Transcript Immersed Tunnels - Middle East Technical University

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.
• 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.
• The most profound effect of an immersed tunnel on the
environment concerns the element it is meant to
bypass—water.
• 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.
• 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.
Foundation
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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:
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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.
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Construction of watertight and durable tunnel elements.
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Installation of the tunnel elements in the tunnel trench.
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Construction of watertight and durable joints between the tunnel
elements.
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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:
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A trench is dredged in the bed of the water channel.
Tunnel Trench Dredging
The dredging works required for the construction of an
immersed tunnel will normally comprise some, or all, of the
following items:
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Dredging of a casting or launching basin.
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Dredging of test pits in the waterway for evaluation of siltation of
tunnel trench.
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Widening of the existing navigation channel in order to provide
temporary navigation channels outside the marine working area.
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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.
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Dredging of the tunnel trench for the immersed tunnel section.
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Dredging of an access channel between the casting/launching
basin and the tunnel trench.
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Maintenance dredging.
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.
Construction of Tunnel Elements
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The tunnel elements are made fully or partially buoyant by means of
temporary bulkheads installed at the element ends.
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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.
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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.
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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.
Casting Basins
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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.
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The typical casting sequence is bottom/walls/roof, but sometimes all at once, in 15-20 m
segments.
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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.
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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.
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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.
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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.
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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.
Cooling of concrete in outer tunnel walls
• 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.
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.
• The tunnel element is lowered to its final place on the bottom of the
dredged trench.
• 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.
• 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.
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.
• 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.
• 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.
• 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.
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.
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.
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.
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 aboveground construction of the elements. Underwater joints depend
on robust rubber seals which have proved effective in dozens
of tunnels to date.
: THE SUBMERGED FLOATING TUNNEL
A NEW DEVELOPMENT
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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.
Design Aspects of Immersed Tunnels
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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.
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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).
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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.
Design for floating
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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.
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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.
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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 client’s
requirements, and safety against sinking ships and dropping anchors.
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)
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 cablestayed 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.
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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 predredged 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.
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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.
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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.