Types of Tunnels-Guide to Classification of Tunnel Construction in Civil Engineering

Classification of tunnels

The tunnels are classified into,

1. According to alignment
2. According to purpose
3. According to the type of material met with at the time of construction
4. According to the shape of tunnels

We are going to read each of this classification in this article

• Types of tunnel according to alignment

1. Offspur Tunnel

   Short length tunnels to negotiate minor local obstacles, which cannot be followed with permitted curves.

Offspur tunnels

2.Saddle  or base tunnel

   These are constructed in the valleys along the natural slope. They are mainly constructed for the transportation purposes. Railway tracks and roads can be constructed through these saddle tunnel.They are one of main types of tunnel for transportation.

2.Saddle  or base tunnel

3.Slope  tunnel

   These include the tunnels that are constructed in steep hills. These types of tunnel are also used for transportation purposes.

3.Slope  tunnel

4.Spiral tunnels

• These include the tunnels that are provided in narrow valleys in the form of loops in the interior of the mountain.

4.Spiral tunnels

   This increases the length of the tunnel to avoid the steep slopes.

• CLASSIFICATION ACCORDING TO THE PURPOSE

1.Conveyance tunnels

• These types of tunnel includes the sewer tunnels, water supply tunnels, hydro-electric power tunnels etc.

1.Conveyance tunnels

2.Water conveyance tunnel

3.Traffic tunnels

•  They include highway tunnels, railway tunnels, pedestrian tunnels, navigation tunnels and subway tunnels.

3.Traffic tunnels

TYPES OF TUNNEL ACCORDING TO TYPES OF MATERIAL MET WITHIN THE CONSTRUCTION

• Tunnels in hard rock.
• Tunnels in soft materials.
• Tunnels underneath river bed or in water-bearing soils.

Classification according to shape of tunnels

1.D shaped tunnel

• Where the risk of failure or collapse caused by external pressure from water or loose unstable soil conditions on tunnel lining is non-existent
• These types of tunnel roof also called segmented roof takes up all the load and distributes it to the straight walls
• Suitable for subways
• 

  D shaped tunnel

2.Circularly shaped tunnel

• This shape of tunnels is strong in offering resistance to external pressure caused by water, water-bearing soils or soft grounds
• Not suitable for rails and roadways as more filling will be required to provide a flat base
• For tunnels which may have to withstand heavy internal or external radial pressures
• Used as a sewage pipeline, to transport oil etc

Circularly shaped tunnel

 Strongest and economical – having more cross-section and least perimeter

3.Horseshoe-shaped types of tunnel

•   Having a semi-circular rood with arch sides
•   Commonly used in railways and roadways
•  Can withstand the internal and external pressure
•   Combines the advantages of D shaped and circular tunnels

Horseshoe tunnel

4.Rectangular shaped tunnel

•   Rectangular types of tunnels are suitable for hard rocks
•   Suitable for pedestrian passage
•   costly

Rectangular shaped tunnel

5.Egg-shaped tunnel

•   Egg-shaped types of tunnels are best suited for sewage line as it is having a self-cleaning velocity in dry weather
•   Egg shaped tunnel can resist both internal and external pressure

Egg shaped tunnel

Lining and Ventilation of Tunnels-Tunnel Engineering

We know that tunnels are underground passages used for underground transportation facilities.For the Lining and Ventilation of Tunnels is an important process in the construction of tunnel ventilation system.

Nowadays transportation in huge hilly regions is avoiding by construction a tunnel through hills.Therefore the importance of tunnels is widely increasing in the modern era.

The two important process in tunnel ventilation requirements are lining and tunnel ventilation is briefly described below.

1.Tunnel Lining

• The tunnel lining definition means the finishing touch given to the cross-section of a Tunnel.

Lining in tunnels

a.Necessity of tunnel lining

• When it is desired to permanently protect the material surrounding the tunnel.
• When the cohesion between masses of particles surrounding the tunnel is not sufficient.
• When the tunnel is subjected to internal or external pressure or heavy ground pressure.
• To increase the strength of tunnel cross-section.

b.Tunnel Lining Types

Lining and Ventilation of Tunnels

1.Permanent lining

    Provided in soft ground prone to disintegration

2.Temporary lining

–For supporting roof & walls of the tunnel while construction.

An ideal lining should be

• Easy to maintain
• Economical
• Durable
• Simple to construct
• Stable

c.What are the Materials used for tunnel lining?

• The types of materials used for shield tunneling is chosen depending upon the strength, purpose to be served and resistance to weather conditions
• Materials used are:

                   1.Brick and stone masonry

                   2.Cement mortar

                   3.Timber

                   4.Cast iron

2.Tunnel ventilation

Tunnel ventilation system is the technique of providing freshness of air inside the tunnel during and after construction

a.Main objects of tunnel ventilation system

• To supply fresh air to the working crew
• To remove injurious and obnoxious fumes and gases of explosion
• To safely remove the dust caused by drilling, blasting and mucking
• To reduce the temperature in tunnel situated at a great depth

Ventilation in tunnels

Types of tunnel ventilation

Temporary ventilation

• When ventilation is to be provided at the time of construction only

Permanent ventilation

• When ventilation is to be provided after construction work is over, such ventilation system must be permanent

Methods of Tunnel ventilation

1.Natural ventilation

• When the tunnel is a short one and of large diameter
• achieved due to the difference in temperature inside and outside the tunnel
• effective only when the orientation of tunnel is along the wind direction.

Length of tunnel up to which natural ventilation would suffice depends on

• If the tunnel alignment contains curves or the grades are changing, the quantity of natural air expected to provide natural ventilation is considerably reduced
• The flow of traffic i.e. in one direction or in both directions will also affect the tunnel length for natural ventilation. Normally for one directional traffic in the tunnel, comparatively longer length of a tunnel may not require mechanical ventilation
• In general, for straight reaches with uniform grade, the tunnels up to about 100 m length would not require any mechanical ventilation

Volume of air considerations

• 6 m3 to 14 m3 of fresh air per minute constantly in the working area
• Any compressed air used for drills is contaminated with oil and dust and when released will be contaminated
• After each explosion, air near the face is filled with fumes and dust and is unfit for breathing
• This foul air has to be replaced with fresh air before the workmen start removing the debris from the explosion

Conditions determining the form and capacity of the ventilating system

• Length and size of the tunnel
• Amount of explosive and frequency of blasting
• Temperature inside the tunnel
• Gases encountered during driving operations

2.Mechanical ventilation

• Provided by one or more electric fans or blowers, which may blow fresh air into a tunnel or exhaust the dust and foul air from the tunnel

The various system of ventilation are:
            1.Blowing process

            2.Exhausting or vacuum process

            3.Combination of blowing and exhausting

Advantage

• Supplying fresh air right near the working face

Disadvantage

• Foul air, smoke & dust slowly moves out, fogging the atmosphere inside the tunnel, especially in the long tunnel

Exhausting Process in Tunnels

The foul air & dust are drawn into an exhausting duct near the working face, thereby creating a flow of fresh air naturally into the tunnel from the entrance or portal

Advantage of exhausting in tunnels

• Quick removal of dust & smoke from the working face

Disadvantage of exhausting in tunnels

• Fresh air has to travel a long distance before it reaches the working face & its quite likely that it may absorb moisture, heat etc

COMBINATION OF BLOWING & EXHAUSTING PROCESS

• Have the advantage of both the process.
• Immediately after blasting operation, the exhausting system is operated for 15-30min, to immediately remove the objectionable air
• After which, the blowing system operates for the rest of working period to supply fresh air.