A tunnel construction is an underground passage provided beneath earth surface or water. Different methods of tunnel construction and their details are discussed.
In most of the cases tunnel construction is expensive but it saves time and provides comfort. Large excavation of soil or rock etc. is necessary for a tunnel construction. With the availability of modern equipment, excavation and backfilling has become easier.
Tunnels can be used for roadways, railways and even as waterways also. In many cities, underground metro rail networks work inside a tunnel.
There are various types of construction techniques developed for construction of tunnels which are discussed below:
Cut and cover method
Bored tunnel method
Clay kicking method
Pipe jacking method
Box jacking method
Cut and Cover Method of Tunnel Construction
Cut and cover method of tunnel construction is generally used to build shallow tunnels. In this method, a trench is cut in the soil and it is covered by some support which can be capable of bearing load on it.
The cutting can be done by two methods. One is bottom up method in which a tunnel is excavated under the surface using ground support. Another method is top-down method in which side support walls are constructed first by slurry walling method or contiguous bored piling.
Then roof is located on the top of the walls and excavation is carried out. Finally, base slab is constructed. Most of the Underground metro rail stations are constructed using cut and cover method.
Bored Tunnel Method
Bored tunnel method is modern technology. In this case, tunnel boring machines are used which automatically work and makes the entire tunneling process easier. It is also quicker process and good method to build tunnel in high traffic areas.
Tunnels boring machines (TBM’s) are available in different types suitable for different ground conditions. These machines can be used in difficult conditions such as below the water table etc.
A special pressurized compartment is provided for TBM to work in below water table conditions. The workers should not enter that compartment except for repair works.
Care should be taken while TBM is in working conditions. The only difficulty with this TBM is its heavy weight. So, transportation is difficult and costlier.
Clay Kicking Method of Tunnel Construction
This method is used for strong clayey soil conditions. This is an old method and used for small works like sewage pipes installations etc.
In this method, a hole is excavated into the ground and after some depth tunnel is excavated which is done by the clay kicker which lies 0n a plank at 45o angle.
An excavating tool is provided under clay kicker foot. The excavated using that tool is collected by other workers. This is well famous because it is the method used by Englishmen to put mines under the German empire during First World War.
Shaft Method of Tunnel Construction
In this method tunnel is constructed at greater depth from the ground surface. The shaft is built up to the depth where tunnel is required.
Shaft is a permanent structure which is like well with concrete walls. At required depth, tunnels are excavated using TBM’s. Shafts are provided at both inlet and outlet of tunnels.
Intermediate shafts are also provided if tunnel is too long. After the construction process, these shafts can also be used for ventilation purpose as well as emergency exits.
Pipe Jacking Method of Tunnel Construction
Pipe jacking method is used to construct tunnels under existing structures like road ways, railways etc. In this method, specially made pipes are driven into underground using hydraulic jacks. Maximum size of 3.2-meter diameter is allowed for tunnels.
Box Jacking Method of Tunnel Construction
Box jacking method is similar to pipe jacking, but in this case instead of pipes, specially made boxes are driven into the soil. A cutting head is provided at the front side of the box. Excavated matter is collected within the box. Larger size tunnels can be excavated using box jacks up to 20 meters.