Tunnelling Methods

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Tunnelling Methods
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• 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.

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• 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.)

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• 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

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• 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• These craft-based methods are no longer
  applicable, although some of their principles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 worlds great tunnels
  were built with these methods.

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• 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.

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• 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.

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• 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.

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The Rove Tunnel near Marseille measured 22 x
15,40 m, and was excavated with multiple drifts.
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Classical multiple face excavation
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Mechanical Drilling and Cutting-Crushing Strength
of rock
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Roadheaders
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Cut and Cover Method
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• 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.

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