In search of sustainable fire safety

With estimates predicting the expansion of the tunnel boring machine (TBM) market from US$1.59bn in 2019 to US$2.37bn by 20271, the future is looking bright for the tunnel construction sector. As population growth and increasing urbanisation continue to fuel government investment in road and rail infrastructure, boring companies are facing ever-increasing demand, and with this upsurge in activity inevitably comes a heightened focus on fire prevention and safety.

Tunnel construction sites are almost universally high-pressure environments where extreme friction from TBMs and super-heated engine parts operate in close proximity to flammable hydraulic fluid and vehicle fuel, often deep underground and miles from emergency service support. For all these reasons, a fast-acting and effective vehicle fire suppression system is essential – not only to ensure the safety of construction crews, but to protect expensive and highly specialised equipment from costly fire damage.

The main factors shaping boring contractors’ decisions surrounding fire suppression solutions have typically been cost and crucially, performance – with engineers demanding systems that are optimised for use in high heat, high humidity underground environments, and can withstand the extreme vibrations generated by TBMs and other equipment. But as awareness grows of the impact suppression agents containing per- and polyfluoroalkyl substances (PFAS) can have on soil and water sources when discharged underground, companies must increasingly consider emerging environmental and regulatory requirements.

TALKING TUNNEL RISK FACTORS

Much of the current literature regarding tunnel fire safety focuses on the protection and evacuation of finished, fully operating road and rail tunnels. The impact of fire in these scenarios can be massive, as in the case of the 2008 Channel Tunnel fire, which burned for a total of 16 hours reaching temperatures of 1,000°C before a team of more than 300 firefighters could extinguish the blaze². The fire left 32 passengers trapped in the tunnel for hours, 14 of whom needed to be treated for minor injuries, while an almost 700m section of the North Tunnel was permanently damaged, causing it to be closed for almost six months for structural repairs³. Eye-witness reports from this incident demonstrate the uniquely dangerous progression of tunnel fires: thick smoke and intense heat from the initial fire source quickly fill the tunnel from the roof downwards, limiting visibility and clean airflow while increasing the chances of injury or death from heat radiation⁴. (Figure 1)

Depending on the size of the tunnel and the location of the fire, the nearest exit routes could potentially be located miles away, making both evacuation and firefighting activities much more challenging⁵. These factors mean the threat of fire is already incredibly serious in finished tunnel structures, but during the construction phase, these risks are magnified and accompanied by a host of additional safety concerns.

Prior to breakthrough, multiple boring faces can be under construction independent from one another, while stemming back to the original access tunnel, which acts as a main throughfare for deliveries, excavated rock and machinery⁶. This busy, often crowded route can be the only escape path for both people and smoke in the early stages of construction, increasing the likelihood of workers becoming trapped if a fire breaks out in this vital access tunnel.

Tunnel construction sites are also noisy and dark in the early stages of construction, limiting visibility for workers and meaning they are less likely to hear fire alarms7. This issue of communication is further compounded by the fact that sites of this nature, especially large infrastructure or government-funded projects, often involve a diverse range of companies, teams and contractors of multiple nationalities⁸. To complete the picture, many tunnel sites are situated in remote, hard-to-reach areas such as mountain ranges or countryside, potentially delaying arrival times and access for emergency services.

FOCUS ON VEHICLE FIRES

In this already hazardous environment, high-power, constantly running heavy machinery poses yet another challenge to fire safety. TBMs, scalers, sealers and even conventional transport trucks all present significant fire hazards due to their large engines, multiple moving components and their proximity to flammable hydraulic fluid. In order to keep the fire load (the potential severity of a fire taking into account the materials within a given space⁹) to a minimum, the use of gasoline, ethanol or other highly combustible substances is prohibited for TBMs or any other tunnel construction equipment, meaning most motor vehicles are either electric or run on less combustible diesel fuel.¹⁰

Despite these precautions however, vehicle fires may still occur, with electrical faults caused by damaged cable insulation, the atomisation of hydraulic fluid and oil leaks being the main causes of tunnel fires^11,12. In addition to the inherent challenges presented by TBM equipment, the tunnel environment itself can exacerbate fire hazards by rendering traditional suppression systems unsuitable. With varying degrees of humidity and vibration, many systems that are designed for use in buildings or industrial applications may fail in a tunnelling environment or operate according to a shortened lifecycle.

To combat the immense risks posed when a TBM or other vehicle catches alight, various regulations and technologies have been implemented to prevent the outbreak of fire and suppress it quickly before it can cause a major incident. These include light and sound alarms, rescue chambers or safe havens, TBM man-locks, additional ventilation channels and sprinkler systems, all complemented by rigorous evacuation drills and onsite training^13,14. But with TBM fires in particular posing a major threat to both people and projects, installing an effective vehicle suppression system is another essential consideration for tunnel contractors.

SPECIALISED VEHICLE SUPPRESSION SYSTEMS

Vehicle fire suppression systems are designed and tested for use in the unique conditions within a tunnel, taking into account vibration, moisture and electromagnetic wavelengths, as well as the amount of combustible hydraulic fluids present on a site. Unlike many conventional suppression systems, they often include a wetting or liquid agent to help cool the equipment below the flash point (Figure 2) to prevent re-ignition.

Though they are most often used in large off-road vehicles, such systems have been successfully incorporated into TBMs and supporting tunnel equipment for decades. Like many portable fire extinguishers and industrial fire suppression systems, vehicle-mounted fire-safety solutions discharge agents into the engine bay or other high-heat areas to smother flames and cool surrounding components, often leading to foams or wet agents spilling out into the construction site. It is this lack of containment, along with a growing understanding of the environmentally harmful nature of some of the substances that are commonly used in suppression agents, that has led regulatory bodies around the world to highlight another potential risk area that has implications for health, safety and sustainability.

THE CHANGING PERCEPTION OF PFAS

Per- and polyfluoroalkyl substances (PFAS) have been staples in the fire suppression industry for decades, with around 20,000t of these chemicals sold annually in the EU alone¹⁵. Commonly used in Class B firefighting foams and agent concentrates thanks to their hydrophobic (water repellent) and lipophobic (fat/oil repellent) properties which effectively starve flames of oxygen, PFAS are also valued for their extreme stability when exposed to high heat.¹⁶

Despite their long-established effectiveness and widespread usage, it is this stability that – in recent years – has led to growing concern surrounding the impact PFAS can have on the planet. Due to their non-biodegradability and persistence, PFAS have been shown to have low Predicted No Effect Concentration (PNEC) scores, meaning that just a small amount of the chemicals being present in soil, ground or surface water can significantly damage nearby plant and animal populations¹⁷. In light of these concerns, national authorities and regulatory bodies are starting to consider harsher restrictions on the use of PFAS, particularly in fire suppression applications.

REACTING TO TOUGHER REGULATIONS

Since 2009, steps have been taken to restrict the use of PFAS to vital firefighting applications¹⁸. In 2018 however, regulations were revised to significantly limit both the production and use of PFAS to only fire-fighting foams or liquid fuel suppression systems designed to suppress Class B fires¹⁹. In addition, the companies using PFAS had to show that they intended to use the agents only in scenarios where they could be disposed of in an environmentally responsible manner, and that they intended to limit their use of PFAS to fully contained applications by no later than 2025²⁰. A variety of international governments subsequently implemented controls on the sale of PFAS following this agreement, including the European Union (EU), the US Environmental Protection Agency (EPA) and the Swedish Chemical Agency (KEMI)²¹. The Australian government too, has taken a particularly harsh stance on the use of perand poly-fluoroalkyl substances through its recent Chemicals Act 2019 (IC Act), which requires importers and manufacturers of PFAS to register their company with the Australian Industrial Chemicals Introduction Scheme (AICIS) and submit any new PFAS chemical for toxicity review before it can be legally used²².

Restrictions on the sale and use of PFAS are unlikely to end here, with a report commissioned by the European Chemicals Agency last year recommending either a complete ban on the use of PFAS-based fire fighting foams, or at least a gradual phasing out of the chemicals over a period of three to six years for any applications other than petrochemical fires²³. A seismic industry shift is clearly on the horizon for all industries, but with potentially increased significance for the tunnelling sector which not only requires high-performance, reliable suppression agents, but often necessitates these to be discharged in undeveloped, natural locations, passing through or near to the water table. With this in mind, non-fluorinated alternatives to per- and polyfluoroalkyl substances present an attractive and necessary solution for tunnel contractors looking to safeguard their operations. However, as with all equipment commissioning, close attention must be paid to the advantages and drawbacks of these agents before deciding which is the right option.

SEARCHING FOR SAFER SOLUTIONS

Most fluorine-free fire suppression agents are based on hydrocarbon surfactants – detergents (a group of hydrocarbons that are both hydrophilic and hydrophobic) and more rarely a siloxane or protein²⁴. These chemicals have similar hydrophobic and lipophobic properties to the fluorinated surfactants found in PFAS agents, while, in most cases, being far less persistent and therefore less harmful to the planet²⁵.

In terms of disadvantages, the main issues cited with PFAS-free foams are cost, availability and effectiveness in certain applications. According to an industry analysis conducted for the European Chemicals Agency, the upfront cost of replacing PFAS-foams with non-fluorinated alternatives is between 0-30%, however this initial investment must be set against a potential saving of tens of millions of euros per incident in environmental clean-up operations.²⁶

With non-PFAS agents currently making up only 32% of the market, there are also concerns surrounding availability, though this percentage is on the rise as more stringent regulations encourage the transition away from fluorinated substances²⁷.

Finally, the most pressing concern some in the construction or industrial production sectors have raised is the effectiveness of PFAS-free foams in large fires involving petrochemicals. To extinguish a petrochemical fire (such as one in a large fuel tank) as safely and quickly as possible, the suppression agent must be able to flow along the burning liquid and form a seal with the hot metal container to smother the flame and prevent re-flash²⁸. In this specific use, there are no recorded companies who have made the switch to 100% PFASfree foams, as the risk margin is slightly lower when using traditional suppression agents²⁹. There is robust evidence to suggest however that non-fluorinated foams are equally effective as PFAS agents in all other suppression applications, and crucially, have a far lower net impact on the environment than their longer-established counterparts, making them a safer and significantly more sustainable choice.³⁰

NEXT GENERATION FIRE SUPPRESSION

So, how can tunnelling contractors choose a fire suppression solution that is both safe to discharge into the underground environment and can be relied upon every day to ensure that a spark on a TBM or scaler does not become an uncontrollable blaze? A useful pneumonic for this scenario is PERFECT:

? P – Prevents reflash: in large engine fires it is essential that the suppression agent can cool the area to below the flash point (450°C) in order to reduce the risk of re-ignition.

? E – Extinguishes quickly: a tunnel can fill with smoke in a matter of seconds, so a swift response is vital for ensuring safety and preventing escalation.

? R – Regulation compliant: look for an agent that features the full range of approvals needed for a specific project, including Factory Mutual (FM) and CE mark, as well as regional requirements like Australian Standard (AS) 5062-2016.

? F – Fluorine-free: Ensure your solution protects the environment and is future-proofed against shifting PFAS regulations.

? E – Ease of use: the best suppression systems are as simple as they are effective. With language barriers, overlapping shifts and contract workers, it is vital that a solution is quick and easy to implement.

? C – Compatible with older vehicle models: linked with the above, backwards compatibility can help ensure all vehicles are protected, without the cost of extensive upgrades.

? T – Tried and tested: make sure to choose an experienced manufacturer capable of performing the rigorous quality assurance testing needed to create a solution that tunnel contractors can trust.

An example of a suppression solution that meets all these requirements is Johnson Controls’ ANSUL LVS Non-Fluorinated Liquid Suppression Agent, a new fluorine-free solution specifically designed to protect heavy motor vehicles. Offering rapid flame knock-down, followed by blanketing to cut off oxygen and help prevent reflash, this suppression agent combines the performance and ease of use of a traditional PFAS-based solution, with the forward-looking environmental responsibility that government regulations are increasingly demanding.

A SAFER, GREENER FUTURE FOR FIRE SUPPRESSION

With new regulations and innovations on the horizon, the future of tunnel fire suppression will be one in which contractors and boring companies no longer need to choose between safety and sustainability. The emergence of high-performance non-fluorinated suppression systems means that tunnelling contractors can now achieve fast, decisive and above all reliable fire suppression – thereby protecting people, profitability, and the planet.