706 Civil Booster (Civil Ki Goli Publication 9255624029)

Tunnel Engineering 17
It is an engineering structure, artificial gallery, passage or roadway beneath
the ground, under the bed of a stream or through a hill or mountain.
Open cuts:
 It is open to sky passage excavated through huge soil mass of obstacle
like hill, run in required direction to connect.
 Open cuts are provided up to depth 18m economically, if more than
18m depth.Tunneling is preferred. (60 feet)
 For highways, tunneling is preferred to open cut if the depth of open
cut exceeds 15m.
 It will require less time & cost for construction as compared to tunnels.
Advantages of tunnel
(a) For carrying public utilities like water, gas, railway lines or roads etc.
(b) Lesser maintenance &operating cost.
(c) Aerial warfare & bombing of cities have given important values to
tunnel.
(d) Cost of hauling is decreased due to lighter grades, possible in tunnels.
Classification of Tunnels
Based upon Purpose Based upon construction Based on Position or
material Alignment
Traffic Conveyance
Railway Tunnel in Saddle or
Hydro Electric hard Rock Base tunnel
Pedestrain power tunnel
Tunnel in Quick Off spur tunnel
Highway Sewer tunnel Sand or loose sand Spiral tunnel
Water supply
tunnel Open cut tunnel Slope tunnel
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According to Alignment
(a) OFF - Spur Tunnels
 Short length tunnels to negotiate minor local obstacles, is very
high projections on the way, which cannot be followed with
permitted
(b) Slope Tunnels:
 Tunnel constructed in steep hills for economic and safe opera-
tions of roads and railways.
(c) Saddle or Base Tunnels:-
 Tunnel constructed in the valleys along the natural slope till slope
does not exceed ruling gradient
(d) Spiral Tunnels:
 Tunnel provided in narrow valley in the form of loops in the inte-
rior of mountain so as to increase length of tunnel to avoid steep
sloper
Shape of the tunnel

D-Section or Rectangular Circular Egg-Shaped Horse-Shoe

Segmental roof

D 2D
5D

D
1 .2

2D
1.2

5D

D D D
1 in 24 1 in 24 D D

(a) ‘D’ Section (b) Rectangular © Circular (d) Egg. Shaped (e) Horse-shoe
Section Section section section
(i) In Rock tunnels: It is suitable for subway/Navigation channel.
(ii) Egg shaped section is used in sewers as it maintains required self
cleansing velocity. It provide least crosectional area at bottom.
(iii) Rectangular section is suitable only in case of hard rocks.
(iv) Circular section: It is most suitable for sewer & water carriage. It
is best suited for materials without cohesion (Quick sands) where the
pressure acts normal to the line of profile.
(v) Horse shoe shape is best suited for traffic purposes. It is most popular
for tunnel.
 It is suitable for soft rock but difficult to construct.
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Investigations in tunnel construction

Information collected are
(i) Origin of soil mass.
(ii) Location of faults, folds etc.
(iii) Temperature of soil near by.
(iv) Hydrology in surrounding proposed tunnel site.
(v) Presence or foul gases.
(vi) Bearing capacity of soil mass at proposed site.
Sequence of operation
(a) Locating centre line on ground.
(b) From centre line to inside tunnel.
(c) For required grade at the bottom of tunnel.
(d) Checking tunnel cross-section.
Grades in tunnels
 Grades in tunnel should be lesser than surface grade.
 It is not favorable to continue tunnel approach grades in tunnel especially
in railway.
 For Tunnels > 300m length, grade should be provided below 75% of the
ruling gradient.
 In order to maintain desired shape of tunnel the C/S must be check at
regular 2m to 3m interval.
Setting of centre line
Curve linear tunnel Long tunnels Short tunnels
(a) Method of tangent offsets By triangulation using Theodolite using
(b) Method of chords & micrometer & wooden stakes
deflection angles transit theodolite

 Setting out is defined as marking the centre line or alignment of any

construction work on ground.
 It is done indirectly in different stages using theodolites.
 Modern way of providing horizontal control on ground surface is by
running an open traverse between two ends of proposed tunnel by
using theodolite of precision and laser unit pole.
 Laser pole unit is used to define a thin visible vertical plane upto large
height.
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 This plan is bisected by theodolite for measurement of traverse liner.

 Lengths of survey lines are measured by direct method of stepping
with invar or steel tapes.
 Weisbach triangle method is used to connect two ends of an under-
ground tunnel.
 Tunnel alignment is carried out by Triangulation.
 For transfer of centre line from surface to underground, shafts are to
be constructed at regular interval of 500 m along traverse line.
Shafts
There are vertical wells sunk along the centre line of tunnel to permit
excavation, removal of excavated material & to provide adequate ventilation
during construction.
Advantages of shafts :
(1) They help in correct alignment and to carry the centre line into the
tunnel proper.
(2) They can be used as pumping shafts in case of large influx of water.
(3) They afford outlets for excavated material and means of access into the
tunnel for building materials.
(4) They greatly expedite the work by adding two faces per shaft for driving.
(5) In long tunnels, they afford ventilation and are useful for exhausting
smoke & foul air out of the tunnel.
Size of shafts
It depends on following factors :-
1. Quantity of muck to be handled
2. Size of muck car
3. Space required to carry pipes & wires and nature of hoisting system
4. Nature of equipment used in tunneling process
5. No. of labours
6. Eventual use of the shaft

Types of shafts: According to location and position :-

1. Shafts over centre-line :-
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 These have advantage of hoisting out the materials easily from the
tunnel.
2. Side shafts :-
 These are shifted on one side of centre-line.
 A transverse gallery or a pilot tunnel or passage is required for
connecting it with main tunnel.
 The turning of machinery at sharp curve of the gallery is the greatest
drawback of this shaft.
According to alignment
1. Vertical shafts:
 Laid in vertical direction only as they easy to construct and cheap.
2. Inclined shafts :-
 For small depth, inclined shaft with suitable gradient of 45o or less
are constructed to achieve removal of muck by gravity.
According to time of filling material
1. Permanent shafts:
 These are left open even after construction of tunnel and helps in
providing ventilation.
2. Temporary shafts:
 The shaft to be excavated can be refilled if its sole purpose is to
get information is called temporary shafts.
 They are located in valley.
 Mainly provided for the supply of construction material and for the
removal of muck.
Protection round shaft opening:
 It is usual to construct 900 mm high wall around the edge of the shaft
opening, to form a protective fencing.
 This prevents surface water flowing down the shaft.
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Drilling Equipments
Percussion Drills Fusion piercing Abrasion Drills
 Drills Breaks  Produces mixture  Grinds rock
rock by impact of oxygen & flux due to
due to repeated blows bearing fuel like as abrasive effect
 Tripod drill for kerosene at the end  Can drill upto
very hard rock of blow pipe 100 m.

Blasting
It is the operation perform to loosen rock so that it may be excavated or
removed from its existing position.

Theory of blasting Cut Type of Explosives

Impact Initiat blast must release Straight dynamites

suitable amount of rock
Abrasion & free face for blast to Ammonia dynamites
follow
Fusion & vapourized Amonia gelatine
Water gels
Chemical reaction
Angle cut Burn cut Blasting agents
Thermally induced
spalling Semi-gelatine

For blasting, type of explosive used may be:

1. Ammonia dynamites: two types are as :
(a) High velocity High density (HVHD)
(b) Low velocity low density (LVLD)
2. Straight dynamites:
 Not suitable for rocks because poor fume chart.
3. Ammonia gelatin: can be fired in presence of water.
4. Blasting agents:
 Ammonia Nitrate (NH4NO3) 94.5% + fuel oil 5.5% or other car-
bonaceous matter is used as blasting agent.
 It is low in cost & safe but highly sensitive to water.
5. Semi - gelatine: This can be used in water better than above.
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6. Slurries or water gels:

 Recently developed and have high % NH NO in aqueous solution.
4 3
They have resistance to water.
 A good blast with good yield obtained if cut hole inclined at 45o to face.

Methods of Tunneling
In Hard Rock In soft Rock
Without Timber With timber
Cantilever car dump
Linear plate method Austrian method
Full face method Fore poling method
Shield method
Heading and Needle beam method
bench method Compressed air
tunneling Belgian method
Drift method
Army method
English method
German method
American method

Method of tunnelling in rock:

1. Drift Method:
 It consists driving small sized beading, centrally at top or botton of
face, which is later enlarged by widening and benching.
 It is suitable for large size tunnels in difficult or incompetent rock
(hard rock).
 Top drift method is popular & involves operation as :-
(a) Boring and blasting a top centre of drift, end to end.
(b) Widening and enlarging the drift.
(c) Benching in stages.
 This method also known as multiple drift method.
 Advantages:
1. A small preliminary section for the full length has been accurately
drives, which economise construction.
2. During heading work, mucking work goes on undsitrubed.
 Tunnel Engineering 713

3. Elaborate supporting platforms are not necessary for drilling open.

4. It provides good ventilation for workers.
 Disadvantages:
1. The enlargment can not started until central hole constructed for full
length
2. Extremely costly
3. Mucking tracks required to shift frequently, bench to bench
2. Heading and Benching Method:

A B
Heading
D C
Drill Holes

Bench
(a) In hard rock
(b) In soft rock requiring support
Heading and benching method

 This method involves the drilling of the top portion in advance of bottom
portion.
 If rock is hard & self-supporting, the top heading advances ahead by
one round over the bottom, so that heading & benching follow each
other.
 If rock is badly broken, the top heading will need support & bench will
afford platform for this, in such case heading is excavation & supported
to the full length or part length before benching started.
 For full face method excavation is divided into 3 sections & the first
operation in this method relates to excavation being done along the
perimeter.
 Heading is always ahead of benching by convenient length & formed
by excavating full width above spraining line.
 In one blast, one Heading & one Benching are blasted with few
seconds time lag.
 Less quantity of explosive required as compared to full face method.
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 It is generally suitable for soft rock tunnelling of medium/average size.

 It is advantageous such that drilling and mucking is done simultaneously.
 Removal of muck from heading is difficult.
3. Cantilever car dump method:
 This method provides an improved arrangement of throwing muck
from bench to the floor of the tunnel.
 Two plate girders about 23m length & 1.8 c/c distance.
 A belt-conveyor is fitted with no. of jack and running on plate girders.
 The ends of plate girders project beyond the full face of the bench &
front ends are suitable supported by means of struts from the top of
the tunnel.
4. Full face method:
 It is convenient adopted for tunnel of small cross section area through
stable and self-supporting rocks and whole section/full face attacked
at a time.
 This tunnel is continous.
 At present techniues shield & tunnel boring machine (TBM) are
popular and well suited for full face excavation in different ground
condition and various shapes and sizes.
 It is popular due to result of equipment developement.
 It useful when diameter < 6m & face area < 19 m2. As further increase
in dia & area resuts in difficult construction & costly.
 Advantages:
1. Simplicity as minimum equipments are required.
2. Easily and speedy complection.
3. Minimize the total magnitude of ground disturbance and settle-
ment.
4. The mucking track laid once for full operation.
Mehtods of tunnelling in soft grounds
1. Forepoling method:
 This is probably the only system which could be advocated for running
ground & similar soils.
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 The process is slow and required skilled labours.

 Tunnels of small dimensions, for laying sewers, gas pipes etc, at
ordinary depth advantageously constructed by this method
 Forepoles or spikes, horse head etc. are used for supporting.
2. Army method or case method:
 It is discovered by US Army for construction of small tunnel at fairly
shallow depth.
 It is mainly used for laying under ground sewers.
 It is simple and economical as only few planks and jocks are required.
 Also done with unskilled labour without any difficulty.
 The drawback of this method is only that its use is limited for con-
struction of short tunnels of small cross section.
3. Needle beam method:
 Suitable for firm ground & Brick lining.
 3m to 4m length advanced per day.
 Needle beam = 5m to 6m length
• Large no. of jacks are required.
4. American method :
 It is suitable for large sized highway or railway tunnels.
 It does not required heavy timbers, involves simple operation.
 However unsuitable for tunnels of flat bottom.
5. English method :
 It involves the use of a lot of timber and frequent shifting for heavy
timber logs back and forth is greatest disadvantage of this method.
 Other disadvantage is, the mason & excavators to work alternatively
and this the process required more time as compared to other methods.
 If the soil is unstable, the excavation of full section will increase
danger of caving.
• Advantages.
 Simple method of hauling can be adopted.
 The lining is built in one piece from foundation to the crown and it
thus, grants strong & homogeneous construction.
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6. Compressed air method:

 It is most suitable for clays.
 In this working is done under pressure.
(i) When working zone pressure  1.25 kg/cm2, working hours = 8
hrs & breathing internal = 4 hrs.
(ii) Wen P > 1.5 kg/cm2, working = 4 hrs & breathing interval = 2 hrs.
 In it, vol. of free air provided is 6 m/min / m2 of face area.
7. Belgium method :
 Advantages :-
(a) Use lighter timber section.
(b) Completing first the construction or roof arch which is exposed
to the greatest pressure and then constructing the side walls and
the invert.
(c) Starting of work from different points simultaneously.
(d) Reduction in cost as the muck cars can be taken upto the farthest
end in the centre trench.
 Disadvantages :
(i) Settlements and cracks in arch masonary due to the system of
underpinning of the built arch, particularly when the avoidable
subsidence of the soil may takes place.
 But settlement is reduce by using R.C.C arches.
8. Austrian method:
 In it, a centre is taken for full height of tunnel and it is then widened
to full face to permit short sections of the masonry to be completed.
 Adopted for long tunnels at great depths.
9. German method:
 In this method of supporting the roof & sides, three drifts are
employed - one at the crown & two at the bottom along the wall.
10. Liner plates method:
 It is suitable & used when the strata is sub-aqueous
 Standard sized pressed steel plates, plain or corrugated are used to
support the soil and the work is carried on in short lengths
corresponding.
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 Liner plates are generally made 400 mm × 900 mm with 50 mm

flange on all 4 sider.
 When diameter of tunnel  3m, stiffening of the liner plates for tunnels
becomes necessary to withstand pressure.
11.Shield tunnelling:
 Shield is a metal cylinder of hard steel plates bent to required curvature
and thickness, by rivetting 2 to 3 plates with counter sunk headed
rivets.
 Shield is an equipment which acts as bridge with roof for workers to
work on extension of lining on one hand and face of attack on other
hand.
Hauling of Muck
It is the operation of loading & removing excavated or hosted materials and
then dumping it at predetermined sites. it is done to make enough working
room & final cleaning of bottom to place inverts. It is either done by hands
or machines.

Ventilation in tunnel
Natural Ventilation Mechanical Ventilation
Due to the difference in inside Exhausting Blowing Combination of
& outside temperature Blowing & exhausting

(i) Blowing or Plenum process

· In it, fresh air is blown by blower fans.
· It is not employed for tunnel more than few hundred metres long
(ii) Exhausting
· It involves extraction of the vitiated (spoil) air from the tunnel by
means of one or more exhaust fans.
· Fresh air being drawn into the tunnel through its portal ‘or’ through
inlet shafts.
(iii) Combination of blowing and exhausting
· The most efficient ventilation system depend upon the combination
of blower and exhaust fans.
Immediately after blasting, exhausted system is used to drop smoke &
gases and during rest of the time, it is reversed and supplies fresh
air alongwith blowing system.
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Tunnel

Air tight
value

Fan Fan

Ventilation in tunnels

Special point: A light intensity of 260 lumens/ m2 in the working area is

considered satisfactory in tunnelling operations.
The empirical formula commonly adopted to obtain a working station is
T = 82D
D = Diameter of tunnel in m
T = Thickness of lining in mm
But this thickness is limited to a minimum of 230 mm irrespective of the
material used for lining. A thumb rule for obtaining approximate thickness
of the tunnel lining is to keep 25mm thickness of lining for every 300 mm
of the bore diameter of the tunnel.
 For constructing the tunnel, the preferred tool to excavate very soft ground,
is water jets.