Supports, lining and ventilation

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Supports, lining and ventilation
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Support needs

• In soft ground tunnelling immediate support must
  be provided by a stiff lining. In such a case,
  the ground usually participates actively by
  providing resistance to outward deformation of
  the lining.
• In medium-hard rock or in more cohesive soils,
  the ground may be strong enough to allow a
  certain open section at the tunnel face. Here,
  certain amount of stress release may permanently
  be valid before the supporting elements and the
  lining begin acting effectively. In this
  situation only a fraction of the primary ground
  pressure is acting on the lining.
• Hard rock tunneling In hard rock the ground
  alone may preserve the stability of the opening
  so that only a thin lining, if any, will be
  necessary for surface protection.

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Rock Quality Designation
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Competence FactorThe term competence factor (Fc)
is defined as the ratio of the compressive
strength of the ground under uniaxial load to the
net pressure of the overburden (Muir-Wood, 1972).
He recognized the three following conditions
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• Geomechanics Classification (RMR System)
• The Geomechanics Classification or the rock mass
  rating (RMR) system was developed by Bieniawski
  (1979). It utilizes the following six parameters
  all of which are measurable in the field and can
  also be obtained from borehole data.
• a) Uniaxial compressive strength of intact rock
  material
• b) Rock quality designation (RQD)
• c) Spacing of discontinuities
• d) Condition of discontinuities
• e) Groundwater conditions
• f) Orientation of discontinuities

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ÖNORM B 2203Due to the overwhelming success of
the New Austrian Tunnelling Method (NATM) there
has been a trend towards evaluation of the rock
mass quality according to the criterions of
Austrian Standard ÖNORM B 2203. The ground is
grouped into several classes each class being
given a specific type and amount of temporary
support, in addition to specific excavation steps.
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Example
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• The L2 ground condition was encountered in some
  phases of the Bolu tunnel construction, requiring
  very substantial supports in the form of a
  combination of shotcrete, rock bolts and I-beams.
• The Austrian Tunnelling Standard ÖNORM B 2203 is
  compiled on the qualitative base, i.e. it does
  not evaluate any parameters of rock environment
  by system of points. As a result, the rock mass
  quality evaluation depends on experience of a
  geologist and his subjective observation of rock
  mass behaviour. Though this evaluation is
  relatively simple and prompt, ÖNORM B 2203 has
  been seen used as a business standard used to
  arrange working relations between investment
  organization and supplier of underground
  structures. The basis of rock mass quality
  evaluation according to ÖNORM B 2203 is also the
  assessment of the financial demand of the
  technical works. Categorization of rock mass part
  into certain class of ÖNORM B 2203 pledges the
  contractor to the relevant technological
  procedures agreed in the contract (e.g. the
  method of driving and transport of muck, as well
  as method of temporary and permanent lining).
  Each deviation from the agreed procedures
  complicates relations between investment and
  realizing (design/construct) organisations.
• Principally the above rock classification systems
  relate to defects or potential defects of the
  rock mass and not to the inherent properties of
  the rock material. For weak rocks, the
  contribution from a rock classification system is
  more limited since behaviour of the rock will
  depend as much, or more, on the rock material
  than upon the discontinuities. Attempts to base
  support requirements for weak ground on rock
  classification figures have been notably
  unsuccessful. Generally, the evaluation of
  support needs for weak rock is more difficult
  that for strong rock.

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Types of support

• Steel arches
• Steel ribs are used for reinforcement of weaker
  tunnel sections, and give rigid to semi-rigid
  support. The ribs are made from I-beam or H-beam
  structural steel bent to conform to the
  requirements of a particular tunnel
  cross-section.
• The design of steel arches based on the notion of
  the unstable rock wedge in the crown, or possible
  asymmetrically, to be supported by the arch. The
  arch is buttressed against the rock around the
  remainder of the periphery of the tunnel, to
  limit bending stresses. The design of the
  foot-blocks is vital to the success of the system
  of support, in relation to bearing capacity of
  the ground, which may be weakened by the
  disturbance caused by the tunnel excavation. The
  weakness of steel arch support concerns the load
  at which failure may occur by lateral buckling
  and torsion.
• Timber may be used for packing between the beams
  and the rock. However, providing continuous
  bedding against the rock may considerably
  increase the load-bearing capacity of the arches.
  A means for achieving this objective is the
  inserting between the rock and arch a bolster
  made of porous fabric filled with a weak
  sand/cement grout.

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• Rock bolts
• Steel bolts are frequently set in holes drilled
  into the rock to assist in supporting the entire
  roof or individual rock slabs that tend to fall
  into a tunnel. Rock bolts maintain the stability
  of an opening by suspending the dead weight of a
  slab from the rock above by
• providing a normal stress on the rock surface to
  clamp discontinuities together and develop beam
  action
• by preventing key blocks becoming loosened so
  that the strength and integrity of the rock mass
  is maintained.
• If the characteristics of the rock are such that
  the bolts will suffice in supporting the roof or
  parts thereof, the use of bolts is both safe and
  economical.
• The effective use of bolts requires some
  understanding of the natural forces that exist
  underground. In an underground excavation all
  downward-acting forces are transmitted to the
  walls of the excavation. Most of the rock above
  the excavation is supported by natural arch
  action that bears on the walls. The arch suspends
  the remaining rock below the arch. If this
  suspended rock lacks sufficient strength, it sags
  and tension cracks develop. As the cracks work up
  into the roof, weakening the suspended strata,
  rock begins to fall all at once or over an
  extended period of time. If the rock is strong
  enough and free of large slips and cracks, the
  rock that is subject to falling usually should
  not exceed one-third of the width of the roof. It
  is this rock that bolts can support.

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Rock Bolting
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• Shotcrete
• Pneumatically applied mortar and concrete are
  increasingly being used for the support of
  underground excavations. The effectiveness of a
  shotcrete is determined by its compressive
  strength, bond strength, flexural strength and
  modulus of elasticity. A layer of shotcrete 150mm
  thick around a tunnel 10m in diameter can carry a
  load of 500 kN/m² corresponding to a burden
  exeeding 20m of rock. A combination of rock bolts
  and shotcrete has proved an excellent temporary
  support for all qualities of rock.
• Shotcrete is best known in tunnelling as an
  integral component of the NATM method.
  Quick-setting concrete is sprayed onto the bare
  rock surface immediately after excavation, and
  rapidly hardens to form a preliminary support
  until the final lining of conventional poured
  concrete can be installed.
• Shotcrete has advantages and disadvantages.
  Traditionally, shotcrete's quick-setting
  properties have been achieved by the injection of
  high-alkaline additives at the spraying nozzle.
  However, this method has always had its
  drawbacks. The resulting concrete is highly
  porous, and lacks strength. Caustic dust from the
  additives can cause skin and lung problems, and
  represents a health hazard to construction
  workers.
• The German water authorities were concerned about
  the environmental problems associated with
  conventional shotcrete. Due to its porous nature,
  large quantities of groundwater seep through
  causing caustic alkalines to be leached out of
  the concrete. These are washed into aquifers and
  rivers, where they constitute a serious polluant.
  Leaching causes problems for tunnel owners as
  well, because hardened leachate rapidly blocks
  the tunnel's drainage systems.

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• Wire mesh
• Wire mesh is used to support small pieces of
  loose rock or as reinforcement for shotcrete. Two
  types of wire mesh are commonly used in
  underground excavations chainlink mesh and
  weldmesh. The chainlink mesh is commonly used for
  fencing and it consists of a woven fabric of
  wire. The wire can be galvanized for corrosion
  protection and it tends to be flexible and
  strong. Weldmesh is commonly used for reinforcing
  shotcrete and it consists of a square grid of
  steel wires, welded at their intersection points.

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• Tunnel Lining
• Permanent lining is required in most tunnels,
  always in soft ground and frequently in rock. The
  purpose of a lining is partly structural, to
  contain and support the ground and control inflow
  of water, as well as to provide an internal
  finishing suitable for the equipment of the
  tunnel. The principal materials and construction
  methods for permanent lining of bored tunnels
  are 1) in-situ concrete, 2) sprayed concrete
  (shotcrete), 3) segments in prefab concrete or
  cast-iron.
• The process of placing concrete in situ was
  incompatible with timber supports. In
  consequence, the first uses of concrete were for
  tunnels in good rock and it was only with the
  introduction of steel supports that concrete
  became the norm for a tunnel lining material.
  In-situ forms used for lining tunnels are, with
  few exceptions, of the travelling type,
  constructed of steel. The travelling type form is
  constructed of steel members which are lines with
  steel plate or wood to give a surface which
  conforms with the shape of the inside surface.

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• Precast Concrete Segments
• In recent years a number of tunnels driven
  through earth by using shields or TBMs have been
  lined with prefabricated concrete segments,
  which, in general, were placed immediately behind
  the excavating operation, either with e.g. the
  tailpiece of an excavating machine, or in the
  case of TBMs a special segment erector. The
  number of segments used to produce a ring has
  varied from two to eight or more, and the width
  of the rings has been in the range of 60 150
  cm. This method of driving a tunnel has in
  general proved very satisfactory and economical.
• Precast concrete segments used to line large
  diameter running tunnels on the Lisbon Metro in
  Portugal were designed to withstand highly
  aggressive ground conditions. The ground water
  beneath the historic centre and below the old
  dockyards along the Tagus River is contaminated
  with chlorides, nitrates and sulphates, which are
  all aggressive to elements to steel reinforced
  concrete. Corrosion attacks to the concrete were
  a major design consideration. To comply with a
  120-year design lifespan, the concrete of the
  precast segmental linings has a permeability of
  no more than 10mm and B40 quality minimum
  strength requirement of 40 Mpa after 28 days. The
  segments also require an early strength of 10-12
  Mpa to allow a strike of the formwork within
  seven days. Meeting these specifications began
  with a low W/C ratio, which was not easily
  achieved with the SRMR (sulphate resistant
  cement) available in Portugal.

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• Ventilation of tunnels
• Mechanical ventilation systems provide the
  temperature, humidity and air velocity conditions
  necessary to give tunnel users a reasonable
  degree of comfort during normal operation. When a
  fire occurs in a tunnel, the system must also
  provide a safe evacuation route for tunnel users
  and access for fire fighting services.
• The choice and design of a ventilation system
  depends on these main factors
• tunnel length, number of tubes, urban or rural
• fresh air requirement under normal and special
  traffic situations
• admissible air pollution around tunnel portals
• fire safety considerations.
• Key pollutants include carbon dioxide, nitrogen
  oxides, nitrogen dioxide, hydrocarbons PM10 and
  lead.
• The increase in the number of long tunnels has
  created demand for better understanding of
  ventilation techniques and aerodynamics behaviour
  of vehicles. In addition to demand for design and
  operational efficiency, this is driven by the
  need to comply with new safety and environmental
  legislation.

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• Ventilation during construction
• During construction it is necessary to ventilate
  a tunnel for various reasons
• To furnish fresh air for the workers
• To remove the dust caused by drilling, blasting,
  mucking, diesel engines, and other operations
• To remove obnoxious gases and fumes produced by
  explosives
• Mechanical ventilation is usually supplied by
  electric fans, as for example axial flow pressure
  fans. If air is blown into a tunnel, it may be
  forced through a lightweight pipe or fabric duct.
  If air is exhausted, it is necessary to use a
  rigid duct that will not collapse.
• The exhaust method has the advantage of more
  quickly removing objectionable air from spaces
  occupied by the workers.