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Technical Manual for Design and Construction of Road Tunnels - Civil Elements
Chapter 2 - Geometrical Configuration
2.4 Cross Section Elements
2.4.1 Typical Cross Section Elements
Although many road tunnels appear rectangular from inside bordered by the walls, ceiling and pavement (Figure 2-3), the actual tunnel shapes may not be rectangular. As described in Chapter 1, there are generally three typical shapes of tunnels – circular, rectangular, and horseshoe/ curvelinear. The shape of a tunnel section is mainly decided by the ground condition and construction methods as discussed in Chapter 1.
Figure 2-3 A Typical Horseshoe Section for a Two-lane Tunnel (Glenwood Canyon, Colorado)
Figure 2-4 Typical Two-Lane Road Tunnel Cross Section and Elements
A road tunnel cross section must be able to accommodate the horizontal and vertical traffic clearances (Section 2.3), as well as the other required elements. The typical cross section elements include:
Additional elements may be needed under certain design requirements and should be taken into consideration when developing the tunnel geometrical configuration. The requirements for travel lane and shoulder width, sidewalks/emergency egress, drainage, ventilation, lighting, and traffic control are discussed in the following sections. Other elements cited above are required for fire and safety protection for tunnels longer than 1000 ft (300m) or 800 ft (240m) long if the maximum distance from any point within the tunnel to a point of safety exceeds 400 ft (120m) (NFPA, the latest). Fire and safety protection requirements are not within the scope of this manual. Refer to Appendix A, and the latest NFPA 502 Standard for requirement for fire and safety protection elements.
2.4.2 Travel Lane and Shoulder
As discussed previously, for planning and design purposes, each lane width within a road tunnel should be no less than 12 feet (3.6 m) as recommended in the 5th Edition of Green Book (AASHTO, 2004).
Although the Green Book states that it is preferable to carry the full left- and right-shoulder widths of the approach freeway through the tunnel, it also recognizes that the cost of providing full shoulder widths may be prohibitive. Reduction of shoulder width in road tunnels is usual. In certain situations narrow shoulders are provided on one or both sides. Sometimes shoulders are eliminated completely and replaced by barriers. Based on a study conducted by World Road Association (PIARC) and published a report entitled "Cross Section Geometry in Unidirectional Road Tunnels" 2001; shoulder widths vary from country to country and they range from 0 to 2.75 m (9 ft). They are generally in the range of 1 m (3.3 ft). It is suggested for unidirectional road tunnels that the right shoulder be at 4 ft (1-2 m) and left shoulder at least 2 ft (0.6 m).
Figure 2-2(A) does not show a minimum requirement for a shoulder in a tunnel, except it requires that a minimum 2 feet (0.6m) be added to the travel lane width of the approach structure. The Green Book also recommends that the determination of the width of shoulders be established on an in-depth analysis of all aspects involved. Where it is not realistic (for economic or constructability considerations) to provide shoulders at all in a tunnel, travel delays may occur when vehicle(s) become inoperative during periods of heavy traffic. In long tunnels, emergency alcoves are sometimes provided to accommodate disabled vehicles.
To prevent errant vehicles from hitting the walls of the tunnel, a deflecting concrete barrier with a sloping or partially sloping face is commonly used. The height of the barrier should not be so great that it is perceived by drivers of low vehicles to be narrowing the width to the wall nor should it be too low to allow vehicles to mount it. A barrier of 3.3 ft (1 m) is common. A reduced shoulder width from a traveled way to the face of the adjacent barrier ranging between 2 and 4 feet (between 0.6 and 1.2 m) has been found to be acceptable.
Figure 2-5 illustrates an example of a typical tunnel roadway section including and two standard 12 ft lane widths and two reduced shoulder widths. Refer to Section 2.4.3 for the requirements for the barriers when used as the raised sidewalks or emergency egress walkways.
2.4.3 Sidewalks/Emergency Egress Walkway
Although pedestrians are typically not permitted in road tunnels, sidewalks are required in road tunnels to provide emergency egress and access by maintenance personnel. The 5th Edition of Green Book recommends that raised sidewalks or curbs with a width of 2.5 ft (0.7 m) or wider beyond the shoulder area are desirable to be used as an emergency egress, and that a raised barrier to prevent the overhang of vehicles from damaging the wall finish or the tunnel lighting fixtures be provided.
In addition, NFPA 502 requires an emergency egress walkway within the cross-passageways be of a minimum clear width of 3.6 ft (1.12 m).
Figure 2-5 Typical Tunnel Roadway with Reduced Shoulder Widths
2.4.4 Tunnel Drainage Requirements
Road tunnels must be equipped with a drainage system consisting of pipes, channels, sump/pump, oil/water separators and control systems for the safe and reliable collection, storage, separation and disposal of liquid/ effluent from the tunnels that might otherwise collect. Drainage must be provided in tunnels to deal with surface water as well as water leakage. However, drainage lines and sump-pumps should be sized to accommodate water intrusion and/or fire fighting requirements. They should be designed so that fire would not spread through the drainage system into adjacent tubes by isolating them. For the safety reason, PVC, fiberglass pipe, or other combustible materials should not be used.
Sumps should be provided with traps to collect and remove solids. Sand traps should be provided, as well as oil and fuel separators. It may be assumed in sizing sumps that fires and storms do not happen simultaneously. Sumps and pumps should be located at low points of a tunnel and at portals to handle water that might otherwise flow into the tunnel. Sumps should be sized to match the duty cycle of the discharge pumps such that inflow does not cause sump capacity to be exceeded. Sumps should be designed to be capable of being cleaned regularly.
2.4.5 Ventilation Requirements
The ventilation system of a tunnel operates to maintain acceptable air quality levels for short-term exposure within the tunnel. The design may be driven either by fire/safety considerations or by air quality; which one governs depends upon many factors including traffic, size and length of the tunnel, and any special features such as underground interchanges.
Ventilation requirements in a highway tunnel are determined using two primary criteria, the handling of noxious emissions from vehicles using the tunnel and the handling of smoke during a fire. Computational fluid dynamics (CFD) analyses are often used to establish an appropriate design for the ventilation under fire conditions. An air quality analysis should also be conducted to determine whether air quality might govern the design. Air quality monitoring points in the tunnel should be provided and the ventilation should be adjusted based on the traffic volume to accommodate the required air quality.
Environmental impacts and air quality may affect the locations of ventilation structures/buildings, shafts and portals. Analyses should take into account current and future development, ground levels, the heights and distances of sensitive receptors near such locations and the locations of operable windows and terraces of adjacent buildings to minimize impacts. Ventilation buildings have also been located below grade and exhaust stacks hidden within other structures.
The two main ventilation system options used for tunnels are longitudinal ventilation and transverse ventilation. A longitudinal ventilation system introduces air into, or removes air from a road tunnel, with the longitudinal flow of traffic, at a limited number of points such as a ventilation shaft or a portal. It can be sub-classified as either using a jet fan system or a central fan system with a high-velocity (Saccardo) nozzle. The use of jet fan based longitudinal system was approved by the FHWA in 1995 based on the results of the Memorial Tunnel Fire Ventilation Test Program (NCHRP, 2006). Generally, it includes a series of axial, high-velocity jet fans mounted at the ceiling level of the road tunnel to induce a longitudinal air-flow through the length of the tunnel as shown in Figure 2-6.
Figure 2-6 Ventilation System with Jet Fans at Cumberland Gap Tunnel
A transverse ventilation system can be either a full or semi-full transverse type. With full transverse ventilation, air supply ducts are located above, below or to the side of the traffic tube and inject fresh air into the tunnel at regular intervals. Exhaust ducts are located above or to the side of the traffic tube and remove air and contaminants. With semi-transverse ventilation, the supply duct is eliminated with its "duties" taken over by the traffic opening. When supply or exhaust ducts are used, the flow is generated by fans grouped together in ventilation buildings. Local noise standards generally would require noise attenuators at the fans or nozzles.
Selection of the appropriate ventilation system obviously has a profound impact on the tunnel alignment, layout, and cross section design. Detailed discussion of tunnel ventilation design is not within the scope of this manual.
2.4.6 Lighting Requirements
Lighting in tunnels assists the driver in identifying hazards or disabled vehicles within the tunnel while at a sufficient distance to safely react or stop. High light levels (Portal light zone) are usually required at the beginning of the tunnel during the daytime to compensate for the "Black Hole Effect" that occurs by the tunnel structure shadowing the roadway as shown on Figure 2-7. These high light levels will be used only during daytime. Tunnel light fixtures are usually located in the ceiling, or mounted on the walls near the ceiling. Tunnel lighting methods and guidelines are not within the scope of this manual. However, the location, size, type, and number of light fixtures impact the geometrical requirements of the tunnel and should be taken into consideration.
Figure 2-7 "Black Hole" (Left) and Proper Lighting (Right)
The tunnel lighting documents issued by the IESNA (ANSI/IESNA RP-22 Recommended Practice for Tunnel Lighting) and the CIE (CIE-88 Guide for the Lighting of Road Tunnels and Underpasses) offer comprehensive approaches to tunnel lighting. The AASHTO Roadway Lighting Design Guide provides some recommendations for road tunnels as well.
For improved safety during a fire, it is suggested that strobe lights be placed to identify exit routes. If used they should be placed around exit doors, especially at lower levels which might then be under the smoke level. The strobe lights would be activated only during tunnel fires. Emergency lighting in tunnels including wiring methods and other requirements are included in NFPA 502 "Standard for Road Tunnels, Bridges and Other Limited Access Highways", PIARC "Fire and Smoke Control in Road Tunnels", and in the findings of the 2005 FHWA/AASHTO European Scan Tour (Appendix A).
2.4.7 Traffic Control Requirements
The latest NFPA 502 Standard mandates that tunnels 300 ft (90 m) in length should be provided with a means to stop traffic approaching portals (external). In addition, the NFPA 502 also specifies that traffic control means within the tunnel 800 feet (240 m) in length are required. These should include lane control signals, over-height warning signals, changeable message signs (CMS), etc. Traffic control may be required to close and open lanes for maintenance and handling accidents, and for monitoring of vehicles carrying prohibited materials. Incident control systems linked to CCTV cameras should be installed. It is recommended that 100% coverage of the tunnel with CCTV be provided. Refer to the latest NFPA 502 for more detailed requirements. Traffic control requirements should be taken into consideration when developing the cross sectional geometry.
2.4.8 Portals and Approach
Tunnel portals may require special design considerations. Portal sites need to be located in stable ground with sufficient space. Orientation of the portals should avoid if possible direct East and West to avoid blinding sunlight. Ameliorating measures should be taken where drivers might otherwise be blinded by the rising or setting sun. Intermittent cross members are sometimes provided across the approach structure above the traffic lanes as an amelioration measure. A central dividing wall sometime is extended some distance out from the portal to prevent recirculation of polluted air, i.e. vented polluted air from one traffic duct is prevented from entering an adjacent duct as "clean" air.
Tunnels with a high traffic volume and long tunnels should be equipped with emergency vehicles at each end with potential access to all traffic tubes. Wrecker trucks should be capable of pushing a disabled vehicle as well as the more traditional method of towing. These vehicles should preferably be equipped with some fire-fighting equipment, the extent of which depending upon the distance to the nearest fire department. At least, they should carry dry chemical fire extinguishers.
If the tunnel is in a remote rural area where responses of nearby fire companies and emergency squads are not available in a timely matter, a larger portal structure as shown in Figure 2-8 may be required to host the operation control center, as well as the fire-fighting and emergency-responding personnel, equipment and vehicles.
In determining portal locations and where to end the approach structure and retaining walls, protection should be provided against flooding resulting from high water levels near bodies of water and tributary watercourses, or from storm runoff. The height of the portal end wall and the approach retaining walls should be set to a level at least 2 ft (0.6m) higher than the design flood level. Alternatively a flood gate can be provided. Adequate provision should be made for immediate and effective removal of water from rainfall, drainage, groundwater seepage, or any other source. Portal cross drain and sump-pump should be provided.
Figure 2-8 Portal Structure for Cumberland Gap Tunnel