How to construct Earth Tunnels?
In this article, we will discuss the procedure of the construction of the Earth Tunnels in a stepwise manner. It will be useful to you all in understanding and remembering the procedure.
We would be studying the construction method of tunnel in two types of soils:
- Firm soil
- Soft soil
Step1:
Excavation of the ground from below. It is similar to the process used in mining.
Step2:
Check the type and bearing capacity of soil. The method of construction would depend upon the ability of soil to support itself temporarily during construction.
It also depends on the ability of the soil to withstand the magnitude of pressure imposed on it during construction.
Step3:
It is important to understand the factors that affect these parameters. They are as follows:
- Depth of the tunnel
- Diameter of the tunnel
- Properties of the soil
- Ground water table
Support to the Tunnel
Most of the time, the tunnel requires temporary support during construction and a permanent support after construction.
Step 4:
Soil type assessed: Firm soil
Construction Method:
The excavation is started at the roof level of the tunnel and progresses downwards.
Step5:
For the initial support of the incremental excavation, the liner plate is used.
What are Liner Plates?
Liner plates are flanged steel plates. When multiple liner plates are bolted together, they form a continuous wall.
An alternative to Liner plates
Sometimes, laggings are used in place of liner plates. Laggings are wooden or steel horizontal planks braced by steel frames.
Step6:
After placing the liner plates and bolting them together, the excavation is carried on to a further step and the procedure of placing is repeated until the complete section of the tunnel has been excavated.
In short;
First, we excavate
Second, place the liner plates for support and carry on further excavation
Step7:
Type of soil: Soft soil
Type of support used for constructing a tunnel: Cyclindrical support
It is jacked through the soil. It is also called shield.
The face of the tunnel is supported by a bulkhead fitted with doors through which excavation of a limited portion of the face can be done.
Step 8:
A permanent lining (for permanent support of the tunnel) is constructed inside the shield after some excavation has been done.
The procedure is then repeated; that is,
First, shield is jacked through the soil
Second, permanent lining is constructed inside the shield after some amount of excavation
Third, shield is jacked further down,
Excavation is done
Permanent lining is done until the complete section is obtained.