure prominently in the decision to intervene when other means of avoidance are exhausted. Different cultural outlooks exist in North America, South America, Europe, Asia Pacific and the Middle East when it comes to preventive maintenance of expensive, complicated equipment of any kind. Suffice it to suggest here that the North American approach is to foster the preventive maintenance approach vs. running machines until they break with the ensuing costly time and mechanical complications.

It is difficult to argue against continuous shallow air saturation as the intervention method of choice given its effectiveness and safety vs. multiple “bounce” interventions requiring as many post-exposure decompressions of compressed gas workers. In the scientific diving universe the added value and efficiency of saturation vs. bounce diving was validated during the TEKTITE saturation habitat program (Collette and Earle, 1972). In the end, while the costly decision to intervene is contemplated, pre-planning of the hardware contingencies and a hyperbaric intervention partner saves project time and money. And if all else fails, whom are you going to call to keep that TBM functioning and moving forward?

Dr. Michael A. Lang  is Senior Vice President of the OxyHeal Health Group and Adjunct Faculty at the UC San Diego Department of Emergency Medicine. He can be reached at mlang@oxyheal.com.

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