Tunnels are artificial
passages constructed for transportation purposes. They are required for
1) Highways
2) Railways
3) Sewerage
4) Water supply
5) Public utility
6) Canals
Categories of
obstacles:-
1) Hill as an obstacle can be passed by
1) Sleepy sloping surface roads
2) Open cuts
3) Tunnels
2) I) body of still water mass like lake by
a) surface road partly
around the lake
b) bridge
c) tunnel
II) body of flowing
water mass like river or sea creak by
a) bridge
b) tunnel
3) When obstacle is marshy land at the time of
construction by
a) bridge
b) tunnel
c) embankment
4) a) when obstacle is a road junction at same
level can be passed by
construction of
bridge or tunnel
b) when obstacle is a
road junction at different
levels it can overcome
by construction of
1) tunnel
2) bridge
3) flyover
Definitions:-
1) 1) Tunnel: It is an
artificial underground passage to bypass
obstacles safely without disturbing the overburden i.e., land above it, to
carry freights, passengers, sewage, water, etc.
2) Open
cut: It is open to sky passage excavated through huge soil mass of
obstacle, like hill, run in required direction to connect two roads or
railways.
3) Bridge: It
is an overground construction to cross over obstacles safely without much
disturbing the natural way below it.
4) Surface
road: It is a long spiralling surface road or railway constructed to
by pass the obstacle like hill.
Advantages of tunnels: -
1) Tunnels are more economical than open cuts beyond
certain depths.
2) Tunnels avoid disturbing or interfering with
surface life and traffic during construction.
3) In case of aerial warfare and bombing the tunnels
would grant better protection aas compared to bridges.
4) Tunnels avoid the dangerous open cut very near to
the structure.
5) Tunnels prove to be cheaper than bridges or open
cuts to carry public utility services like water, sewer, gas, electricity and
telephone lines.
6) If tunnels are provided with easy gradients, the
cost of hauling is decreased.
7) The safety of tunnel construction has considerably
increased by the improved modern methods of construction and construction
equipments.
8) There is freedom from snow and iceberg hazards.
9) There is overall reduction in cost because of
shortening the distance.
10)Tunnels avoid interference with surface and air rights.
Disadvantages of tunnels:
1) The initial cost of construction of a tunnel is
high as compared to an open cut.
2) It is necessary to have skilled labour and
technical supervision of high order for the construction of a tunnel.
3) It takes long time for the successful completion of
a tunnel under normal conditions.
4) The construction of tunnel requires specialized and
sophisticated equipments.
Advantages of open cuts:
1) It will require less time for construction.
2) The construction cost will be less as compared to
the cost of tunnel.
Disadvantages of open
cuts:
1) It will require greater lengths due to approaches.
2) The maintenance cost of highways and railways are
very high.
3) It requires locomotives and automobiles with high
tractive force due to grades.
4) It proves unsafe during aerial warfare or bombing
of cities.
5) The wear and tear of pavements due to natural
forces like snow, rains, etc. are high.
TERMS AND DEFINATIONS
RELATED TO TUNNEL
Abutments:- these are
outermost support of an tunnel, from which tunnel springs.
Piers:- These are end
supports of a of tunnel.
Tunnel ring:- This is the curved
ring of masonry forming the tunnel.
Voussoirs:- These are the
wedge shaped or tapered units of bricks, stones or concrete works, forming the
courses of tunnel.
Arcade:- These are the ends
supporting a wall above and being supported by piers.
Skew-backs:- These are the
inclined or splayed surfaces of the abutments or piers, prepared to receive the
tunnel. Arch work actually starts from skew back.
Springing points:- These are the
points of the intersection between the skewbacks and the intrados and from
these points only, the curve of tunnel springs are commences.
Springing line:- This is the
imaginary horizontal line joining the two springing points.
Intrados:- The inner curved
surface of the tunnel is known as intrados.
Extrados:- Outer curved
surface of the tunnel ring is known as extrados. It is also
known as back of the tunnel.
Soffit or Bottom:- This is the
inner or under surface of the tunnel. Soffit and Intrados terms indicate same
thing.
Crown:- The highest
point of extrados of tunnel.
Key stone:- This is the uppermost
or central voussoir of tunnel. It is sometimes made prominent by making it
larger and projecting it above and below the outline of tunnel this is
inserted in the centre of many types of tunnels to improve the appearance but it
does not carry structural significance.
Span:- Clear horizontal
distance between the tunnels is
known as span of the tunnel.
Depth or height:- This is the
perpendicular distance between the intrados and extrados.
Thickness ( Breadth of
the soffit):- The horizontal distance measured perpendicular to the front and
back faces of tunnel is known as breather thickness of soffit.
Rise:- It
is the clear vertical distance between the springing line and the highest point
on the intrados.
Centre (or
striking point):- This is the geometrical centre point of the curve
of the tunnel.
Springers:- These are the extreme
or lowest voussoirs of tunnel, which are placed at springing level on either
side immediately adjacent to the skew- backs
Haunch:- This is lower half
portion of the tunnel between the crown and the skew-back or Springer.
Spandril:- This is the
triangular space formed between the extrados and the horizontal line down
through the crown.
Jambs:- These are the sides
of the Abutments or piers below the springing line.
Import:- The projecting course
at the upper part of piers and an abutment to stress the springing line.
Bed joints:- These are the joints
between the voussoirs which radiate from the centre.
