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Bridges & Structures


Tunnel Research Projects

Integrated Design of Fixed Firefighting Systems and Emergency Ventilation Systems

In many industry sectors, water-based fixed firefighting systems (FFFS) and emergency ventilation systems (EVS) are routinely implemented as part of a facility's solution for fire protection and life safety (FPLS) to include preserving human life, preventing substantial injury, and protecting structures against serious damage. EVS provides fresh air to tunnel users during the evacuation phase; and it exhausts smoke and super-heated gasses from enclosed spaces to help ensure a viable period of tenability for both evacuees and rescuers; however, EVS also fans the flames with oxygen rich air. FFFS suppresses fire growth while substantially cooling the fire site to protect structures from damage due to excess heat. However, previous misconceptions within the highway tunnel community about the effectiveness of FFFS have limited the full utilization of FFFS and resulted in knowledge gaps that must be overcome. At best, independent FFFS-EVS designs have been implemented sporadically in tunnels, but more commonly, EVS has been deployed as a standalone system without FFFS. To overcome the knowledge gaps within the highway tunnel community, the coupled behavior of FFFS-EVS within a highway tunnel environment must be better understood using established methods such as computer simulation, laboratory experiments, and full-scale testing.

There are four main classes of fires including Class A - fires involving normal solid combustibles like wood, paper, trash or similar to things one might find in a building and these items are brought into tunnels as cargo on HGV; Class B - fires involving flammable or combustible liquids like oil, gasoline, and diesel fuel, or things one typically doesn't find in a building, but perhaps these items are brought into tunnels as fuel for trucks, busses, and automobiles and in rarer instances as cargo onboard tanker trucks; Class C - fires involving energized electrical fires like hot wiring; and Class D - fires involving combustible metals like when batteries catch fire in laptops in buildings or even cars in tunnels. This research is concerned with Class A and B fires in highway tunnels. Water works best to extinguish Class A fires, but with foam additives, FFFS may be able to effectively prevent Class B fires from growing dangerously large in the tunnel enclosure.

When fully integrated, the benefits of FFFS-EVS are anticipated to be significant in terms of public safety, tenability, structural protection, asset resiliency, facility robustness, infrastructure sustainability, disaster recovery, road network reliability, and cost savings. The magnitude, scale, and cost of FPLS equipment in tunnels can be significant; therefore, a cost efficient but risk equivalent design needs to be explored for reasons of safety, resiliency, and sustainability. It is noted that EVS alone is not effective in protecting life and structure against fires from heavy goods freight vehicles and tanker trucks. FFFS is the only technology that can substantially suppress the fire and cool the structure.

The reliability of FFFS in the harsh tunnel environments is also an unknown; infrequent mishaps whether real or perceived may negatively impact the deployment of FFFS in tunnels. Over-height vehicles can strike FPLS equipment, which are overhanging the roadway rendering it inoperable. Reliability also touches upon maintenance and inspection issues needed to keep the FPLS systems in good running condition and available for use at a moment's notice. Information that is uncovered as part of this work can be used to supplement the TOMIE Manual and develop a reliability index design parameter, or uncertainty factor, for FFFS.

Important stakeholders for this project include FPLS design engineers, tunnel owners and operators, state fire marshals, emergency responders, and technical committee members of relevant professional organizations, as well as federal, state, and local transportation officials. Professional organizations generally represent an important cross-section of industry stakeholders. Historically, the National Fire Protection Association (NFPA 502), the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE TC5.9), the National Cooperative Highway Research Program (NCHRP), and the Federal Highway Administration (FHWA) have made significant research contributions in the fields of FFFS-EVS.

Since a fully-integrated FFFS-EVS design is an emerging technology in highway tunnels, the FHWA has been working with a wide cross-section of industry stakeholders throughout the tunnel community to obtain their perspectives. One of the major reasons for highlighting the importance of industry acceptance is for strengthening design, implementation, and deployment techniques while also reducing the potential for failures, disputes, and costly litigation. Additionally, a basis of design is needed because, in the event of a tragedy, intense public scrutiny and considerable government authority may sometimes be leveraged against the responsible stakeholders.

Precast Concrete Segmental Liners for Large-Diameter Highway Tunnels

Highway tunnels are some of the largest tunnels built using modern mechanized tunnel boring machines (TBMs); as such, the science and technology tends to lag slightly behind the smaller diameter applications such as those in rail, water, and sewer. Since the tunnel industry is trending towards the increased use of mechanized tunneling methods, large-diameter highway tunnels are more often being constructed with these methods; and with the current trend of one pass lining, the final liner is assembled in the aft of the machine under its shield hull to form circular rings of tunnel without the use of either temporary support or junk segments. Since this type of bore is slightly over dug for the machine's hull, the segmental rings must be grouted into place in contact with the in-situ ground; the grouting process provides the necessary strength to thrust the TBM forward and to steer the machine along its intended alignment. Filling in hid annulus or gap between the liner and the ground also helps minimize settlement and ground movement above and around the tunnel; the grouting also produces more uniform loading around the liner, which reduces undesirable stress concentrations and peak loads. While strength and durability are routine considerations for all tunnel liners; highway tunnels must also be robust against damage caused by fires, collisions, blasts, and earthquakes. Since many tunnels are critical elements of the national highway systems, they must provide reliable levels of service for extended durations and be robust to allow future expansions and improvements.

Precast concrete segments are loaded throughout their lifecycle at different curing strengths; it's important that both temporary and permanent loads be considered. While in service, the final liner must be relatively watertight, resilient, robust, and durable to provide long service-life while striking a balance with costs, production, and worker safety. It's also important to review past practices and evaluate the and potential impact on future trends. When recommending or implementing change, some level of disruption is inevitable, therefore, it's important to take many factors into account such as production, human error, quality, materials, costs, and industry preferences as well as the perspectives of the owner, designer, contractor, segment producer, TBM manufacturer, and the industry to include professional organizations such as ACI and AASHTO. By understanding past practices, considering current perspectives, and analyzing future trends, the appropriate balance can be achieved.

Some example situations to consider include the following: Designers often use the worst-case loading conditions along the entire alignment to proportion all the segments, which might not be cost effective. Additionally, rebar and welded wire fabric are historically used as reinforcing; however, their cages pose nonnegligible safety risks to workers, add significant labor costs, and increase production time. Moreover, fiber reinforcing outperforms conventional reinforcing in terms of surface cracking, crack distribution, corrosion, and durability. Furthermore, some modern approaches for evaluating the service life of a reinforced concrete structures focus on the flow of chloride ions through uncracked concrete while ignoring the effects of cracking. There is also debate in the industry about when to use epoxy-coated black steel versus stainless-steel reinforcing; many respected researches express doubt about the effectiveness of coated rebar; while other practitioners have expressed concerns about the availability, supply, and costs of stainless steel reinforcing.

Hybrid reinforcing designs can be used to improve conventional reinforcing designs, and in some cases, fiber-only reinforcing designs can replace conventional reinforcing. Fibers are beneficial in that they tend to limit the depth and thickness of surface cracks, which enhances durability. In transportation tunnels, steel fibers are used due to concerns about excessive heat from fires, but other sacrificial fiber materials show promise with regards to protecting the concrete against violent spalling during fires; shredded tire fragments are also gaining interest from researcher as a “green” technology for tire waste in lieu of synthetic fibers.

One major drawback to a fiber-only design is the lack of industry experience and acceptance. With conventional reinforcing, industry accepted mechanical analysis has been long used with great success; whereas, fiber-only designs are more recent and draw upon experience and empiricism with some approaches not proven. It is important to conduct tests on the segment and to assemble the ring, which helps understand and validate the performance of fiber-only designed segment. While fiber dosing rates have usefulness, this parameter alone isn't adequate to specify a design for highway tunnels. A major concern for many TBM tunneling projects is determining the best type of reinforcing for any given project.

Conventional reinforcing has a proven record; it is ideal as reinforcing in the sense that it yields when overstressed, which provides some level of notice. A major drawback of conventional reinforcing is the propensity for corrosion since the bars in the cage are not electrically isolated and they tend to produce large deep cracks in the concrete. As such, conventionally reinforced structures may not be as durable as fiber reinforcing applications. Conventional reinforcing is deemed to be more expensive than fiber only reinforcing but less expensive than a hybrid.

For many applications, fiber reinforcing can be used to eliminate, or otherwise reduce, traditional reinforcing, which results in substantial cost savings, increased production, and improved worker safety.

  • When peak performance is warranted, a hybrid product might be the best approach, which is accomplished by combining both conventional and fiber reinforcing. With this approach, the reinforcing cage handles the heavy loads and distributes throughout the mass of concrete while the fibers add confinement near the surfaces to minimize surface cracking; designers and owners like this approach of increased performance, however, it is typically the most expensive solution. It should also be noted that for seismic applications, many designers prefer conventional or hybrid approaches over fiber-only designs, because the latter has not been sufficiently industry vetted for this type of application.
  • Fiber-only designs are ideal for segment manufacturers and contractors because there is no need to deal with the heavy reinforcing cages. One area of concern with fiber-only designs is that most fibers tend to pull-out during failure, which gives less warning of impending problems. However, the latest generation of fibers have been proclaimed to yield instead of pull-out. In any case, the failure mechanisms are different; and the constitutive models need to be evaluated.

Knowledge gaps exist that limit the full-utilization of precast concrete segment technology within the highway tunnel community to include how to select the best type of reinforcing for the project, evaluating different reinforcing schemes along the alignment, understanding the failure mechanism and constitutive models of reinforced concrete segments, and ensuring that the segments are both fire resistant and corrosion resistant for reasons of durability, resilience, and robustness. It is also important to have more meaningful approaches for evaluating service life based on the flow of water through cracks and the electrical isolation of the reinforcing steel instead of the current method that focuses on the flow of chloride ions through the uncracked concrete. Materials need to be evaluated such as black steel, stainless steel, polymers, fiberglass, and coatings. In addition to the concrete segments, a review of segmented gaskets, joints, and hardware is warranted to capture the latest technological advancements. The effects of fire, seismic, and water tightness need to be evaluated.

Important stakeholders for this research include tunnel design engineers, tunnel owners, contractors, precast segment manufacturers, suppliers of fiber reinforcing, and technical committee members from relevant professional organizations. Professional organizations generally represent an important cross-section of industry stakeholders. The American Concrete Institute (ACI 544 and ACI 533), the Fédération internationale du béton (fib WP I.4.I), International Tunneling and Underground Space Association (ITAtech Report No.7), and the International Union of Laboratories and Experts in Construction Materials, Systems and Structures (RILEM TC 261-CCF) have all made significant contributions towards the design of reinforced concrete for tunnel applications. Recent contributions have been made to ACI 544 and soon to be published ACI 533 on precast tunnel segment designs. Experiments and testing of precast concrete segments are being conducted around the world to advance industry knowledge. Major tunnel projects are beginning to utilize fiber-only-reinforcing designs such as the new Parallel Thimble Shoals (TBM) tunnel project.

Since the design of precast concrete segmental linings is an emerging technology for the highway tunnel community, FHWA has been working to obtain perspectives from a wide cross-section of industry stakeholders throughout the tunnel community. One of the major reasons for highlighting the importance of a wide variety of industry participation is for strengthening design, implementation, and deployment techniques while reducing the potential for failures, disputes, and costly litigation. Additionally, a basis of design is needed because, in the event of a tragedy, intense public scrutiny and considerable government authority can be leveraged against the responsible stakeholders.

Updated: 08/21/2018
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000