Tunneling (Transportation Engineering)
A tunnel is needed when an obstacle in the form of a hill or rising ground is met within the alignment of a highway or railway track. The first tunnel , about 4000 years ago of a cross section of 3.6 m * 4.5 m; 910 meter long, was built in Babylon connecting two buildings.
In Europe the first tunnel was built by Roman Emperor Cloudius in 54 AD for carrying spring water through Apennines. The cross of the tunnel was 3.0 m * 1.8 m and its length was 5.8 Km. It was completed in 12 years by 30, 000 laborers.
This article covers the following aspects of Engineering related with the Tunneling:
- Sections of tunnels: advantages, limitations and suitability of each section
- Shaft, pilot tunnel.
- Driving tunnel in rocks, sequence of construction operations
- Full face method, heading and bench method, drift method
- Driving tunnels in soft ground, sequence of construction operations, needle beam method
- Shield tunneling, compressed air tunneling.
- Sections of tunnels, suitability of each section:
In general you will find the four types of tunnel sections which are famous in tunneling:
- Circular shaped tunnels
- Horse-Shoe shape tunnels
- Vertical walls with arched roof(D-Shaped tunnels).
The shape of the cross section of a cement concrete lined tunnel will depend on the pressure of the ground which the lining must be able to resist and the purpose for which the tunnel is to be constructed.
If the ground is solid rock, then any shape may be adopted but for soft ground such as soft clay or sand, the pressure from the sides as well from the top is to be resisted. In such difficult situations, circular cross section is best suited if it is able to serve the purpose of the tunnel.
A circular tunnel, in general is not suitable for the highways because highways need a flat surfaces. Circular sections are more suitable for aqua-ducts.
D-shaped tunnels have a horizontal base( flat invert), two vertical sides and an arched roof. Such tunnels are best suited as the vehicular tunnels.
The semicircular top acts as an arch, and thus takes and shifts the top soil load to the vertical sides and thus to the bed. If the ground bed is broken, subjected to the horizontal pressure, vertical wall sections of a vehicular tunnel may be replaced with a horse shoe section as it can resist the external pressure better.
Circular and elliptical tunnels are popular for water(irrigation) and sewage conduits while the horse shoe and vertical wall sections (D-Shaped) sections are popular for vehicular tunnels depending upon the condition and type of ground.
- Shaft, Pilot Tunnel:
Shafts are the vertical tunnels, generally circular in section. In case of the hydro projects you have to construct the surge shafts to prevent the water hemorrhage. In the highway projects surge shafts are constructed from the top to reach down to the main tunnel and provides the access path to the main tunnels.
A numbers of shafts may be constructed at places more than one in a long tunnel project, and work may be started from those numbers of places. Diameter of a shaft depends upon the purpose of the shaft, if a TBM is to be lowered to the main tunnel than it is necessary to make the shaft of the required size.
Similar to a shaft Pilot tunnels serves as the access tunnels to the main tunnels. The cross section of a pilot tunnel is usually 240 cm or a little bigger and are driven parallel to the main tunnel. The pilot tunnel is first driven to the full length of the tunnel and is connected to the center line of the main tunnel at many points. From these points, the work of the main tunnel may be started and also they make is easy to take out the muck. Uses of the pilot tunnels may be summarized in the following points:
- It helps in providing proper ventilation to the main tunnel.
- It helps in removing the muck from the main tunnel quickly.
- It helps in providing proper lighting in the main tunnel.
Pilot tunnels also offers a path to reach to the main tunnel so that you can access it to go for the further construction. Pilot tunnels are constructed generally parallel to the main tunnel, and when in connects to the main tunnel path, you get two faces/two directions to excavate your main tunnel.
- Driving Tunnel in rocks, sequence of construction operations:
Actual sequence of the tunneling operations, will depend upon the site conditions, size of the tunnel and method of construction. However, the general sequence of the operations carried out during construction of a tunnel is as given below:
- Setting up and drilling
- Loading holes with explosives and firing them.
- Ventilation and removing the dust after the explosion.
- Loading and hauling muck.
- Removing the ground water if necessity arises.
- Erecting supports for sides and roofs in necessity arises.
- Placing reinforcement.
- Placing concrete lining.
There are several methods of tunneling, the method to be selected for a particular site will depend upon the size of the bore, the equipment available, the condition of the formation and the extent to which the timbering is required. Tunneling may be grouped into two groups:
- Tunneling in hard rocks
- Tunneling in soft rocks.
Tunneling in hard rocks is carried by one the following methods:
- Full face method
- Heading and benching method
- Drift Method
- Pilot tunnel method
- Perimeter method
Here we will discuss the first three methods in details.
- Full Face Method
This method of tunneling is adopted when the length of the tunnel is more than 3 meters. Large sized tunnels in rocks are always driven by this method. With the development of drill carriage this method is becoming more and more popular. In this method vertical columns are fixed to the face of the tunnel to which a large number of drills may be mounted or fixed at any suitable height as shown in the figure below. A series of drill holes are drilled at about 120 cm center to center in any number of desired rows, preferably in two rows. The size of the holes may vary from 10 to 40 mm. These holes are then charged with explosives and ignited. The muck is removed before the next operation of drilling holes.
- Heading and Benching Method
Tunnel cross section is divided into two parts, the top portion of the tunnel is known as the heading and the bottom portion as bench. Usually this method is adopted for railway tunnels. In this method of tunneling, top portion or heading will be about 3.70 to 9.6 m ahead of the bottom portion as shown in the figure below. In hard rock which may permit the roof to withstand without supports, the top heading generally is advanced by one round of bottom portion. If the rock is broken then heading may be driven well ahead of the bottom portion and after giving proper support to the roof, the bottom portion is completed. In hard rock the heading is bored first and the holes are driven for the bench portion at the same time as the removal of the muck. This required less explosive than the full face method, but due to the development of the drill carriage or jumbo, the use of this method is decreasing.
|Heading and Benching Method (Tunneling)|
- Drift Method:
Drift is a small tunnel, usually its size is 3m*3m. In driving a large tunnel it has been found advantageous to drive a drift first through the full length or in a portion of the length of the tunnel prior to the excavating the full bore.
|Drift Method (tunneling)|
The drift may be provided at the center, sides, bottom or top as desired. In this method after driving the drift, the drill holes are drilled all round the drift in the entire cross section of the tunnel, filled with explosives and ignited. The rock shatters, the muck removed and the tunnel expanded to the full cross section.
While tunneling in soft grounds, explosives are not used and tunneling is done with the hand tools such as pick-axes, shovels etc. During excavation operation supports for soil are required immediately depending upon the type of soil. In the old days, timber was the only material used for supporting soft ground till the introduction of the steel liner plates few years ago. As heavy supporting system is needed to support the roof and sides, there is more obstruction in the movement inside the tunnel, which reduces the progress of the work. Care should taken to ensure that all struts should be sufficiently strong to bear the pressure coming on them. The method to be adopted in the soft ground tunneling depends upon the type of ground.
- Needle beam method, sequence of construction operations
This method is useful for tunneling in the soft ground whose roof soil can stand without support for few minutes. In this method 5 to 6 meters long R.S. joist or timber beams are required in addition to other timber boards and struts. This method requires large number of jacks which cause obstruction in the efficient working of the laborers. For tunneling in soft ground it is more economical than other methods.
- Sequence of Working:
- First of all a small drift of size of about 1*1 m is prepared on the working face of the tunnel.
- The needle beam consisting of two I girders, bolted together with a wooden block at the center, is inserted in the drift and its roof is supported on lagging carried on the wooden segment as shown in the figure below. These segments are supported by jacks resting on the needle beam.
Needle beam method- 1
- As shown in the figure below, the needle beam is placed horizontally, whose front end rests on the drift itself and the rear end is supported on the vertical stout post, resting on the lining of the tunnel.
Needle beam -2
- The jack is placed on the top of the beam(Needle Beam) to support the roof with lagging and then drift is widened side ways and the whole section is excavated. After excavating lining may be provided.
- Compressed Air Tunneling Method:
This method is considered as most modern method of tunneling in soft grounds having water bearing stratum. A compressed air is forced into the enclosed space to prevent the collapse of the roof and sides of the tunnel.
Usually air is used in conjunction with a shield and air-tight locks. However, numerous small tunnels have been driven using only linear plates or wood cants only. This method can be safely adopted if the air pressure is approximately 1 kg/cm^2. If the pressure is more than 1 kg/cm^2, the working hours should be reduced considerably which will increase the cost of tunneling.
Application of the air pressure to the tunneling is not so simple due to the following reasons:
- The earth pressure varies from the top of the tunnel to the bottom of the tunnel.
- As the pressure in the floor of the tunnel depends upon the nature of the strata, it is difficult to ascertain it theoretically.
- The value of pressure varies with the moisture content in different strata, which is difficult to ascertain.
- The compressed air will escape through the pores of the soil, hence air pressure will diminish continuously. Thus the value of air pressure will have to vary from time to time to get a balanced value and the determination of this value depends more on experience than theoretical considerations.
This method is ideally suitable for clay formations which do not contain large number of pores and the pressure does not vary much from top of the tunnel to its bottom.
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