Road Tunnels or Vehicular Tunnels
Road tunnels or vehicular tunnels are defined as enclosed roadways through ground, sea or mountains with vehicle access restricted through portals or tunnel ends, independent of structure type, method of construction or the crossing medium. The carriageways covered by bridges are not considered as tunnels. Some of the most common tunneling methods are listed below.
- Cut and-cover tunnels,
- Mined and bored tunnels in rock,
- Soft ground, and difficult ground tunnels,
- Immersed tunnels
- Jacked box tunnels.
Advantages of Road Tunnel Structures
Advantages of Tunnel Structures are many. If technically feasible, Tunnels are best option to cross a water bodies, mountains, existing roads and junctions, rails or other obstacles. Some key features and highlights of vehicular tunnels are that they
- Minimize potential environmental impact such as traffic congestion,
- Minimize congestion due to pedestrian movement,
- Reduce impacts on air quality,
- Reduce noise pollution as they are covered,
- Minimize visual intrusion;
- Land acquisition not required.
- Tunnels are adopted to protect areas of special cultural/historical importance
- Can be adopted to avoid the impact on natural habitats
Considerations involved in Planning & Design
Since planning and construction of a tunnel requires multi-disciplinary contribution, it is one the most expensive option when it comes to total installed cost. Planning of tunnel involves various considerations, such as
- Geo-technical feasibility
- Construction methodology
- Drainage Aspects
- Life safety
- Operation and maintenance
- Capital construction cost,
- Life cycle cost
Design life of tunnels are significantly longer compared to other transportation facilities like viaducts or roads.
Different Shapes of Vehicular Tunnels
Shape of tunnel depends on the ground conditions and the method of construction. At the same time, only certain shapes can be adopted for certain method of construction. Most common shapes of a transportation tunnel and adopted method for the shape are listed below
- Rectangular or square Tunnels – > Cut and cover tunnels and jacked box tunneling.
- Circular tunnels – > Tunnel boring machine (TBM) or by drill and blast in rock.
- Horseshoe tunnels -> Drill and Blast in rock, Sequential Excavation Method (SEM) and New Austrian Tunneling Method (NATM)
Lining for Tunnels
Except for cut and cover tunnels and jacked box tunnels, the most important structural component of the Road tunnel is its Lining. For rock tunnels, linings are provided at portals and wherever the rocks are not stable. Rock reinforcement and rock anchors like dowels and bolts are adopted at these locations.
Shotcreting or sprayed concrete is applied to prevent spalling of small fragments of rock. Wire meshes or a thin concrete lining are also used. For TBM construction, precast segmental lining are used in conjunction where the ground condition is soft rock. Segments are sequentially installed after boring. Segments can be made of cast iron, steel and mostly concrete. Linings are also important for waterproofing requirements. Linings also suffice safety and maintenance requirements.
Interior Finishes for Road Tunnels
Providing proper finishes based on the function is very necessary for road tunnel. The carriageways are paved with bitumen and walls are coated with finishes. Finishes are usually mounted using epoxy metal panels or tiles.
Functions of tunnel finishes are as follows
- Enhanced tunnel lighting
- Improve visibility
- Provide fire protection for the lining,
- Attenuate noise
- Provide a surface easy to clean.
Various support systems are required based on the function, rating and design classification of tunnels. Some of them are
- Ventilation – Jet fans and Circulation systems
- Lighting Systems
- Drainage and Flood Control systems
- Communication Systems
- Fire Life Safety – Fire Hydrants, Deluge systems
- Traffic Operation – CCTV, Signals, Road Markings
- Operation and control – SCADA, Power supply