Tunnels are underground passages used for transportation. They could be used for carrying freights and passengers, water, sewage, etc Tunnels are more economical than open cuts beyond certain depths. Tunnels avoid disturbing or interfering with surface life and traffic during construction. Tunnels prove to be cheaper than bridges or open cuts to carry public utility services like water, sewer and gas.  Feasibility of these constructions in natural materials, such as rock and soil, causes the geological conditions to play a major role in their stability. Aspects of major importance and that is decisive for the feasibility of a tunnel project is geological conditions, construction time and costs. The objective of this lesson  is to provide the general aspects of importance in tunnels, their types and methods of tunnelling.   

Tunnel construction for transport routes is becoming increasingly important worldwide. Transport is accelerated and optimum protection is provided for the environment and the landscape. Many tunnels are considered technological masterpieces and governments have honored tunnel engineers as heroes. Constructing a tunnel, however, is one of the most complex challenges in the field of civil engineering. Tunnels are  attractive solutions  for railways, roadways, public utilities and telecommunications. 

1.Basics of Tunnels:  

Tunnel is an artificially constructed underground passage to by- pass obstacles safely without disturbing the over burden.  Tunnels are created by the process of excavation.  

Open Cut is an "open to sky" passage excavated through huge soil mass of obstacle, in the required directions to connect two roads or railways.   Bridge is an over-ground construction to cross over obstacles without disturbing the natural way below it.  Tunnels are  underground passages for Road or rail traffic, Pedestrians, Utilities, Fresh water or sewer lines. A tunnel must be completely enclosed on all sides along the length.  

Ratio of length to width, in a tunnel, should always be at least in 2: 1.    

Tunnelling is desirable when  rapid transport facilities are required which need to avoid  acquisition of land for roads. Tunnels are also erected when shortest route connection is needed in cities. Tunnels  permit  easy gradient & encourage high speed on  strategic routes.  

Some structures may require excavation similar to tunnel excavation, but are not actually tunnels.  Shafts, for example, are often hand-dug or dug with boring equipment. But unlike tunnels, shafts are vertical and shorter.

Often, shafts are built either as part of a tunnel project to analyze the rock or soil, or in tunnel construction to provide headings, or locations, from which a tunnel can be excavated. The diagram shows the relationship between these underground structures in a typical mountain tunnel.

The opening of the tunnel is a portal.

The "roof" of the tunnel, or the top half of the tube, is the crown.

The bottom half is the invert.  The basic geometry of the tunnel is a continuous arch.  Because tunnels must withstand tremendous pressure from all sides, the arch is an ideal shape. In the case of a tunnel, the arch simply goes all the way around.

2.Physics of tunnelling: 

Tunnel engineers, like bridge engineers, must be concerned with an area of physics known as statics. Statics describes how the following forces interact to produce equilibrium on structures such as tunnels and bridges:

  1.  Tension, which expands, or pulls on, material
  2.  Compression, which shortens, or squeezes material
  3.  Shearing, which causes parts of a material to slide past one another in opposite directions
  4.  Torsion, which twists a material

3.Economics of Tunnelling depend on:  

Nature of Soil/ rock,    

Requirements of fill,    

Depth of cut > 18m –tunnelling.  

4.Tunnel Design Criteria  

Tunnelling requires proper design.  Every  tunnel will have its own geometry, design, alignment, and construction methods.  The tunnel design criteria include the following aspects:  

1. Spatial Requirements;

2. Alignment;

3. Underground Stations;  

4. Fire Life Safety; and  Tunnel Systems and Operation.   

Every tunnel should  have its own  Horizontal and Vertical Alignment, Tunnel Ventilation, Tunnel Lighting, Electrical and Safety Equipment, Tunnel Drainage, Fire Life Safety, and Security.    

5.Factors to be considered in tunnelling:  

The following factors should be taken into consideration when selecting the  method: - Tunnel dimensions,   - Tunnel geometry - Length of tunnel,  - Total volume to be excavated - Geological and rock mechanical conditions - Ground water level and expected water inflow - Vibration restrictions &  - Allowed ground settlements.  

6.Tunnel alignment  

Tunnel alignment is an important aspect in engineering constructions. The primary objectives of the tunnel alignments are to: 1. Reduce transit trip times; 2. Increase quality and reliability of service; and 3. Minimize impacts of surface transit operations in sensitive locations.   

7.Selection of Tunnel alignment depends  on   

1. Topography of area & points of entrance and exit,

2. Selection of site of tunnel to be made considering two points.  

3. Alignment Restraints

4. Environmental Considerations.  

8.Classification of Tunnels:  

The types of tunnels are  classified based on three aspects:

1. Based on purpose (road, rail, utilities),

2. Based on Alignment

3. Based on surrounding material (soft clay vs. hard rock ) &

4. Submerged tunnels.      

Egyptians and Babylonians  constructed tunnels about 4000 years ago with a   length of 910 m , width –of 3600 mm and a height of 4500mm.  The Channel Tunnel  was constructed by linking Britain & France, way back in 1994.  The total  length was about 50 km. The undersea component itself is about 39 km.  

9.Classification of Tunnels :   

The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.  There are three basic types of tunnel construction in common use: Cut-and-cover tunnel, constructed in a shallow trench and then covered over;  Bored tunnel, constructed in situ, without removing the ground above. There are also  Conveyance Tunnels  and  Traffic Tunnels. Shallow tunnels are of a cut-and-cover type (if under water of the immersed-tube type). 

Deep tunnels are excavated, often using a tunnelling shield. For intermediate levels, both methods are possible.

10. Based on Alignment  

1.  Off-Spur tunnels : Short length tunnels to negotiate minor obstacles.  

2.  Saddle or base tunnels : tunnels constructed in valleys along natural slope .

3.  Slope tunnels : constructed in steep hills for economic and safe operation .

4.  Spiral Tunnels : constructed in narrow valleys in form of loops in interior of mountains so as to increase length of tunnel to avoid steep slopes.  

11.  Based on type of material met with in construction  

1. Tunnels in Hard Rock

2. Tunnels in Soft materials

3. Tunnels in Water Bearing Soils  

12.Investigations for tunnelling:   

The major Investigations to be carried out prior to planning are:

A. Geological Investigations –relation between bed rock and top soil.          

B. Morphology, Petrology, Stratigraphy C. Electrical Resistivity Methods –positions of weak zones -faults, folds and shear zones.  Investigations made at time of planning are as follows: 

1. Drilling holes by percussion, rotary percussion and rotary

2. Rotary or Rotary Percussion methods –loose soils

3. Rotary Drilling –rocky soils

4. Spacing –300-500m ; reduced to 50-100 m in geologically disturbed areas.  

5. Lateral Spacing –10-15m from C/L of tunnel

6. Depth –20-50 m deeper than proposed invert level of tunnel.   For detailed undisturbed observations, shafts can be excavated .  

7. Shafts are vertical or inclined tunnel excavated to reach and to get information for the area surrounding proposed tunnel and tunnel section.

8. Section of 3m x 1.5 m to 3 m x 2m is preferred.

9. Minimum depth of excavation is yet another factor in this selection.  

10. Temporary and Permanent Shafts are made depending upon the circumstances.  

13.Setting out of a tunnel  

1. Setting Out refers to the making the centre line or alignment of any construction work on ground.

2. Setting out centre line of tunnel is made in  4 stages: 

a. Setting out tunnel on ground surface

b. Transfer of Centre line from surface to underground

c. Underground setting out d. Underground Leveling  

Setting out of the tunnel on the ground surface: Running an open traverse between two ends of proposed tunnel.

Curved alignment:   Heading consist of short tangent to curve alignment .  

Offsets measured from these tangents.  

14. Transfer of centre line from the surface to the underground:  

1. Underground shafts –interval of 500 m along transverse lines

2. Rectangular Horizontal frame set at proposed location along AB

3. On two sides of the frame, iron plates are fixed and screwed down & holes are drilled along A and B at X & Y

4. Plumb bobs are suspended to define vertical lines

15.Shape of tunnels  

D or Segmental Roof Section

1. Suitable for sub-ways or navigation tunnels

2. Additional Floor Space and flat floor for moving equipment  

Circular Section

1. To withstand heavy internal or external radial pressures

2. Best theoretical section for resisting forces

3. Greatest C/s Area for least perimeter

4. Sewers and water carrying purposes  

Rectangular Section Suitable for hard rocks, Adopted for pedestrian traffic Costly & difficult to construct  

Egg shaped Section=  Carrying sewage Effective in resisting external and internal pressures  

Horse –shoe Section

1. Semi-circular roof with arched sides and curved invert

2. Best shape for traffic purposes

3. Most suitable for soft rocks and carrying water or sewage

4. Most widely used for highway and railway tunnels  

16.Size of the Tunnel 

1. Determined from utility aspect,  

2. Road tunnels –No. of traffic lanes.  

3. Railway tunnels –Gauge & No. of tracks

4. Thickness of lining.    

5. Provision for drainage facilities

6. Clear opening required for traffic.  

7. Nature of traffic  

17.Tunnelling methods:   

TABLE-1.1 Construction methods versus typical shape

construction method circular rectangular horseshoe oval
cut and cover   x    
shield driven x x    
immersed tube        
drill and blast     x x
sequential excavation     x x

Mechanical methods  

can be split further into partial face (e.g. road headers, hammers, excavators) or full face (TBM, shield, pipe jacking, micro tunneling).  

The drill & blast method is still the most typical method for medium to hard rock conditions.    It can be applied to a wide range of rock conditions.    Hard-rock TBMs can be used in relatively soft to hard rock conditions, and best

when rock  fracturing  &  weakness zones are predictable.  The TBM is most economical method for longer tunnel lengths.  

Drilling and blasting:  

Drilling Pattern Design:

The drilling pattern ensures the distribution of the explosive in the rock and desired blasting  result.   Several factors must be taken into account when designing the drilling pattern: rock  drillability and blastability, the type of explosives, blast vibration restrictions and accuracy  requirements of the blasted wall etc.  

Excavation of Tunnel:

1. Percussion drills (penetrate rock by impact action alone)

2. Rotary drills (cut by turning a bit under pressure against the rock face)

3. Rotary-Percussion drills (combine rotary and percussion action)

4. Abrasion Drills –Shot, Diamond

5. Fusion Piercing 6. Special Drills –Implosion, Explosion  


Primary blasting vs Secondary blasting.  

Types of Explosives:

Straight Dynamites,  

Ammonia Dynamites,    

Ammonia -Gelatine.  

Semi –Gelatine.  

Blasting Agents,   

Theory of Blasting: Impact, Abrasion, Thermally Induced Spalling, Fusion and Vaporization, Chemical Reaction.  

18.Nature of substratum:  

A.  Hard Rock or fully self-supporting

B.   Soft Soils –requiring temporary supports during and after construction.  

Tunnelling in soft soils:    Challenges,  Preventing soil movements: Soil pressure,  Water seepage.      

19.Tunnelling Techniques:  

A.  Cut and Cover( Supporting Beams, Roof lining) 

Tunnelling in Hard rocks

Influencing Factors

Type of rock, Igneous, Sedimentary, Metamorphic Rock Hardness ,   Rock Brittleness.     Extent of existing fractures and planes of weakness.   

Tunnelling Methods in Hard Rocks:  

Heating and quenching (old technique).  

Immersed Tube

A body of water such as a canal, channel, bay, or river can be crossed using immersed tube tunnel technology.  

20.tunnel Boring Machine (TBM) 

Other Considerations:

• Protection of Structural Elements

• Fire Detection,   

Fire Protection (i.e., standpipe, fire hydrants, water supply, portable fire extinguisher, fixed waterbase,  fire-fighting systems, etc.),

• Communication Systems,

• Traffic Control ,

• Tunnel Drainage System

• Emergency Egress,

• Electric, and  Emergency response plan.  

21.Tunnel Drainage:    

During the construction of underground concrete lined structures like tunnels, it is necessary to prevent water from coming into contact with the completed tunnel lining that can cause both safety problems and increased maintenance costs.   

A drainage system will include all the components needed to ensure that the substructure is properly drained, and may be formed of components such as open ditches, closed ditches with pipe drains and drainage through stormwater drainage pipes, channels and culverts.    

Permanently functional tunnel drainage must maintain the operation and keep operating costs low. This includes a primary drainage system to capture the hill water and a secondary drainage system to channel this mountain water in the tunnel cross section.  

The secondary drainage systems  include: - Pipes for floor / base course drainage - Side drainage - Floor drainage - Cross-collectors and mountain water collecting main - Conveyor pipes.  

22.Tunnel Portals: 

Tunnel portal is the interface point of the open cut and the cut and cover tunnel.   Portals and ventilation shafts should  satisfy environmental and air quality.     Tunnel should be hazard free.  

23.Causes for Tunnel collapse:  

Tunnel collapse can happen for a number of reasons such as: inadequate ground investigation, shallow ground conditions, inadequate support measures, cost optimization, inexperienced contractors, inadequate supervision, delays of excavation and support erection.

• Construction failure

  1.  Ground and Groundwater Conditions
  2.  Preliminary investigation carried out without any drilling
  3.  Probe drilling was not performed during tunnelling
  4.  No stabilization measures to support a large swelling clay section before blasting.  

24. Conclusion:  

In addition to bring a certain risk to tunnel projects, geological conditions are highly influencing construction time and costs.  Basically, constructing a long tunnel is a time consuming and very expensive project. Adverse tunnelling conditions will increase the costs and construction time due to increased need for rock support. The major part lies in the geology and structure of the area in which we are planning to align the tunnel.  In Engineering geology, a lot more things are to be understood like this. thank u