BRIDGE ENGINEERING
INTRODUCTION
Bridge is a structure designed to provide continuous
passage over an obstacle. They commonly carry highways, railway lines and
pathways over obstacles such as waterways, deep valleys and other
transportation routes. They may also carry water pipe lines, support power
cables, or house telecommunications lines. Some special types of bridges are
defined according to function. An overpass allows one transportation route,
such as a highway or railway line, to cross over another without any traffic
interference between the two routes. The overpass elevates one route to provide
clearance to traffic on the lower level. An. aqueduct transports water.
Aqueducts have been used historically to supply drinking water to densely
populated areas. A viaduct carries a railway or highway over a land
obstruction, such as a valley.
The earliest bridges were simple structures created
by spanning a gap with timber or rope. Designs became more complex as builders developed
new construction methods and discovered better construction materials. The
stone arch was the first major advance in bridge design. It was used by the
ancient Greeks, Etruscans and Chinese. The Romans perfected arch design and
used arches to build massive stone bridges throughout the Roman Empire. Stone
arch construction remained the premier bridge design until the introduction of
the steam locomotive in the early 19th century.
Between 1830 and 1880, as railway construction
expanded throughout the world, bridge design and construction also evolved to
carry these heavy vehicles over new obstacles. Designers experimented with a
wide variety of bridge types and materials to meet the demand for greater
heights, spans and strength. Locomotives were heavier and moved faster than
before, requiring stronger bridges. The basic beam bridge, a simple beam over a
span, was strengthened by adding support piers underneath and by reinforcing
the structure with elaborate scaffolding called truss. During the period of
railway expansion, iron trusses replaced stone arches as the engineers
preferred to design large bridges.
In 1855, British inventor Sir Henry Bessemer
developed a practical process for converting cast iron into steel. This process
increased the availability of steel and lowered production costs considerably.
The strength and lightness of steel revolutionized bridge building. In the late
19th century and the first half of the 20th century, many large-scale steel
suspension bridges were constructed over major waterways. Also, in the late
19th century, engineers began to experiment with concrete reinforced with steel
bars for added strength.
More recently, reinforced concrete has been combined
with steel girders, which are solid _beams that extend across a span. The last
three decades of the 20th century saw a period of large-scale bridge building
in Europe and Asia. Current research focuses on using computers,
instrumentation, automation and new materials to improve bridge design,
construction and maintenance.
(1) Superstructure: Superstructure consists of
structural members carrying a communication route. Therefore, handrails, guard
stones and flooring supported by any structural system such as beams, girders,
arches and cables constitutes the superstructure.
(2) Substructure: Substructure of a bridge
supports its superstructure. It consists of the following components.
(i) Abutments : Abutments are the end supports
of the superstructure, retaining a their back. They are built either with stone
masonry or brick masonary or ordinary mass concrete or reinforced concrete. The
to of the abutment is made flat when the superstructure is of trusses or griders
or serni-circular arch. In case of segmental or elliptical arch type of
superstructure, the abutment is made skew. To drain off the retained earth ,
weep holes are provided at different levels through the body of abutment.
(ii) Piers: The intermediate supports of a
bridge substructure are called piers. The function of providing piers is to
divide the total length of the bridge into suitable spans with minimum
obstruction to the stream or river.
In case of an arch bridge having number of spans,
every fourth or fifth pier is designed to resist the inclined reaction due to
dead load on one side and no load on other side. Such a pier is known as an
abutment pier. By providing an abutment pier, the construction of an arch
bridge can be carried out in sections resulting in speedy and economical
construction. Moreover, the effect of any failure of an arch, may be due to
earthquake or flood or any other reason, can be easily localized because the
effect of the failure does not extend from one end of the bridge to the other
but ends at the abutment pier itself.
(iii) Wing walls: Wing walls are provided at both
of the abutments to retain the earth filling of the approach road. (Approach
roads are the roads at both ends of the bridge.) They are constructed of the
same materials as those of the main abutment. The design of the wing wall
depends upon the nature of banks. Depending upon the type of material used, the
wing walls are categorized as masonry wing walls and reinforced concrete wing
walls.
The main function of a wing wall is to provide a
smooth entry into the bridge site and to support and protect the embankment.
(iv) Foundations: Foundations are required to
distribute the total load of the bridge equally and uniformly on the subsoil.
The bridge foundations can be divided into the following three categories.
(a) Spread
foundations:
Spread foundations are also referred to as open foundations as the construction
work is carried out in open excavation. This type of foundation is adopted
where the water is not very deep and good soil is available at a shallow depth.
(b) Pile
foundations:
Pile foundations are adopted when the loose soil extends to a great depth. The
load of the structure is transmitted by the piles to the hard stratum below or
it is resisted by the friction developed on the sides of the piles. R.C.C.
piles are the most popularly used piles.
(c) Caissons: The word caisson is derived from
the French word caisse meaning 'a box'. In civil engineering, a caisson is
defined as a structure, which is sunk through the ground or water to exclude
water and mud slurry during the process of excavation of foundations and later
it becomes an integral part of the substructure.
Caissons are of three types :
(1) Box caissons: A box caisson is open at the top
and closed at the bottom. It may be built of reinforced concrete, steel or
timber. It is suitable when the velocity of flow of water is slow and the depth
of water is 6 m to 8 m.
(2) Wells: A well is a caisson which is
open at the top as well as at the bottom. It is provided with a cutting edge at
the bottom to facilitate sinking. The shape of the well may be rectangular,
circular or of any other shape.
(3) Pneumatic
caissons: A
pneumatic caisson is open at the bottom but closed at the top. Compressed air
is used to remove water from the working chamber and the foundation work is
carried out in dry conditions. Pneumatic caissons become useful when it is not
possible to adopt wells.
Bridge designs differ in the way they support loads.
These loads include the weight of the bridges themselves (the weight of the
material used to build the bridges), and the weight and stresses of the
vehicles crossing them. There are basically eight types of common bridge
designs.
Each design differs in appearance, construction
methods, materials used and overall expenses. Some designs are better for long
spans while others are suitable for short spans.
(1) Beam bridges: Beam bridges represent the
simplest of all bridge designs. A beam bridge consists of a rigid horizontal
member called a beam that is supported at both ends, either by a natural land
structure such as the banks of a river, or by vertical posts called piers. Beam
bridges are the most commonly used bridges in highway construction.
Single-piece, rolled-steel beams can support spans of 15 m to 30 m (50 to 100
ft.). Heavier reinforced beams and girders are used for longer spans.
(2) Cantilever
bridges:
Cantilever bridges are a more complex version of the beam-bridge design. In a
cantilever design, a tower is built on each side of the obstacle to be crossed
and the bridge is built outward or cantilevered from each tower. The towers
support the entire load of the cantilevered arms. The arms are spaced such that
a small suspended span can be inserted between them. The cantilevered arms
support the suspended span while the downward force of the span is absorbed by
the towers.
Cantilever bridges are self-supporting during
construction. They are often used in situations where the use of scaffolding or
other temporary supports is difficult. The Forth Bridge, a railway bridge
across the Firth of Forth in Queensferry, Scotland, has two main spans of 521 m
(1,710 ft.) each. The Howrah Bridge in Calcutta, India, Suspension was opened
in 1943, with a main span of 457 m (1,500 ft.). The Quebec Bridge was the
world's across the Saint Lawrence River in Canada has a span of 549 m (1,800
ft.).
(3) Arch bridges: Arch bridges are characterized
by their stability. In an arch, (1,595 ft. 6 the force of the load is carried
outward from the top to the ends of the arch, in Japan was where abutments
prevent the arch ends from spreading apart. The rise of the While susin arch is
kept as high as possible to reduce the horizontal thrust and to economize
drawback. It is the design of the piers and abutments. Arch bridges have been
constructed of bending in the stone, brick, timber, cast iron, steel and
reinforced concrete.
Steel and concrete arches are particularly well
suited for bridging ravines or chasms with steep, solid walls. The New River
Gorge Bridge in West Virginia is the longest arch bridge, spanning a gap of 518
m (1,700 ft.). Other long arch bridges include the Bayonne Bridge between New
York and New Jersey, and the Sydney Harbor Bridge in Australia, with main spans
of 504 m (1,652 ft.) and 503 m (1,650 ft.), respectively.
(4) Truss bridges: Truss bridges utilize strong,
rigid frameworks that support these bridges over a span. Trusses are created by
fastening beams together in a triangular configuration. The truss framework
distributes the load of the bridge such that each beam shares a portion of the
load. Beam, cantilever and arch bridges may be constructed of trusses. Truss
bridges can carry heavy loads and are relatively light weight. They are also
inexpensive to build. The Astoria Bridge over the Columbia River in Oregon has
a span of 376 m (1,232 ft.).
(5) Suspension
bridges: As
shown in fig. 41-10, suspension bridges consist of two large, or main cables
that are hung (suspended) from towers. The main cables of a suspension bridge
drape over two towers with the cable ends buried in enormous concrete blocks
known as anchorages. The roadway is suspended from smaller vertical cables that
hang down from the main cables. In some cases, diagonal cables run from the
towers to the roadway and add rigidity to the structure. The main cables
support the weight of the bridge and transfer the load to the anchorages and the
towers.
Suspension bridges are used for the longer spans.
The Brooklyn Bridge, which was the world's longest suspension bridge at the
time of its completion in 1883, crosses the East River in New York City and has
a main span of 486 m 31 cm (1,595 ft. 6 in). The Akashi Kaikyo Bridge between
Honshu and Awaji Island in Japan was completed in 1998, with a span of 1,990 m
(6,529 ft.).
While suspension bridges can span long distances,
this design has a serious drawback. It is very flexible and traffic loading may
cause large deflections, or bending in the bridge roadway. Suspension design is
rarely used for railway bridges because trains are heavier and travel faster
than highway traffic.
(6) Cable-stayed
bridges:
Cable-stayed bridges represent a variation of the suspension bridge. Cable-stayed
bridges have tall towers like suspension bridges, but in a cable-stayed bridge
the roadway is attached to the towers by a series of diagonal cables. A
cable-stayed bridge is constructed in much the same way as a suspension bridge
is, but without the main cables.
Cable stayed bridge FIG. 41-11 Cable-stayed designs
are used for intermediate-length spans. Advantages a cable-stayed bridge has
over a standard suspension bridge include speed of construction and lower cost
(since anchorages are not necessary). There are no massive cables, as with
suspension bridges, making cable repair or replacement simpler. The Pont de
Normandie (Normandy Bridge) over the Seine river near La Havre in France,
opened in 1995, has a span length of 856 m (2,808 ft.).
(7) Movable
bridges: Movable
bridges make up a class of bridges in which a portion of the bridge moves up or
swings out to provide additional clearance beneath the bridge. Movable bridges
are usually found over heavily travelled waterways. They are generally
constructed over waterways where it is either impractical or too costly to
build bridges with high enough clearances for water traffic to pass underneath.
The three most common types of movable bridges.
(i) Bascule
bridges: In case
of a bascule bridge, the entire superstructure is rotated in the vertical plane
about the horizontal axis. Suitable rack and pinion arrangement and counter
weights are provided for easy operation of hinge at the back end of the bascule.
Modern bascule bridges usually have two movable
spans that rise upward, opening in the middle. They are used for the short
spans. A bascule bridge over the Black river in Lorain, Ohio, has a length of
102 m. The main draw back of this bridge is that its working is seriously
disturbed when the wind is blowing with high velocity.
(ii) Vertical-lift bridge: The bridges whose span
can be lifted vertically high enough for tall ships to pass underneath are
called vertical-lift bridges. A typical vertical-lift bridge, having a suitable
arrangement of the pulley and the counter weight.
The towers are provided on either side with sheaves
or grooves. The rigid trusses of the bridge move up and down by the cables,
which pass over the pulleys mounted at the top of the towers. The pulleys in
turn are connected to the counter weights at the other end. When the truss is
lifted up in the vertical plane, it allows navigation in the channel. The
vertical-lift bridge over Arthur Kill between Staten Island in New York City,
New Jersey has a span of 170 m (558 ft.) and can be raised 41 m (135 ft.) above
the water.
(iii) Swing
bridges: Swing
bridges are mounted on central pier provided with suitable bearings or rollers.
The superstructure consists of a pair of steel trusses and it can be rotated in
the horizontal plane about the vertical axis by some suitable system. The plan
and the elevation of a swing bridge. The dotted lines indicated the position of
the bridge after rotation.
Swing bridges have the advantage of not limiting the
height of passing vessels, but they do restrict the horizontal clearance, or
width, of passing ships. The longest swing-bridge span is that of a railway and
highway bridge crossing the Mississippi river at Fort Madison, Iowa. This
bridge has a span of 166 m. (8) Floating
bridges: Floating bridges are formed by fastening together sealed, floating
containers called pontoons and placing a roadbed on top of them. A pontoon
typically contains many compartments so that even if a leakage occurs in one
compartment, the pontoon will not sink. Some floating bridges are constructed
using boats or other floating devices rather than pontoons.
Floating bridges were originally developed and are
most widely used as temporary structures for military operations. For everyday
use, floating bridges are popular when deep water, bad riverbed conditions, or
other conditions make it difficult to construct traditional bridge piers and
foundations. A concrete-pontoon bridge carries a highway across Lake
Washington, near Seattle, Washington. it consists of 25 floating sections
bolted together and anchored in place and a span that can be opened to permit
the passage of large ships. The floating section of the bridge is 2.3 km (1.4
mile) long.
BRIDGE PLANNING AND CONSTRUCTION
New bridges are built either to replace old
structures that no longer meet the demands of modern traffic or to cross
obstacles on a new transportation route. Old bridges are replaced when repairs
cannot be made economically or when traffic becomes too heavy for the old bridge.
New transportation routes are built when traffic levels have outgrown the
capacity of existing routes or simply to make it faster to get from one busy
place to another. Often, new transportation routes are part of government
programmes to promote regional economic development.
There are three basic steps involved in the planning
and construction of bridges.
(1) Design selection: Engineers consider several
factors when designing a bridge.
(i) Engineers
consider the distance and the feature (such as a river, bay or canyon) to be
crossed.
(ii) They
anticipate the type of traffic and the amount of load the bridge may have to
carry.
(iii) They
work out the minimum span and height required for traffic travelling across and
under the bridge.
(iv) They
also consider the temperature, environmental conditions, and the physical
nature of the building site (such as the geometry of the approaches, the
strength of the ground, and the depth to firm bedrock).
(v) They
finally determine the best bridge design for a particular situation.
Once engineers have the data they need in order to
design a bridge, they create a work plan for constructing it. Factors to be
considered include availability of materials, equipment and trained labour;
availability of workshop facilities; and local transportation to the site.
These factors, in combination with the funding and time available for bridge
design and construction, are the major requirements and constraints on design
decisions for a particular site.
(2) Design
decisions: There
are four basic categories of design decisions.
(i) Bridge type: The bridge type (such as beam,
arch, truss, etc.) depends largely on the required dimensions for the bridge
and the type of traffic to be carried. The required length and clearances
needed by traffic are the major considerations in bridge design. Many bridges
are long enough to require several intermediate supports or piers. The location
of piers is usually a crucial factor, whether in water or on land.
(ii) Materials
required:
Materials historically used for bridge building include rope and other fibres,
wood, stone and masonry, iron, steel and concrete. Fibre, timber, stone and
masonry are now used occasionally; steel and concrete are the materials used
for most modern bridge building. Fibre rope is occasionally used for short
pedestrian bridges. Timber is suitable only for short spans that carry minimum
traffic. loads. Stone and masonry are sometimes used as facing materials on
concrete and steel bridges, if appearance is important enough to justify the
additional expense.
When deciding between steel and concrete, designers
evaluate the trade-offs among weight, strength and expense to determine which
material is best for a particular bridge. The major advantages of concrete are
that it is considerably cheaper than steel and can be formed into a greater
variety of shapes. For short bridges, the weight of material is not an
important concern, and so concrete is an economical choice: However, as span
increases, the weight of the structure increases substantially, and greater
strength is needed to support the overall structure. Steel tends to be
preferred for large bridges because less material has to be handled and
supported during the construction process. The distinction between steel and
concrete is not absolute, as most steel bridges have concrete decks, and all
concrete is reinforced with steel to provide greater tensile strength.
(iii) Type of
foundation: All
bridge piers rest on foundations that transfer loads from the bridge structure
into the ground. The foundations support the bridge, and their • design is
critical. Difficult conditions, such as deep water or soft ground, can make
foundation construction complicated and expensive. In such circumstances
designers may choose to decrease the number of piers by increasing span length.
Of course, greater span lengths often require a more expensive bridge type, and
therefore the trade-offs must be evaluated carefully.
If the ground is very strong at a bridge site, a
foundation is formed by pouring a simple concrete mat beneath each of the
piers. If the soil is weak, it may be excavated down to bedrock, and the piers
can then be built directly on the solid rock. Alternatively, a group of
vertical posts or piles, can be driven through the soil to bedrock, and piers
can be built on top of the piles.
(iv) Construction
methods: Bridges
are erected using a variety of construction methods. Some techniques are
associated with a particular bridge type, and care must be taken not to select
a design that requires construction methods unsuitable for the site. Concrete
and steel bridges are generally built using similar techniques, although
concrete bridges are built in shorter sections than steel bridges because of
the greater weight of the material.
One of the simplest construction methods for bridges
is to assemble a span away from the bridge site and then transport it to the
site. The span can then be lifted into position as one piece. This method is
most often suitable for small truss bridges or for the suspended span of a
cantilever truss.
Another approach is to use falsework or temporary
scaffolding to support the incomplete parts of a bridge before they are joined
and able to support themselves. The use of falsework is not always possible,
owing to strong river currents, interference with river traffic, or great
distances to the ground. If .falsework is impractical, bridges can be
constructed by the cantilever method.
With the cantilever technique, a bridge is built
piece by piece, with the entire structure supported from the section previously
completed. Thus, the structure is self-supported throughout the entire
construction process. The use of cantilever construction methods saves material
and therefore expense, but it is very complex as great care must be taken not
to unbalance the structure during construction. Most arch bridges, and of
course cantilever bridges, are built using cantilever methods.
The large towers and cable anchorages of suspension
bridges are built without the use of falsework, and then the suspension cables
are spun. Many individual wires are dropped over the towers and are then
squeezed together into a circular shape and clamped at intervals to create a
main cable. Suspension wires are dropped from the cables to support the
roadway, and the roadway is completed.
For all bridge types, underwater foundations require
unique construction methods. Builders use cofferdams and caissons to obtain access
to ground that is normally under water.
A cofferdam is a temporary watertight enclosure
constructed on the spot where a pier is to be built. A cofferdam usually
consists of sheets of steel driven into the ground to create a walled chamber.
The cofferdam is then pumped dry to expose the river bed. Soil can be excavated
to bedrock or piles can be driven to create the pier foundation. The cofferdam
is removed after the foundation and piers are constructed.
A caisson is a large cylinder or box chamber that is
sunk into the river bed. The excavation and foundation work takes place within
the submerged caisson. Some caissons are removed after construction, while
others are left in place, filled with concrete, and used as part of a permanent
foundation.
(3) Safety: In bridge design, engineers
strive to plan an economical structure that will safely transmit loads to the
ground without collapsing or deforming excessively. Since it is difficult to
predict the exact loading and circumstances that a bridge must withstand, all
bridge designs include a substantial margin of safety. Design standards vary
throughout the world, but all aim at ensuring that new bridges will provide
many years of service and will maintain an adequate margin of safety against
failure. Of course, the safety of a structure when it is first erected does not
ensure that it will remain safe for all the time. All structures require both
periodic inspection and proper maintenance to keep them safe. Notable bridge
failures include the collapse of the Firth of Tay Bridge in Scotland in 1879,
the collapse of the Quebec Bridge in Canada while under construction in 1907,
and the collapse of the Tacoma Narrows Bridge, nicknamed Galloping Gertie, in
Washington State in 1940.
TUNNELS ENGINEERING
INTRODUCTION
A tunnel is an engineering structure, artificial
gallery, passage or roadway beneath the ground, under the bed of a stream or
through a hill or a mountain. Tunnels are used for highway traffic, railways
and subways; to transport water, sewage, oil and gas; to divert rivers around
dam sites while the darn is being built; and for military and civil-defence
purposes. Following are the advantages of the tunnels:
(i) Tunnels avoid the dangerous open cut adjacent to
a structure.
(ii) They save' tearing up of expensive pavements by
providing protection from snow, rain and other material influences. This
reduces the maintenance and operating costs of the tunnels.
(iii) For carrying public utilities like water or
gas across a stream or a mountain, it is generally economical to construct a
tunnel rather than a bridge which can be used as a highway or a railway.
(iv) Aerial warfare and bombing of cities have given
intangible values to tunnels.
(v) Modern methods of construction have eliminated
the danger of settlement of overlaying ground, thus, providing safety in tunnel
construction.
The only disadvantage of tunnel is that it requires
a lot of time in completion as well as it requires specialized equipments and
methods for its construction.
DIFFERENT TYPES OF TUNNELS
The shape of the sectional profile of a tunnel
should be such that the lining is able to resist the pressures exerted by the
unsupported walls of the tunnel excavation. These pressures are both lateral
and vertical in direction and vary with the cohesion and internal friction of
the material. On the basis of these considerations, following tunnel shapes are
commonly adopted.
(1) Polycentric
tunnel:
Polycentric tunnels are those tunnels whose different portions of the
cross-section have different curvatures.
Salient features:
(i) Such tunnels suit to the majority of conditions
as the section can be adjusted to suit the existing conditions.
(ii) The execution of the design is difficult and
hence, they are not popularly preferred.
(2) Circular tunnels: Circular tunnels are those
tunnels whose cross-section is having a constant curvature throughout the
perimeter.
Salient features:
(i) Theoretically, circular tunnels are the best
tunnels for resisting external or internal forces.
(ii) They provide the greatest cross-sectional area
for the least perimeter.
(iii) They are best suited for non-cohesive soils.
(iv) They are popularly used for carrying water. If
these tunnels are to be used as traffic tunnels, a lot of filling is required
to get a flat base.
(v) Application of concrete lining is difficult
because of the circular shape.
(3) Horse shoe
shaped tunnel:
Horse shoe shaped tunnel is a tunnel whose one-half cross-section is circular
and the other half is arched.
Salient features:
(i) The floor of the horse shoe shaped tunnel is
flat, which gives more working space.
(ii) The external pressure is resisted by the arch
action.
(iii) It is suitable in soft rocks.
(iv) It is best suited for traffic purpose and is
commonly used for railways and highways in all countries.
(4) Egg shaped
tunnels: Tunnels
having varying curvature throughout the perimeter of the cross-section,
resembling an egg are called egg shaped tunnels.
Salient features:
(i) Egg shaped tunnels are popularly used as sewers.
This is because of the typical cross-section which is smaller at the bottom and
to an extent goes on increasing with respect to the height. Such a type of
cross-section maintains self cleansing velocity both in dry and storm weather
flow.
(ii) The circular walls resist both internal and
external pressure.
(5) Elliptical
tunnels: Tunnels
having the variation in the curvature of the cross-section as an ellipse are
called elliptical tunnels.
Salient features:
(i) Elliptical tunnels are suitable for softer
materials.
(ii) The circular walls resist both internal and
external pressure.
(6) Rectangular
tunnels: Tunnels
having straight and plain cross-sections are called rectangular tunnels.
Salient features:
(i) Rectangular tunnels have less depth.
(ii) The bending moment and stresses exerted at the
roof is resisted by a steel girder.
(iii) They are suitable as pedestrian tunnels.
(iv) Concrete lining is difficult in these tunnels
and hence, are rarely used.
(7) Segmental
tunnels: Tunnels
whose roof is a segment of a circle and sides are vertical with flat floor are
referred as segmental tunnels.
Salient feature:
Segmental tunnels are chiefly used for subways or
for navigation because of the typical cross-section.
MODERN TUNNELLING METHODS
The building of a tunnel is known as driving a
tunnel. It involves advancing the passageway by blasting or boring and
excavating. Tunnels through mountains or underwater are usually worked from the
two opposite ends, or faces of the passage. In the construction of a very long
tunnel, vertical shafts may be dug at convenient intervals to excavate from
more than two points. Improved boring and drilling machineries now allow a
tunnel to be driven four to five times faster than older techniques.
The rock drill that is driven by compressed air has
helped most in reducing the time of tunnelling in recent years. A number of
these drills may be positioned on wheeled vehicles, called jumbos, and rolled
to the face of the tunnel. Many holes are then drilled concurrently in
predetermined places on the rock face. Blasting material is inserted into the
holes, the area is cleared, and the explosives are detonated. Broken rocks are
then removed and the process is repeated.
Another recently developed tunnelling machine is the
mole, a long machine with a circular cutting head that rotates against the face
of the tunnel. Attached to the cutting head is a series of steel disk cutters
that gouge out the rock on the face as the machine rotates and is pushed
forward by hydraulic power.
Moles provide several advantages over drilling and
blasting.
(i) The tunnel can be bored to the exact size as
desired, with smooth walls, thus eliminating the condition called overbreak,
which results when explosives tear away too much rock.
(ii) The use of moles also eliminates blasting
accidents, noise and earth shocks. Workers need not be concerned with fumes or
noxious gases and can clear away broken rock without stopping for blasting
intervals.
(iii) A mole can advance about 76 m a day, depending
on the diameter of the tunnel and the type of rock being bored.
Despite these advantages, moles have some drawbacks.
(i) The cost runs in cores of rupees.
(ii) The cutting head must be of the same diameter
as required for the tunnel.
(iii) They are useless in soft ground and mud, which
collapse as soon as the machine digs in.
(iv) Until recent years, when improved cutting
surfaces came into use, extremely hard rock quickly wore out cutting disks.
In addition to blasting and boring machines, several
other methods are used to dig tunnels.
The cut-and-cover method involves digging a trench;
building the concrete floor, walls and ceiling or installing precast tunnel
sections; and then refilling the trench over the tunnel. In built-up areas in
cities, use of this method is sometimes impossible. In soft earth or mud, a large-diameter
pipe like device can be driven through the ground by jacks or compressed air.
Workers remove the earth as the pipe moves forward, its edge cutting into the
earth. The method was used in driving the Lincoln Tunnel through the muddy
bottom of the Hudson River between the states of New York and New Jersey.
Underwater sunken tube tunnels, such as the Baltimore Harbor Tunnel, have been
built by fabricating short tunnel sections in a trench in the river-bed or
sea-floor. Each section, after sinking, is attached by oversized bolts to the
previously sunk section in line. Heavy, thick concrete walls prevent the tunnel
from floating.
Another method of underwater tunnel construction
uses a caisson, or watertight chamber, made of wood, concrete or steel. The
caisson acts as a shell for the building of a foundation. The choice of one of
three types of caissons, the box caisson, the open caisson or the pneumatic
caisson, depends on the consistency of the earth and the circumstances of
construction. Difficult conditions generally require the use of the pneumatic
caisson, in which compressed air is used to force water out of the working
chamber. Pneumatic caissons were used to construct railway tunnels under the
Hudson River.
HAZARDS OF TUNNEL CONSTRUCTION
New tunnelling techniques have not eliminated all
the hazards of tunnel digging. Following are the hazards of the tunnel
construction:
(i) Water, sometimes as much as 72,000 litres (about
19,000 gallons) a minute, can pour into tunnels not yet lined with concrete or
plastic sealers.
(ii) The water, which must be pumped out
continuously, inconveniences causes tunnel roofs and walls to collapse, damages
equipment, and causes delays in digging. In recent tunnel projects, attempts
have been made to freeze the tunnel area before blasting or digging to prevent
flooding before the walls can be sealed and lined.
(iii) Dust from blasting is another serious hazard,
causing illness among workers and delay in digging. A machine that sprays a
fine curtain of water to settle the dust following a blast has recently been
used.
FAMOUS TUNNELS OF THE WORLD
Following are some of the notable tunnels of the
world, with the years of completion given in parentheses.
(i) Orwigsburg (1821), water tunnel at Orwigsburg
Landing, near Auburn, Pennsylvania 137 m long; the first tunnel of its kind dug
in the U.S.
(ii) Mont Cenis (1871), through the Alps between
France and Italy, 13.7 km long; the first railway tunnel built.
(iii) Simplon (1922), railway tunnel through the
Alps between Switzerland and Italy, 19.8 km long.
(iv) New York City highway tunnels, each more than
1.6 km long: Holland Tunnel (1927) and Lincoln Tunnel (centre tube, 1937; north
tube, 1945; south tube, 1957), under the Hudson river, Queens-Midtown Tunnel
(1940), under the East River; and Brooklyn-Battery Tunnel (1950), under New
York Bay.
(v) Yerba (1936), through Yerba Buena Island, San
Francisco Bay, California, 165 m long, 23 m wide and 15 m high; the
largest-diameter bore tunnel in the world, carrying two decks for traffic.
(vi) Delaware Aqueduct (1944), in New York State,
137 km long; starts at Roundout Reservoir in the Catskill Mountains and ends at
Hillview Reservoir, Yonkers; the longest tunnel in the world.
(vii) Mont Blanc (1965), highway tunnel through the
Alps between Chamonix-Mont-Blanc, France and Courmayeur, Italy 11.6 km long.
(viii) Snowy Mountains Scheme (1972), in Australia,
a complex of tunnels totalling about 145 km long, linking reservoirs and
power-houses; among the complex, Eucumbene-Snowy (1965), 23.5 km long, is the
longest.