Civil engineers who are a specialist in tunnel engineering are the ones that take full responsibility for the designing, planning, construction, maintenance and safety of tunnels.
What are tunnels?
Tunnels are horizontally shaped underground passages which are generally used for transporting men and materials. Making of caves and tunnels are both ancient activities of mankind dating back to 4000 B.C. Nowadays, tunnels are used for different purposes such as Underground traffic,
- Water supply and drainage,
- Diversion of river water,
- Digging of precious materials like gold, coal, iron,
- Military purposes like underground fortification¸
- Underground parking places, power plant, warehouse, Subways, and expressways.
Generally, tunnels are a few meters to several kilometers. A tunnel diameter has a range of less than a meter to 16-17metres. It may be close to the ground or probably at large depths below the surface of the ground, for example, tunnels in mountains.
Before starting the process of making a tunnel, there are some investigations that need to be carried out meticulously – they include:
- Hydrological survey
- Rock temperature and gasses
- Geological survey and exploration
- Factor affecting the diameter of the tunnel
- Influencing factors of the depth of the tunnel below the ground level
- Geological characteristics along the axis of the tunnel
The above factors are important for consideration when planning to design a tunnel, but a site engineer also needs to investigate the parameters before starting out the design. Also, a construction engineer should ensure to verify the available data for design.
Construction Methods of a tunnel:
Cut and Cover Tunneling
Cut and cover tunneling is popular, tested and trusted technique for constructing shallow tunnels. This method has the ability to accommodate any form of changes in tunnel width and non-uniform shapes. This method is often adopted when constructing underground stations. Several overlapping works need to be carried out in the process of using this tunneling method. The three major integral parts of this tunneling method are trench excavation, tunnel construction and soil covering of excavated tunnels.
Drill and Blast
This method of tunneling requires the use of explosives. Drilling rigs are used for drilling blast holes on the intended tunnel surface to a certain depth prior to blasting. Timed detonators (Delay detonators) and explosives are later placed in the blast holes. After blasting, soils and waste rocks are taken out of the tunnel before proceeding for further blasting.
Most tunneling construction in rock is performed on the ground located between two critical conditions of soft ground and hard rock. Hence it is vital to use an adequate structural support measure for this tunneling method.
Bored Tunneling by Tunnel Boring Machines (TBM)
Bored tunneling using a Tunnel Boring Machine (TBM) is useful in the process of excavating long tunnels. An effective use of the TMB method involves the selection of the right equipment for various rock mass and geological conditions. TBM is perfect for excavating tunnels containing competent rocks that have the ability to offer geological stability to bore a long section tunnel without requiring any form of structural support. However, extremely hard rock can cause wearing of the TBM rock cutter which in turn may slow down the progress of the tunneling works resulting in a situation whereby TBM becomes uneconomical and inefficient.
Sequential Excavation Method
This tunneling method is popularly known as the New Austrian Tunneling Method (NATM). The location for excavation of a proposed tunnel is firstly divided into segments. The segments are then sequentially mined with supports. Some mining equipment used for the tunnel excavation includes the road headers and backhoes. The ground for excavation must be tested and ensured to be fully dry before applying the NATM. Also, ground dewatering is a vial process prior to excavation. This method can be relatively slow but it is a perfect option for areas where existing structures like subway or sewer cannot be found.