Methods of Tunnelling

Methods of tunnelling
Drill and blast
This is mainly used for hard rock where digging is not possible, and was very common before
the development of tunnel boring machines (TBM). The process tends to have a lower capital cost that
the use of TBM, but is slower.
Roadheaders
These are track-mounted excavating machines that have powerful cutting booms and are commonly used
in coal mining or civil engineering works that require a range of tunneldiameters. They are relatively
flexible and can be fitted with extra equipment such as gathering arms for the spoil, water jets to
reduce dust, and conveyors for removing debris
Open-cut method
Sometimes referred to as ‘cut-and-cover’, this is suitable for shallow tunnels. It involves

the construction of an open trench within which the tunnel is constructed. The trench is then backfilled.
Depending on the ground conditions, the side walls may be constructed before the trench is created, or
after.
Cover and cut
Cover and cut involves creating the ‘roof’ slab of the tunnel first, within a shallow trench. The trench can
then be backfilled and the tunnel constructed underneath the ‘cover’. The has the advantage of releasing
the site above the tunnel for other uses.
Immersed-tube method
This method is suitable for tunnels that cross deep water. Prefabricated sections of
either concrete or steel tunnel are lowered into a prepared trench at sea or riverbed level. The trench is
then backfilled and any necessary protection constructed above.
Pipe-jacking
Pipe-jacking is a trenchless technology in which a drive pit is constructed and

then sections of steel or concrete tube are hydraulically jacked forward from the pit to form the tunnel
lining. This is particularly suitable for installing services under canals, railway embankments
and roads where it is important that there is little disturbance. Pipe jacking has been adopted widely since
its first recorded use in 1892 in USA by the Northern Pacific Railroad Company. It is often preferred
because of its simplicity and because it avoids settlement of ground.
Box-jacking
This is a similar process to pipe-jacking, but a box shaped section is jacked forward rather than a pipe,
and this can allow a larger tunnel to be created.
Auger-boring
This uses a non-steerable rotating cutting head and is used mainly for conduit installation. A rail-mounted
machine augers the soil as it pushes a sleeve or tube into the ground. The auger cuts the face of the
borehole as it turns, transporting the excavated spoil back down the sleeve into the shaft. A length of
sleeve is inserted by the machine which then withdraws to have another length of sleeve welded on and
the auger flight extended. This process is then repeated.
Guided auger boring systems are also now available.
Microtunnelling
Microtunnelling machines are primarily used to bore tunnels that are too small for operatives to enter,
such as pipelines, and have been developed to be used in almost any type of ground
condition. Microtunnelling machines can be controlled from ground level using laser equipment, and spoil
is removed from the cutting face by an auger running through the pipelineas it is installed. This is
commonly used where surface disturbance must be kept to a minimum, such as in
the construction of urban drainage systems.
Kramer defines microtunnelling as ‘those methods that install pipes with a diameter of less than 36 inches
(900mm) to a predetermined line and level by remotely controlling the cutting head.’(Kramer et al. 1992)
Mined tunnel
Can be used where a self-supporting subsurface material such as rock or hard clay is
present. It involves the use of drill and blast techniques or excavation/sprayed concrete lining to advance
the excavation.
Whole face or ‘full-face’ boring / tunnel boring machines
This technique uses purpose-designed tunnel boring machines (TBM) to excavate the full cross-
section required. There are high costs involved in the purchase and maintenance of such a machine,
although it requires fewer operatives and can be very time efficient. Whole face borers grip the side of
the tunnel to transmit thrust forward and can cut through hard rock, with debris being removed by
conveyor, but they can be inflexible in terms of changing direction or cross-section.
Tunnel linings
The lining process will vary depending on the ground conditions. Linings can be preformed segments
or insitu concrete that is sprayed in position or cast in place.
In soft ground, linings must support the loads imposed by the ground as well as
withstanding jacking pressures from the tunnelling equipment. Segmental cast-iron lining is commonly-
used in soft ground as it is durable and has high compressive strength. Precast concrete linings are more
economical, although it can be difficult to achieve water tightness.
In ground that is self-supporting, expanding tunnel linings can be used. The tunnelling shield is pushed
forward leaving behind it an unsupported space. The lining, made up of precast segments, is then erected
and expanded against the ground to predetermined pressure.
In hard ground, the lining can be concrete sprayed between steel ribs or onto mesh which has been fixed
to the rock face.
New Austrian Tunnelling Method (NATM)
Also known as the Sequential Excavation Method, NATM first came to prominence in the 1960s, and
helped to revolutionise the tunnelling industry.
The main principle behind NATM that differs from other tunnelling methods is that it uses the inherent
geological strength available in the surrounding rock mass to stabilise the tunnel and so can be less
expensive. The optimal method of support is determined based on actual observed ground conditions,
which gives it the moniker of a ‘design as you go’ approach. The initial ground support is provided
by shotcrete in combination with fiber or welded-wire fabricreinforcement,
lattice girders and ground reinforcement.
Safety
Tunnelling can be an extremely hazardous operation involving working at height, working in

confined spaces, sprayed material and the operation of heavy plant. Careful risk assessmentand risk
management and a high level of training and supervision are required .
There are a number of safety schemes in place for personnel, and the British TunnellingSociety has
produced a Code of Practice for the management of risk in tunnel works, the adoption of which may be
required by insurers.
Potential hazards include:
 The movement of soil.

 Rock falls or rock burst.
 Collapse.
 Failure of the working face.
 Heave or settlement.
 Flood or inflow of mud.
 Blasting and explosives.
 Dust and fumes.
 Escape of gas.
 Noise.
 Movement of plant.
 High temperatures.
 Seismic action.

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Methods of tunnelling

Drill and blast

Sometimes referred to as ‘cut-and-cover’, this is suitable for shallow tunnels. It involves

Cover and cut

Pipe-jacking is a trenchless technology in which a drive pit is constructed and

Guided auger boring systems are also now available.

Whole face or ‘full-face’ boring / tunnel boring machines

New Austrian Tunnelling Method (NATM)

Tunnelling can be an extremely hazardous operation involving working at height, working in

Potential hazards include:

 The movement of soil.

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