a) Full face heading

b) Heading & bench method

c) Drift method

d) Pilot tunnel method

Full Face Heading:

• 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 in any number of desired rows, preferably in two rows.
• The size of the holes may vary from 10 to 40 cm.
• These holes are then charged with explosives and ignited.
• The muck is removed before the next operation of drilling holes.

Heading and Bench 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.
• 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.

Drift method

• It is not possible to drive the full face of tunnel, a small tunnel, known as drift is driven for a portion of tunnel length or sometimes for full length before taking up the excavation of full cross section of the tunnel.
• 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.
• 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.

Advantages:

(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.

Pilot Tunnel method

• A small rectangular tunnel known as pilot tunnel is driven at the centre of the proposed tunnel.
• For enlarging the section the holes are drilled readily along a ring, each ring is spaced about 1 to 1.2 m apart. About 4 such rings .i.e. 4 to 5 metres length of tunnel, is blasted in one blasting operation.
• While mucking is carried out in this portion, another set of 4 rings are drilled and loaded.
• As soon as the mucking is over, four more rings are blasted and thus the cycle of mucking is continuous. This method helps in achieving very high rate of progress.
• This type of tunneling is used in inclined portions of the penstock tunnels, since in this case full face drilling is not possible.