Tunnel engineering 

In this article we will  discuss Tunnel Engineering

After reading this article you will be able to define the following

1. what is tunnel ?
2. Necessity of tunnels
3. requirements of good tunnel of good tunnel 
4. classification of tunnel
5. Operations Involved in Bored Tunnelling
6. shapes of tunnel
7. methods of tunnel constructions
8.  methods of tunneling 
9. Types of TBM
10. Tunneling in hard rocks
11. tunneling n soft soil
12. hauling and mucking
13. drainage in tunnel 
14. ventilation of tunnel and method of tunnel ventilation
15. methods of ventilations 
16. lighting of tunnel
17. method of dust control
18. advantage and disadvantage of tunnel etc.

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Introduction 

•A tunnel is an underground passageway completely enclosed apart form opening and exit, commonly at each end.

 • A tunnel could also be for road traffic, rail traffic, canal, hydroelectric station sewer etc.

• It is more efficient than bridges (as during wars it's much difficult to destroy the tunnel compared to bridge)

 Necessity of Tunnelling Arises?

• The main reason behind provision of a tunnel is to eliminate a circuitous route for reaching the opposite side of a hill the length of railway line are going to be considerably reduced)

• In large cities, tunneling would offer the foremost rapid and unstructured transportation (leading to reduction in traffic congestion)

• Tunnels provide flat gradient in hilly areas 

• Tunnels under river bed is usually economical and convenient than a bridge over the river

Classification of Tunnel

1. Classification supported purpose:

(a) Traffic Tunnel:

Railway Tunnel 

Highway Tunnel 

Pedestrian Tunnel

(b) Conveyance Tunnel 

 Hydroelectric Power Tunnel

 water system Tunnel 

Sewer Tunnel

2. Classification supported quality of material:

(a) tunnel in hard rock 

(b) Tunnel in loose sand and quick sand 

(c) Open cut tunnel

3. Classification supported position or Alignment:

(a) Saddle or Base Tunnel: 

To minimize the length of tunnel, the track is led through valleys as long as natural slope of the valley

(b) Spiral Tunnel:

 In narrow valleys, the extra length for minimum permissible radius is obtained by forming loop into the inside of mountain.

(c) off spur tunnels:

 Tunnels that are made to short out minor local obstacles

They are very short long 

Operations Involved in Bored Tunnelling

1 ) Probe Drilling

This type of drilling is completed so as to seek out out suitable method for drilling

 it's necessary a part of all drilling operations

(2) Grouting :-

It is the method to supply additional support to drilled mine is completed by a thick emulsion (a suspension of non mixing small droplets of 1 liquid during a second squid) card grout

Grout consists of water, cement colour tint and sometime Grouting results in achievement of an honest surface

3)Excavation

It is the digging and recording of artifacts at an archaeological site.

It is necessary to understand the archaeological importance of a site before digging this is often performed by experts in scientific way) Many government grant permission for tunneling ater finding a go certificate in excavation.

(4) Supporting

After Initial mining, tunnel needs supports for faster processing.

Earlier, timber and masonry were usually applied Now, support is provided by injecting building completely before further tunneling 

(5) Transportation of Muck

 Earlier, transportation was done by external-combustion engine or manul transport Today, it's done by arrangements made in TBM (Tunnel boring machines) for transportation of muck

(6) Lining or Coating 

Lining is completed by modern methods like polishing, painting, to stop wear and tear and corrosion 

(7) Draining

 it's the method to get rid of the water or the liquid from working site (having status water level) with the assistance of pumps and pipes.

(8) Ventilation

 Ventilation is completed by proper checking of oxygen and water parameters (in order to supply safety of workers) by proper installations for exit at hazardous gases beginning from tunnel

Shape of Tunnels

The shape of the sectional profile of a tunnel should be such that the road is in a position to resist the pressure exerted by the supported walls of the tunnel excavation In quick sands or material without cohesion the pressure will act normal to the line of profile Circular section is most ordinarily used

• Commonly adopted sections are 
• Polycentric 
• Egg shaped 
• Circular section 
• Elliptical
• Horse shoe
• Rectangular

with order of preference best choice is polycentric ,The horse shoe and therefore the circular shaped tunnels

• Horse shoe section is suitable for soft rocks.
• Horse shoe section is best fitted to traffic operations utilized in railways and highway
• Egg shaped is employed for sewers

Methods of Tunnel Construction 

Shield Tunnelling Method:- 

• TBM has a shield (a big metal cylinder) and trailing supporting mechanisms 
• Tunneling Shield may be a protective structure utilized in the excavation of tunnel through the soil that's too soft or unstable.
• Shield is a short lived support structure
• At the front at the shield is rotating cutting wheel is found 

Behind the cutting wheel, there's a chamber where, counting on the sort of the TBM, the excavated soil is either mixed with slurry (so called slurry TBM) or left because it is (earth pressure balance) Modern TBM typically contains the rotating cutting wheel called a putter head followed by a main bearing, thrust system and trailing support mechanism.

Behind the chamber there's a group of hydraulic jacks supported by the finished a part of the tunnel which are wont to push the TBM forward

Once a particular distance has been excavated (roughly 1.5-2 metres), a replacement tunnel ring is made using the erector

The sector may be a rotating system which picks up precast concrete segments and places them within the desired positions

Behind the shield, inside a part of the finished tunnel, several support mechanisms (part of TBM) can be found

These support mechanism are dirt removal slurry pipelines, control rooms, rial for transport of precast segments

Tunnel Lining

Tunnel lining is  the wall of the tunnel 

it always consists of precast concrete segments which form ring

Additives like bentonite, polymers and foam are injected into the bottom to further stäbilize it

  Types of TBM 

1) Mechanical support TBM

It has a full face cutterhead which provides the support by constantly pushing the excavated material before the cutter head against the encompassing ground

Tunnel Lining Tunnel lining is that the wall of the lunnel. it always consists of precast concrete segments which form Finge

Additives like bentonito. polymers and foam are injected into the bottom to further stäbilize it

2) compressed gas TBM

 It can have either a full face cutterhead or excavating arms

3) Slurry shield TBM

 •it features a full cutterhead

Confinement is achieved by pressurising boring fluid in side the clutter chamber

 4)Earth pressure balance machine

 it's a full face cutterhead 

Confinement is achieved by pressurizing the excavated material within the cutterhead chamber 

(v) Mix face shield TBM 

It has full face cutterhead 

It can add closed and open mode and with various confinement techniques.

Cut and canopy Tunnelling Method

during this construction method, where by the location is fully excavated the structure is made and covered over uses diaphragm wall with during a site area

First we construct diaphragm wall,

Decking and pin piles at both left and right area 

Then, excavation is completed within the diaphragm wall installing struts on work progress

The, construction of permanent floor slab and walls takes place

 Finally, the interior structures are fitted out backfilling is completed and therefore the surface structure are reinstated

Drill and Blast 

Before the arrival of TBM, drilling and blasting was only economical way of excavating tunnel through hard rock where digging wasn't possible

Drill hole normal to the quarry face when exploded with a correct charge will escape a crater whose sides are inclined at 45 as shown in figure it's desirable to settle on a depth such the entire round of drilling, mucking, clean-up, fixing drills, firing and clearing smoke are going to be complete in one shift operation

Immersed Tunnels 

These sorts of tunnels are partly or wholly underwater

In this sort of method, there's no problem of congestion of traffic (as they are doing not block the route for ships), compared to bridges over rivers and seas

Tunnel Jacking 

may be a process to form tunnels in already existing bodies like roads and railway area.

In this method, specially made pipes are pushed by hydraulic pump in ground

Maximum diameter of tunnel by this method is around 2.4 metre.

Tunneling in hard rocks

The major methods of tunneling in hard rock are:

1 )Full Face Method

Whole section of the tunnel is attacked at an equivalent time

It is suitable just for small tunnels.

2)Heading and Bench Method

After the heading (8 to 10 ft. deep) has been formed as shown in Fig. 26 all the drill holes within the head and bench are loaded together for blasting

Bench is fired a moment before the highest heading holes are fired

Most of the muck from the heading ABCD is thrown from the bench and therefore the rest is cleared by labour The advantage of this method is that while mucking operation is ongoing . drilling can continue the heading thus providing simultaneous drilling and mucking operation

When bad ground is encountered, longer headings (50-75 ft. deep) are provided and therefore the roof arch is supported by shuttering While excavating the bench posts are provided to support the roof arch shuttering

(3)Drift Method 

This method is split into central drift method and side drift method.

(a) Central drift method

 When the smaller section of the tunnel chosen for mining lies within the centre. it's referred to as central drift method.

Central drift is administered from portal to portal or between two exits

 This method is economical in use of dynamite and provides good ventilation during widening in bad ground, when central drift method is used , the arch portion is excavated immediately after the centre drift

(b) Side drift method

 The side drift represented as 1 in Fig 2.7(b) is extended along the edges as shown by 2

Core no. 3 which is mined last, provides a working platform for mining core no 2 on the brink of the roof.

Driving Tunnels in Soft Ground 

The material which may be excavated with tools like picks, shovel etc. and no heavy or costly equipments required is termed as soft ground

Methods of Tunneling (Soft soils)

 the foremost popular methods of tunneling through soft ground is as shown below 

• Forepoling method  
• Belgium method 
• Army method or case method 
• German method 
• Needle beam method 
• American method 
• English method

The above methods require timbering for supporting the excavation and that they generally differ in respect of sequence, or placing, lining order during which out and heading is driven, different patterns and sort of wooden members

Other method (not requiring timber)

1)Linear plate method

2) Shield method 

3)Compressed air tunneling method

Forepoling Method

Forepoling method is extremely old used for Tunneling grounds

Tunnels of small dimensions. for laying sewars, gas pipes etc . ordinary depths, can advantageously be constructed by this method.

The process of forepoling method is slow and tedious requiring skilled labour.

 Needle Beam Method

This method is suitable for soil during which the roof might be depended upon to face for jiffy without support

By this method, 3m to 4 m length per day might be advanced and it's more suitable for brick lining 

It consists of a stout timber beam or a composite Witched beam and form the most temporary support during the excavation of the tunnel

 Belgium Method 

This is popular method and is suitable for all classes of moderately trim or hard soils, where height of overburden is extremely small 

 American Method

 this is often a way suitable for giant sized railway or highway tunnels,

English Method

 This method involves the use of lot of timber.

The distinctive characteristics of the method are the excavation of full section of the tunnel at once and alternate masonry work

Army Method

 it is useful method for driving small tunnels through soft around

This method is mainly used for laying underground sewers.

 Germen Method

Germen method consist in excavating first a gallery in side walls,roof arch is excavated before taking out the centre drift

This method employs 3-drift

 Other Method

1. Linear Plate Method

Pressed steel plates are used to support the soil, during excavation of short length

 The standard size of plate is 0.91 m 0.41 m The corrugated plates have high section modulus can ba used for the larger turnels without ribs.

2. Shield Method

 A shield is a movable frame which supports the face  of a tunnel and the ground behind the face and provides protection under which the tunneling is carried out

• The components of shield are

• The skin 
• Propelling juck
• Cutting edge 
• The port holes 
• The hood
• The tail 
• Employed when strata is subaqueous

 

Compressed Air Tunneling 

The recent method of tunneling in compressed air very useful in many soft and water bearing soils.

The air pressure required to force back the percolating water at water mixed soil and keep the tunnel dry varies with the nature of ground and the depth of the cover

Hauling or Mucking 

Mucking and Hauling refers to the operation of loading and removing excavated or blasted material sand dumping at predetermined side

Method of mucking

a) Mucking by hand

(b) Mucking by machine

 In all big tunnels this is a major expenditure and efficiently and quick removal of muck minimizes the costs.

In modern tunneling practice, various types of machines like the convey loader and elimco are employed to quicker the work and save labour

 Drainage of Tunnels 

In tunnel water comes from the following two sources:

Wash water:-

which is used for washing drill holes and the cuttings inside the tunnel at the time of

Construction

Ground or sub-soil water from the ground through which the tunnel is driven

The simplest method for the removal water a by drawing through open drains or ditches constructed along the tunnel on both sides at the Invert 

This system is useful in hard rock bases and water resistant soils

Tunnel Ventilation

 The ventilation means supply of air, light and keeping the level of noise to bearable to human beings 

Requirements of a Ventilation Systems

• To supply fresh air to the working crews
• To remove noxious and obnoxious fumes and gases of explosion
• To remove safely the dust caused by drilling, blasting and mucking 
• To decrease temperature in tunnel found to be at great depth

Method of Ventilation

1. Natural Ventilation:

•  Where drift is driven  from portal to portal, it provides fair ventilation naturally during the enlarging operations particularly when the tunnel is a short one and of large diameter
• Natural ventilation is achieved due to difference in temperature inside and outside the tunnel
•  it will be effective only when the orientation of the tunnel is along the wind direction

2. Mechanical Ventilation:

Mechanical ventilation is usually supplied by one or more electric motor driver fans which may blow fresh air into a tunnel or exhaust the dust or foul air from the tunnel

There are three techniques of mechanical ventilation:

Blowing or plenum process 

Exhausting or vacuum process 

Combination of blowing and exhausting  

Blowing or Plenum process 

 in blowing method fresh air is blown by blower fans mounted in one or more input shafts

This method is not employed for tunnel more than few hundred metres long.

 Exhausting 

This method 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 via its portals or through inlet shafts

Combination of blowing and exhausting

 The most efficient ventilation system relies upon combination of blower and exhaust fans

Soon after blasting, exhaust system is used to drop smoke and gases and rest of the time it is reversed and supplies fresh air along with blowing system

Lighting of tunnel 

tunneling operation activities can't be carried out effectively if the lighting is bad

A light intensity of 260 lumens per square meter in the working area is considered satisfactory 

Following are the types of light which are in common use in tunneling work:

• lanterns and lamp burning oil 
• Acetylene gas lighting
• Coal gas lighting
• Electric lighting

Dust Control

Various operations involved in tunnel excavation such as drilling blasting and handling muck cause dust accumulation in the tunnel atmospheres 

This dust laden constitutes  a serious health risk, unless the dust concentration in limited

Methods of dust control

Various methods used to minimize dust accumulation are as follows.

Wet drilling

use of vacuum hood 

Use of respirators 

Advantages of Tunnel 

• For carrying public utilities like water or gas, railway lines or roads across a stream or mountain tunnels could also be cheaper than bridge or open cut
• Tunnel avoid the damaging open cut adjacent to the structure
• Tunnels save leading from expensive pavements then lesser maintenance costs and operating expenses
• Cost of hauling is decreased thanks to lighter grades, possible in tunnels.
• Aerial warfare and bombing of cities have given intangible values to tunnels.
• It is generally assumed that when the cut required will have a vertical depth exceeding 20 m, it's less costly to create tunnel.