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Technical Manual for Design and Construction of Road Tunnels - Civil Elements
Chapter 1 - Planning
Road tunnels as defined by the American Association of State Highway and Transportation Officials (AASHTO) Technical Committee for Tunnels (T-20), are enclosed roadways with vehicle access that is restricted to portals regardless of type of the structure or method of construction. The committee further defines road tunnels not to include enclosed roadway created by highway bridges, railroad bridges or other bridges. This definition applies to all types of tunnel structures and tunneling methods such as cut-and-cover tunnels (Chapter 5), mined and bored tunnels in rock (Chapter 6), soft ground (Chapter 7), and difficult ground (Chapter 8), immersed tunnels (Chapter 11) and jacked box tunnels (Chapter 12).
Road tunnels are feasible alternatives to cross a water body or traverse through physical barriers such as mountains, existing roadways, railroads, or facilities; or to satisfy environmental or ecological requirements. In addition, road tunnels are viable means to minimize potential environmental impact such as traffic congestion, pedestrian movement, air quality, noise pollution, or visual intrusion; to protect areas of special cultural or historical value such as conservation of districts, buildings or private properties; or for other sustainability reasons such as to avoid the impact on natural habit or reduce disturbance to surface land. Figure 1–1 shows the portal for the Glenwood Canyon Hanging Lake and Reverse Curve Tunnels – Twin 4,000 feet (1,219 meter) long tunnels carrying a critical section of I-70 unobtrusively through Colorado's scenic Glenwood Canyon.
Figure 1-1 Glenwood Canyon Hanging Lake Tunnels
Planning for a road tunnel requires multi-disciplinary involvement and assessments, and should generally adopt the same standards as for surface roads and bridge options, with some exceptions as will be discussed later. Certain considerations, such as lighting, ventilation, life safety, operation and maintenance, etc should be addressed specifically for tunnels. In addition to the capital construction cost, a life cycle cost analysis should be performed taking into account the life expectancy of a tunnel. It should be noted that the life expectancies of tunnels are significantly longer than those of other facilities such as bridges or roads.
This chapter provides a general overview of the planning process of a road tunnel project including alternative route study, tunnel type and tunneling method study, operation and financial planning, and risk analysis and management.
1.1.1 Tunnel Shape and Internal Elements
There are three main shapes of highway tunnels – circular, rectangular, and horseshoe or curvilinear. The shape of the tunnel is largely dependent on the method used to construct the tunnel and on the ground conditions. For example, rectangular tunnels (Figure 1-2) are often constructed by either the cut and cover method (Chapter 5), by the immersed method (Chapter 11) or by jacked box tunneling (Chapter 12). Circular tunnels (Figure 1-3) are generally constructed by using either tunnel boring machine (TBM) or by drill and blast in rock. Horseshoe configuration tunnels (Figure 1-4) are generally constructed using drill and blast in rock or by following the Sequential Excavation Method (SEM), also as known as New Austrian Tunneling Method (NATM) (Chapter 9).
Figure 1-2 Two Cell Rectangular Tunnel (FHWA, 2005a)
Figure 1-3 Circular Tunnel (FHWA, 2005a)
Figure 1-4 Horseshoe and Curvilinear (Oval) Tunnels (FHWA, 2005a)
Road tunnels are often lined with concrete and internal finish surfaces. Some rock tunnels are unlined except at the portals and in certain areas where the rock is less competent. In this case, rock reinforcement is often needed. Rock reinforcement for initial support includes the use of rock bolts with internal metal straps and mine ties, un-tensioned steel dowels, or tensioned steel bolts. To prevent small fragments of rock from spalling, wire mesh, shotcrete, or a thin concrete lining may be used. Shotcrete, or sprayed concrete, is often used as initial lining prior to installation of a final lining, or as a local solution to instabilities in a rock tunnel. Shotcrete can also be used as a final lining. It is typically placed in layers with welded wire fabric and/or with steel fibers as reinforcement. The inside surface can be finished smooth and often without the fibers. Precast segmental lining is primarily used in conjunction with a TBM in soft ground and sometimes in rock. The segments are usually erected within the tail shield of the TBM. Segmental linings have been made of cast iron, steel and concrete. Presently however, all segmental linings are made of concrete. They are usually gasketed and bolted to prevent water penetration. Precast segmental linings are sometimes used as a temporary lining within which a cast in place final lining is placed, or as the final lining. More design details are provided in the following Chapters 6 through 10.
Road tunnels are often finished with interior finishes for safety and maintenance requirements. The walls and the ceilings often receive a finish surface while the roadway is often paved with asphalt pavement. The interior finishes, which usually are mounted or adhered to the final lining, consist of ceramic tiles, epoxy coated metal panels, porcelain enameled metal panels, or various coatings. They provide enhanced tunnel lighting and visibility, provide fire protection for the lining, attenuate noise, and provide a surface easy to clean. Design details for final interior finishes are not within the scope of this Manual.
The tunnels are usually equipped with various systems such as ventilation, lighting, communication, fire-life safety, traffic operation and control including messaging, and operation and control of the various systems in the tunnel. These elements are not discussed in this Manual, however, designers should be cognizant that spaces and provisions should be made available for these various systems when planning a road tunnel. More details are provided in Chapter 2 Geometrical Configuration.
1.1.2 Classes of Roads and Vehicle Sizes
A tunnel can be designed to accommodate any class of roads and any size of vehicles. The classes of highways are discussed in A Policy on Geometric Design of Highways and Streets Chapter 1, AASHTO (2004). Alignments, dimensions, and vehicle sizes are often determined by the responsible authority based on the classifications of the road (i.e. interstate, state, county or local roads). However, most regulations have been formulated on the basis of open roads. Ramifications of applying these regulations to road tunnels should be considered. For example, the use of full width shoulders in the tunnel might result in high cost. Modifications to these regulations through engineering solutions and economic evaluation should be considered in order to meet the intention of the requirements.
The size and type of vehicles to be considered depend upon the class of road. Generally, the tunnel geometrical configuration should accommodate all potential vehicles that use the roads leading to the tunnel including over-height vehicles such as military vehicles if needed. However, the tunnel height should not exceed the height under bridges and overpasses of the road that leads to the tunnel. On the other hand, certain roads such as Parkways permit only passenger vehicles. In such cases, the geometrical configuration of a tunnel should accommodate the lower vehicle height keeping in mind that emergency vehicles such as fire trucks should be able to pass through the tunnel, unless special low height emergency response vehicles are provided. It is necessary to consider the cost because designing a tunnel facility to accommodate only a very few extraordinary oversize vehicles may not be economical if feasible alternative routes are available. Road tunnel A86 in Paris, for instance, is designed to accommodate two levels of passenger vehicles only and special low height emergency vehicles are provided (Figure 1-5).
Figure 1-5 A-86 Road Tunnel in Paris, France (FHWA, 2006)
The traveled lane width and height in a tunnel should match that of the approach roads. Often, allowance for repaving is provided in determining the headroom inside the tunnel.
Except for maintenance or unusual conditions, two-way traffic in a single tube should be discouraged for safety reasons except like the A-86 Road Tunnel that has separate decks. In addition, pedestrian and cyclist use of the tunnel should be discouraged unless a special duct (or passage) is designed specifically for such use. An example of such use is the Mount Baker Ridge tunnel in Seattle, Washington.
1.1.3 Traffic Capacity
Road tunnels should have at least the same traffic capacity as that of surface roads. Studies suggest that in tunnels where traffic is controlled, throughput is more than that in uncontrolled surface road suggesting that a reduction in the number of lanes inside the tunnel may be warranted. However, traffic will slow down if the lane width is less than standards (too narrow) and will shy away from tunnel walls if insufficient lateral clearance is provided inside the tunnel. Also, very low ceilings give an impression of speed and tend to slow traffic. Therefore, it is important to provide adequate lane width and height comparable to those of the approach road. It is recommended that traffic lanes for new tunnels should meet the required road geometrical requirements (e.g., 12 ft). It is also recommended to have a reasonable edge distance between the lane and the tunnel walls or barriers (See Chapter 2 for further details).
Road tunnels, especially those in urban areas, often have cargo restrictions. These may include hazardous materials, flammable gases and liquids, and over-height or wide vehicles. Provisions should be made in the approaches to the tunnels for detection and removal of such vehicles.
1.2 Alternative Analyses
1.2.1 Route Studies
A road tunnel is an alternative vehicular transportation system to a surface road, a bridge or a viaduct. Road tunnels are considered to shorten the travel time and distance or to add extra travel capacity through barriers such as mountains or open waters. They are also considered to avoid surface congestion, improve air quality, reduce noise, or minimize surface disturbance. Often, a tunnel is proposed as a sustainable alternative to a bridge or a surface road. In a tunnel route study, the following issues should be considered:
Often sustainability is not considered; however, the opportunities that tunnels provide for environmental improvements and real estate developments over them are hard to ignore and should be reflected in term of financial credits. In certain urban areas where property values are high, air rights developments account for a significant income to public agencies which can be used to partially offset the construction cost of tunnels.
It is important when comparing alternatives, such as a tunnel versus a bridge or a bypass, that the comparative evaluation includes the same purpose and needs and the overall goals of the project, but not necessarily every single criterion. For example, a bridge alignment may not necessarily be the best alignment for a tunnel. Similarly, the life cycle cost of a bridge has a different basis than that of a tunnel.
1.2.2 Financial Studies
The financial viability of a tunnel depends on its life cycle cost analysis. Traditionally, tunnels are designed for a life of 100 to 125 years. However, existing old tunnels (over 100 years old) still operate successfully throughout the world. Recent trends have been to design tunnels for 150 years life. To facilitate comparison with a surface facility or a bridge, all costs should be expressed in terms of life-cycle costs. In evaluating the life cycle cost of a tunnel, costs should include construction, operation and maintenance, and financing (if any) using Net Present Value. In addition, a cost-benefit analysis should be performed with considerations given to intangibles such as environmental benefits, aesthetics, noise and vibration, air quality, right of way, real estate, potential air right developments, etc.
The financial evaluation should also take into account construction and operation risks. These risks are often expressed as financial contingencies or provisional cost items. The level of contingencies would be decreased as the project design level advances. The risks are then better quantified and provisions to reduce or manage them are identified. See Chapter 14 for risk management and control.
1.2.3 Types of Road Tunnels
Selection of the type of tunnel is an iterative process taking into account many factors, including depth of tunnel, number of traffic lanes, type of ground traversed, and available construction methodologies. For example, a two-lane tunnel can fit easily into a circular tunnel that can be constructed by a tunnel boring machine (TBM). However, for four lanes, the mined tunnel would require a larger tunnel, two bores or another method of construction such as cut and cover or SEM methods. The maximum size of a circular TBM existing today is about 51 ft (15.43 m) for the construction of Chongming Tunnel, a 5.6 mile (9-kilometer) long tunnel under China's Yangtze River, in Shanghai. See Figure 1-6 showing the Chongming Tunnel. Note the scale of the machine relative to the people standing in the invert.
Figure 1-6 Chongming Tunnel under the Yangtze River
When larger and deeper tunnels are needed, either different type of construction methods, or multiple tunnels are usually used. For example, if the ground is suitable, SEM (Chapter 9) in which the tunnel cross section can be made to accommodate multiple lanes can be used. For tunnels below open water, immersed tunnels can be used. For example, the Fort McHenry Tunnel in Baltimore, Maryland accommodated eight traffic lanes of I-95 into two parallel immersed units as shown in Figure 1-7.
Figure 1-7 Fort McHenry Tunnel in Baltimore, MD
Shallow tunnels would most likely be constructed using cut-and-cover techniques, discussed in Chapter 5. In special circumstances where existing surface traffic cannot be disrupted, jacked precast tunnels are sometimes used. In addition to the variety of tunneling methods discussed in this manual, non-conventional techniques have been used to construct very large cross section, such as the Mt. Baker Ridge Tunnel, on I-90 in Seattle, Washington. For that project, multiple overlapping drifts were constructed and filled with concrete to form a circular envelop that provides the overall support system of the ground. Then the space within this envelop was excavated and the tunnel structure was constructed within it (Figure 1-8)
Figure 1-8 Stacked Drift and Final Mt Baker Tunnel, I-90, Seattle, WA
There are times when tunneling is required in a problematic ground such as mixed face (rock and soft ground), squeezing rock or other difficult ground conditions requiring specialized techniques, as discussed in Chapter 8.
1.2.4 Geotechnical Investigations
As discussed in Chapter 3, geotechnical investigations are critical for proper planning of a tunnel. Selection of the alignment, cross section, and construction methods is influenced by the geological and geotechnical conditions, as well as the site constraints. Good knowledge of the expected geological conditions is essential. The type of the ground encountered along the alignment would affect the selection of the tunnel type and its method of construction. For example, in TBM tunnel construction mixed ground conditions, or buried objects add complications to the TBM performance and may result in the inability of the TBM to excavate the tunnel, potential breakdown of the TBM, or potential ground failure and settlements at the surface. The selection of the tunnel profile must therefore take into account potential ground movements and avoid locations where such movements or settlements could cause surface problems to existing utilities or surface facilities and mitigation measures should be provided.
Another example of the effect of the impact of geological features on the tunnel alignment is the presence of active or inactive faults. During the planning phase, it is recommended to avoid crossing a fault zone and preferred to avoid being in a close proximity of an active fault. However, if avoidance of a fault cannot be achieved, then proper measures for crossing it should be implemented. Such measures are discussed in Chapter 13 Seismic Considerations. Special measures may also be required when tunneling in a ground that may contain methane or other hazardous gasses or fluids.
Geotechnical issues such as the soil or rock properties, the ground water regime, the ground cover over the tunnel, the presence of contaminants along the alignment, presence of underground utilities and obstructions such as boulders or buried objects, and the presence of sensitive surface facilities should be taken into consideration when evaluating tunnel alignment. Tunnel alignment is sometimes changed based on the results of the geotechnical to minimize construction cost or to reduce risks. The tunnel profile can also be adjusted to improve constructability or accommodate construction technologies as long as the road geometrical requirements are not compromised. For example, for TBM tunnels the profile would be selected to ensure that sufficient cover is maintained for the TBM to operate satisfactorily over the proposed length of bore. However, this should not compromise the maximum grade required for the road.
If the route selection is limited, then measures to deal with the poor ground in terms of construction method or ground improvement prior to excavation should be considered. It is recommended that the geotechnical investigation start as early as possible during the initial planning phase of the project. The investigation should address not just the soil and rock properties, but also their anticipated behaviors during excavation. For example in sequential excavation or NATM, ground standup time is critical for its success. If the ground does not have sufficient standup time, pre-support or ground improvement such as grouting should be provided. For soft ground TBM tunneling, the presence of boulders for example would affect the selection of TBM type and its excavation tools. Similarly, the selection of a rock TBM would require knowledge of the rock unconfined compressive strength, its abrasivity and its jointing characteristics. The investigation should also address groundwater. For example, in soft ground SEM tunneling, the stability of the excavated face is greatly dependent on control of the groundwater. Dewatering, pre-draining, grouting, or freezing are often used to stabilize the excavation. Ground behavior during tunneling will affect potential settlements on the surface. Measures to minimize settlements by using suitable tunneling methods or by preconditioning the ground to improve its characteristics would be required. Presence of faults or potentially liquefiable materials would be of concern during the planning process. Relocating the tunnel to avoid these concerns or providing measures to deal with them is critical during the planning process.
The selection of a tunnel alignment should take into consideration site specific constraints such as the presence of contaminated materials, special existing buildings and surface facilities, existing utilities, or the presence of sensitive installations such as historical landmarks, educational institutions, cemeteries, or houses of worship. If certain site constraints cannot be avoided, construction methodologies, and special provisions should be provided. For example, if the presence of contaminated materials near the surface cannot be avoided, a deeper alignment and/or the use of mined excavation (TBM or SEM) would be more suitable than cut and cover method. Similarly, if sensitive facilities exist at the surface and cannot be avoided, special provisions to minimize vibration, and potential surface settlement should be provided in the construction methods.
Risk assessment is an important factor in selecting a tunnel alignment. Construction risks include risks related the construction of the tunnel itself, or related to the impact of the tunnel construction on existing facilities. Some methods of tunneling are inherently more risky than others or may cause excessive ground movements. Sensitive existing structures may make use of such construction methods in their vicinity undesirable. Similarly, hard spots (rock, for example) beneath parts of a tunnel can also cause undesirable effects and alignment changes may obviate that. Therefore, it is important to conduct risk analysis as early as possible to identify potential risks due to the tunnel alignment and to identify measures to reduce or manage such risks. An example of risk mitigation related to tunnel alignment being close to sensitive surface facilities is to develop and implement a comprehensive instrumentation and monitoring program, and to apply corrective measures if measured movements reach certain thresholds. Chapter 15 discusses instrumentation and monitoring.
Sometimes, modifications in the tunnel structure or configurations would provide benefits for the overall tunnel construction and cost. For example, locating the tunnel ventilations ducts on the side, rather than at the top would reduce the tunnel height, raise the profile of the tunnel and consequently reduce the overall length of the tunnel.
1.2.5 Environmental and Community Issues
Road tunnels are more environmentally friendly than other surface facilities. Traffic congestion would be reduced from the local streets. Air quality would be improved because traffic generated pollutants are captured and disposed of away from the public. Similarly, noise would be reduced and visual aesthetic and land use would be improved. By placing traffic underground, property values would be improved and communities would be less impacted in the long term. Furthermore, tunnels will provide opportunities for land development along and over the tunnel alignment adding real estate properties and potential economical potential development.
In planning for a tunnel, the construction impact on the community and the environment is important and must be addressed. Issues such as impact on traffic, businesses, institutional facilities, sensitive installations, hospitals, utilities, and residences should be addressed. Construction noise, dust, vibration, water quality, aesthetic, and traffic congestion are important issues to be addressed and any potentially adverse impact should be mitigated. For example, a cut-and-cover tunnel requires surface excavation impacting traffic, utilities, and potentially nearby facilities. When completed, it leaves a swath of disturbed surface-level ground that may need landscaping and restoration. In urban situations or close to properties, cut-and-cover tunnels can be disruptive and may cut off access and utilities temporarily. Alternative access and utilities to existing facilities may need to be provided during construction or, alternatively, staged construction to allow access and to maintain the utilities would be required. Sometimes, top-down construction rather than bottom-up construction can help to ameliorate the disruption and reduce its duration. Rigid excavation support systems and ground improvement techniques may be required to minimize potential settlements and lateral ground deformations, and their impact on adjacent structures. When excavation and dewatering are near contaminated ground, special measures may be required to prevent migration of the groundwater contaminated plume into the excavation or adjacent basements. Dust suppression and wheel washing facilities for vehicles leaving the construction site are often used, especially in urban areas.
Similarly, for immersed tunnels the impact on underwater bed level and the water body should be assessed. Dredging will generate bottom disturbance and create solid turbidity or suspension in the water. Excavation methods are available that can limit suspended solids in the water to acceptable levels. Existing fauna and flora and other ecological issues should be investigated to determine whether environmentally and ecologically adverse consequences are likely to ensue. Assessment of the construction on fish migration and spawning periods should be made and measures to deal with them should be developed. The potential impact of construction wetlands should be investigated and mitigated.
On the other hand, using bored tunneling would reduce the surface impact because generally the excavation takes place at the portal or at a shaft resulting in minimum impact on traffic, air and noise quality, and utility and access disturbance.
Excavation may encounter contaminated soils or ground water. Such soils may need to be processed or disposed in a contained disposal facility, which may also have to be capped to meet the environmental regulations. Provisions would need to address public health and safety and meet regulatory requirements.
1.2.6 Operational Issues
In planning a tunnel, provisions should be made to address the operational and maintenance aspects of the tunnel and its facilities. Issues such as traffic control, ventilation, lighting, life safety systems, equipment maintenance, tunnel cleaning, and the like, should be identified and provisions made for them during the initial planning phases. For example, items requiring more frequent maintenance, such as light fixtures, should be arranged to be accessible with minimal interruption to traffic.
Tunnels by definition are sustainable features. They typically have longer life expectancy than a surface facility (125 versus 75 years). Tunnels also provide opportunities for land development for residential, commercial, or recreational facilities. They enhance the area and potentially increase property values. An example is the "Park on the Lid" in Mercer Island, Seattle, Washington where a park with recreational facilities was developed over I-90 (Figure 1-9). Tunnels also enhance communities connections and adhesion and protect residents and sensitive receptors from traffic pollutants and noise.
Figure 1-9 "Park on the Lid" Seattle, Washington