Tunnels & Terminalogy

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

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.

·       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.