Five life-saving best practices preserve the integrity of tunnel liners and their permanent structures

Today's complex underground structures and tunnels must account for probable incidents affecting tunnel services, especially tunnel fires. Proper tunnel design considers the effects of design fire events and related temperature impacts on tunnel liners and other essential permanent structures. These structures must be designed to protect users and operators during a fire event. Design also should allow for repairs to be implemented in a timely manner so the facility can return to normal operation as soon as possible and economic impacts are minimized. In addition, design must be efficient and economical.

Following are five best practices that can provide proper fire engineering and preserve tunnel structural integrity during and after a fire event:

1. Using the proper approach to fire engineering. It can save lives and protect property.

Minimizing the impact of fire on tunnel permanent structures and systems, especially those supporting life safety, is a top priority for the engineering community nationally and internationally. Structural fire durability, including fire protection and/or hardening of tunnel structural and system components, has become an important fire engineering aspect in recent years.

This trend is expected to continue for several reasons:
• Advances in design of tunnel permanent liner is evolving rapidly and resulting in stronger, more durable concrete liner. However, such design affected negatively the liner behavior during high temperature fires. Severe impacts of recent tunnel fires accelerated this trend. Increasingly, owners are becoming aware of current industry standards of practice and taking active roles to make sure designs fully meet those standards. This practice protects users, operators and the general public by delivering a structurally and functionally safe tunnel facility.
• Aging infrastructure is being (or expected to become) expeditiously retrofitted to meet rapidly fading service requirements and/or to continue operating under demands for increased transportation capacity. Retrofitted structures are expected to meet the latest fire life safety guidelines.
• Federal funding initiatives require engineers to demonstrate, primarily through design criteria and security/safety checklists, that their designs incorporate state-of-the-art fire engineering practices.

In recent years, the resurgence in tunnel construction has given the industry an opportunity to greatly advance the design of tunnel fire life safety aspects. Engineers have succeeded in designing tunnel structures that work in conjunction with modern fire life safety systems to:
• Provide passengers with a safe, smoke-free egress in case of a fire event.
• Provide measures to aid firefighting activities and protect first responders.
• Protect the tunnel structure from explosive spalling of concrete during fire.
• Minimize material strength loss, ensuring that during and after a fire, the structure still withstands service loads.
• Ensure damage to the tunnel structure is repairable to minimize economic impacts.
2. Screening and designing the underground structures to satisfy National Fire Protection Association guidelines.

Classifying an underground structure as a tunnel is a critical first step that prompts a series of design decisions. Initial structure classification guides tunnel size, which accommodates the vehicle clearance envelope, facility system and maintenance provisions, including required equipment, and the emergency egress layout that works with proper egress scenarios.
Officials at Fort Lauderdale-Hollywood International Airport were well aware of this and willingly invested the time and resources necessary to determine their runway undercrossing project was a tunnel, which often is the case. A highway threading under an airport taxiway or a railroad box structure passing under an urban area both would classify as tunnels if they meet a certain length. Owners are increasingly becoming vigilant about these guidelines, so their transportation infrastructure is properly classified at the onset of the planning process and subsequently designed with the appropriate structural protection and fire life safety provisions.
3. Considering project-specific customized fire curves.

Another emerging best practice is to invest in sophisticated tunnel fire engineering. This includes structural fire durability analyses and the use of customized project-specific fire curves versus standard off-the-shelf fire curves.
Customized fire curves are becoming the industry norm due to an increased focus on designing a tunnel for an actual fire event, based on the type of vehicles and cargo loads it will carry.

Project-specific fire curves help engineers better estimate the design fire impacts on the facility’s structural and fire hazard mitigation systems. For example, the San Francisco MTA’s Central Subway developed a project-specific fire curve and customized system design, which led to efficient facility layouts.
There are two types of fire curves:
1. Fire heat release curves define the thermal energy release from the start of the fire event to its subsiding period. This thermal energy is transferred into the structure through conduction, convection and radiation.

2. Time-temperature curves define the temperature over the life of the fire event. These curves attempt to anticipate the hot air temperature at the concrete surface.
Using fixed fire suppression systems will alter the fire curves and improve the resulting heat and temperature output. The Alaskan Way SR99 Tunnel’s overhead fire protection sprinkler system was incorporated in the development of project-specific heat release and time-temperature curves to reduce infrastructure equipment size for both fire durability and fire hazard mitigation systems.

The Fort Lauderdale-Hollywood International Airport identified project-specific time-temperature curves taking into consideration the impact of overhead fire protection sprinkler systems. The revised curves allowed the owner to approve project-specific fire durability design requirements, which contributed to the efficient planning of the facility.
There are some fundamental differences, however, between the fire curve used for fire safety/hazard analysis and the fire time-temperature curve used for structural fire durability:

• For fire safety/hazard analyses, for example, HNTB uses fire heat release curves focusing on smoke flow direction, visibility and smoke propagation. This information can be used to determine tenable time and improve the fire hazard mitigation systems designs to save lives quickly through self and/or assisted rescue.

• For structural fire durability analyses, HNTB uses fire time-temperature curves. These analyses are focused on the damage the fire would cause to the tunnel structure. The analyses account for the adopted fire incident, including fire growth, peak heat release rate, fire decay, cooling and any modification to the fire characteristics due to active fire suppression systems and ventilation systems.

Of course, standard fire curves, such as the Rijkswaterstaat time-temperature curve or the International Organization for Standardization time-temperature curve, are still valid and many engineers rely on them when additional fire engineering is not available or planned. However, standard fire curves typically yield more conservative results and are implemented when a supplemental fireproofing layer is to be installed over a tunnel structure or liner.
Using standard fire curves, engineers risk under-designing or over-designing the tunnel structural system. A project-specific, performance-based approach uses a structural durability analyses to achieve economy and efficiency. The analyses considers the tunnel’s specific characteristics and its fire safety systems, including active (fire suppression systems, tunnel ventilation, etc.) and passive (fire proofing) fire safety systems for mitigating hazards generated by the design fire event.
HNTB’s projects that have used a performance-based approach include:
• Seattle’s SR99 Alaskan Way Viaduct and Seawall Replacement
• San Francisco Presidio Parkway tunnels
• Fort Lauderdale-Hollywood International Airport’s access tunnel
• Istanbul Strait Road Tube Crossing

These owners concluded that to meet NFPA 502 requirements on steel and concrete temperatures and to prevent explosive spalling, specific use of the tunnel safety systems (ventilation and fire suppression) is necessary to respond to a project-specific fire event.
4. Collaborating with the authority having jurisdiction (AHJ). It will pay off in the long term and save project schedule time.

Timely collaboration with the AHJ is essential in both cases, when using a customized project-specific fire curve or when using a specified off-the-shelf curve.
A critical component of a project management work plan is the approval of any adopted design fire scenario, which depends on the recommendations of the local authority having jurisdiction, such as fire department. Proactive, fruitful collaboration early in the project development phase can secure timely buy-ins, save project schedule and reduce design costs.
The Washington State Department of Transportation began a dialogue with the local deputy fire marshal during the environmental impact statement phase of the SR99 Tunnel. The fire marshal developed a memorandum of understanding, which communicated his expectations for the fire load. From there, final design of the ventilation system progressed smoothly. Collaboration enabled the tunnel’s design to incorporate synergistic impacts of fire suppression and ventilation systems, providing cost-effective sizing of fire protection and hazard mitigation systems.
Similarly, for the San Francisco Central Subway, collaboration and coordination with the fire marshal were essential to a successful design.
5. Incorporating advances in structural hardening and protection.
Adding polypropylene (PP) microfibers to a concrete mix for final tunnel structure or liner construction is a well-adopted industry design strategy for reducing spalling. Both the NFPA 502 standard in road tunnels and an NFPA 130 standard for fixed guideway transit acknowledge the engineering analysis process is required for this approach to be implemented.
As the PP microfibers’ synthetic resin begins to melt during a fire, it creates channels or voids into which the steam from trapped moisture in the concrete can expand or escape without increasing the concrete’s internal pressure, which reduces or eliminates spalling. According to tests, fiber-modified concrete exhibited less or no spalling.
London’s Channel Tunnel Rail Link project was the world’s first major tunnel project to incorporate PP microfibers. Many other major tunneling projects followed suit, including the Tom Lantos Tunnels in California, the Alaskan Way Tunnel in Seattle, the Presidio Parkway in San Francisco and the Istanbul Strait Road Tube Crossing, among others.
Other measures, such as a sacrificial layer of concrete cover, also are used.
Another newer material is super concrete. Still in the laboratory stage, fire-resistant super concrete has been shown to have a compressive strength of about 2 to 3 times that of high-strength concrete and a tensile ductility of more than 300 times that of high-strength concrete, according to the University of Michigan.

High-strength, high-ductility super concrete makes for more resilient construction and extreme loading than the concrete widely used today, according to the university.

Furthermore, recent research at the TU Braunschweig, Germany, showed precast segmental liner with a sophisticated concrete mix, an optimized selection of aggregates consisting of basaltic gravel and quartzite, and the addition of 2 to 3 kg/m3 polypropylene fiber, can withstand significant fire loads with temperatures up to 1,200 degrees Celsius for 90 minutes.
Other fire-protection materials, such as sprayable cementitious fireproofing layers, manufactured boards or intumescent coatings, are under active development and use and incorporate the latest technology for effective heat insulation.
By developing project-specific fire curves, assessing each fire event for its specific features, assessing the structural durability of the tunnel liner for a project specific fire, and incorporating proper fire suppression methods and operational aspects, engineers are well equipped to design safer and more cost-efficient tunnels.