Engineering Geology Lecture 6

1. Chapter 6
Engineering Geology of Tunnel

2. Chapter Outline
2
6.1 Introduction to tunnels
6.2 Methods of tunnel excavation
6.3 Geological Considerations for Tunneling
6.4 Geological Structure Considerations for Tunneling
6.5 Site investigation for Tunnels
6.6 Support for Underground Openings

3. End Objective
Students will be acquire knowledge about tunneling and
problems
Students will be able to seek engineering solutions for
tunneling on difficult ground
Will be familiar with tunnel site investigation
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4. Introduction
What is tunnel?
 It is underground passages or routes (or
passages through hills or mountains) for
different purposes.
 Unlike other civil engineering
constructions, tunnel lie underground (i.e..
within the rocks).
 For this reason, the needs for their safety
and stability is much more important
 It is usually made to over come the natural
obstacle in transportation systems mainly
roads and railways, for underground
transport to link areas of interest.
• However the cost of tunneling is more than its surface alternation,
but it help to reduce the distance, improve safety and less
environmental impact.

5. Effects Of Tunneling On The Ground
The tunneling process deteriorates the physical
conditions of the ground.
This happens because due to heavy and repeated blasting
excavation, the rocks gets shattered to great extent and
develop numerous cracks and fractures.
This reduces the cohesiveness and compactness of rocks.
In other words, rocks becomes loose and more fractured
and porous.
This naturally adversely affects the competence of the
rocks concerned.
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6. Tunneling purpose
Tunnel is constructed for different purpose, some of them are;
 Passage ways
– Is a tunnel that used as a passage for trains and motor vehicles.
Tunnel that used as transportation passage have two or three
large parallel line for opposite direction and emergency exist
routine.
 Underground installation: they are used for installation of
structures such as underground power plant.
 Water supply
 Mining/ underground mining
 River diversion/ for flood control or irrigation
 Pipeline/ utilizing water to a given area
 Secret tunnels/ where the on surface road is difficult for security
issues, under ground passage is an alternative.

7. Types of Tunnel
Tunnel can be classified into four types depending on
its purpose;
i. Traffic tunnel is a tunnel that constructed
underground for the passage of roads and railways
ii. Hydropower tunnel is used to pass water under
pressure and produce power by colliding with
generators.
iii. Public utility tunnels: this is relatively small and
construct for carrying utility lines for routing
power, pipeline and telecom cables.
iv. Diversion tunnel: this tunnel is used for flood
control or supplying water for different purpose.

8. Methods of tunnel excavation
In excavation of tunnel, especially for long tunnel different lithological
units can be encountered. They may be hard rocks to soft soils (non-
cohesive)
In some cases, buried channels can be encountered, which can supply
excessive water to the tunnel.
Depending on geology of different section of tunnel, different methods
of excavation can be used.
The excavation methods also depends on excavation depth, depth of
water table and accessibility of the site.
As a general the choice of tunnelling method may be dictated by:
 Geological and hydrological conditions,
 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.

9. The excavation methods that commonly used are;
– Cut-and-cover
– Boring machine
– Drill and blasting
I. Cut-and-cover method
This construction method, whereby the site is fully excavated,
the structure built and then covered over, uses diaphragm walls
as temporary retaining walls within the site area.
Step one :Construction of diaphragm walls, pin piles, and
decking.
Step two :Excavation within the diaphragm walls, installing
struts as work progresses.
Step three :Construction of permanent floor slabs and walls.
Step four : Fitting out the internal structures, backfilling, and
reinstating the surface structures.

11. II. Tunneling by tunnel boring machine (TBM)
 Bored tunneling by using a Tunnel Boring Machine (TBM) is
often used for excavating long tunnels.
This methods involves horizontal cylindrical metal that rotate and
pressurized to excavate long tunnel.
An effective TMB method requires the selection of appropriate
equipment for different rock mass and geological conditions.
The TBM may be suitable for excavating tunnels which contain
competent rocks that can provide adequate geological stability for
boring a long section tunnel without structural support.
However, extremely hard rock can cause significant
wear/crushed of the TBM rock cutter and may slow down the
progress of the tunneling works to the point where TBM becomes
inefficient and uneconomical and may take longer time. 11

12. 12
Types of Tunnel Boring Machine (TBM)
Open Beam TBM
Shielded TBM

13. Advantages using TBM excavation are:
 It requires less rock support.
 It gives smoother tunnel walls and reduced head loss in water
tunnels.
 Longer tunnel sections can be excavated between adits.
 It has higher tunneling capacity.
 It gives better working conditions for the crew.
The disadvantages of TBM are:
o More (better) geological information from the pre-investigation
stage is required.
o More sensitive to tunneling problems in poor rock mass
conditions.
o Fixed circular geometry, limited flexibility in response to
extremes of geologic conditions, longer mobilization time, and
higher capital costs.
o Only longer tunnel sections can be bored more economically
(because of larger investment and rigging costs)
o The TBM may get stuck under squeezing rock conditions.
o It is difficult to perform / install rock support at the tunnel face. 13

14. 14
III. Drill and Blast Method
This tunneling method involves the use of explosives. Drilling rigs are
used to bore blast holes on the proposed tunnel surface to a designated
depth for blasting.
Explosives and timed 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.
Most tunneling construction in rock involves in ground that is
somewhere between two extreme conditions of hard rock and soft
ground.
Hence adequate structural support measures are required when
adopting this method for tunneling.
A temporary magazine site is often needed for storage of explosives.

15.  Compared with bored tunneling by TBM, blasting generally
results in higher cost but lesser duration of vibration levels.
Advantages:
 The drill & blast method has several advantages mentioned below:
 Almost any type and cross sectional shapes can be made.
 It can be applied to nearly any type of rock.
 It gives great flexibility in the performance of the excavation.
 The rock support can be installed easily and quickly.
Some of the disadvantages are:
o Production of toxic gases and smoke from the explosives.
o Vibrations on nearby structures from the blasting;
o Rough surface gives head loss for water tunnels;
o The blasting creates new cracks in the rocks, which leads to
increased need of rock support;
o Potential hazard associated with establishment of a temporary
magazine site for storage of explosives shall be addressed
through avoiding populated areas in the site selection process.
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16. 16
Reading Assignments
1. Tunneling in hard Rocks
2. Tunneling in Soft Ground or Soft Rock

17. 17
Site Investigations for Tunnels
Geological, geotechnical, and hydrological factors are
more than any other factors determine the degree of
difficulty and cost of constructing an underground facility.
 Geologic profile (stratigraphy, structure, and
identification of principal rock types and their
general characteristics).
 Rock mass characteristics and geomechanical
properties.
 Hydrogeology (groundwater reservoirs, aquifers,
and pressures).
 Exposure to construction risk (major water-bearing
faults, methane gas, etc.).

18. 18
Explorations for reconnaissance and feasibility studies
Methods of data acquisition include the following:
 Available data acquisition and study.
 Remote sensing.
 Preliminary geologic field mapping.
 Geophysical explorations if appropriate.
 Selected exploratory borings in critical locations.

19. 19
I. Sources of available information:
 Topographic maps.
 Geologic maps.
 In urban areas and where site improvements have
been made (e.g., highways), private and public owners
will frequently have information about past geotechnical
and geologic investigations.
 Local geotechnical firms regularly maintain files
of such information.
 Case histories of underground works in the region,
or in similar types of rock, are sometimes available
and are very useful additions to the geotechnical
database.
II. Remote sensing techniques:

20. 20
III. Field mapping:
(a) Initial onsite studies should start with a careful
reconnaissance over the tunnel alignment, paying
particular attention to the potential portal and
shaft locations.
 Features identified on maps and air
photos should be verified.
 Rock outcrops, often exposed in road
cuts, provide a source for information
about rock mass fracturing and bedding
and the location of rock type
boundaries, faults, dikes, and other
geologic features.

21. 21
the field survey should pay attention to
 Slides, new or old, particularly in portal areas.
 Major faults.
 Sinkholes and karstic terrain.
 Hot springs.
 Volcanic activity.
 Anhydrite, gypsum, or swelling shales.
 Caves.
 Stress relief cracks.
 Zones of deep weathering or talus.

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(b) Once alignment and portal site alternatives have been
set, a detailed geologic mapping effort should be carried
out.
 Joints,
 Faults, and
 Bedding planes should be mapped and their
orientations plotted by stereographic projection so
that statistical analysis can be performed (often,
today, with computer assistance).

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IV. Hydrogeology
• Groundwater has the potential to cause great
difficulties for underground work, and a special
effort should be made to define the groundwater
regime aquifers, sources of water, water quality and
temperature, depth to groundwater.
• A hydrological survey is necessary to ascertain
whether tunnel construction will have a deleterious
effect on the groundwater regime and the flora and
fauna that depend on it.

24. 24
• As a part of the hydrogeological survey, all
existing water wells in the area should be
located, their history and condition assessed, and
groundwater levels taken.
• Additional hydrogeological work to be carried
out at a later stage includes measurements of
groundwater levels or pressures in boreholes,
permeability testing using packers in boreholes,
and sometimes pumping tests.

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V. Geophysical explorations from the surface
 The most common geophysical explorations carried
out for underground works are seismic refraction or
reflection and electric resistivity surveys.
 Seismic explorations can measure the seismic velocity
of under ground materials and discover areas of
velocity contrasts, such as between different kinds of
rock or at fault zones.
 They are also useful in determining the depth of the
groundwater table.

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VI. Additional explorations during feasibility
studies
• It is often appropriate to conduct initial field
explorations in the form of borings or trenching at
this early stage, primarily to verify the presence
or location of critical geologic features that could
affect the feasibility of the project or have a great
effect on the selection of tunnel portals.

27. Geological Considerations for Successful
Tunneling
As already stated, the safety, success and economy of
tunneling depend on the various geological conditions
prevailing at the site.
As usual, the important geological factors which interfere
with this civil engineering project (i.e. tunneling) are also
lithological, structural and ground water conditions.
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28. Importance of Rock Types
The nature of rock types which are encountered along
the tunnel alignment is very important for the safety
and stability of the tunnel.
the competent rocks (i.e. those which are strong, hard
and massive) will lead to safe but slow tunneling and
incompetent rocks (which are loose or soft or
fractured), through amenable for easy tunneling, will
be unstable and require lining.
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29. Suitability of Igneous Rocks at the Tunnel Site
Massive igneous rocks, i.e. the
plutonic and hypabyssal varieties, are
very competent but difficult to work.
No need lining or any special
maintenance.
Because they are very strong, tough,
hard, rigid, durable, impervious and
tunneling, do not succumb to
collapse, floor bumps, side bulges or
to any other deformation.
The volcanic rocks, too in spite of
their vesicular or amygdaloidal
character are competent and suitable
for tunneling.
Further, by virtue of frequently
present vesicular or amygdaloidal
structure, they are more easily
workable than intrusive rocks.
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30. Sedimentary Rocks at the Tunnel Site
In general, sedimentary rocks are less
competent than igneous rocks.
Thick bedded, well-cemented and siliceous
or ferruginous sandstones are more
competent and better suited for tunneling.
They will be strong, easily workable and,
moreover, do not require any lining.
Thus, they possess all the desirable
qualities for tunneling, provided they are
not affected adversely by any geological
structures and ground water conditions.
Shales, by virtue of their inherent weakness
and lamination, may get badly shattered
during blasting. But being soft, they can be
easily excavated and hence tunneling
progresses faster through shale formations.
Proper lining is necessary for tunnels built
in Shales.
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31. Among limestones, dolomitic limestone are harder and
more durable.
They are better than other varieties.
On the other hand, calcareous limestones or porous
limestones are naturally weaker and softer.
In a majority of the cases, sedimentary rocks being
relatively softer, facilitate fast progress of work, but by
virtue of their weakness requires suitable lining
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32. Metamorphic Rocks at the Tunnel Site
Gneisses are nearly similar to granites
in terms of their competence, durability
and workability.
They are capable of withstanding the
tunneling process without requiring any
lining.
The gneissose structure may be
advantageous in the excavation process.
Phyllites, Schists, etc, which are highly
foliated and generally soft, are easily
workable but necessarily require good
lining.
Quartzite are very hard and hence very
difficult to work they are more brittle
too.
They are competent and need no lining. 32

33. Water bearing rocks
• Excavation a tunnel through the water bearing rock
is difficult since ground water rushes into the tunnel
and causes flooding during excavation.
Swelling rocks
• Shale, unconsolidated tuff and anhydrite are
examples of swelling rocks.
• They absorb moisture and swell when they are
exposed to water saturation
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34. Geological structure condition in Tunneling
(I) Inclined strata, vertical and horizontal strata (IV) Jointed rocks
(II) Folded rocks
(III) Fault Zones
Inclined strata
1. Tunnel along the strike line:
• When a tunnel is driven
parallel to the strike
direction, there is
tendency in the rocks to
slide into the tunnel.

35. 2.Tunnel across the strike of the rocks:
When a tunnel is made across the strike of the rocks, it will pass
through different beds of rocks. In such cases, there will be arching
action or down ward pressure from the roof. There is also the failure
of incompetent layers from the roof.
Fig. Tunnel across the strike line of the rocks.

36. Horizontal Strata
For small tunnels or for short
lengths of long tunnels, horizontally
layered rocks might be considered
quite favorable but
When horizontally bedded rock lies
above the roof, the thin strata near
the opening will tend to detach from
the main rock mass and form
separated beams.
The stability of such beams is great
if:
 There is horizontal stress;
 The span-to-thickness ratio is
fairly small.
• Thin beds just above the opening
will tend to fall down unless there is
immediate support in the form of
rock bolts or sets.
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37. Tunnel excavations in the slopes:
the discontinuities
(layers, fissures)
inclined inside or
outside of the slope are
very important
regarding the stress
and strength of the
tunnel.
horizontal, vertical and
inclined layers have
different kinds of
loading conditions for
tunnels 37

38. Folded rocks
1.Tunnels along troughs: This encounters unfavorable conditions,
because rock masses along trough are harder and more resistant.
There also seepage problem of groundwater
2.Tunnels along crests: The rock masses along the crest may be in
a highly fractured condition due to development of tension joints.
As a consequence of this, if tunnels are driven in such places,
there may be frequent fall of rocks from the roof.
Fig. A= Tunnel along crest and, B= Tunnel along trough

39. 3.Tunnel aligned parallel to fold axis through limbs: This is
desirable if competent rock is selected because similar rocks
with similar properties are encountered along the course of the
tunnel. But if there is a problem in one place, it can face all part
Fig. Tunnel aligned parallel to fold axis through limbs

40. 4.Tunnel aligned perpendicular to fold axis through limbs:
This is undesirable because, under such a condition, different rock
formations are encountered from place to place along the length of
the tunnel. This results in difficulty of excavation, instability of the
tunnel and need of support.
Fig. Tunnel aligned perpendicular to fold axis through limbs.

41. Fault zones:
Faults are commonly found associated with a zone of highly
crushed rock or fault gouge. The crushed rocks, being highly
permeable, allow the ground water to seep into the tunnel.
Jointed rocks
In one way, the jointed rocks facilitate, easy tunneling.
But in the other way they present many difficulties, because the
roof of a tunnel can not withstand with out support & there is a
water seepage
.

42. As a general the geological condition to be suitable
for tunneling should be;
– There should be one type of rocks
– There should be no faults and intrusion
disturbance.
– The rocks should be soft but stiff enough not to
need immediate support near the face
– The rock should be impermeable and not adversely
affected up on air exposure.
– The rocks or the soil should not be changed its
behavior under the exposure to water (non-
expandable)
– Not be highly weathered and resulted in collapse.

43. 43
The bedding planes in stratified
sedimentary rocks (Golden,
Colorado).

44. 44
Failure
mechanisms
In Tunnels

45. Consideration during tunnel excavation
i. Natural state of stress:
 Due to the weight of the overlying rock and overburden, natural
stresses increase with depth below the ground surface.
 Tunneling experiences shows that, for surface tunnels horizontal
stress (h) becomes negligibly small and at depths (more than
3Km) h approaches the vertical stress (v) which is called litho-
static pressure. Their ratio should be considered.
ii. Stress around tunnel openings:
No Shape of opening Zone of influence
along vertical axis
1 Square 4.5*a
2 Circular 4*a
3 Elliptical (W0/H0)=
0.5(a=radius; W0/=
Width of opening;
H0=Height of
opening)
4.7*a
 Shape of opening in relation to
stress concentration: in elliptical
and square tunnel forms the stress
concentration factors can rise to
higher values than for circular
forms.
 Effect of the shape of an
underground opening on its zone
of influence is given ff table.

46. iii) stand-up time
Stand-up time is the length of time a tunnel will support itself without
any added structures.
Knowing this time allows the engineers to determine how much can be
excavated before support is needed.
The longer the stand-up time is the faster the excavating will go.
Generally certain configurations of rock and clay will have the greatest
stand-up time whereas sand/ fine soils will have a much lower stand-
up time.
Tunnel shape is very important in determining stand-up time.
The force from gravity is straight down on a tunnel.
the circular have longer stand-up time than the square or rectangle.
And also, if the tunnel is wider than its high it will have a harder time
supporting itself, decreasing its stand-up time. And if a tunnel is higher
than its width the stand up time will increase making the project easier.

47. iv) Tunnel support:
Support and ground reinforcement may be applied before, during and
after excavation.
This depends on the stand up time of tunnel.
Its purpose is to strengthen and support the ground surrounding the
tunnel and to prevent falling of ground or flow of water into the tunnel.
Some of the tunnel support methods include
a)Ground improvement ahead of the tunnel face: excavation conditions
may be improved by:
Injection of cement into the ground (grouting)
Freezing of the water saturated zone.
Drainage of water out of the area to be tunneled.
b) Support during excavation:
Shield tunneling in very soft ground.
Bentonite tunneling with boring machine.
Caisson tunneling to counteract water pressure.

48. C) Support after excavation:
 Bolts, Anchor, Steel ribs, Shot crete, Wire mesh or steel
mats, Perforated concrete + backfill mortar, Formed
concrete.
 Underground openings have a relatively limited zone of
influence on the stress distribution in the surrounding
rock.
 The stress concentration values along a tunnel wall
depend on the shape and on the ratio of vertical
stress/horizontal stress not the size of the tunnel.

49. v) Moisture in tunnel
The effect of water on tunnels reveals itself in three respects:
Static and dynamic pressure head (loading action).
Physical: dissolving and chemical (modifying action).
Decomposing and harmful against certain linings (attacking action).
Generally seeping and moving water exerts more harmful action, than
standing or banked up backwater.
Which quantities and what kind of water will enter the tunnel during
construction depends primarily on the character and distribution of
water-conveying passages (aquifer).
The length and depth below the terrain surface of the cavities,
precipitation and local geological conditions are also important.

50. To reduce the effects of moisture;
– A governing principle of tunnel alignment:
waterlogged areas and spots should be possibly
avoided by any underground cavity/drainages
– If possible, the tunnel should not be located under
the groundwater table.
– However, where construction in such a layer is
unavoidable special tunnelling methods and
techniques must be resorted to (shield driving,
dewatering by compressed air).
– If the tunnel can be located above the groundwater
table then only drainage of periodically percolating
meteoric water should be provided.

51. vi. Gasses in tunneling
Carbon monoxide (CO)
Carbon dioxide (CO2)
Methane (CH4) Highly explosive with air (marsh gas
Hydrogene sulphide (H2S) Highly toxic and explosive
Sulphur dioxide (SO2)
Owing to the enclosed space of a tunnel, fires can have very
serious effects on users.
The main dangers are gas and smoke production, with even low
concentrations of carbon monoxide being highly toxic.

52. 52
Support for Underground Openings
 Support and ground reinforcement may be applied before,
during and after excavation.
 Its purpose is to strengthen and support the ground
surrounding the tunnel and to prevent falling of ground
or flow of water into the tunnel.
 Some of the tunnel support methods include the following:
(a) Ground improvement ahead of the tunnel face:
excavation conditions may be improved by:
 Injection of cement milk into the ground
(grouting).
 Freezing of the water saturated zone.
 Drainage of water out of the area to be tunnelled.

53. 53
(b) Support during excavation:
 Shield tunnelling in very soft ground.
 Bentonite tunnelling with boring machine.
 Caisson tunnelling to counteract water pressure.
 With shotcrete (sprayed grout) on the tunnel face
and freshly excavated tunnel sides.
(c) Support after excavation:
 Bolts.
 Anchore.
 Steel ribs
 Shotcrete.
 Wire mesh or steel mats.
 Perforated concrete+backfill mortar.
 Formed concrete.

54. What is tunnel ?