A brief overview on tunnelling.

2. Definition
Tunnels are artificial underground
passages opened at both ends
constructed for different purposes.
Required for highways, railways,
sewerage, water supply, public utilities
and canals.
Necessitated when open excavation of
strata becomes uneconomical, not only
construction cost but for maintenance as
well.
Continued
Tunneling

3. History
The oldest tunnel was constructed 4000
years ago in ancient Babylon to underpass
the bed of river Euphrates and to establish
an underground connection between the
royal palace and Temple of Jove.
The length of the tunnel was 1 km and it was
built with the considerable cross-section
dimensions of 3.6 m by 4.5 m.
In the second half of the 20th century, with
the progress in development of both
explosives and equipment, underground
construction became feasible.
Continued

4. Geological Considerations or ground
properties before constructing any tunnel
project:
The engineering properties of a rock
generally depend not only on the matrix
(structure formed by the minerals) but
also imperfections in the structure such
as voids (pore space ),cracks ,inclusions,
grain boundaries and weak particles.
Pore spaces are largely made up of
continuous irregular capillary cracks
separating the mineral grains.

5. Physical discontinuities are present in all
rock masses as a result of geological
activities.
Introduction of defects into the rock mass
due to human activities that alter the
properties of the rock material.
The mechanical breaks in the rock have
zero or low tensile strengths, increase
rock deformability and provide pathways
for water to flow.

6. Crown: The uppermost
part of the tunnel
Transportation tunnel cross-sections
Wall
Crown
Core (strozze) Springline
Invert
•Springline : The
line at which the
tunnel wall breaks
from sloping
outward to sloping
inward toward the
crown
Top heading
Bench
Bottom
Circular Horseshoe Vertical walls arch roof
Invert : The bottom Heading : The excavated face of the tunnel
(floor) of the tunnel
Drift : A
horizontal
excavation
Station : The distance measured from the portal (chainage)
Wall : The side of the tunnel Portal : The tunnel entrance

7. The choice of tunnelling method
may be dictated by:
geological and hydrological
conditions
Weak rock under high stresses
leads to squeezing ground
conditions.
Brittle strong rock high stress
conditions may lead to rockbursting.
 cross-section and length of
continuous tunnel
local experience and time/cost
considerations (what is the value
of time in the project)
limits of surface disturbance,
and many others factors
Choice of method
depends upon
nature of strata
and geometry of
tunnel section
Continued

8. Methods divided into three categories
Firm ground- reasonable time available for
installing conventional support.
Soft ground- Conventional support can not be
installed.
Running ground- Special treatment required
before starting excavation
Continued
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Rough classification of methods
 Excavation: An excavation process without
removing the overlying rock or soil
• Steps: excavating-remove muck-supporting-lining-
ventilation-draining
• Drill & Blast, hammer, roadheader, ....
Cut and cover: trenching to excavate and construct a
tunnel, and then backfilling earth over it.
Immersed tunnel: lowering prefabricated tunnel
elements into a dredged channel and joining them
up under water
• Concrete
• Steel
Continued

10. Bottom-up method: The main site is excavated,
with ground support as necessary, and the tunnel
is constructed within. The tunnel may be of in situ
concrete, precast concrete, precast arches, or
corrugated steel arches. The trench is then
carefully back-filled above the tunnel roof and the
surface is restored.

14. Coffer Dam and
hydroelectric Tunnel

17. Top-down method: Here side support walls and
cap beams are constructed from ground level
typically with slurry walls, or secant piling. Then a
shallow excavation allows making the tunnel roof
of precast beams or in situ concrete. The surface is
then restored except for access openings. This
allows early reinstatement of roadways, services
and other surface features. Excavation then takes
place under the permanent tunnel roof, and the
base slab is constructed.

22. Tunnelling in Firm ground
Traditional methods. Involving drilling and blasting
Full Face Method. In firm soils where full
face excavation can hold it self for
sufficient time to allow mucking and
supporting operations.
In the full-face method, workers excavate
the entire diameter of the tunnel at the
same time.
This is most suitable for tunnels passing
through strong ground or for building
smaller tunnels.
Continued

25. Top Heading and benching. When full face
excavation is not possible. Heading should be
sufficient distance ahead of benching.
In this technique, workers dig a smaller tunnel
known as a heading. Once the top heading has
advanced some distance into the rock, workers begin
excavating immediately below the floor of the top
heading; this is a bench.
 One advantage of the top-heading-and-bench
method is that engineers can use the heading tunnel
to gauge the stability of the rock before moving
forward with the project.

27. Excavation by tunnelling Methods
 Drift Method. In case of large or pilot tunnel , Drift is made
and then expanded. There can be more than one drifts.
 Different ‘drift methods’ depending on location of drift can be
used.

28. Tunnelling in Soft Strata
Tunnels constructed in soft materials
require temporary support immediately or
shortly after excavation. Way to provide
support are:
Suitably spaced bents of wood
Suitably spaced bents of steel for lagging
(covering)
Liner plates
Fore poling placed to retain material
between adjacent bents
Temporary supports must be designed for
higher working stress compared to those in
permanent designs.

29. Tunnelling in Soft Ground
Instantaneous support is required
No drilling and blasting.
Forepoling is done.
 Boards are driven ahead of the last ‘rib’, around
periphery
 Forepoles act as cantilevers beyond breasting
 Soil is excavated after removing the breast board and
new rib is erected
Continued

30. Because stand-up time is generally short when
tunneling through soft ground, cave-ins are a
constant threat. To prevent this from happening,
engineers use a special piece of equipment called a
shield.
A shield is an iron or steel cylinder literally pushed
into the soft soil. It carves a perfectly round hole and
supports the surrounding earth while workers remove
debris and install a permanent lining made of cast
iron or precast concrete.
When the workers complete a section, jacks push the
shield forward and they repeat the process.

32. Tunnelling in Running Ground
Many methods
Principle is, Linear plates are pushed in starting
from crown
Cavity is excavated
Additional linear plates are pushed one by one and
bolted
Arch section gradually widened down to springing
line.
Continued

33. Tunnelling in Rocks
Tunnels are driven in rocks by drilling holes in the
rock face, loading the holes with explosives, blasting
and removing the broken rock
Each sequence full cross section of the tunnel may be
excavated, or one or more drifts may be there.
Drilling pattern for holes of explosives may differ
depending upon rock type, cross section, experience
expertise and type of explosive suggested.
Methods are similar in principle like
 Full Face Method
 Top Heading and benching
 Drift Method
Continued

34. Tunneling through hard rock almost always involves
blasting. Workers use a scaffold, called a jumbo, to place
explosives quickly and safely.
The jumbo moves to the face of the tunnel, and drills
mounted to the jumbo make several holes in the rock. The
depth of the holes can vary depending on the type of rock,
but a typical hole is about 10 feet deep and only a few
inches in diameter.
Next, workers pack explosives into the holes, evacuate the
tunnel and detonate the charges. After vacuuming out the
noxious fumes created during the explosion, workers can
enter and begin carrying out the debris, known as muck,
using carts.
 Then they repeat the process, which advances the tunnel
slowly through the rock.

37. Fire-setting is an alternative to blasting. In this
technique, the tunnel wall is heated with fire, and then
cooled with water. The rapid expansion and contraction
caused by the sudden temperature change causes large
chunks of rock to break off. The Cloaca Maxima, one of
Rome's oldest sewer tunnels, was built using this
technique.
The stand-up time for solid, very hard rock may measure
in centuries. In this environment, extra support for the
tunnel roof and walls may not be required. However, most
tunnels pass through rock that contains breaks or pockets
of fractured rock, so engineers must add additional
support in the form of bolts, sprayed concrete or rings of
steel beams. In most cases, they add a permanent concrete
lining

38. Sequence of operation in Rocky
Strata (Drill and Blast)
Marking tunnel profile.
Setting up and drilling
Loading explosive and blasting
Removing the foul gases
Checking
Scaling
Mucking
Rock bolting and lining
Continued

39. Modern Tunnel Construction Methods:
 Drill and blast
 Mechanical drilling/cutting
 Cut-and-cover
 Immersed tunnels
 Tunnel boring machines (TBMs)
 New Austrian Tunnelling Method
(NATM)
Continued

40. Road headers

43. Types of support
Steel arches
Steel ribs are used for reinforcement of weaker tunnel
sections, and give rigid to semi-rigid support. The ribs are
made from I-beam or H-beam structural steel bent to
conform to the requirements of a particular tunnel cross-section.
Timber may be used for packing between the beams and
the rock. However, providing continuous bedding against
the rock may considerably increase the load-bearing
capacity of the arches.
Continued

45. Rock bolts
Steel bolts are frequently set in holes drilled into the rock
to assist in supporting the entire roof or individual rock
slabs that tend to fall into a tunnel. Rock bolts maintain
the stability of an opening by suspending the dead weight
of a slab from the rock above
Continued

46. Shotcrete
Pneumatically applied mortar and concrete are
increasingly being used for the support of underground
excavations.
A combination of rock bolts and shotcrete has proved an
excellent temporary support for all qualities of rock.
Shotcrete is best known in tunnelling as an integral
component of the NATM (New Austrian Tunnelling
Method).
Quick-setting concrete is sprayed onto the bare rock
surface immediately after excavation, and rapidly hardens
to form a preliminary support until the final lining of
conventional poured concrete can be installed.
Continued

47. Wire mesh
Wire mesh is used to support small pieces of loose rock
or as reinforcement for shotcrete.
Two types of wire mesh
chain link mesh commonly used for fencing and it
consists of a woven fabric of wire, tends to be flexible
and strong
weld mesh. commonly used for reinforcing shotcrete
and it consists of a square grid of steel wires, welded
at their intersection points.

49. Ventilation during construction
Why required?
To furnish fresh air for the workers
To remove the dust caused by drilling, blasting,
mucking, diesel engines, and other operations
To remove obnoxious gases and fumes of explosives
How done?
Mechanical ventilation is usually supplied by electric
fans, as for example axial flow pressure fans.
 If air is blown into a tunnel, it may be forced through a
lightweight pipe or fabric duct.
 If air is exhausted, it is necessary to use a rigid duct that will
not collapse.
The exhaust method has the advantage of more
quickly removing objectionable air from spaces
occupied by the workers.
Continued

50. Ventilation of tunnels
Mechanical ventilation systems provide the temperature,
humidity and air velocity conditions necessary to give
tunnel users a reasonable degree of comfort during normal
operation.
 When a fire occurs in a tunnel, the system must also
provide a safe evacuation route for tunnel users and
access for fire fighting services.
The choice and design of a ventilation system depends on
these main factors:
tunnel length and volume
admissible air pollution around tunnel portals
fire safety considerations.
Key pollutants include carbon dioxide, nitrogen oxides,
nitrogen dioxide, hydrocarbons PM10 and lead.
Better understanding of ventilation techniques
Awareness of new safety and environmental legislation.

55. Geological Survey For Tunneling
 A geological survey is the systematic investigation
of the subsurface and surface of a given piece of
ground for the purpose of creating a geological
map ,model and feasibility studies.
 A geological survey employs techniques from the
traditional walk-over survey, studying outcrops
and landforms, to intrusive methods, such as hand
auguring and machine driven boreholes, use of
geophysical techniques and remote sensing
methods, such as aerial photography and satellite
imagery etc.

56. Types Of Geological Survey
 Mainly geological surveys are classified into
two types:
Surface Geological Survey
Subsurface Geological Survey
 Surface Geological Survey: includes on land
geology and geological
structures,landforms,hydrology,outcrop
pattern ,engineering properties etc
 Subsurface Geological Survey: : includes on
underground geology and geological
structures,Geo-hydrogeology,subsurface rock
or soil pattern etc

57. Surface Geological Survey
Geological profile is prepared along line of tunnel.
Geological observations are done along this profile
like engineering properties of rock/soil, geological
structures like fold,faults,joints,spring,stream,river
alignment and any seepage etc
Trial boring plan is prepared along the tunnel line.

63. Subsurface Geological Survey
Trial boring is done along the tunnel line.
Different rock and soil samples are collected
from subsurface and their physical and
chemical properties are observed.
Some samples are sent to laboratory for
determination of engineering properties like
shearing,strenght,permeability,porosity,compr
essive strenght,and other test are performed.
GWT(Ground Water Table) is noted and water
bearing strata is marked in geological profile.

70. Different geophysical surveys like refraction,
resistivity, GPR(Ground Penetrating Radar) and
bor hole logging etc are also done.
In situ testing like Packer-testing, hydro
fracturing, load-testing etc are performed.
A geological/feasibility report comprising all
information ,photographs, profile and others
necessary data are presented for final tunnel
design and construction.

80. Subsurface
Profiling/3 D
modelling