Methods of Tunnelling

Twin tunnel

•        Twin tunnel is useful in railways.

•        Sometimes it proves more advantageous to have two small tunnels instead of single large tunnel.

•        Each of twin tunnel is excavated separately & tunnels are so spaced that disturbance caused in ground from construction of one tunnel does not affect the other .

 

Advantages of twin tunnel

•        The facility for transportation is increased.

•        If one tunnel is blocked up due to any reason the other tunnel can be conveniently used.

•        If there is trouble of hot water it can be easily & effectively   managed.

•        There is considerable improvement in the ventilation & drainage of tunnels.

 

 

Portals

•        The actual doorways or main entrance of the tunnel are known as portals.

•        Portals indicates the intersection point between the underground opening & the ground surface.

•        The portals should be made massive in appearance. They  should look beautiful & attractive.

•        Such a construction would encourage a layman to enter the tunnel with confidence of safety.

 

 

Pilot tunnel

•        Sometimes it may be found that a horizontal approach to the tunnel line may be to closer & shorter compared to the deep vertical shafts .

•        In such circumstances, a tunnel of small size called a pilot tunnel is driven parallel &close to the proposed main tunnel & short cross connecting tunnels are driven from it reach the proposed main tunnel to create operational faces.

 

 

Advantages of pilot tunnel

•        The cross headings are convenient places to store tools & material during the construction work.

•        It is found to be less costly than the shaft.

•        The chances of any material failing accidentally down the shaft in to the tunnel during the construction work are considerable reduced

•        It avoids the dislocation of the strata at the sides of the tunnel after the work is completed, the cross headings may be used as passage for the workers

•        The shaft can be used as means of artificial ventilation by the use of fans.   

 

 

Following are methods of tunneling in soft

•        Forepoling method

•        Needle beam method

•        Army method or case method

•        American method

•        British method

•        Belgian method

 

 

Factors affecting methods of tunneling in soft soils

•        Factors affecting in Forepoling method

•        The process is slow & tedious requiring skilled labour

•        Tunnels of small dimensions, for laying sewers , gas pipes,etc.,at ordinary depths, could advantageously be constructed by this method .

•        it is very important that the sequence of operations has to be strictly adhered to , in the correct order.

 

 

Needle beam method

•        Factors affecting in Needle beam method:

•        It makes use of beams & bracings .Hence,there is difficulty in mechanical methods of concrete lining.

•        The pushing of heavy beam by hand is laborious & tedious.

•        It requires large number of trench jacks & they interfere with the efficient working of the gang.

•           

 

 

Factors affecting Army method or Case method

•        The only drawback of this method is that its use or application is limited for the construction of short tunnels of small cross-section.

•        Factors affecting American method:

•        The method is unsuitable for tunnels having flat bottoms.  

 

 

English method

•        This method involves the use of  a lot of timber & frequent shifting of heavy timber logs back.

•        The masons & excavators have to work alternatively & thus , the process requires more time as compared to other methods of tunneling.

•        If the soil is unstable , the excavation of full section of tunnel will increase the danger of caving.

 

 

Factors affecting belgian method

•        The disadvantage is due to the system of underpinning of the built arch .

•         particularly when the avoidable subsidence of the soil may take place , causing settlements can be reduced by adopting R.C.C.arches.

 

 

Liner plates methods

•        The liner plates method is somewhat  expensive.

•        It is also relatively weak as plates are attached with nuts & bolts.

 

 

Factors affecting tunneling in rock compared with tunneling in soft soil

•        It is more costly than in soft ground.

•        It requires extreme care in carrying out the work as slight mistake may result in heavy loss of money.

•        Timbering required will be less as rocks are self-supporting.

•        Over-cutting of section has to be avoided.

•        It allows the tunneling operations to be carried out at many sections along the tunnel length.

 

 

The following are different methods  of tunneling in rock

•        Drift method

•        Heading &bench method

•        Full face method

•        Cantilever car dump method

 

 

Factors affecting in drift method

•        The enlargement work cannot be started until the central holr is constructed for the full length.

•        That as the enlarging & benching work commences , mucking tracks have to be shifted frequently from bench to bench.

•        This method is extremely costly & it is generally recommended for ground conditions which are difficult to solve.

 

 

Factors affecting in heading & bench method

•        In this method enlargement work cannot be started until the central hole is constructed for full length.

•        This method is extremely costly .

•        It is generally recommended for ground conditions which are difficult to solve.

 

 

 

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

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.

Overview of Tunnel Lining

Specification of Tunnel Lining

Methods of Tunneling

Singly reinforced beam(Limit state method of design)

Different methods of design of RCC
1.Working Stress Method
2.Limit State Method
3.Ultimate Load Method
4.Probabilistic Method of Design
 
Limit state method of design

• The object of the design based on the limit state concept is to achieve an acceptable probability, that a structure will not become unsuitable in it’s lifetime for the use for which it is intended,i.e. It will not reach a limit state
• A structure with appropriate degree of reliability should be able to withstand safely.
• All loads, that are reliable to act on it throughout it’s life and it should also satisfy the subs ability requirements, such as limitations on deflection and cracking.
• It should also be able to maintain the required structural integrity, during and after accident, such as fires, explosion & local failure.i.e. limit sate must be consider in design to ensure an adequate degree of safety and serviceability
• The most important of these limit states, which must be examine in design are as follows      Limit state of collapse

             - Flexure

                       - Compression

             - Shear

               - Torsion

This state corresponds to the maximum load carrying capacity.

Types of reinforced concrete beams
a)Singly reinforced beam
b)Doubly reinforced beam
c)Singly or Doubly reinforced flanged beams
Singly reinforced beam
In singly reinforced simply supported beams or slabs reinforcing steel bars are placed near the bottom of the beam or slabs where they are most effective in resisting the tensile stresses.

LABORATORY TESTS on Mild Steel (Reinforcement Steel)

 
TENSILE TEST ON MS SPECIMEN
• Equipment required: Universal Testing Machine, Specimen and extensometer.
                                
• In a tensile test of mild steel specimen, usually a round or flat bar is gradually pulled in a testing machine until it breaks.
• Two points, called gauge points, are marked on the central portion. The distance between these points, before the application of the load, is called gauge length of the specimen.
• The Load is applied gradually and at regular interval of loads extension is measured.
• The strains corresponding to the recorded extensions are calculated by dividing the extensions by the gauge length, while the stresses are calculated by dividing the loads by the original area of cross-section of the specimen.
• Stresses so arrived at is called nominal stress to distinguish it from actual stress which is obtained by dividing the load at a particular instant by the area of the cross-section at that instant
 
• Limit of Proportionality(A):It is the limiting value of stress upto which stress is proportional to strain.
• Elastic Limit:This is the limiting value of stress up to which if material is stressed and released, strain disappears completely and original length is regained. This point is slightly beyond the LOP.
• Upper Yield Point(B):This is the stress at which the load is decreasing and strain increases. This phenomena is called Yielding of Material.
Stress=250kn/mm²
Strain=0.125%
• Lower Yield Point(C): At this stage the stress remains constant but strain increases for some time.
• Ultimate Stress(D): This is maximum stress the material can resist. At this stage C/S area at a particular section starts reducing very fast. This is called Neck formation. After this stage load resisted and hence the stress developed starts reducing.(370-400N/mm²)
• Breaking Point(E):The stress at which finally the specimen breaks is called breaking point.
Strain= 20-25%

MARINE PILING - PROCEDURE & COMPONENTS

Pile Foundations
Pile foundations are the part of a structure used to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface. The main components of the foundation are the pile cap and the piles. Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity The main types of materials used for piles are Wood, steel and concrete. Piles made from these materials are driven, drilled or jacked into the ground and connected to pile caps. Depending upon type of soil, pile material and load transmitting characteristic piles are classified accordingly.
Classification of piles
Classification of Pile with respect to Load Transmission and Functional Behaviour.

• End bearing piles (point bearing piles)
• Friction piles (cohesion piles )
• Combination of friction and cohesion piles

Classification of Pile with respect to type of material. Timber

• Concrete
• Steel
• Composite Piles

Classification of Pile with respect to effect on the soil.

• Driven Pile ( Displacement pile)
• Bored Pile ( Non Displacement pile)

Classification of Pile with respect to Shore.

• On Shore ( Land Pile)
• Off Shore (Marine Pile)

Marine Piling
Marine piling work differs from land piling work in many respects.

• Distance from land
• Depth of water
• Hydrostatic pressure and buoyancy
• Underwater currents
• Wave and swells
• Tidal variation
• Wind and storm
• Cyclone
• Existing navigation and possibility of diversion etc.
• Marine Piling

Offshore Piling Works:
This kind of piling works are mostly carried out in construction of various marine structures like jetties, harbours, ports, wharfs and bridges on river/sea that are away from land.
Marine Piles can be installed by

• Tripod Rig.
• Rotary Rig ( Wirth Rig or Ordinary Crawler Mounted Hydraulic Rig)

Tripod Rig
Marine Piling
Different methods commonly used for advancing the bore holes.

• End on Piling Gantry ( Temporary movable gantry )
• Self elevated Platform ( Jack-up platform )
• Temporary fixed platform supported on temporary/ permanent piles.

Jack Up Platform with Bailer & Chisel

Temporary Movable Gantry with Bailer & Chisel

Temporary Movable Gantry with Bailer & Chisel

Basic Concepts for Civil Engineers for Interview and Campus preparation- Cement

The manufacture of cement consists of grinding the raw materials (calcareous and argillaceous stones consisting silica, alumina and iron oxide ) and mixing them intimately in a certain proportion. it is then burnt in a large rotary kiln at temeperature of 1500C, when the material sinters and partially fuses into balls known as clinker. The clinker is cooled and ground to fine powder with some gypsum added, and the resulting product is Commercial Portland cement.
The main Components of Cement are
 Lime - 63%
Silica - 22%
Alumina - 6%
Iron Oxide - 3%
Magnesium Oxide - 2.5%
Sulphur trioxide - 1.5%
Alkalies - 0.5%

 The lime, silica and iron oxide imparts strength to cement , while alumina gives quick setting property. The alkalies when in excess, causes efflorescence.

The Bougue's Components (Components of Cement Clinker)
Tri Calcium Silicate -C3S- 40%
Di Calcium Silicate - C2S- 32%
Tri Calcium Aluminate - C3A - 10.5%
Tetra Calcium Alumino Ferrite - C4AF- 9%

Basic Concepts for Civil Engineers for Interview and Campus preparation- Bricks

Stones

• Stones are geologically classified as Ingenious, Sedimentation and Metamorphic rocks.
• Chemically they are classified as Siliceous, Argillaceous(Clay or alumina), Calcareous stones
• Physically they are classified as Stratified and Un-stratified rocks. 

Bricks
The composition of Bricks are

• Alumina 20-30%
• Silica 50-60%
• Magnesium, manganese, lime, sodium.

Excess alumina in bricks may cause cracks and warp on drying.
Excess silica in bricks may cause brittle and weak.
Excess lime causes melting and distortion during burning process of bricks.
Presence of Alkaline salts causes efflorescence in Bricks
Classification of Bricks

• First class bricks – 20% of water absorption only when bricks are immersed in water for 24 hours and compression strength of 10.5 MN/sq.m.
• Second class bricks – 22% of water absorption only when bricks are immersed in water for 24 hours.
• Third class bricks – 25% of water absorption only when bricks are immersed in water for 24 hours and not properly burnt.
• Jhuma Bricks- Over burnt, Bluish or dark colour and irregular shape.

Size

Nominal size of Bricks are 19x9x9 cms

CPWD India – 23x11.5x7.5 cms.

Notes:

• The specific Gravity of bricks is 2-2.1
• In 1 cubic metre volume around 550 bricks are used