Tunnel construction is an underground passage constructed beneath the earth’s surface or water. In most cases, tunnel construction is expensive, but it provides comfort as well as saves time. A lot of excavation of rock or soil is required for tunnel construction. However, excavation and backfilling have become easier over the years, thanks to the availability of modern equipment.
Cut and Cover Tunnelling
Cut and cover tunnelling is a prevalent technique used for constructing shallow tunnels. The advantage of this method is that it can accommodate changes in tunnel width, have non-uniform shapes and is quite commonly adopted in the construction of underground stations. Many overlapping works need to be carried out for the use of this tunnelling method. Tunnel construction, soil covering of excavated tunnels and trench excavation are three integral parts of the tunnelling method. Most of these are quite similar to other road construction projects except for the fact that the excavations involved are much more profound. Bulk excavation is undertaken under a road deck in order to reduce environmental impacts in the form of dust and noise emissions as well as minimise traffic disruption.
Drill and Blast
This tunnelling method is characterised by the use of explosives. Blast holes are drilled on the proposed tunnel surface up to a designated depth after which explosives and timed detonators are placed in the blast holes. After the blasting has been carried out, waste soil and rock are moved out of the tunnel before further blasting. Most tunnel construction done on rock involves ground which is somewhere in between two extreme conditions of soft soil and hard rock. Therefore adequate structural support measures need to be taken while adopting this tunnelling method.
Bored Tunnelling by Tunnel Boring Machines (TBM)
Bored tunnelling using a Tunnel Boring Machine (TBM) is usually used for the excavation of long tunnels. A functional TMB method requires the appropriate selection of equipment for different geological conditions and rock mass. TBM can be suitable for excavating tunnels that contain competent rocks that can give adequate geological stability for boring a lengthy section of the tunnel without any structural support. However, there is a chance of encountering extremely tough rock which can cause significant wear and tear of the TBM rock cutter and slow down the progress of the tunnelling work to the point where TBM is inefficient, uneconomical and may take a longer time than the drill-and-blast tunnelling method.
Sequential Excavation Method
Also known as the New Austrian Tunnelling Method (NATM), this method involves dividing the excavation location of a proposed tunnel into segments. These segments are then mined in sequential order with supports. Some mining equipment such as backhoes and road-headers are commonly used for this method of tunnel excavation. The ground to be excavated must be completely dry for applying this method, and ground dewatering is an essential process before the excavation.