Tunnels, types & importance

Tunnel is an artificially constructed underground passage to by- pass obstacles safely without disturbing the over burden. This module explains about tunnels, their parts, types and importance.

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TUNNELS, TYPES & IMPORTANCE

by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
UNIVERSITY OF MYSORE
MYSORE-6

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Introduction:
Tunnel is an artificially constructed underground
passage to by- pass obstacles safely without
disturbing the over burden.
Open Cut is an open to sky passage excavated through
huge soil mass of obstacle in 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.

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Tunnels are underground passages for Road or rail
traffic, Pedestrians, Utilities, Fresh water or sewer
lines.

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Ratio of length to width, in a tunnel , should always
be at least in 2: 1.
A tunnel must be completely enclosed on all sides
along the length.
The types of tunnels are classified based on three
aspects: Based on purpose (road, rail, utilities), Based
on surrounding material (soft clay vs. hard rock ) &
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.

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The Channel Tunnel was constucted by linking
Britain & France way back in 1994. The total length
was about 50 km ; undersea component itself is about
39 km.
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.

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Economics of Tunnelling depend on:
Nature of Soil/ rock,
Requirements of fill,
Depth of cut > 18m –tunneling.

Tunnelling is desirable when Rapid transport
facilities are required which needs 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.

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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:
Spatial Requirements;
Alignment;
Underground Stations;
Fire Life Safety; and
Tunnel Systems and Operation.

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Every tunnel will have its own horizontal and vertical
alignment, Tunnel Ventilation, Tunnel Lighting,
Electrical and Safety Equipment, Drainage, Fire Life
Safety, Security.
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.

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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.

Tunnel portal – the interface point of the open cut
and the cut and cover tunnel.

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Classification of Tunnels= 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.

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Classification of Tunnels=
Based on purpose
Conveyance Tunnels & Traffic Tunnels.
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

Investigations:
Investigations prior to planning
A. Geological Investigations –relation between bed
rock and top soil.

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B. Morphology, Petrology, Stratigraphy
C. Electrical Resistivity Methods –positions of weak
zones -faults, folds and shear zones

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Investigations made at time of planning

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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. Lateral Spacing –10-
15m from C/L of tunnel.
5. Depth –20-50 m deeper than proposed invert level
of tunnel. For detailed undisturbed observations,
shafts can be excavated .

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Shafts –vertical or inclined tunnel excavated to reach
and to get information for the area surrounding
proposed tunnel and tunnel section
6. Section of 3m x 1.5 m to 3 m x 2m. Minimum
depth of excavation. Temporary and Permanent
Shafts

Setting out of a tunnel
1. Setting Out -Making the centre line or alignment of
any construction work on ground
2. Setting out centre line of tunnel by 4 stages:

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3. Setting out tunnel on ground surface
4. Transfer of Centre line from surface to
underground
5. Underground setting out
6. 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.

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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

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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.

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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

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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

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Size of the Tunnel
1. Determined from utility aspect, Road tunnels –No.
of traffic lanes. Railway tunnels –Gauge & No. of
tracks
2. Thickness of lining. Provision for drainage
facilities
3. Clear opening required for traffic. Nature of traffic

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Tunnelling methods:

Mechanical methods can be split further to 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

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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.

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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)

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4. Abrasion Drills –Shot, Diamond
5. Fusion Piercing
6. Special Drills –Implosion, Explosion
Blasting=Primary blasting vs Secondary blasting.

Types of Explosives= Straight Dynamites, Ammonia
Dynamites, Ammonia -Gelatine. Semi –Gelatine.
Blasting Agents, Slurries (mixture of explosives, gel
and water)

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

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Nature of substratum=
A. Hard Rock or fully self-supporting
B. Soft Soils –requiring temporary supports during
and after construction.
Tunneling in soft soils:
Challenges, Preventing soil movements=Soil
pressure, Water seepage.

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

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B. Tunnel Shields

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Tunnelling in Hard rocks-Influencing Factors
Type of rock, Igneous, Sedimentary,Metamorphic
Rock Hardness, Rock Brittleness.
Extent of existing fractures and planes of weakness.
Tunneling 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.

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Tunnel Boring Machine (TBM)

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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 ,

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• Tunnel Drainage System
• Emergency Egress,
• Electric, and Emergency response plan.

Tunnel Drainage: imp aspect to be studied.

Tunnel Portals=
Portals and ventilation shafts should satisfy
environmental and air quality.
Tunnel should be hazard free.