Rock mechanics and tunnelling module 2

1. Tunnelling

2. Site exploration
Site investigation or Sub-Soil explorations are done for obtaining the information about
subsurface conditions at the site proposed for construction.
The results assess the feasibility of a project, to plan and design appropriate foundations, and
to draw up bills of quantity for excavation.
The project design normally requires the following information:
• What rocks and soils are present, including the sequence of strata, the nature and
thicknesses of superficial deposits and the presence of igneous intrusions;
• How these rocks are distributed over, and under, the site (that is, their structure);
• The frequency and orientation of joints in the different bodies of rock and the location
of any faults;

3. • The presence and extent of any weathering of the rocks, and particularly of any soluble
rocks.
• The groundwater conditions, including the position of the water table, and whether the
• Groundwater contains noxious material in solution, such as sulphates, which may affect
cement with which it comes in contact;
• The presence of economic deposits which may have been extracted by mining or
quarrying, to leave concealed voids or disturbed ground; and
• The suitability of local rocks and soils, especially those to be excavated, as construction
materials.

5. Geologic site investigations and soil mechanics tests are done in sufficient detail and
intensity for planning, design, and implementation of conservation engineering practices.
Geological investigations help in:
a) Selection of alignment
b) Selection of Excavation Method
c) Selection of Design for the Tunnel
d) Assessment of Cost and Stability
e) Assessment of Environmental Hazards

6. a) Selection of alignment:
Many alternate alignments that could connect two points through a tunnel.
The final choice/alignment having least geologically negative factors would be the a choice.
b) Selection of Excavation Method:
Excavation methods are linked with the type of rocks to be excavated.
Choice of the right method depends on nature of the rocks and the ground all along the
alignment.
c) Selection of Design for the Tunnel:
The ultimate dimensions, shape and design parameters of a proposed tunnel dictated by the
geology of the alignment than by any other single factor.

7. d) Assessment of cost and stability:
The line of actual excavation, the method of excavation and the dimensions of excavation as
also the supporting system depend mainly on local geology.
The cost of the project would also depend on the geological details.
Assessment of Environmental Hazards:
The process of tunnelling involves disturbing the environment of an area. For example,
vibrations induced through blasting or ground cutting and drilling, abnormal quantities of
dust and interference with water supply system of the nearby areas.

8. Systematic exploration may involve up to five procedural stages:
• Geologic reconnaissance
• Preliminary site investigation
• Detailed site investigation

9. a) Reconnaissance: Reconnaissance is the preliminary examination of the general geological
features and characteristics of a region.
Limited on-site investigations assess the feasibility and suitability of the site, recognize the
associated hazards, verify the accuracy or adequacy of existing information and required
additional data.
The reconnaissance stage comprises of geological mapping, outcrop sampling, wide-spaced
geochemical sampling, and preliminary geophysical survey.

10. Data gathered during the reconnaissance are primarily descriptive and include the following:
• General geology of the site: surface and subsurface earth materials
• Geologic conditions that may affect erosion, sedimentation, groundwater movement and
recharge
• General character of topography and stream system
• Engineering properties of soil and rock materials
• Presence and activity of faults
• General relationship between the existing geologic conditions and design requirements for the
proposed conservation.
• Practice or structure
• Ongoing or projected activities near the area (other development plans/projects, tourism, roads,
mining, oil or gas extraction)

12. b) Preliminary site investigation
Purpose: The purpose of a preliminary site investigation is to establish the geologic feasibility
of the site and to determine the extent and precision of detailed subsurface investigations
required to obtain the information needed for design and construction.
Before beginning a field study of a site, available geologic, physiographic, and engineering-
experience data are reviewed. Stages involve in preliminary site investigation:
• Assembly of data
• Use of imagery
• Field study
• Mapping
• Report of preliminary investigation

13. Detailed site investigation
It is made to verify and supplement reconnaissance level and preliminary level investigations to
provide the designer with specific and quantifiable information for use in design and construction.
A detailed geologic investigation may include any combination of the following:
• Conducting subsurface investigations, including exploration holes, trenching, pitting, geophysical and seismic
evaluations.
• Obtaining soil and rock samples for laboratory analysis and performing in situ testing
• Analysing remotely sensed data and imagery
• evaluating the geomorphology, geologic units, and structures at and near the site
• Developing sediment budgets and sediment storage requirements or sediment management, including
sediment production, transport, and yield
• Defining structural or cultural features in the area of interest (may include water wells, oil and gas wells,
mine shafts and adits, quarries)

14. Joint Studies
A thorough study of the rock joint systems in the terrane through which the tunnel will pass will be of prime importance to
the tunnel engineer who must evaluate tunnel excavation techniques and estimate the quantity and suitability of various
support methods. In the analysis of the joint data, an attempt is made to answer the following:
• How many distinguishable joint systems occur in the area?
• What are the attitudes of these joint systems relative to the proposed tunnel centreline?
• What is the spacing of the joints within a joint system?
• What is the typical 2-dimensional extent of joints within a system?
• What are the widths of the joints in a system?
• Do joints have a filling, and, if so, what is the filling material?
• Do joints of one system tend to intersect but not continue across the joints of another system?
• Has slippage occurred along any joint or system of joints?

15. Excavations have been classified into two broad groups depending on whether they are
made at the surface or below ground.
Open excavation: those at the surface are termed open excavations and they may be formed
in the course of the construction of foundations, tunnels and under-ground chambers for a
variety of purposes.
Subsurface excavations includes: Excavation for the installation and/or repair of utilities
(water, sewage, gas, or communications).
The geological conditions affects the formation of excavations affecting the method of
excavation, stability of the opening and the stability of the surrounding, or overlying, ground.
Excavation

16. Constructional problems include the maintenance of the stability of the excavations, surface
subsidence and difficulties with the excavation operations.
The method of boaring appropriate for a particular situation depends on:
• The ground conditions,
• The water conditions and
• other factors connected with location of the excavation itself.
Seismic velocities for determining diggability
The diggability of ground affects, selection of
excavating equipment and depends primarily upon its
intact strength, bulk density, bulking factor and natural
water content.

17. Tunnelling methods
Tunnelling methods depends on:
• 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.

19. New Austrian Tunnelling Method
Also known as Sequential Excavation Method.
For shorter tunnel sections, non-circular tunnels, or tunnels with variable geometry, it provides cost effective,
flexible, and safe tunnelling option.
The excavation location of a proposed tunnel is divided into segments first. The segments are then mined
sequentially with supports.
The tunnel is sequentially excavated and support is provided by shotcrete, in combination with fibre or welded-
wire fabric reinforcement, steel lattice girder arches.
Ground improvement methods such as jet grouting, dewatering, ground freezing, and grouted pipe spilling are
also available to stabilize the face.
The ground for excavation must be fully dry for applying the NATM.
This method is relatively slow but is found useful in areas where existing structures such as sewer or subway
could not be relocated.

20. Advantages:
Similar to the drill-and-blast and bored tunnelling methods, only localised potential
environmental impacts would be generated;
Disadvantages:
As the method is relatively slow, duration of potential environmental impacts would be
longer than that of the other methods.

21. Tunnel Jacking
Tunnel jacking is a method for constructing monolithic, rectangular concrete box sections
under surface areas with critical uses such as railways, major roadways, and airport runways.
Due to its expense, it is generally only used when there is no other option, and so examples
are rare.
This method is generally applied in cases where the underground crossing is relatively short,
is located in soft ground, and with shallow cover.
Special construction techniques are required to minimize friction, reduce settlement, and
maintain the tunnel alignment.

22. Cut and Cover
Cut and cover tunnelling is a common, well-proven technique for constructing shallow
tunnels.
This technique consists of an in-situ cast concrete structure in an excavated trench, which is
covered afterwards.
This method accommodates changes in tunnel width and non-uniform shapes and is often
adopted in the construction of underground transit stations.
To minimize surface disruption, cut and cover tunnelling can be accomplished using the
traditional bottom up method or as a top-down construction.
Early underground metro rail stations in London were built using this method.

23. Construction Methodology 1

24. Construction Methodology 2

25. Disadvantages:
More dust and noise impact may arise, though these can be mitigated through
implementation of sufficient control measures;
Temporary decks are often installed before bulk excavation to minimise the associated
environment impacts;
Larger quantity of C&D materials would be generated from the excavation works,
requiring proper handling and disposal.

26. Bored or Mined Tunnelling:
The tunnels are built without excavating the ground surface. This method is enabled by the
use of Tunnel Boring Machines (TBM’s).
Tunnels to be excavated in a wide variety of ground conditions and beneath towns, cities,
rivers and even tall buildings.
They do this by maintaining the static pressure of the existing ground to virtually eliminate
settlement, thereby avoiding damage to surface buildings or structures.
TBM’s are also able to install a concrete ring for permanent ground support when the
tunnel is in operation

29. Advantages:
Potential environmental impacts in terms of noise, dust and visual on sensitive receives are
significantly reduced and are restricted to those located near the launching and retrieval
shafts.
Compared with the cut-and-cover approach, disturbance to local traffic and associated
environmental impacts would be much reduced.
Compared with the cut-and-cover approach, quantity of C&D materials generated would be
much reduced.

30. Drill and Blast
Drill and blast is an excavation method used frequently in hard-rock tunnelling because its flexibility
allows for different layout setups.
Involves the use of explosives.
Drilling rigs are used to drill blast holes on the proposed tunnel surface to a designated depth for
blasting. Explosives and timed detonators (Delay detonators)are then placed in the blast holes.
Once blasting is carried out, waste rocks and soils are transported out of the tunnel before further
blasting.
Adequate structural support measures are required when adopting this method for tunnelling.
Commonly used mining excavation method in the world.

33. Advantages:
Noise, dust and visual on sensitive receives are significantly reduced and are restricted to those located
near the tunnel portal.
Compared with the cut-and-cover approach, quantity of C&D materials generated would be much
reduced.
Compared with the cut-and-cover approach, disturbance to local traffic and associated environmental
impacts would be much reduced.
Blasting would significantly reduce the duration of vibration, though the vibration level would be
higher compared with bored tunnelling.
Disadvantages:
Potential hazard associated with establishment of a temporary magazine site for overnight storage of
explosives shall be addressed through avoiding populated areas in the site selection process.

34. Relationship between Shape of tunnel and Method of tuneeling
• Circular Tunnels: usually constructed using either Tunnel Boring Machine Method (TBM) or by
Drill and Blast Method.
• Rectangular Tunnels: by the Cut and Cover method, by the immersed method, or by the
jacked box tunneling.
• Horseshoe Configuration: The drill and blast in method, or the Sequential Excavation Method
(SEM), also known as the New Austrian Tunnelling Method (NATM)

35. Open face construction without a shield:
Timber heading: One of the oldest methods of tunnel construction involves the use of
timbering to provide the temporary support for the ground during tunnelling and is still used
extensively in all underground works.
Timber heading as used
in London Clay for
constructing a
connecting passage
between the
two running tunnels

36. Open face tunnelling with alternative linings
Ground has some stand-up time, open face tunnelling can be adopted in association with
segmental linings, such as precast concrete and spheroidal graphite iron segmental lining.
Construction of an escalator
tunnel at The Angel Islington
Station, London Underground,
UK (completed in 1990).

37. Partial face boring machine (Road header)
As the name suggests, a partial face boring
machine works on discrete areas of the face rather
than excavating the whole face in one go.
Depending on the direction of the rotation of the
cutterhead, it is possible to differentiate between
a partial cutting machine with a cutterhead which
rotates along the axis of the cutter machine boom
(axial) and one in which the cutter rotates at right
angles to the cutter machine boom (transverse)

39. Tunnelling shields
In its simplest form, a tunnelling shield is a steel frame with a cutting edge on the forward
face.
For circular tunnels, this is usually a circular steel shell under the protection of which the
ground is excavated and the tunnel support is erected.
A shield also includes back-up infrastructure to erect the tunnel support (lining) and to
remove the excavated spoil.