Tunnelling Methods
The choice of tunnelling method may be
dictated by:
• geological and hydrological conditions,
• cross-section and length of continuous
tunnel,
• local experience and time/cost
considerations (what is the value of time in
the project),
• limits of surface disturbance, and many
others factors.
tunnel construction methods:
• Classical methods
• Mechanical drilling/cutting
• Cut-and-cover
• Drill and blast
• Shields and tunnel boring machines
(TBMs)
• New Austrian Tunnelling Method (NATM)
• Immersed tunnels
• Special methods (Tunnel jacking, etc.)
• The process for bored tunnelling involves
all or some of the following operations:
• Probe drilling (when needed)
• Grouting (when needed)
• Excavation (or blasting)
• Supporting
• Transportation of muck
• Lining or coating/sealing
• Draining
• Ventilation
Classical Methods
• Among the classical methods are the
Belgian, English, German, Austrian,
Italian and American systems. These
methods had much in common with
early mining methods and were used
until last half of the 19th century.
• Excavation was done by hand or simple
drilling equipment.
• Supports were predominantly timber,
and transportation of muck was done on
cars on narrow gauge tracks and
powered by steam.
• Progress was typically in multiple
stages i.e. progress in one drift, then
support, then drift in another drift, and
so on.
• The lining would be of brickwork.
These craft-based methods are no
longer applicable, although some of
their principles have been used in
combination up to present day.
Nevertheless some of the world’s great
tunnels were built with these methods.
The English method (crown-bar
method, figure left) started from a
central top heading which allowed
two timber crown bars to be
hoisted into place, the rear ends
supported on a completed length
of lining, the forward ends propped
within the central heading.
Development of the heading then
allowed additional bars to be
erected around the perimeter of
the face with boards between each
pair to exclude the ground. The
system is economical in timber,
permits construction of the arch of
the tunnel in full-face excavation,
and is tolerant of a wide variety of
ground conditions, but depends on
relatively low ground pressures.
• The Austrian (cross-bar) method
required a strongly constructed
central bottom heading upon
which a crown heading was
constructed. The timbering for
full-face excavation was then
heavily braced against the
central headings, with
longitudinal poling boards built
on timber bars carried on each
frame of timbering. As the lining
advanced, so was the timbering
propped against each length to
maintain stability. The method
was capable of withstanding high
ground pressures but had high
demand for timber.
• The German method (core-leaving method) provided a series of box
headings within which the successive sections of the side walls of the
tunnel were built from the footing upwards, thus a forerunner of the
system of multiple drifts. The method depends on the central dumpling
being able to resists without excessive movement pressure transmitted
from the side walls, in providing support to the top 'key' heading prior to
completion of the arch and to ensuring stability while the invert arch is
extended in sections.
• The Belgian system (underpinning or flying arch method) started from
the construction of a top heading, propped approximately to the level of
the springing of the arch for a horseshoe tunnel. This heading was
then extended to each side to permit construction of the upper part of
the arch, which was extended by under- pinning, working from side
headings. The system was only practicable where rock loads were not
heavy.
• The first sizeable tunnel in soft ground was the Tronquoy tunnel on the
St Quentin canal in France in 1803, where the method of construction,
based on the use of successive headings to construct sections of the
arch starting from the footing, was a forerunner to the German system
described above.
The Rove Tunnel near Marseille measured 22 x 15,40 m,
and was excavated with multiple drifts.
Classical multiple face excavation
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Mechanical Drilling and Cutting Crushing Strength of rock
Roadheaders
Cut and Cover Method
• The principal problem to be solved in connection with
this construction method is to how to maintain surface
traffic, with the least disturbance during the construction
period. One method is to restrict traffic to a reduced
street width, another to direct traffic to a bypassing
street.
• Another way of supporting the sidewalls of open
trenches is to substitute sheet-pile walls by concrete
curtain walls cast under bentonite slurry (ICOS method),
and using steel struts. This is especially a requisite in
narrower streets trimmed with old sensitive buildings with
their foundation plane well above the bottom level of the
pit. This type of trench wall becomes a requirement for
maintenance of surface traffic due to the anticipation of
vibration effects potentially harmful to the stability of
buildings with foundations lying on cohesionless soils.