There are various methods of tunnelling. The selection of a method depends upon the size of the bore, the condition of the ground, the equipment available, and the extent to which timbering is required. Tunnelling may be basically divided into two main groups.
(a) Tunnelling in hard rocks
(b) Tunnelling in soft rocks
These are described in detail in the subsequent sections. Tunnelling through water-bearing strata and compressed air tunnelling are discussed subsequently.
30.4.1 Tunnelling in Hard Rocks
The following methods are generally employed for tunnelling in hard rocks.
Full face method
The full face method is normally selected for small tunnels whose dimensions do not exceed 3 m. In this method, the full face or the entire facade of the tunnel is tackled at the same time. Vertical columns are erected at the face of the tunnel and a large number of drills mounted or fixed on these columns at a suitable height as shown in Fig. 3.2. A series of holes measuring 10 mm to 40 mm in diameter with about 1200 mm centre-to-centre distance are then drilled into the rock, preferably in two rows. These holes are charged with explosives and ignited. Next the muck is removed before repeating the process of drilling holes.
(a) Since an entire section of the tunnel is tackled at one time, the method is completed expeditiously.
(b) Mucking tracks, which are tracks used for collecting muck, can be laid on the tunnel floor and extended as the work progresses.
(c) With the development of the ‘jumbo’ or drill carriage, this method can be used for larger tunnels too.
(a) The method requires heavy mechanical equipment.
(b) It is not very suitable for unstable rocks.
(c) It can normally be adopted for small tunnels only.
Heading and bench method
In this method, the heading (top or upper half) of the tunnel is bored first and then
the bench (bottom or lower half) follows. The heading portion lies about 3.70 m to
524 Railway Engineering
4.60 m ahead of the bench portion (Fig. 30.3). In hard rock, the drill holes for the bench are driven at the same time as the removal of the muck. The hard rock permits the roof to stay in place without supports.
Fig. 30.3 Heading and Bench method
(a) The work of drilling of holes for the explosives and the removal of muck can progress simultaneously.
(b) This method requires the use of lower quantities of gunpowder than the full face method.
A drift is a small tunnel measuring 3 m x 3 m, which is driven into the rock and whose section is widened in subsequent processes till it equates that of the tunnel. A number of drill holes are provided all around the drift and these are filled up with explosives and ignited so that the size of the drift expands to become equal to the required cross section of the tunnel.
The position of the drift depends upon local conditions; it may be in the centre, top, bottom, or side as shown in Fig. 30.4. Field experience has shown that the central drift is the best choice, as it offers better ventilation and requires lower quantities of explosives. The side drift, however, has the advantage that it permits the use of timber to support the roof.
(a) If the quality of the rock is bad or if it contains excessive water, this is detected in advance and corrective measures can then be taken in time.
(b) A drift assists in the ventilation of tunnels.
(c) The quantity of explosives required is less.
(d) A side drift allows the use of timber to support the roof.
(a) It is a time-consuming process, as the excavation of the main tunnel gets delayed till the drift is completed.
(b) The cost of drilling and removing the muck from the drift is high, as the work has to be done using manually operated power-driven equipment.
Fig. 30.4 Drift method
Pilot tunnel method
This method normally involves the digging of two tunnels, namely, a pilot tunnel and a main tunnel. The cross section of the pilot tunnel usually measures about 2.4 m x 2.4 m. The pilot tunnel is driven parallel to the main tunnel and connected to the centre line of the main tunnel with cross cuts at many points. The main tunnel is then excavated from a number of points. The pilot tunnel offers the following advantages.
Fig. 30.5 Pilot tunnel
(a) It helps in removing the muck from the main tunnel quickly.
(b) It helps in providing proper ventilation and lighting in the main tunnel.
The method, however, requires the construction of an additional tunnel and therefore the time and cost of construction are higher as compared to the methods described before.
Perimeter method of tunnelling
In this method, the excavation is carried out along the perimeter or periphery of the section. The method is also known as the German method.
Tunnelling in soft ground or soft rock is a specialized job. It does not involve the use of explosives and the requisite excavation work is done using hard tools such as pickaxes and shovels. In recent times, compressed air has also been used for this purpose. During excavation, the rail requires support at the sidewalls and the roofs depending upon the type of soil. The support could be provided in the form of timber or steel plates or other similar material. The various operations involved in soft rock tunnelling are as follows.
(a) Excavation or mining
(b) Removal of excavated material
(c) Scaffolding and shuttering
(d) Lining of tunnel surface
The nature of the ground is the most important factor in deciding the method to be used for tunnelling. The types of ground which are generally encountered in the field are detailed in Table 30.3.
Table 30.3 Types of grounds
Nature of ground
Typical quality of ground
Requires instant support throughout the excavation. Examples include dry sand, gravel, silt, mud, and water bearing sand.
Requires instant support for the roof but the walls can do without support for a few minutes. Examples include damp sand, soft earth, and certain types of gravel.
The sidewalls and face of the tunnel can do without support for one or two hours, but the roof can last only a few minutes. Examples include firm clay, gravel, and dry earth.
Excavation of the tunnel section can be carried out without support for small lengths ranging from 2 to 5 m. Examples include sand stone, hard clay, etc.
In the case of soft rock, the selection of the method of tunnelling depends upon the following important factors.
(a) Nature of ground
(b) Size of tunnel
(c) Equipment available
(d) Sequence of operations
Some of the important methods of tunnelling in soft rock are described in the following sections.
Forepoling is an old method of tunnelling through soft ground. In this method, a frame is prepared in the shape of the letter A, placed near the face of the tunnel, and covered with suitable planks. Poles are then inserted at the top of the frame up
to a viable depth. The excavation is carried out below these poles, which are supported by vertical posts. The excavation is carried out on the sides and the excavated portion is suitably supported by timber. The entire section of the tunnel is covered thus. The process is repeated as the work progresses.
Fig. 30.6 Forepoling method
Forepoling is a slow and tedious process and requires skilled manpower and strict supervision. The method has to be meticulously repeated in sequence and there is no short cut for the same.
Linear plate method
In the linear plate method (Fig. 30.7), timber is replaced by standard size pressed steel plates. The use of pressed steel plates is a recent development. The method has the following advantages.
(a) The linear plates are light and can be handled easily.
(b) The number of joints is less, as the linear plates are bigger in size, and as such the maintenance cost is low.
(a) The steel plates are fireproof and can be safely used while working in compressed air condition.
(d) The necessary work can be done by semi-skilled staff.
(e) There is considerable saving in terms of the excavation and concrete required.
Needle beam method
The needle beam method (Fig. 30.8) is adopted in terrains where the soil permits the roof of the tunnel section to stand without support for a few minutes. In this method, a small drift is prepared for inserting a needle beam consisting of two rail steel (RS) joists or I sections and is bolted together with a wooden block in the centre. The roof is supported on laggings carried on the wooden beam. The needle beam is placed horizontally with its front end supported on the drift and the rear end supported on a vertical post resting on the lining of the tunnel. Jacks are fixed on the needle beam and the tunnel section is excavated by suitably incorporating timber. This method of tunnelling is more economical compared to other methods.
Fig. 30.8 Needle beam method
In this method (Fig. 30.9), a drift is driven into the top of the tunnel. The drift is supported by laggings, caps, and two vertical posts. The sides of the drift are then widened and additional support is provided using timber planks and struts. The process of widening is continued till it reaches the springing level. Wall plates are fixed at the springing level, which in turn are supported by vertical posts. The vertical posts now occupy the entire roof level. The posts supporting the drift can then be removed and tunnelling work continued further in a similar manner.
This method is similar to the American method except that the roof load is supported by underpinning instead of using vertical posts. A drift is driven into the top of the tunnel about 5 m ahead of the existing arch lining. The drift is subsequently widened
on both sides and supported by crown bars and posts. The work is carried on till the springing level is reached. The sill is then extended across the tunnel and the extended piece is supported by underpinning. This method requires good quality timber as well as simultaneous and frequent shifting from place to place.
This method is used for long tunnels, particularly those at great depths, where the walls of the excavation may yield under the weight of the cover. It involves excavating the whole section for a short length and furnishing with sidewalls and an arch.
This method is particularly suitable for areas where the height of the overburden is less and the surface is not to be disturbed. In this case, the heading is excavated first and supported by crown bar posts and laggings. The sides are excavated next and supported by crown bars and posts. Finally, the work of lining the arch is carried out and further excavation is done.
30.4.3 Tunnelling Through Water-bearing Strata
Tunnelling through subaqueous or water-bearing strata is quite a different job. Shield tunnelling is generally preferred in such cases. A shield is a movable frame that is used to support the face of a tunnel. The tunnel is excavated and lined under the protection of the shield.
A shield is a device meant for excavation that is to be carried out beneath waterbearing strata. It basically consists of a cutting edge, a skin plate in the form of a shell structure, and a hood of jacks, ring girders, stiffening steel plates, ports as well as port doors, and a tail. The various methods of shield tunnelling through different types of soils are enumerated in Table 30.4.
Type of soil
Method of tunneling
One or two port doors are opened. The material is excavated and deposited at the bottom of the tunnel.
One or two ports are opened and the material flows continuously into the tunnel. Excavation is carried out and the soil is removed immediately after the excavation.
In this case, tunnelling is of the open type. The sand settles on the floor of the shield and it should be continuously removed. Proper care should be taken to ensure that the material does not block the propelling jacks and other equipment.
The bulk head shield is used in this case. Other details regarding tunnelling in such a soil are the same as for sand.
Tunnels constructed using the shield method usually have a circular section because of the following considerations.
(a) The rotation of the shield is easy in a circular section.
(b) It grants protection to the primary lining.
(c) The circular section provides the maximum cross-sectional area with the smallest perimeter.
(d) The circular section is ideally suited to resist the semi-fluid pressure exerted by the soft ground.
30.4.4 Compressed Air Tunnelling
This method is possibly the most modern method of tunnelling. The compressed air, which has a pressure of about 1 kg/cm2, is forced into the enclosed space within the tunnel so that the sides and top of the tunnel do not collapse and remain in their position. The equipment for tunnelling consists of a bulk head, which is an airtight diaphragm with an airlock. The airlock is an airtight cylindrical steel chamber with a door at each end opening inwards.
Tunnelling by means of compressed air is quite a difficult process because of the following reasons.
(a) The pressure inside the earth varies from the bottom to the top of the tunnel.
(b) It is not possible to ascertain the pressure on the floor of the tunnel as it depends upon the nature of the strata.
(c) The pressure varies from strata to strata depending upon the moisture content, which is difficult to ascertain.
(d) The compressed air normally escapes through the pores and the air pressure diminishes continuously. The application of air pressure has to be varied from time to time in order to achieve a balanced value. The determination of this value depends more on experience than on technical considerations.