- Tunnels are underground passages used for transportation. They could be used for carrying freights and passengers, water, sewage, etc.
- The methods involved are underground operations known as tunnel driving and the surface is not disturbed.
- It is understood that first tunnel was constructed by Egyptians and Babylonians about 4000 yrs ago. It was built to connect two buildings in Babylon. The length, width and height of this tunnel were 910m, 360cm, 450cm respectively.
Selection of Route of Tunnel
- Alignment restraints: Underground space is a heterogeneous mass and in addition, problems like water table, position of fractured rocks etc. are to be tackled. A through detailed inspection and evaluation of the existing alignment restraints of underground space should therefore be made & correlated with the tunneling technology to be adopted for the project.
- Environmental considerations: The site of tunnel should be selected in such a way that the least difficulty is experienced for various environmental factors such as disposal of exhaust gas, groundwater, muck,etc.
Tunneling through Rock
- Full Face Method: The full face method is adopted only for small tunnels whose dimensions do not exceed about 3m. The vertical columns are fixed at suitable height. A series of drilholes about 10mm to 40mm diameter are drilled at about 1200mm centers.
- Heading & Bench System: This is the method usually adopted for all railway tunnels. The heading is the top portions which will be 3700mm to 4600mm ahead of the bottom portion know as bench.
- Cantilever Car Dump Method: This arrangement consists of two plate girders about 23m long and fixed at 1800mm centers. A belt conveyor fitted with a number of jacks is running on these plate girders, the ends of which project beyond full face of the bench.
- Drift System: In this system, a drift is first drive of appropriate size, usually of 3000mm by 3000mm. The drilholes are provided all round the drift in entire cross section of tunnel.
- Pilot Tunnel Method: The pilot tunnel which is first driven to full size is connected to the centre-line of the main tunnel, can be started from a number of holes.The pilot tunnel also helps in removal of muck and the lighting and ventilation of the main tunnel.
Tunneling Through Soft Ground
- Forepoling Method: In this method, a frame in the shape A is prepared and placed near the face of the tunnel covered with suitable planks. The poles are then inserted at top and continued to a depth upto which they can be easily taken up. The fore poling is an old method and it can be used successfully for carrying out tunneling operations through ground.
- Needle Beam Method: This method is useful when the soil is hard enough to stand for few minutes. A small drift is prepared for inserting a needle beam consisting of two I-girders and bolted together with a wooden block in the centre.
- Five-piece set Method: In this method, the widening of the tunnel is carried out by using a set of timbering consisting of five pieces.
- Liner Plates Method: In this method, the timbering is replaced by pressed steel plates of standard sizes. The advantages of this method are. They are light, Easy to handle, Larger than timber pieces, require less number of joints. They are fire-proof, they can be erected by unskilled labour.
Drainage of Tunnels
- Sumps & Pumps: The sumps connected by a pipe line are provided at a distance of about 300m & water is pumped from one sump to another until it is thrown out of tunnel opening.
- Grouting: The above method cannot be used, if water is percolating from the top of the tunnel. In such cases, the grouting is adopted to make the seams water-tight.
- Pilot Tunnel: In cases where pilot tunnel at a lower level than the main tunnel is constructed parallel to it for drainage of water.
Lighting of Tunnels
- The situations which demand adequate light can be obstructions in tunnel, drilling & mucking zones, bottoms of shaft, storage points, pumping stations, underground repair shops.
- The spacing of lights will depend on various factors such as tunnel dimentions, size of light source, nature of rock surface.
- The common types of lights used in tunneling work are acetylene gas lighting, electric lighting & lanterns.
Ventilation of Tunnels
- Blow in Method: In this method fresh air is forced by a fan through a pipr & is supplied near face. This method has the advantage that a positive supply of fresh air is guaranteed where it is required.
- Exhaust Method: In this method the foul air is pulled out through a pipe & is exhausted by a fan. This sets up a current of fresh air to enter the tunnel. This method has the advantage that the foul air is kept out from working place.
Maintenance of Tunnel
- The slops of portals at entry & exit should be checked.
- It should be seen weather the masonry has crushed, cracked or deteriorated.
- The track through the tunnel should be in the line and level.
- The dimensions of tunnel should confirm to original dimensions.
- The lining of tunnel should be examined & checked if it is in a satisfactory consitions.
Advantages of Tunneling
- Tunnels are more economical than open cuts beyond certain depths.
- Tunnels avoid disturbing or interfering with surface life and traffic during construction.
- Tunnels prove to be cheaper than bridges or open cuts to carry public utility services like water, sewer and gas.
- If tunnels are provided with easy gradient, the cost of hauling is decreased.
- In case of aerial warfare and bombing of cities, the tunnels would grant better protection as compared to bridges.