Immersed Tunnels

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Immersed Tunnels

• Typically, an immersed tunnel is made by
• sinking precast concrete boxes into a dredged
  channel and joining them up under water.
• Tunnel sections in convenient lengths, usually 90
  to 150 meters, are placed into a pre-dredged
  trench,
• joined, connected and protected by backfilling
  the excavation.
• The sections may be fabricated in shipyards, in
  dry docks, or in temporary construction basin
  serving as dry docks.

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• Immersed tunnels are,
• more advantegous as a subaquous solution in soft
  soils
• increasingly used alternative to traditionally
  used shield tunnelling, without having the risks
  associated with pressure chambers and inrush of
  water.
• also suitable in water deeper than it is possible
  with the shield method, which essentially is
  restricted to less than 30 m of water (concerning
  the maximum air pressure at which workers can
  safely work).
• Advantegous as there is less loss in height than
  with tunnelling deeply under the riverbed, and
  the tunnel may therefore be shorter overall.

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• The world's longest and deepest application to
  date is the twin-tube subway crossing of San
  Francisco Bay, constructed between 1966 and 1971
  with a length of 3.6 miles (5,8 km) in a maximum
  water depth of 41 m. The 100 m long, 15 m wide
  sections were constructed of steel plate and
  launched by shipbuilding procedures.
• The first tunnel of the concrete type was
  constructed in 1940 in Rotterdam under the Maas
  estuary.

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• The most profound effect of an immersed tunnel on
  the environment concerns the element it is meant
  to bypasswater.
• The influence of the tunnel on the groundwater
  and the surface water in the area plays a
  predominant role in the tunnel design and
  construction methods.
• An aspect of more recent concern affecting
  construction is the possible presence of
  contaminated soils that must be removed for the
  tunnel trench. Ways of removing these soils and
  transporting them to depositories that are
  especially equipped to receive them are
  environmental problems requiring novel techniques
  and quality control procedures.
• The more traditional environmental aspects are
  those encountered on any construction job noise,
  dirt, and traffic hindrance.

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• The top of the tunnel should be protected by
  adequate protective backfill, extending about 30
  m on each side of the structure and confined
  within dykes or bunds. The fill must be protected
  against erosion by currents with a rock blanket,
  protective rock dykes or other means.
• Tides and current effects of the waterway must be
  evaluated to determine conditions during dredging
  and tube sinking operations.
• Importantly, dredging and backfilling operations
  should be executed in such a manner as to limit
  disturbance in the natural ecological balance at
  the construction site.
• Governmental agencies having jurisdiction over
  environmental protection, natural resources or
  local conditions must be consulted and approval
  of authorities should be obtained in the
  preliminary design stage.

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• Foundation
• The foundation method to be used must be chosen
  with due consideration, first of all, for the
  subsoil conditions and the degree to which the
  tunnel will be subject to dynamic loadings, and
  earthquake loadings in particular. Pile
  foundations are an option but this solution has
  been used for a few tunnels only.
• For both steel and concrete types of tunnels, the
  main tasks are
• Excavation of a tunnel trench to specifications
  and to keep it free of siltation that may be
  detrimental to the permanent foundation until
  this foundation has been constructed and the
  tunnel has been brought to rest on it.
• Construction of watertight and durable tunnel
  elements.
• Installation of the tunnel elements in the tunnel
  trench.
• Construction of watertight and durable joints
  between the tunnel elements.
• Construction of a durable foundation for the
  tunnel.

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• Once completed, an immersed tunnel is no
  different operationally from any other tunnel.
  However, it is built in a completely different
  way.
• The Construction technique
• A trench is dredged in the bed of the water
  channel.

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• Tunnel Trench Dredging
• The dredging works required for the construction
  of an immersed tunnel will normally comprise
  some, or all, of the following items
• Dredging of a casting or launching basin.
• Dredging of test pits in the waterway for
  evaluation of siltation of tunnel trench.
• Widening of the existing navigation channel in
  order to provide temporary navigation channels
  outside the marine working area.
• Compensation grouting to make up for the
  reduction of the waterway cross section caused by
  the permanent tunnel works, and thereby avoiding
  changes in hydrographical and biological
  conditions in the waterway.
• Dredging of the tunnel trench for the immersed
  tunnel section.
• Dredging of an access channel between the
  casting/launching basin and the tunnel trench.
• Maintenance dredging.

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• The dredging volume is generally in the order of
  l million m3 per km for a typical four-lane
  motorway tunnel.
• The excavation must provide space for the
  prefabricated tunnel body the sand or gravel
  foundation under the body as wells as the
  protective backfill on the sides and on the top
  of the tunnel.
• Because the top of the backfill has to be kept
  below the existing or future navigation channel
  profile, a trench bottom level at between 25 and
  30 m below Low Water level is quite common.
  Immersed tunnels in deep or open sea may require
  specially built dredgers.

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• Except for cases where very soft subsoil, deemed
  unsuitable for support of the tunnel, has to be
  removed and replaced by suitable materials, the
  general requirements for the dredging of the
  trench bottom are
• A clean, even surface, as close as possible to
  the upper acceptable limit in order to avoid the
  economic consequences of having to fill
  overdredged areas
• A minimum disturbance of the remaining exposed
  upper soil layers in the trench bottom, in order
  to limit the changes in the geotechnical
  characteristics of the subsoil.
• The possible physical disturbance and softening
  of the exposed soil layers in the trench bottom,
  particularly in cohesive subsoils, can have a
  considerable influence on the geotechnical
  behaviour of these soil layers later-and, hence,
  on the quality of the tunnel support as a whole.
  This in turn influences the design of the
  structural tunnel body and, thus, eventually the
  overall economy. These technical requirements
  are met by
• Using the proper type of dredger(s).
• Careful controlling the position of the cutting
  tool, bearing in mind that the dredging normally
  has to be done in tidal waters and sometimes in
  waters subject to swell and waves.
• Careful planning the dredging operation in order
  to avoid undesirable failures of the slopes.
• Timing of the dredging operation, in order to
  limit the time that the trench bottom is exposed
  and, at the same time, to limit the sedimentation
  caused by subsequent dredging nearby.

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• Construction of Tunnel Elements
• The tunnel elements are made fully or partially
  buoyant by means of temporary bulkheads installed
  at the element ends.
• In addition to providing proper structural
  strength and controlling the weight of the
  element, the main design and construction task of
  the reinforced concrete tunnel is to provide a
  watertight structure. For many years, the answer
  was to wrap the tunnel element in a watertight
  membrane composed of steel on the bottom, outer
  walls and even on the roof. Alternatively,
  bituminous membrane has been used on the outer
  walls and roof.
• In recent years, reinforced concrete tunnels
  without a membrane at all are being used. Above
  all, this will require sophisticated control of
  concrete temperature during hardening to avoid
  cracking. In order to reduce the development of
  cracks during hardening, primarily in the walls
  when they are cast after the bottom slab, cooling
  of the lower part of the walls has been the
  practice for many years. Insulation of the
  formwork and careful sequencing of stripping of
  the forms are also used to control the concrete
  temperature.
• Improved field concrete technology aimed at
  minimising the development of cracks during
  hardening, combined with moderate prestressing,
  seems to be the course to follow.

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• Tunnel elements are constructed in the dry, for
  example in a casting basin, a fabrication yard,
  on a ship-lift platform or in a factory unit.

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• Casting Basins
• The tunnel elements can be prefabricated in a
  casting basin or in a dry dock. For shorter
  roadway and railroad tunnels, the elements are
  normally cast in one batch in a casting basin. A
  programme for control of concrete density and
  concrete dimensions is required in order to
  control the weight and displacement of the tunnel
  elements.
• The typical casting sequence is
  bottom/walls/roof, but sometimes all at once, in
  15-20 m segments.
• The tunnel elements can be monolithic, or they
  can be provided with flexible joints between
  tunnel segments within the elements. The latter
  arrangement minimises longitudinal bending
  moments caused by compression of the subsoil in
  the permanent stage, but is unsuitable for
  railway tunnels in soft ground and in seismic
  regions.
• Normally the tunnel elements will be buoyant and
  need to be ballasted prior to flooding of the
  casting basin in order to make sure that they
  remain parked until they are to be brought to
  the immersion location. This ballasting is
  normally done with water contained in
  purpose-built ballast tanks inside the tunnel
  element. Pumps and associated pipelines allow
  charging and removal of the ballast. A number of
  lifting eyes and bollards must be provided on the
  element roof.
• Watertight, temporary bulkheads are installed at
  the ends of the element, and rubber gaskets are
  mounted around the periphery of the one end of
  the tunnel element, while a plane steel plate is
  provided at the opposite end. Later, when the
  tunnel element is joined to the previously placed
  tunnel element, this gasket provides a watertight
  seal between the two tunnel elements.
• As the casting basin is flooded or as the tunnel
  is launched from the dock, the tunnel element is
  checked for watertightness, the attention being
  directed principally towards the temporary
  bulkheads and pipe let-ins.
• Recently a system of constructing concrete tubes
  on floating pontoons is developed. By removing
  the need for casting basins on the river or canal
  costs are reduced, and the process is more
  environmentally friendly.

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Cooling of concrete in outer tunnel walls
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• The ends of the element are then temporarily
  sealed with bulkheads.
• Each tunnel element is transported to the tunnel
  site - usually floating, occasionally on a barge,
  or assisted by cranes.

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• Installation of tunnel elements in the trench
• For transportation of the element from the
  flooded casting basin or dock to the tunnel
  trench, conventional towage is normally used.
• The warping, which ends with the tunnel element
  being moored for immersion, is normally carried
  out by the contractor's organisation responsible
  for the subsequent sinking and joining, whereas
  towing normally is done by experienced towage
  companies.
• The immersion of the tunnel element is carried
  out after the tunnel element bas been moored and
  the element has been ballasted as necessary to
  provide adequate loads in the immersion tackles.

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• The tunnel element is lowered to its final place
  on the bottom of the dredged trench.

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• The new element is placed against the previous
  element under water. Water is then pumped out of
  the space between the bulkheads.
• Water pressure on the free end of the new element
  compresses the rubber seal between the two
  elements, closing the joint.

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• Once placed, the elements are joined first by
  bringing rubber gasket at the joint into contact
  with the steel face of the previously placed
  tunnel element, and then draining the joint
  chamber, thereby mobilising the full hydrostatic
  water pressure on the tunnel cross section remote
  end.

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• Backfill
• material is placed beside and over the tunnel to
  fill the trench and permanently bury the tunnel,
  as illustrated in the figures.

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• Approach structures can be built on the banks
  before, after or concurrently with the immersed
  tunnel, to suit local circumstances.

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• Immersed tunnels have been in widespread use for
  about 100 years. Over 150 have been constructed
  all over the world, about 100 of them for road or
  rail schemes. Others include water supply and
  electricity cable tunnels. The examples given
  indicate the diversity of projects that have been
  realized.

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• Immersed tunnels do not suit every situation.
  However, if there is water to cross, they usually
  present a feasible alternative to bored tunnels
  at a comparable price, and they offer a number of
  advantages, such as
• Immersed tunnels do not have to be circular in
  cross section. Almost any cross section can be
  accommodated,
• making immersed tunnels particularly attractive
  for wide highways and combined road/rail tunnels.
  Some examples of realised cross sections are
  shown below.

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• Immersed tunnels can be placed immediately
  beneath a waterway. In contrast, a bored tunnel
  is usually only stable if its roof is at least
  its own diameter beneath the water. This allows
  immersed tunnel approaches to be shorter and/or
  approach gradients to be flatter - an advantage
  for all tunnels, but especially so for railways.

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• expensive, such as the soft alluvial deposits
  characteristic of large river estuaries. They can
  also be designed to deal with the forces and
  movements in earthquake conditions, as in the
  example illustrated above, to be placed in very
  soft ground in an area prone to significant
  earthquake activity.

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• Bored tunnelling is a continuous process in which
  any problem in the boring operation threatens
  delay to the whole project.
• Immersed tunnelling creates three operations -
  dredging, tunnel element construction and tunnel
  installation, which can take place concurrently,
  thus moderating programme risk considerably.
• Partly for this reason, an immersed tunnel is
  generally faster to build than a corresponding
  bored tunnel.

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ARE THERE ANY SPECIAL PROBLEMS ?

• Immersed tunnels are sometimes perceived by
  newcomers to the technology as "difficult" due to
  the presence of marine operations. In reality
  though, the technique is often less risky than
  bored tunnelling and construction can be better
  controlled. The marine operations, though
  unfamiliar to many, pose no particular
  difficulties.

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The perceived problems include

• DREDGING
• Dredging technology has improved considerably in
  recent years, and it is now possible to remove a
  wide variety of material underwater without
  adverse effects on the environment of the
  waterway.

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• INTERFERENCE WITH NAVIGATION
• Interference with navigation On busy waterways,
  it is sometimes assumed that construction of an
  immersed tunnel would be impractical as it would
  interfere with shipping. In fact, such tunnels
  have been successfully built in some
  exceptionally busy waterways without undue
  problems.

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• WATERTIGHTNESS
• It is often assumed that the process of building
  a tunnel in water, rather than boring through the
  ground beneath it will increase the likelihood of
  leakage. In fact, immersed tunnels are nearly
  always much drier than bored tunnels, due to the
  above-ground construction of the elements.
  Underwater joints depend on robust rubber seals
  which have proved effective in dozens of tunnels
  to date.

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A NEW DEVELOPMENT THE SUBMERGED FLOATING TUNNEL

• Traditional immersed tunnelling results in a
  tunnel buried beneath the waterway which it
  traverses. A new development- the submerged
  floating tunnel - consists of suspending a tunnel
  within the waterway, either by tethering a
  buoyant tunnel section to the bed of the
  waterway, or by suspending a heavier-than-water
  tunnel section from pontoons.
• This technique has not yet been realised, but one
  project, in Norway, is currently in the design
  phase. The submerged floating tunnel allows
  construction of a tunnel with a shallow alignment
  in extremely deep water, where alternatives are
  technically difficult or prohibitively expensive.
  Likely applications include fjords, deep, narrow
  sea channels, and deep lakes.

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Design Aspects of Immersed Tunnels

• The starting point of an immersed tunnel design
  is required cross-sectional area i.e. the hollow
  space. The tunnel must have the same number of
  traffic lanes as the road.
• Dimensional requirements vary from country to
  country generally speaking the lanes should be
  3,5 m wide with headroom above, depending on
  local regulations (e.g. 4.5 m for Holland).
• There should also be a clearance from the
  carriageway to the walls of 0.8 to 1.0 m, for
  broken down cars. The clearance will also reduce
  the wall effect drivers shying away from the
  wall thereby reducing the capacity of the road.
  Above the headroom there should be adequate room
  for ventilation booster fans, luminaries and
  signal equipment. In the dual-carriageway tunnels
  there is often a service gallery for cables
  located between the traffic tubes.

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• Design for floating
• After construction, the elements are floated to
  their final position. The element is then made
  heavier than its displacement by means of
  temporary ballast (often water), after being
  temporarily supported by the immersion rigs. At a
  later stage this ballast is replaced by
  definitive ballast in the shape of non-reinforced
  concrete below the future carriageway or
  externally, or other secondary interior
  structural concrete. By this time the immersion
  equipment and bulkheads will have to be removed.
  The element must now weigh sufficiently more than
  its buoyancy to remain in place.
• The pressure head of the groundwater below the
  tunnel base may lag behind the water level in the
  river. At low tide this may result in an
  additional upward force. To compensate for
  effects of this kind, the design criterion often
  adopted at this stage is that the weight of the
  tunnel must exceed the water displacement by an
  absolute minimum margin against flotation when
  all removable items and backfill are removed.
  This floation margin may be in the range of
  1.075, but is determined on a project basis. The
  safety margin is later increased because the
  sides and top of the dredged trench into which
  the tunnel was placed is then backfilled.
• This results in the first place in a load on the
  roof whereas friction on the walls is ignored.
  Erosion protection is placed to continuously
  maintain a 1.15 or 1.2 factor of safety against
  flotation, depending upon the clients
  requirements, and safety against sinking ships
  and dropping anchors.

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• At the transport stage ,
• Weight 0.99 maximum water displacement or
• 2.46 S 3.0 0.99 (B H S) (1)
• A value of 2.46 will be taken as the specific
  weight of reinforced concrete in the flotation
  stage, and a density of 2.42 in the final stage.
• In the final phase a counter-flotation margin
  should apply,
• assume this margin to be 7,5 per cent, hence
• Weight 1.075 water displacement, or
• 2.42 S 2.25 B 1.075 (B H S) (2)

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Case study Øresund Link Strait crossing

• The Øresund link connects Copenhagen on Zealand,
  Denmark to Malmö in Sweden thus establishing a
  land traffic corridor from Scandinavia to the
  continent. The link comprises a 3.5 km immersed
  tunnel, 4 km artificial island, and 8 km bridge,
  including a 490 m span cable-stayed bridge. The
  link incorporates approximately one million m³ of
  concrete, of which more than two thirds
  constitute the immersed tunnel.
• The Øresund Tunnel was motivated by the fact that
  one of the main shipping lane is very close to
  the Copenhagen international airport, making a
  high bridge over the nearest navigational channel
  unfeasible.

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• The tunnel cross-section accommodates two tubes
  for the two-track railway and two tubes for the
  four-lane motorway. A central installation
  gallery between the motorway tubes doubles as a
  safe and smoke-free escape route in case of
  emergency.
• The immersed part of the tunnel consists of 20
  elements, each approximately 175 m long,
  resulting in a total immersed tunnel length of
  3,510 m. Each element is made from 8 segments,
  joined together by temporary prestressing, and
  weighing approximately 56,000 t. The outer
  cross-sectional dimensions are 8.6 m by 38.8 m,
  the height being governed by the railway
  clearance profile. The track is fastened directly
  to the bottom slab, the omission of the ballast
  reducing the required tunnel height. The elements
  were placed in a pre-dredged trench, and founded
  on a gravel bed. Backfilling along the sides and
  on the roof was designed to offer a permanent
  cover and protection of the tunnel in all
  situations.
• The final tunnel profile is in general below
  seabed level, and at the Drogden navigation
  channel the top of the cover is 10 m below water
  level. The rock cover was designed to withstand a
  falling or dragging anchor, or a sunken ship.
  Furthermore the protective layer is stable
  against scour and erosion caused by currents or
  ship propellers.

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• All 20 Tunnel elements were fabricated in a
  purpose-built casting yard. The precast facility
  applied production techniques developed and
  tested in the construction of bridges over the
  last 20 years, but it was the first time these
  techniques were applied to immersed tunnel
  construction, involving casting and in
  incrementally launching segments weighing up to
  7,000 t and elements of 56,000 t.
• Factory conditions were achieved by the erection
  of sheds where the reinforcement was assembled
  and prefabricated. A central shed covered two
  production lines, where two segments were made
  simultaneously per week, in order to meet the
  time schedule. Each 22 m segment was constructed
  on specially prepared formwork, being cast in one
  single pour of 2,800 m³ of concrete over a
  30-hour period. By casting an entire segment in a
  single operation production was sped up, and
  thermal cracking of the concrete was minimized. A
  crack-free concrete is essential, since the
  tunnel design does not include a watertight
  membrane.

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