WU_Engineering geology_lecture note_Elias.pdf

Wollo University
College of Natural Sciences
Department of Geology
Engineering Geology (Geol 4112)
By: Elias Assefa (MSc)
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Wollo University, Ethiopia Elias A.

Course Title: Engineering Geology
Course Code: Geol. 4112
Credit hours: 4 credit or 6 ECTS
Course Category: Core course
Instructors: Elias Assefa (MSc)
eliasassefa29@gmail.com
Cellphone no. +251910110141
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Course Aim/Rationale
The course is aimed to increase your
knowledge of application of geology in civil
engineering practice.
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Course Learning Outcomes
Upon successful completion of this course, you will be able to:
 Acquire a basic understanding of the principles of site investigation and able to
conduct geotechnical site investigation
 Acquire a basic understanding to prepare an engineering geological map.
 Acquire a basic understanding of different engineering structures (dam, tunnel,
road, bridge, building and others) and evaluate theirs suitable site and foundation
condition.
 Acquire a basic understanding to identify the suitable potential source of
geological construction material and characterize the geological construction
material for different engineering structures.
 Acquire a basic understanding to access and evaluate different geological hazards
on different engineering structures and recommend their mitigation measures.
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COURSE OUTLINE
Engineering Site Investigation and Exploration
Hazardous of Earth Processes and Engineering Works
 Subsurface Water and Engineering Works
 Introduction to Dams and dam sites
 Introduction to engineering geology of tunnel working
Engineering Geology of River Engineering and Hydraulic Structures
 Engineering Geology and Shallow Foundation Structures
Geological Construction Material
Engineering geological mapping
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Recommended References
 Ayenew T. (2004), Fundamental of Engineering Geology, teaching book,
department of geology and geophysics, Addis Ababa University.
 Bell F.G. (2007), Engineering Geology, pub Elsevier.
 Blyth F.G.H & Freitas D.H. (2007), A Geology for engineers, pub Elsevier,
Delhi, India.
 Deerman W. R (1991), Engineering geological mapping. Oxford, Pp. 1-23.
 Franklin J.A. and Dusseault M. (1991), Rock Engineering Applications),
McGraw Hill, New York, 431p.
 Garg S.K. (2008), physical and engineering geology, Khanna pub, Delhi, India,
pp. 30- 257.
 Goodman, R.E. (1989) Introduction to Rock Mechanics, 2nd Edition, John
Wiley & sons.
 Hoek E. & Bray J.W. (1991) Rock slope engineering.
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Chapter one:
Introduction to engineering geology
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General overview and definition of Engineering Geology
What is engineering geology?
Definition:-
a) The science which deals with the physical structure and substance of
the earth, their history, and the processes which act on them.
b) The geological features of a district.
c) The geological features of a planetary body.
• Engineering Geology provide geological and geotechnical
recommendations, analysis and design related to human development
and different types of structures. The engineering geologist’s realm is
essentially about earth-structure interactions or investigating how
earth or earth processes impact human-made structures and human
activities.
• Engineering geologists are involved in processes that modify surface
and sub-surface geology for the built environment.
• They may also be involved in the related disciplines of engineering
geophysics, hydrogeology and mineral exploration.

Functions of Engineering
Geology
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• Description of the geologic environments pertinent to
the engineering practice.
• Description of earth materials, their distribution and
general physical/chemical characteristics.
• Deduction (investigate) of the history of pertinent
events affecting the earth materials.
• Forecasting of future events and conditions that may
develop.
• Recommendations of ways to handle and treat various
earth processes.

cont’
d
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In engineering geology; there are three premises:
1. All engineering works are built in or on the ground.
2. The ground will always react to the construction of the
engineering works.
3. The reaction of the ground (engineering behaviour) to
the engineering work must be accommodated (within
allowable limit).

cont’
d
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Toarrive at the engineering behaviour of the ground, there are
common relations or equations between rock, rock mass and
engineering/structure:
1. Material properties + mass fabric = Mass property
2. Mass property + environment = Engineering geological
condition
3. Engineering geological condition+ changes produced by
engineering work = Engineering Behaviour of the Ground.
Let’s see all the variables in the equations above and their
significance:
• Material: rock, soil, and fluids and/or gas
• Material property: density, shear strength, deformability, etc.

cont’
d
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• Mass fabric: beds, dykes, veins, joints, faults, etc.
• Mass: ground mass, volume of ground which will be influenced
by or will influence the engineering work.
• Environment: includes
– climate,
– stress condition,
– Natural and man-made hazards and earthquakes, etc.
– Time: immediate after construction, after construction and
through its life time.
In the three equations all the factors leading up to the description
of the engineering geological situation/ condition may be
established by the process of site investigation (chapter two).

CHAPTER I
1
Engineering geological site investigations
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Engineering Site Investigation and Exploration
 Site investigation (stages, tools, methods)
 Disturbed and undisturbed samples (samples and
samplers)
 Boring and sampling
 Cores sizes and core recovery
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Site investigation and soil exploration:
 Objectives - planning - reconnaissance - depth and lateral extent of
explorations –
 methods of subsurface exploration - test pits - Auger borings - rotary drilling
 Types of soil samples-split spoon samplers- Standard penetration test-
hand cut samples- boring log - soil profile- geophysical methods.
OBJECTIVES OF SOIL INVESTIGATION
● Determination of
– The nature of the deposits of soil
– The depth and thickness of the various soil strata and their extent in the horizontal
direction
– The location of ground water and fluctuations in GWT
– Engineering properties of the soil and rock strata by conducting laboratory tests
– In-situ properties of soil by performing field tests
● Obtaining soil and rock samples from the various strata

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Geotechnical investigations are to be carried out by
– engineering geologists,
– geological engineers,
– geotechnical engineers,
– geologists
– civil engineers
 those with education and experience in geotechnical investigations….
Factors influencing the selection of methods of investigation include:
a) Nature of subsurface materials and groundwater conditions.
b) Scope of the investigation, e.g., feasibility study, formulation of plans and
specifications.
c) Size of structure to be built or investigated.

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d. Purpose of the investigation, e.g., evaluate stability of existing
structure, design a new structure.
e. Complexity of site and structure.
f. Topographic constraints.
g. Difficulty of application.
h. Degree to which method disturbs the samples or surrounding
grounds.
i. Constraints….(Budget, Time, Political constraints)
k. Environment requirements/consequences.

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RECONNAISSANCE
● Inspection of the site and study of the topographical features
● Study of maps and other relevant records.
● Collect details about proposed constructions
● Collect already existing data and then examine for soils and geological
conditions
● Collect details required for economic designs
● Helps in deciding future programme of site investigations, scope of work,
methods of exploration to be adopted, types of samples to be taken and the
laboratory testing and in-situ testing.

PRELIMINARY INVESTIGATIONS
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● To determine the depth, thickness, extent and composition of each
stratum at the site.
● The depth of bed rock and the ground water table is also
determined.
● Generally in the form of test pits and few borings
● Tests are conducted with cone penetrometers and sounding rods to
obtain information about the strength and compressibility of soils.
● Geophysical methods are used for locating the boundaries of different
strata.

There are two ground characterization model
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2
Stages in Site Investigation
1. Planning
2. Implementation
3. Interpretation [analysis]
4. Reporting
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Stage 1: desk study

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Stage 1: walkover survey

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Stage 2: shallow geophysical survey

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Stage 3: main ground investigation

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Stages in Site Investigation
1. Desk Study and Walk over Survey
2. Sub‐surface investigation
– In‐situ testing and sampling
– Laboratory testing
3. Report writing
4. Monitoring
Sub‐surface investigation
• to learn the specific geology underneath site.
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Borehole
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Post hole auger Helical auger
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Percussion Boring
 Dry boring or water circulated to remove loose soil
 Heavy drilling bit or chisel is dropped while inside the casing to
chop the hard soil.
 Percussion drilling rods may be replaced by cables.
Number of Boring
 There are no hard and fast rules for the number and spacing of the
boreholes.
 The tables give some general guidelines for borehole spacing.
 can be increased or decreased, depending on the subsoil condition.
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Spacing Boring
Approximate Spacing of Boreholes
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Depth of Boring
 When deep excavations are anticipated, the depth of boring should be at, least
1.5 times the width of excavation.
 depth of core boring into the bedrock is about 3m.
 If the bedrock is irregular or weathered, the core borings may have to be
extended to greater depths.
Depth of Boring
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Sampling and Sampler in Boreholes
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Soil Sampling
Disturbed vs. Undisturbed
Two types of soil samples can be obtained during sampling:
disturbed and undisturbed.
 The most important engineering properties required for foundation design are
strength, compressibility, and permeability.
These tests require undisturbed samples.
 Disturbed samples can be used for determining other properties such as
Moisture content, Classification & Grain size analysis, Specific Gravity, and
Plasticity Limits.
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Sample storage, handling and transportation
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Common Sampling Methods
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Laboratory testing
 Advantage: tests can be precise controlled and measurement of tests is
possible.
 Disadvantage: is to bring samples to laboratory without changing (or
disturbing)
Disturbed Samples: Natural soil structure is modified or destroyed during
sampling
 Representative Samples:
Natural water content and mineral constituents of particular soil layer are
preserved
Good for soil identification and water content
Non-representative Samples:
Water content altered and soil layers mixed up
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Undisturbed Samples:
• Soil structure and the other mineral properties are preserved to an
extent.
• Some disturbance is always there, e.g. due to stress release.
• However it should be minimized in order to have suitable sample for
our analysis.
 It needed to determine
• shear strength parameters in-situ density and water content
• coefficient of permeability consolidation parameters
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In‐situ testing
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IN‐SITU TESTING
 Advantages: soil/rock sample is not disturbed by bringing it to the laboratory and
is being tested in its natural state in the ground.
 Disadvantage: the test cannot be precisely controlled and measured like in the
laboratory.
 Because of these advantages and disadvantages, for most site
In‐situ testing can be grouped into
 Penetration Testing
– SPT, CPT….
 Strength and Compressibility Testing
– Field vane test , Pressuremeter testing , Plate loading tests,
Dilatometer (DMT)…..
 Permeability Testing
– Packer or ‘Lugeon’ test
ASSIGNMENT 1
10%
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PENETRATION TESTING
 Many forms of in situ penetration test are in use worldwide.
 Penetrometers can be divided into two broad groups.
– Dynamic penetrometers (simplest)
– Static penetrometers (more complicated)
The two most common penetration tests, which are used virtually
worldwide, are
– the dynamic SPT, and the static CPT
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1
Chapter II
Hazardous Earth processes and Engineering works
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HAZARDOUS EARTH PROCESSES AND ENGINEERING WORKS
1. Land‐movements and Flooding
[EXTERNAL FACTORS]
2. Earthquake and Volcanisms
[INTERNAL FACTORS]

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What is geohazard ?
is a geologic event that has the potential to causing
great loss of life and property damage.
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Introduction
What is natural hazard?
• Natural events causing both destroy property and cause a
loss of life or property damage

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Definition and concepts
Forecasting, or predicting, the interaction of engineering
works with earth processes is necessary for safety and
reliability.
i. Natural hazard: means the probability of occurrence within
a specified period of time and within a given area of a
potentially damaging phenomena.

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ii. Vulnerability: means the degree of loss to a given element of set
of elements at risk-resulting from the occurrence of a natural
phenomena of a given magnitude.
 It is expressed on a scale from 0 (no damage) to 1 (total
damage).
iii.Elements at risk: means the population, properties, economic
activities, including public services etc. at risk in a given area.
iv. Specific risk: means the expected degree of loss due to a particular
natural phenomena.

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Major Types of Geo-hazards
(a) Slope failures are landslides, which can occur in almost any
hilly or mountainous terrain, or offshore
 The potential for failure is identifiable, and therefore forewarning
is possible, but the actual time of occurrence is not
predictable.
 Most slopes can be stabilized, but under some
conditions failure cannot be prevented by reasonable
means.

(b) Ground subsidence, collapse, and expansion usually are the result of
human activities and range from minor to major hazards, although loss of life
is seldom great as a consequence.
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Their potential for occurrence evaluated on the basis of geologic
conditions, is for the most part readily recognizable and they are
therefore preventable or their consequences are avoidable.
(c) Earthquakes-represent the greatest hazard in terms of potential
destruction and loss of life. They are the most difficult hazard to assess in
terms of their probability of occurrence and magnitude as well as their
vibrational characteristics, which must be known for a seismic
design of structures.
Recognition of the potential on the basis of geologic conditions and
historical events provides the information for a seismic design.

(d) Volcanic activities is the upcoming of materials from the
interior part of the earth.
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 it can be liquid (lava), solid (pyroclastic materials), and
volatile gases
(e) Floods- have a high frequency of occurrence, and under certain
conditions can be anticipated.
 Protection is best provided by avoiding potential flood areas, which
is not always practical. Prevention is possible under most conditions,
but often at substantial costs.
(f) Health hazards-related to geologic conditions include asbestos,
silica and radon, and the various minerals found in groundwater such as
arsenic and mercury.
Recently, mold has been added to the list of health
hazards. Wollo University, Ethiopia Elias A.

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2.1. Landslide hazards and its mitigation measure:
Common responsible factors for the occurrence of
landslide, Types of landslides, Engineering problems related
to landslide and Mitigation measure of landslide hazards

2.1. Mass movement/mass wasting
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What is landslide
 A “landslide” can be define as a downward movement of rock or soil,
or both, occurring on the surface of rupture-either rotational slide
(curved), free falling or translational slide (planar) rupture-in which
major parts of the material often moves as a coherent or semi-
coherent mass with little internal deformation under the force of
gravity.
 The term landslide is used in alternative with the mass
movement/mass wasting.
• Mass movement:
– Occur in terrain ranging from vertical cliff to gentle slope
– Velocity range from extremely slow to extremely rapid
– In completely dry to completely wetted states of earth’s material
• Materials include natural rock, soil, artificial fills, or combination
these.

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o The speed of the movement may range from very slow to rapid.
o The speed of the landslide will make an even more or less
avoidable and therefore, more or less risky.
o It is important to distinguish the different types of landslides to
be able to understand how to deal with each of them.

• CONCEPTS OF SLOPE STABILITY
• Factors that Influence Slope Stability
• GRAVITY
 The main force responsible is gravity
• On a flat surface the force of gravity acts downward
 On a slope, the force of gravity can be resolved into
• perpendicular to the slope and tangential to the slope
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Slope Stability
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• Safety Factor: = Resisting/Driving Forces If SF >1, then
safe or stable slope
If SF <1, then unsafe or unstable slope
• Driving and resisting force variables depend on:
– Slip surface – “plane of weakness”
– Type of Earth materials
– Slope angle and topography
– Climate, vegetation, and water
– Shaking

Slopes
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CONCEPTS OF SLOPE STABILITY
Factors that Influence Slope Stability
THE ROLE OF WATER
 Dry unconsolidated grains will form a pile with a slope angle
determined by the angle of repose. The angle of repose is the
steepest angle at which a pile of unconsolidated grains remains
stable, and is controlled by the frictional contact between the grains.
 In general, for dry materials the angle of repose increases with
increasing grain size, but usually lies between about 30 and 37o.

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THE ROLE OF WATER
 Slightly wet unconsolidated materials exhibit a very high angle of repose because
surface tension between the water and the solid grains tends to hold the grains in
place.
 When the material becomes saturated with water, the angle of repose is reduced to
very small values and the material tends to flow like a fluid.

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CONCEPTS OF SLOPE STABILITY
Factors that Influence Slope Stability
THE ROLE OF WATER
 Another aspect of water that affects slope stability is fluid pressure.
 In some cases fluid pressure can build in such a way that water can
support the weight of the overlying rock mass.
 When this occurs, friction is reduced, and thus the shear strength
holding the material on the slope is also reduced, resulting in slope
failure.
 Water fills voids and increase weight which increases driving forces
 Water also exerts pore pressures which decrease effective stress and
therefore strength

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Types of landslide:

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COMMON TYPES OF LANDSLIDES
a) Rotational slides move along a
surface of rupture that is curved and
concave
b) Translational slides occurs when the
failure surface is approximately flat or
slightly undulated
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c) Rock Fall:
Free falling of detached bodies of
bedrock (boulders) from a cliff or
steep slope
d) Rock toppling occurs when
one or more rock units rotate
about their base and Collapse.
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e) Lateral spreading
occurs when the soil mass spreads
laterally and this spreading comes with
tensional cracks in the soil mass.
f) Debris Flow:
Down slope movement of
collapsed, unconsolidated material
typically along a stream channel.
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Landslides may also be classified according to their causes.
Deoja et al.,1991 classify landslides into following categories.
1) Rainfall induced landslides
2) Earthquake induced landslides
3) Cloudburst induced landslides – mostly mud flows, debris flows and
flash floods.
4) Landslide dam break
5) Glacial lake outburst flood
6) Freeze and thaw induced rock falls during sunny days in the snow
bound steep rocky mountains.
Causative classification

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Types of Rock Failure
The possible mode of failures in
rock slopes can be classified
into four types;
1. Circular or Rotational
mode of failure,
2. Plane mode of failure,
3. Wedge mode of
failure,
4. Toppling mode of
failure,
5. Raveling slopes or falls and
6. Rock Falls.
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Landslide Features
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EFFECTS AND LOSSES DUE TO LANDSLIDES
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A) Direct Effects:
Physical Damage-Debris may block roads, supply lines (telecommunication,
electricity, water, etc.) and waterways.
Causalities- deaths and injuries to people and animals.
B) Indirect Effects:
Influence of landslides in dam safety- failure of the slopes bordering the reservoir,
Flooding caused by movements of large masses of soil into the reservoir.
Landslides and flooding- Debris flow can cause flooding by blocking valleys and
stream channels, forcing large amounts of water to backup causing backup/ flash
flood.

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C) Direct losses:
Loss of life, property, infrastructure and lifeline facilities, Resources,
farmland and places of cultural importance.
D) Indirect losses:
Loss in productivity of agricultural or forest lands, Reduced property
values, Loss of revenue, Increased cost, Adverse effect on water quality
and Loss of human productivity
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ENGINEERING CONTROLS
• Designing the cut slope.
• De‐pressuring the slope.
• Improving Drainage of the slope.
• Engineering Retaining structures.
• Surface protection.
• Reinforcement of slope.
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Improving Drainage of Slope
 In most of the cases water saturation induce instability in
slope.
 For this reason only most of the slopes fail during rainy
season.
 During rainy season there is a considerable recharge of
ground water.
 In soil slope water saturation can;
 Considerably Increases the weight of soil – Increase in
driving force
 Development of pore water pressure – Increase in driving
force
 Reduction in shear strength
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Improving Drainage of Slope
 In Rock slope water saturation can;
– Develop water forces along potential failure plane –
uplift water force – reduction in normal stress –
reduction in shear strength
– Lubricates the failure surface – ease for sliding mass.
Thus, controlling or improving surface drainage
improves the stability of slope
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Surface drainage of a slope
Internal drainage gallery in
restored slope
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Engineering Retaining structures
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• Generally, retaining structures are not particularly effective methods.
• Difficult to construct on an already moving slide.
• One use of them, though it is used to ensure complete stability of an
existing (old) landslide, which may in the future be reactivated.
• We estimate the force acting on a retaining wall by using the interslice
forces from stability analysis.
• The wall provides additional resistance which is only mobilized by further
deformation of the slope.
• The force then acts along the line of action (see figure) into the soil or rock
beneath the slope.
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Retaining structures

Permanent retaining structures;
1. Gravity retaining wall
2. Semi gravity retaining wall
3. Cantilever retaining wall
4. Counter fort retaining wall
5. Berm below the toe.
Temporary retaining structure
1. Gabbions
2. Bracings in the soil cut
3. Sheet pile (Bulk head) walls
Engineering Retaining Structures
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Engineering Retaining Structures
In soil slopes considerable stability can be attained by providing
retaining structures.
 Permanent retaining structures
 Temporary retaining structures
Permanent retaining structures;
 Gravity retaining wall- These walls
depends upon their weight for
stability.
 Semi gravity retaining wall-small
amount of reinforcement is provided
near the back face
 Cantilever retaining wall -are made of
reinforced cement concrete. The wall
consists of a thin stem and a base slab
cast monolithically.

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Vegetation Cover – Surface Protection
 Plant roots and vegetation cover may stabilize the underlying slope
 by reducing the pore water pressure through Evapo‐transpiration,
 Intercept direct impact of precipitation and reducing the effective
surface area to reduce percolation
The plant roots tightly strengthen the underlying soils.
 The big tree species if planted in the upper slope, it may increase
the load over the critical slopes.
 type of vegetation species to be planted over slope face, should be
identified and supported with site specific scientific research.
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Summary Landslide Mitigation
Before making a choice to adopt a suitable mitigation
measure for a given landslide prone area following has to
be considered;
1. Possible mode of failure –
2. Slope Material Type
3. Technical feasibility of Remedial measures
4. Financial Consideration
5. Degree of Risk
116

Landslide Hazard in Ethiopia
117
• Over 700 landslide sites recorded in
Ethiopia; mostly affecting rural
communities, infrastructures, farm
lands, dwelling houses
(KifleWoldearegay,2013).
Earth slide along Jimma-Agaro road
• landslide has been a frequent
problem in Ethiopia spatially in the
high land north, south, western
and rift escarpment valley (Ayele et
al.,2014).
Landslide Hazard in Ethiopia

Landslide type, factors ,distribution and effects in Ethiopia
118
Landslide type in highland of
Ethiopia
Types of landslides triggered by rainfalls
in the highlands of Ethiopia include:
debris/earth slides,
debris/earth flows, and
rockslides. But
Along Shire-May Tsebri
road
Tarmaber area,
Feresmay area
Jimma
area
Mush area
rock fall& toppling have little
association with rain fall
Kifle Woldearegay(2013)
Adishu
area
(Debreberhan)

landslide controlling factors in the highlands of Ethiopia
119
Most of the slope failures in the highlands of Ethiopia a
happened because of
produced by Ayalew
(1999), Woldearegay
et al. (2005), and
Woldearegay (2005).
rainfall
geological (lithological and structural)settings,
slope shapes,
slope gradients,
Drainage lines(stream incisions/gullying) and
Slope modification, and
vegetation cover.

2.2. Settlement
120
 Settlement is the downward
movement of a building to a
point below its original position.
 Foundation settlement is usually
the result of the shifting or
compaction of the underlying
soil, often due to construction on
backfill or changes in soil
conditions and moisture content.
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Causes of settlement
121
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 The causes of foundation settlement are rarely due to the
design of the structure itself. More commonly, damage is
caused as changes occur within the foundation soils that
surround and support the structure.
 The most common causes of foundation settlements are:
1. Weak bearing soils
• Some soils are simply not capable of supporting the weight or
bearing pressure exerted by a building's foundation. As a result,
the footings sink into the soft soils.
• Majority of settlement problems caused by weak bearing soils
occur in residential construction, where the footings are designed
based upon general guidelines and not site-specific soil
information.

Cont’d
122
43

Cont’d
123
2. Poor compaction
• When fill soils are not adequately compacted, they can
compress under a foundation load resulting in settlement of
the structure.
• In general, before a foundation can be constructed, properly
placed and compacted fill soils can provide adequate support
for foundations.
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Cont’d
124
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3. Changes in moisturecontent
• Extreme changes in moisture content within foundation soils can
result in damaging settlement.
• Excess moisture can saturate foundation soils, which often
leads to softening or weakening of clays and silts. The reduced
ability of the soil to support the load results in foundation
settlement.
• Increased moisture within foundation soils is often a consequence
of poor surface drainage around the structure, leaks in water lines
or plumbing, or raised groundwater table.
• Soils with high clay contents also have a tendency to shrink with
loss of moisture. As clay soils dry out, they shrink or contract,
resulting in a general decrease in soil volume.

Cont’d
125
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Cont’d
126
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4. Maturing trees and vegetations
• Maturing trees, bushes and other vegetation in close proximity to a
home or building are a common cause of settlement. As trees and
other vegetation mature, their demand for water also grows.
• The root systems continually expand and can draw moisture from
the soil beneath the foundation. Again, clay-rich soils shrink as
they lose moisture, resulting in settlement of overlying structures.
Many home and building owners often state that they did not have
a settlement problem until decades after the structure was built.
• This time frame coincides with the maturation and growth of the
trees and vegetation.

Cont’d
127
• Foundations closer to the surface are more often affected by soil
dehydration due to tree roots than are deep, basement level
foundations.
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Settlement
…
128
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5. Soil consolidation
• Consolidation occurs when the weight of a structure or newly-
placed fill soils compress lower, weak clayey soils. The applied
load forces water out of the clay soils, allowing the individual soil
particles to become more densely spaced.
• Consolidation results in downward movement or settlement of
overlying structures. Settlement caused by consolidation of
foundation soils may take weeks, months, or years to be
considered "complete."
• As this occurs, the foundation will experience downward
movement -- sometimes at an uneven rate. This leads to
cracks and structural damage.

Cont’d
129
50

2.3. Subsidence
130

2.3. Subsidence
131
• Displacement of the ground
surface vertically over broad
region or localized areas.
• Natural or human-induced
• Slow settling or rapid collapse
• Causes:
– Extraction of GW, natural
gases and oils
– Underground mining
– Dissolution of LST
– Earthquake
– Faulting induced
– Sediment loading

Cont’d
132
52

1. Dissolution of limestone
133
53
• Dissolution of limestone by fluid flow in the subsurface
causes the creation of caves or karast.
• This type of subsidence can result in sinkholes which can be
many hundreds of meters deep.
2. Mining
• Sub-surface mining which intentionally cause the extracted
void to collapse will result in surface subsidence.
3. Extraction of natural gas
• If natural gas is extracted from a natural gas field the initial
pressure in the field will drop over the years.
• The gas pressure also supports the soil layers above the field. If
the pressure drops, the soil pressure increases and this leads to
subsidence at the ground level.

4. Earthquake
134
54
• Caused displacement of earth’s crust due to internal and
external causes => subsidence
5. Groundwater related subsidence
• Groundwater table fluctuation leads to subsidence
6. Fault induced
• When differential stresses exist in the Earth, either by geological
faulting in the brittle crust, or by ductile flow in mantle.
• Where faults occur, absolute subsidence may occur in the hanging
wall of normal faults. In reverse, or thrust, faults, relative
subsidence may be measured in the footwall.

7. Sediment loading
135
55
• The mass added due to deposition or excavational fill of soil
increases the compaction degree of underlying soft rocks =>
subsidence.
8. Seasonal effects (expansive clays)
• soils containing significant proportions of clay affected by
changes in soil moisture content.
• Seasonal drying of the soil results in a reduction in soil
volume. If building foundations are above the level to which
the seasonal drying reaches they will move and this can result
in cracking the building.
• Shrinking and swelling of soil and soft rock requires two
conditions to be satisfied before it occurs.
1. The soil or rock must have the potential for volume change.
2. Adequate amount of water

Mitigating the effect of subsidence
136
56
• Avoiding withdrawal overdraft from compressible GW aquifer
• Controlling land-use to avoid subsidence in areas underlain by
soluble rocks.
• Extraction of hydrothermal can be re-injected with water
• Sealing sinkholes, restoring the ground surface and promoting of
GW flow away from sinkholes.
• Avoiding founding structures on expansive soils.

J. David Rogers
2.4. Hazards from expansive soils:
Introduction to clay mineralogy, Origin of
expansive soils,
Tropical soils and engineering,
Climate-soil interaction and ASSIGNMENT 3
impacts on engineering works,
Expansive soils in Ethiopia,
Mitigation measures of expansive soil hazards
137

Expansive Soils
138
 Expansive soils are typically clayey soils that
undergo large volume changes in direct
response to moisture changes in the soil.
 Expansive soils are those
containing sufficient quantities of clay
minerals (Montmorillonite, Kaolite,
Illite, vermiculite and soon) which
tend to swell when they absorb
moisture and shrink when they lose
moisture.
 A pattern of polygonal desiccation, or
“shrinkage cracks”, results, as seen at
left.
• These soils possess a high plasticity
index.
• The cracks travel deep into the
ground.

• Sidewalk heave is a common manifestation of expansive soils
at foundations.
• Excessive watering, leaky irrigation systems, and/ or poor
drainage often highlights this problem.
139

• Poor drainage adjacent to
slabs and flatwork is a
common problem is
expansive soils-related
damage
• Difficult to solve in flat-lying
flood plains
140

• Signs of expansive soils behavior include lifting of lighter
structural elements, as opposed to heavy elements, such as
chimneys. Edge lift at corners and shear cracking near corners
is also common.
141

Fig, When clay materials getting dry, Develop deep Cracks around 0.8m
142

8
The entire ground is collapsed due to expansive soil
143

Cont’d
144
Evaluating expansive soils
• Common methods for identifying expansive soils on the basis of volume-change
characteristics or related physical properties are:
1. Atterberg limits test, 2. Free swell test
3. Colloid-content determination, 4. linear shrinkage test
• The geotechnical engineer needs to identify the soils that are likely to collapse and
determine the amount of collapse that may occur.
• Some soils at their natural water content will support a heavy load but when water is
provided they undergo a considerable increment in volume
• The amount of collapse is a function of the relative proportions of each component
including degree of saturation, initial void ratio, stress history of the materials,
thickness of the collapsible strata and the amount of added load.

Swelling Potential
145
The three ingredients generally necessary for potential swelling to
occur are:
1. Presence of montmorillonite/smectite in the soil,
2. The natural water content must be around the PL, and
3. There must be a source of water for potential swelling clay
Table : Probable expansion as estimated from classification test data

Foundations on collapsing soils
146
 Identification of collapsing soil is highly crucial
• All collapsing soil contain an appreciable percentage of air in the voids.
• Collapsing soils compress significantly even during sampling by tubes.
• Collapsible soils usually slake upon immersion, but disintegration by slaking
is not a definitive indicator because other types of soils also slake.
• Swelling potential is related to plasticity index.
• Final decision should be based on consolidation test, load test conducted

Collapsing soil can cause settlement:
147
– Total settlement = immediate settlement + primary consolidation
settlement + secondary compression settlement
– Differential settlement = non-uniform settlement
 The severity of settlement and impact to structures which can result
from collapse of the sub-soils depends on several conditions.

Settlement on soil stratum
148
• Imagine a soil layer with thickness H is loaded.
• The following settlement occurrences can be observed:
1. Rapid reduction of thickness H, due to elastic deformation (immediate
settlement, Si);
2. Further reduction of H, due to expulsion of water from the voids
(primary consolidation settlement, SC). This is a very slow process
and continues over a long period;
3. Further reduction of H, due to plastic re-adjustment of soil (solid)
grains (secondary compression settlement, SS)
• The figure below illustrates the three phases of settlement over
time.

Figure: three phases of settlement for fine-grained soils as function of time.
149

Settlements of foundations
150
No settlement Total settlement Differential settlement
• Uniform settlement is usually of little consequence in a building,
• Differential settlement can cause severe structural damage.

Cont’d
151
Differential Settlement due to variable soil types

152
Expansive soils in Ethiopia
 In Ethiopia expansive soil are formed over the tertiary to recent basaltic volcanic rocks.
 They are contain montmorillonite as principal clay minerals and with accessory kaolinite and
halloysite.
 They are formed from the weathering of basic volcanic rocks which cover the Ethiopia
plateau.
 They usually have high silica-oxide ratio and also high amount of Fe, Ca, and Mg.
 Most of the expansive soils met in nature have clay size fraction (less than 2 micron size)
varying between 40% and 75%, silt size varied between15% to 30%, sand varied between15%
to 30% and gravel size less than 5%.
 Expansive soil is found anywhere in the world and distribution of expansive soil is generally a
result of geological history, sedimentation and local climatic conditions.

153
In Ethiopia, covering nearly 40% surface area of the country, expansive soils are observed in
area such as central Ethiopia, following the major trunk road like Addis Ababa - Ambo, Addis
Ababa - Weliso, Addis Ababa – Debere Birhan, Addis Ababa - Gohatsion, Addis Ababa -Mojo.
Also the cover the area like Mekelle, Bahirdar, Gambela, Arba Minch and the most Southern,
South-west and south-east part of the capital Addis Ababa area in which the most major recent
construction are being carried out. The distribution are showed in figure below.
Distribution of Expansive Soils in
Ethiopia

154
The properties of significance in performance of clays and shales in construction
engineering are:
I. Static properties of earth materials:
A. Particle-size distribution,
B. Unit weight,
C. Void ratio,
D. Specific gravity of the constituents,
E. Fluids content
II. Dynamic properties of earth materials:
A. Consistency,
B. Permeability,
C. Thixotropy,
D. Shear resistance,
E. Compressive strength,
F. Volume change (1. Consolidation with loading, 2. Swelling with hydration, 3. Drying
shrinkage, 4. Syneresis, 5. Frost heaving),
G. Sensitivity to remolding,
H. Slaking,
I. Electro-osmosis,
J. Thermo-osmosis

Mitigation measures of expansive soil hazards (Treatment of the
effects of expansive soils)
155
• The best mitigation method for the effects of expansive soils to avoid
founding on them. However, their extensive occurrence in some areas
make it infeasible.
• Some of the methods are;
1. Removing the expansive soil and replace it with non-expansive soil.
thickness may be too big to permit complete removal. Removal of
expansive soils and replacement with non expansive sand-gravel soil
used to avoid damage.
2. Foundation treatments: applying confining load is one type of foundation
treatment. This involves placing a blanket or embankment of non
expansive soil over expansive soil. The surcharge resists the uplift
pressure of underlying expansive soil. This is effective for large buildings.
3. Another foundation treatment is placing reinforced concrete piers
below the depth of expansive soil.

Cont’d
156
4. Chemical stabilization of expansive soil: to modify the ionic
character of soil and water preventing swelling.
a. Hydrated lime (Ca(OH)): Strong Ca2+ replaces weak Na1+ on the
surface of clay particles. This reduces base-exchange capacity of clay
=> lower volume-change potential.
b. Portland cement: it has two separate effects. The lime within the
cement acts in the same way as hydrated lime. Besides, the hardened
cement matrix in the soil resists movement.
5. Isolating water from expansive soils. Depend on whether surface water
(ditches and pipes) or ground water used to keep water away from the
sensitive area. Sand and gravel is used to break in capillary continuity
when GW is moving upward. Enveloping masses of expansive in
impermeable membrane is good isolating of water.
6. Deep vertical geomembranes/moisture barriers: effective in highways.
Moisture barriers are constructed in trenches filled with gravel or
impervious membrane.

Chapter
Outline
157
CHAPTER III
Engineering works and subsurface water
• Effects of subsurface water on engineering structures
• Water quality and engineering work
• Controlling techniques of subsurface water effect

At the end of this chapter
158
CHAPTER III
Engineering works and subsurface water
GEOTECHNICAL INVESTIGATION
 Geological Media
– Ground
• Soil
• Rock
– Groundwater
• Students will be able to understand the interaction of subsurface water
with earth material and its effect on engineering structures
• Students will be acquired knowledge to reduce the effects of
subsurface water on engineering works

Introduction to Subsurface water
159
• Ground water: the water that lies beneath the ground surface,
filling the pore space between grains in bodies of sediment
(soils) and clastic sedimentary rock, and filling cracks,
discontinuities and cavities in all types of rock.
• The subsurface water can flow in different direction depending
on its level and subsurface structures.
• This subsurface flow is facilitated where there is hydraulic
head.
• The flow can be towards or away from engineering structures,
hence it affects the performance of the structures.

Effects of subsurface water on engineering Structures
160
• Engineering structures like dam, building, highways,
railways, roads and other underground projects such as
mining, tunnels could be affected by the water (surface or
subsurface) in different ways .
• It may pose problems during
• construction stage,
• its performance stage and
• reduce the safe functioning of an engineering project.
• Engineering project can also affects the subsurface water by
altering its quality and flow direction.

The Main Effects of Subsurface Water on Engineering Structures are:
161
• Eroding the foundation of structures
• Volume change of soil or rocks of the foundations which is resulted in
Settlement or collapse.
• Increasing moisture of slope material that resulted in the sliding of slope by
reducing safety factors.
• Affect excavation and construction activities when it flowing towards the
structures to be constructed.
• Reducing the bearing capacity and shear strength of a material on site.
• Lubricating the contacts between layers or weak zones.
• leakage towards the structures and develop uplift pore pressure which
results in the failure of engineering structures.

162
• Generally, sub surface water would be resulted in flooding, swelling
of expansive materials, reduction in bearing capacity, uplift
pressures, chemical attack and difficulties during construction due to
flooding to the site.
activities of subsurface water on Engineering structures.
• The following information should be collected properly during site
investigation
 Distribution and content of sub-surface water.
 Direction and velocity of subsurface water flow in the site
 Depth to water table and its range of fluctuation under
different condition.
 Regions of confined, perched and unconfined water levels.
 Hydro-chemical properties and pollutants that can decompose
the engineering structures.

Effects of Subsurface Water on Dam Site
163
• Subsurface water is the most and critical problems in the
foundation and abutments of dam project.
• Because in most cases, dam foundation will be situated to
placed at great depth below subsurface water in order to
reduce instability problems.
• In this case, there will be an inflow of water into the
excavation, which may block or retard the construction
activities.
• Rock mass contains discontinuity may serve as reservoir
and conduit for ground water that may pose problems
during excavation.

164
 Subsurface water conditions in dam projects will be
causes
• Seepage into the storage.
• Water over flow.
• Failure of a dam and flooding downstream side.
• Increase pore water pressure within foundation and
abutments, which is responsible in the reduction of
cohesion/resisting force .
• Pose problems in excavation and construction activities.
• Erode foundation and damage the structure of the dam.
• lubricate the discontinuity and facilitate the failure of
dam abutment.

165
• Subsurface water fluctuations may cause uplift problems
in the dam foundation area which in turn responsible for
the settlement.
• Sub surface water can bring different dissolved chemical
to the foundation, which can react with construction
material and damage overall structures
Generally, dam failures can be grouped into four
classifications which may or may not related to subsurface
water effect:
– Overtopping,
– Foundation failure
– Structural failure and
– Other unforeseen failures.

166
• The water near the tunnel can develop pore water pressure around
the tunnel and can results in collapsing of a tunnel.
• The water can saturate the roof of the tunnel passage and results in
ground collapse by reducing the withstand capacity of the soils.

Effects of Subsurface Water onTunnel
167

Effects of Subsurface Water on Building Foundations
168
• Temporary or permanent rising and lowering of the groundwater table
from man-made or natural causes an effect on buildings, streets,
underground utilities and other structures.
• Foundation / base of every engineering structure are on or in the soils
or rocks.
• When the rocks and soils exposed to subsurface water their
engineering properties can be changed by saturation and pore pressure
effects.
• This effect is results in the reduction of bearing capacity, shear
strength, durability, hardness of soils and rocks.
• Generally the effects of ground water on the stability of foundations
are pore water pressure/uplift, saturation of foundation rocks and soils,
dissolving cementing material, developing slippery base and swelling
effects. Wollo University, Ethiopia Elias A.

Effect of Sub Surface Water on Pavements
169
• The stability of pavements depend on the presence of
ground water, and types of construction material.
• When the ground water level reaches the base of the
pavement it will have an effect like saturation, reduce the
adhesion in construction material and reduce the strength
of the materials on foundation.
• The fluctuation of subsurface water makes the swelling
and shrinkage of sub grade of the pavements, which in
turn reduce the bearing capacity of the soils.
• During the fluctuation of sub surface water the soil under
the structure is equally saturated and the soils under the
shoulder dry faster than the other and form a crack
parallel to the road on the side of the roads.

170
• Thus subsurface water bring a distress of pavement.
• Moisture variation and frost action are the main cause of
deterioration of the subgrade.
• When the water content is decreased,
shrinkage cracks develops, which
cause differential settlement in the
rigid pavement and cracks in the
flexible pavement.
• Hence the pavement should be
provided with a suitable drainage
system or the pavement must be
constructed above the maximum level
of the ground water table to keep it
dry.

Water Quality and Engineering Structures
171
• Water chemistry- the chemistry of subsurface water can varies
from place to place and from time to time, because it depends on
the material through which it exists or in what chemistry it exist.
• The chemistry of sub surface water are measured in terms of
acidity and total dissolved solid (TDS).
• Depend on the chemistry, subsurface water is the most dissolving
agents on engineering structure which responsible for the formation
of karst, solution cavities.
• This results in the collapsing of structure on the surface above the
karst or solution cavities.
• Also the water can react with carbonate rocks along its path, this
reaction results in the formation of carbonic acid, which is
chemically acidic and easily react with construction materials such
as concrete. Wollo University, Ethiopia Elias A.

172
• Sulfuric acid also formed when water react with some
evaporate rocks such as gypsum.
• The sulfuric acid will facilitates the weathering process of
the native foundation rock causing decrease in strength.
• When Sulfate present in large amount, is aggressive to
concrete, metallic structures, like rock bolts, steel used as
reinforcement etc.
• This ability water to deteriorate, weathering and eroding
of structure due to its composition is known as
corrosivity.
• In corrosive subsurface water conditions, while doing
excavations, a proper precaution has to be taken to reduce
the effect of corrosion, especially in permanent
excavations. Wollo University, Ethiopia Elias A.

173
 Chemistry of ground water affects stability of engineering
structures because of
• Formation of cavern- when water dissolve the carbonate rocks.
– Most caves are formed by the chemical dissolution process.
• Sinkhole-form as a result of lowering the water table by excessive
pumping for human use of the water. Or by dissolving of underground
support.
• Subsidence- results from withdraws of fluids or collapse of underground
caves

Controlling Subsurface Water Effects
174
Why Drainage & Dewatering?
• Carryout construction activity below water table.
• To increase stability of soil.
• To decrease seepage & pore water pressure.
• Reclamation of water logged areas.
• Release of hydrostatic pressure behind the retaining
structures.

Controlling Subsurface WaterEffects
 Shallow well system

 Deep well system
 Vacuum method
 Electro-osmosis method
175
1. Lowering Water Table
 Ditches & sumps
 Well point system
 Sheet pile
 Ground freezing
Grouting
2. Water Exclusion Method

Ditches & Sumps
176

Deep well Drainage System
177

Vacuum Method
178
• Useful for fine grained soils
• (fine, non cohesive soils, Silty sands etc.) particle size D10 is smaller than
0.05mm & its co-efficient of permeability between 10 -3 and 10-5 cm/s.
• It is necessary to apply a suction head in excess of the capillary head to
the dewatering system.
• A hole of 25 cm dia. is created around the well point and the rise pipe by
jetting water under sufficient pressure.
• Vacuum pumps are used to create a vacuum in the sand filling.
•When the vacuum is drawn on the well point, the ground
surface is subjected to unbalanced atmospheric pressure

3. Drainage by Electro-Osmosis
179
• Used in cohesive soils
• +ve water particles electrostatically bound to –ve soil particles
makes dewatering difficult
• Direct current is passed between two electrode in to saturated soil
mass to break attraction and allow water to flow.
• Soil water travel from positive to negative Cathode made in a form
of well point or a metal tube for pumping out the seeping water.
• Natural flow of water is reversed away from the excavation
• Thereby increasing shear strength of the soil and stability of the
slope
• Very costly
• Used where the main purpose is to increase consolidation and
shear strength of the soil

Water Exclusion
180
1. Sheet Piles/Secant Piles/Diaphragm Wall
• Dual purpose (providing
impermanent support to
excavation and excluding
groundwater)
• The pile block the movement of
water towards the excavation/
construction area and support the
side of excavation.
• The water pressure can
develop and result in failure
of the wall, if it is not well
designed.

2. Grouting
181
• Used where permeability is too high or where access is
difficult (tunnelling)
• Grout is injected of cement into the soil under pressure
via boreholes or drill holes
• May be cementitious, chemical (silica based) or bentonite.
• Can strengthen soil and / or form impermeable barrier.

1
CHAPTER IV
182
 Dams and Reservoirs
 Tunnels
 Roads, Bridges and Railways
Significance of Engineering geology in Engineering
Hydraulic structures

183
Hydraulic Structures are engineering constructions designed and mechanically fit for managing and
utilizing water resources to the best advantage of the human being and environment.
Dam is a barrier across flowing water that obstructs, directs or retards the flow, often creating a Reservoir.
Reservoir is an artificial lake created by flooding land behind a dam. Some of the world's largest lakes
are reservoirs.
Spillway is a section of a dam designed to pass water from the upstream side of a dam to the downstream
side. Many spillways have gates designed to control the flow through the spillway.
Flood is an overflow or an expanse of water submerging land.
 Dams differ from all other major civil engineering structures in a number of important regards:
 Every dam, large or small, is quite unique; foundation geology, material characteristics, catchment
flood /hydrology etc. are each site-specific.
 Dams are required to function at or close to their design loading for extended periods.
 Dams do not have a structural lifespan; they may, however, have a notional life for accounting
purposes, or a functional lifespan dictated by reservoir sedimentation.
 The overwhelming majority of dams are of earth fill, constructed from a range of natural soils; these
are the least consistent of construction materials.
 Dam engineering draws together a range of disciplines, e.g. Structural and fluid mechanics, geology
and geotechnics, flood hydrology and hydraulics, to a quite unique degree.
 The engineering of dams is critically dependent upon the application of informed engineering
judgment.
 Hence the dam engineer is required to synthesize design solutions which, without compromise on
safety, represent the optimal balance between technical, economic and environmental considerations.
Hydraulic Structures

2
Dams
184
 Dams are civil engineering structures build across the river valley to impound large
volume of water to be used for single or multipurpose use of; power generation,
irrigation purpose, flood control, ground water recharge and water diversion.
 The most common reasons for building dams are
 to concentrate the natural fall of a river at a given site, thus making it possible to
generate electricity;
 to direct water from rivers into canals and irrigation and water-supply systems;
 to increase river depths for navigational purposes;
 to control water flow during times of flood and drought; and
 to create artificial lakes for recreational use.

Wollo University, Ethiopia Elias A. 185
Important Terminology of The Dam (parts of the dams)
 Heel of the dam: It is the part where the dam comes in
contact with the ground on the upstream side
• Toe of the dam: It is that part where the dam comes in
contact with the ground on the downstream side
• Free board: It is the difference in level between the top of
the dam wall and the highest storage level.
• Galleries: These are small rooms left within the dam for
checking operations.
• Spillway: An arrangement is made in a dam near the top or
inside to allow excess water of the reservoir flow to the
downstream side
• Sluice (outlet conduit): It is an opening in the dam near the
ground level. It is useful in clearing the silt of the reservoir.
• Cut-off wall: It is an underground wall-like structure of
concrete in the heel portion. It is useful in preventing leakage
under the foundation.
• Abutment: These are the sides of the valley on which the
dam structure rests.
• Parapet walls: Low Protective walls on either side of the
roadway or walkway on the crest.
• Dead Storage level: The portion of total storage capacity
that is equal to the volume of water below the level of the
lowest outlet (the minimum supply level).
• Diversion Tunnel: Tunnel constructed to divert or change
the direction of water to bypass the dam construction site.
• Tail water: water at the downstream base of the dam
resulting from the backup of water discharged from the
spillway or powerhouse.

186
 Classification according to material of construction
 Timber dams
 Steel dams
 Concrete dams
 Earth dams
 Rock fill dams
 Combined dams
 Classification according to design criteria
Hydraulic design Stability consideration
Non-overflow dams Gravity dams
Over-flow dams Non-gravity dams
Composite dams
 Classification according to Purpose
Storage dams Stage control dams Barrier dams
Flood control Diversion Levees and dykes
Water supply Navigation Coffer dams
Detention storage
When the size of the dam has been determined, the type of dam envisaged requires certain
geological and topographical conditions which, for the main types of dams, may be stated as
follows.
Concrete Dams Embankment Dams
 Gravity dams Rock fill dams
 Buttress dams Hydraulic fill dams
 Multiple arch dams Earthen embankments
 Thick arch dams Composite dams
 Thin arch dams Wollo University, Ethiopia Elias A.
DAM TYPES

187
A. Gravity
B. Arch
C. Buttress
D. Embankment
5
Concrete, rubble masonry
Concrete
Concrete, also timber & steel
Earth or rock
Dams are classified on the basis of structural form and materials used.
Types Materials of Construction
 The first three types usually are built of concrete.
Classification according to height (H)
 H ≤ 30m low dam
 30 ≤ H ≤ 100m medium
 H ≥ 100m high dam

188
Factors governing selection of types of dam
1. Topography-Valley Shape
A Narrow V-Shaped Valley: Arch Dam
A Narrow or Moderately with U-Shaped Valley: Gravity/Buttress Dam
A Wide Valley: Embankment Dam
2. Geology and Foundation Condition
Solid Rock Foundation: All types
Gravel and Coarse Sand Foundation: Embankment/Concrete Gravity Dam
Silt and Fine Sand Foundation: (earth) Embankment/ low concrete Gravity Dam
but not rockfill
Clay foundation: earth dams
3. Cost
 availability of construction materials near the site; accessibility of
transportation facilities

Cont’d
189
1. Gravity Dams:
• These dams are heavy and massive
concrete wall structure in which the
whole weight acts vertically downwards.
• rigid monolithic structure
• Gravity dams are dams which resist the
horizontal thrust of the water entirely by
their own weight.
• Minimal differential movement tolerated
• Dispersed moderate stress on valley floor
and walls
Reservoir
Force
•As the entire load is transmitted on the small area of foundation, such
dams are constructed where rocks are competent and stable.
•The weight of rock and concrete structure to hold back the water in the
reservoir.
6

190

Cont’d
191
3. Uplift
The water under pressure that comes b/n dam and foundation and results
in upward (uplift) forces against the dam.
h1 = depth of water @ upstream face, “heel” (higher)
h2 = depth of water@ downstream face, “toe” (lower)
Υ = specific weight of water
t = base thickness of dam
4. Ice pressure
t
U  
h1 h2
2
Pressure created by thermal expansion exerts thrust against upstream
face of the dam
5. Earthquake forces
Results in inertial forces that include vertical motion, oscillatory
increase, or decrease in hydrostatic pressure (all put force against dam)
Causes of Failure GD:
1. Sliding along horizontal plane (shear failure)
net force > shear resistance at that level
2. Rotation about the toe
3. Failure of material

192

193
ADVANTAGES
 External forces are resisted by weight of dam
 More strong and stable
 Can be used as overflow dams also with spillway feature
 Highest dams can be made as gravity dam’s because of its high stability
 Specially suited for heavy downpour; slopes of earthen dams might get washed away
 Less maintenance required
 Gravity dam does not fail suddenly but earthen dams
DISADVANTAGES
 Can be made only on sound rock foundation
 Initial cost is high
 Takes more time to construct if materials are not available
 Requires skilled labour.

2. Buttress Dam:
194
This type of structure can be considered even if the foundation rocks are little weaker.
A buttress dam has an upstream face or deck to support the impounded
water, and a series of buttresses or triangular vertical walls built to
support the deck and transmit the water load to the foundation. These
dams are sometimes called hollow gravity dams because they require
only 35 to 50 per cent of the concrete used in a solid gravity dam of
comparable size.

195
Types of buttress dam
I. The flat slab type Buttress Dam. In this
type the concrete deck slab spans the
distance between adjacent buttresses.
II. The Multiple Arch Type Buttresses. In this
type each unit of the water supporting
member is an inclined arch barrel supported
by the buttresses.
III. The Massive Head Type. In this type the
water supporting member is merely an
enlargement of the upstream head of the
buttresses.

196
Advantage
 Retain water between buttress
 Less massive than gravity Dams
 When future increase in reservoir; Future extension is possible by
extending buttress and slab
 Power house can be made B/W buttress; thus reducing cost
 Can be designed to accommodate moderate movement of
foundation without any serious damage.
Disadvantages
 Skilled labor requirements
 Deterioration of u/s as very thin concrete face

 High strength concrete or masonry wall
 convex face upstream.
 Huge stresses imposed on valley walls and floor
 Utilize the strength of an arch to displace the load of water behind it onto the
rock walls that it is built into.
 This shape helps to transmit the major part of the water load to the abutments
 Arch dams are built across narrow, deep river gorges,
3. Arch Dams:
197

198

199
Advantage
 Curved in plan
 Carried load horizontally to it’s by arch action
 Balance of water load is transferred to the foundation by cantilever
action
 Adapted in gorges where length is small in proportion to height
dam require less material can be made in moderate foundation
because of load distribution as compared to gravity dams.
Disadvantages
 Require skilled labor
 Speed of construction is slow
 Require strong abutments of solid rock of resisting arch thrust.

200
Embankment dam can be classified into two
1. Earth Dams
are the most simple and economic (oldest dams)
Types:
I. homogeneous embankment type;
These dams are constructed with uniform and homogeneous materials. It is suitable
for low height dams (up to 10m). These dams are usually constructed with soil and
grit mixed in proper ratios.
II. Zoned embankment type
These are dams with the central portions called core or hearting made from
materials which are relatively impervious. The thickness of the core wall is made
sufficiently thick to prevent leakage of water through the body of the dam.
An Embankment Dam is a freshwater-retaining structure comprising excavated rock, soil, or a
combination of rock and soil from nearby geological formations. In fact, Embankment Dams
are known as an “Earth-fill Dam” when filled with soil, and a “Rockfill Dam” when filled with
rocks. Earth-fill Dams are most common.

201
Homogeneous embankment type
Zoned embankment type

General view of an Earth-fill dam

203
2. The rock fill dam
Consist of three basic elements;
I. a loose rock fill dump, which constitutes the bulk of the dam and resist the thrust
of the reservoir.
II. Impervious facing of the upstream slope with concrete, timber, steel
III. Rubble masonry between (i) and (ii) to act as a cushion for the membrane and
resist destructive deflections
Advantage
• Made of locally available gravels
• Can be made on any type of available foundation
• Can be constructed rapidly
• Cheaper
• Future consideration can be made (raising height)
Disadvantages
 Vulnerable to damage by floods
 Cannot be used as overflow dams not suitable where heavy downpour is more
common
 High maintenance cost

204
While selecting a site for a dam the following points should be taken into
consideration
I. The dam should be as near as possible to the area to be served, hence
conveyance cost and water losses will be minimized.
II. Foundation area should be impervious and should be able to support the
weight of the dam.
III. The topography of the dam and reservoir sites should permit maximum
storage of water at minimum cost.
IV. Materials of construction should be available in sufficient quantity and
good quality at a reasonable distance.
V. The value of property and land which will be submerged by the
reservoir has to be as small as possible.
vi. The cost of relocating roads, buildings etc. should be as small as possible
vii. The cost of stream diversion and dewatering the site should be as small
as possible
viii. Transportation facilities and accessibility of the site
ix. Availability of suitable sites for construction equipment and camps
x. The safety of the structure.

Engineering geological consideration in dam construction
205
I. Reconnaissance study
1. Evaluation of the data having at archives
2. Field investigation for limited time (Reconnaissance Study)
3. Some maps in small scale, for example 1/25.000 or 1/50.000
4. Some hydraulic data about
a. Basin
b. Precipitation area
c. Runoff, maximum discharge {Q=R/t (m3/s)}
5. Some approach to the reservoir area, dam site and type of dam
and height of dam...etc
6. Photogeological studies

Cont’d
206
II. Preliminary Studies At The Reservoir Area And Dam Site
1. Dam site investigations
1. Location of dam axis
2. Location of diversion tunnel
3. Location of spillway
4. Location of powerhouse...etc
2. Geological studies
3. Geophysical surveying
4. Underground investigations
1. Boreholes
2. Drilling tests
5. Surveying for materials
1. Field surveying
2. Laboratory tests
6. Slope stability investigations
7. Earthquake hazard & risk
analysis
8. Environmental studies
9. Leakage possibilities from
reservoir area
10. Leakage possibilities from
dam site
11. Erosion, sedimentation &
siltation

207

208

209

210

211

212
Introduction: definitions and concepts of reservoir
Reservoir: a water body or lake which could be created when a barrieris
constructed across a river or a stream.
Advantages/uses of reservoirs:
 Water supply.
 Irrigation.
 Hydroelectric power generation.
 Recreation.
 Flood control.
 Navigation, and others.
Disadvantages of reservoirs:
 Detract from natural settings, ruin nature's work.
 Inundate the spawning grounds of fish, and the potentialfor archaeological
findings.
 Inhibit the seasonal migration of fish, and even endangersome species of fish.
 Foster diseases if not properly maintained.
 Water can evaporate significantly.
 Induce earthquakes.

213
Factors that affect Reservoir
The most important factor are:
a) Location of the dam
b) Run-off characteristics of the catchment area.
c) Water tightness of the proposed reservoir basin.
d) Reservoir rim stability.
e) Rate of sedimentation in the reservoir.
f) Water quality and
g) Seismic activity induced by the reservoir.

214
 Factors which influence the feasibility and economics of a
proposed reservoir site are:
Location of the dam

215
Reservoir
Run-off characteristics of the catchment area

Reservoirs problems:
watertightness- Seepage, Buried channels
216
Water
added
Leakage from
reservoir
Water
subtracted
-
Rainfall in
river basin
Infiltration
Evaporation
Transpiration
Net amount of water
available for storage
Runoff
-
1. Dam bypass
2. Water table effects

50 km
Ancient river/valley
Modern river/valley
Sautet
dam and
reservoir
Bypass of reservoir in drift
Reservoirs: leakage
217

before
after
water table divide
river
Leakage to next valley
reservoir
Bedrock with a water
table and finite
permeability
new
water
table
Reservoir problem: water table
leakage-1
218

before
Bedrock with low
permeability: aquiclude
High
layer
river
Water table in aquifer
reservoir
High
permeability
layer
Modified water table in aquifer
after
Leakage to next valley
Reservoirs: water table leakage-2
219

Before
Water table
river
reservoir
After - 1
Raised water table
reservoir
After - 2
Failure and
slumping
due to
weakened
rock mass
Reservoirs: raised water
table
220

221
Reservoir rim stability
Reservoir

222
Reservoir
Water quality(the effect of water and its contents) on
building materials, especially concrete)

223
Reservoir
Seismic activity induced by the reservoir
Present land use and social factors

Reservoir problem:
Sedimentation
224

Consequences of Reservoir Sedimentation
225
• Loss of Storage (yield; reliability)
• Upstream: loss of navigable depths
• Downstream: degradation of channel; loss of land and habitats
• Hydropower: downstream deposits can increase and decrease
efficiency HP
• Abrasion of turbines

How do we control sedimentation??
226
1. Reduce sediment inflow
erosion control and upstream sediment trapping.
2. Route sediments
Some or all of the inflowing sediment load may be hydraulically routed
beyond the storage pool by techniques such as off-stream reservoirs,
sediment bypass, and venting of turbid density currents.
3. Sediment removal
Deposited sediments may be periodically removed by hydraulic
flushing, hydraulic dredging, or dry excavation.
4. Provide large storage volume
Reservoir benefits may be considered sustainable if a storage volume
is provided that exceeds the volume of the sediment supply.

Spillway Design
227
Data needed to design spillways;
 Inflow Design Flood (IDF) hydrograph
- developed from probable maximum
precipitation or storms of certain
occurrence frequency.
 Reservoir storage
- storage volume vs. elevation
- developed from topographic maps
 Spillway discharge rating
 Perform hydraulic design of spillway
structures
- Control structure
- Discharge channel
- Entrance and outlet channels
 Selecting spillway type
Types of Spillway
 Overflow type – integral part
of the dam
i. Straight drop spillway
ii. Ogee spillway
 Channel type – isolated from
the dam
i. Side channel spillway, for
long crest
ii. Chute spillway – earth or
rock fill dam
iii. Drop inlet or morning glory
spillway
iv. Culvert spillway

Cont’d
228
39
Side Channel Spillway
Drop Spillway
Drop Chute
Ogee Spillway

Main causes of Dam Failure
229
(I) Failure of concrete dams
• Lack of shear strength and
discontinuity in foundation
• Excessive uplift in the foundation
(inadequate or non-existent drainage)
• Lack of dam stability
• Excessive or differential deformation
of the foundation
• Piping and erosion in the foundation
caused by high permeability
• Flaw in design
• Lack of supervision during
construction
• No monitoring or warning system
(systems were out of order)
 Human error during site
investigation, design,
construction and operation of
concrete dams:
 Inadequate foundation investigation
 Incomplete data on available
material
 Poor design
 Negligible construction supervision
 Incomplete first impoundment
 Incorrect operation of flood gates
 Insufficient monitoring and data
analysis
 Lack of preventive measures or
repair work

Main Causes of Dam Failure
230
(II) Failure mechanisms of Embankment Dam
Failure mechanisms are grouped into four general categories:
1. Slope stability,
2. Piping,
3. Overtopping and
4. Foundation failures
1. Slope stability failures
 For the rapid‐drawdown case, failure occurs on the upstream side of
the embankment as a result of a sudden lowering of the reservoir
level.
• For the seismic case, Liquefaction can occur during an earthquake in
loose, saturated, sandy soils.

231

232

233

234

Environmental impacts of
construction phase of dams
235
 River pollution
 Erosion
 Loss of aesthetic view
 Air pollution
 Noise pollution
 Dust
• Loss of land
• Habitat Destruction :
• Loss of archeological and histrorical
places
• Loss of mineral deposits
• Loss of special geological formations
• Aesthetic view reduction
• Sedimentation
• Change in river flow regime and flood
effects
• Reservoir induced seismicity effects
• Change in climate and plant species
ENVIRONMENTAL IMPACTS of RESERVOIRS

236
CHAPTER FIVE
Introduction to Engineering Geology of Tunnel
working

Tunnels
237
TUNNEL DESCRIPTION
1. Made into natural material (rocks)
2. Empty inside
3. Carry the loads itself
4. Both ends are open to atmosphere
5. Generally horizontal
6. Thick walled structure looks like
cylinder
Plates of the Tarmaber tunnel

238
1- Digging section
2- Support
3- Swelling section
4- Pressurized area
5- Flow direction of water
Tunnel Section for Swelling Ground

239
 There are 4 terms commonly used to
describe the location of the parts of
the tunnel cross section.
1) There is the floor, or invert.
2) The top of the cross section is the
roof, also referred to as back or
crown.
3) The sides of the tunnels are
referred to as tunnel walls, and
4) The spring line is the point where
the curved portion of the roof
intersects the top of the wall.

240
EXPLORATION & INVESTIGATIONS RELATED
to SLOPE STABILITY OF TUNNEL
 Geomorphologic mapping and preparation of longitudinal & cross
sections
 Geological mapping & surveying's (aerial photographs)
 Geophysical surveying
 Underground explorations, boreholes
 Ground water surveying
 Laboratory tests
 Model studies

241
SUBSURFACE EXCAVATIONS
1. GEOLOGY
a) Soil profile or hard rock geology
b) Structure
c) Ground water (hydrogeology)
d) Stability
2. INVESTIGATIONS
a) Mapping (Topographic, geologic, etc...)
b) Geophysical surveying (especially seismic velocity of rocks)
c) Test pits& boreholes
d) General and local stability analysis
e) Decide excavation method

242
FACTORS EFFECTING EXCAVATION of ROCKS
 Mineralogical composition of rocks
 Texture & fabric
 Petrographic features
 Structure
 Rock mass
 Strike & dip of beds in relation to face of excavation
 Intensity of tectonic disturbances
 Degree of weathering

Classification of tunnels
243
Tunnels can be classified according to their position or alignment and purpose.
Based on position and alignment four types of tunnels can be identified.
1) Saddle and base tunnels: constructed at the base of mountains and takes longer
distance (for railways).
2) Spiral tunnels
3) Off spur tunnels: used to shortcut minor local extruding obstacles
4) Slope tunnels: ensure safe operation and protection to railway and highway
routes in steep mountain hangs
Tunnel can be classified into four types depending on it’s 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

244
 Classification of rocks for engineering purposes is needed in analyzing
the project costs and to obtain an economic and reliable solution.
 The classification of the rocks, that the tunnel will be constructed in, is
first done by Terzaghi. But, it is too general and gives qualitative
results.

245
Rock Tunneling Quality Index (Q) or NGI-Q

246
Tunnel support
1. Ground improvement ahead of the tunnel face
Injection of cement milk into the ground
Freezing of the water saturated ground
Drainage of water out of the area to be tunneled
2. Support during excavation
Shield (support) tunneling in soft ground
Bentonite tunneling with boring machine
Caisson tunneling to counteract (protect) water pressure
With shotcrete (sprayed grout) on the tunnel face and freshly excavated tunnel
sides
3. Support after excavation
Bolts
Anchors
Steel ribs
Shotcrete
Wire mesh or steel mats
Preformed concrete and backfill mortar
Formed concrete

Tunnels in different geological formations
247
I. Tunnels in soft ground
 These materials consist of gravels, sands, silts, clays and soft
shale's. They may be dry or water bearing.
 Excavation through such a ground does not require blasting.
Arch supports are always necessary. The soft ground may be
raveling, running or squeezing.
 In shallow tunnels that are driven in the soft ground, the roof
load is enormous. strong lining is required to support it.
II. Tunnels in rocks
 Tunneling through rocks requires blasting.
 If the rocks are structurally poor, support is often placed under
the tunnel ceiling to prevent the rocks from falling during
blasting.

248
The geological factors which influence tunneling are as
follows.
1) Swelling rocks
 If a tunnel is to be constructed through swelling rocks, it will
require special treatment. The examples of swelling rocks are
shale, unconsolidated tuff and anhydrite.
2) Inclined strata - In inclined rock beds when a tunnel is
driven parallel to the strike direction, there is a tendency
in the rocks to fall into the tunnel from the side where the
beds dip into the tunnel.
 This is particularly the case if the hard and soft rocks, such as
sandstone and shale are interbedded. When a tunnel is made
across the strike of rocks, it will traverse beds of different rocks.

249
Tunnel excavations in the slopes
3) Folded rocks - tunnels that are driven through synclinal folds joint
blocks form inverted keystones in the arch and cause rock falls.
In case the rocks happen to be water-bearing, the water flows into the tunnel and
causes great difficulties
Tunnel along the axis of a syncline and an anticline
 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

250
4) Fault zones - Faults are commonly found associated with a zone of highly
crushed rock or clay gouge. The crushed rocks being highly permeable allows
groundwater to seep into the tunnel. Besides this they also form unstable roof
rock.
5) Jointed rocks - Joints at one hand may help in excavating the rocks but on
the other hand they may present difficulties in tunneling.
6) Water-bearing rocks - Driving a tunnel though water-bearing rocks is a
difficult job. During excavation the groundwater rushes into the tunnel and
causes flooding.
Relation between the fault zone and the tunnel

Geological structures
251
(a) Dip and strike
• Influence tunnel excavation
• Three general cases
Unsafe condition
(I) Horizontal strata
Safe condition
 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.

Geological…
252
(II) Moderately inclined strata (<45°)
• Tunnel axis parallel to dip
• Tunnel axis parallel to strike
Tunnel parallel to dip
Tunnel parallel to strike
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.
When a tunnel is driven parallel to the strike
direction, there is tendency in the rocks to slide
into the tunnel.

Geological…
253
(III) A) Steeply inclined strata (>45°)
• Tunnel axis parallel to dip
• Tunnel axis parallel to strike
Tunnel parallel to dip
Tunnel parallel to strike

Geological…
254
(b) Folded rocks
• Anticline fold
• Syncline fold

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.

256
Tunnel construction/excavation methods:
 Mechanical drilling/cutting
 Cut-and-cover: constructed in shallow then covered over
 Drill and blast
 Shields tunnel method
 Tunnel boring machines (TBMs): without removing ground above
 Hard Rock Tunnel –mostly usedTBM
 Soft Ground Tunnel –Use Tunnel Shield

1. SHIELD TUNNELLING METHOD
257
 has protective structure used in the excavation of tunnels
through soil that is too soft or fluid to remain stable.
 Used mostly for deep tunnel
 the shield serves as a temporary support structure for the
tunnel while it is beingexcavated.
 This construction method causes minimal disruption to traffic
and the environment because all the work takes place below
ground and the ground level environment is unaffected.

2. CUT and COVER TUNNELLING METHOD
258
 Cut-and-cover is a simple method of construction for shallow tunnels
where a trench is excavated and roofed over with an overhead support
system strong enough to carry the load of what is to be built above the
tunnel.
 Two basic forms of cut-and-cover tunneling are available.
 Bottom-up method: A trench is excavated, with ground support as
necessary, and the tunnel is constructed in it.
 Top-down method: Side support walls and capping beams are
constructed from ground level.

3. TBM (Tunnel Boring Machine)
259
A. Mechanical-support TBM
B. Compressed-air TBM
C. Slurry shield TBM
D. Earth pressure balance machine

Soft Ground TBM
260

A- Mechanical Support TBM
261
 It has a full-face cutterhead which provides face support by
constantly pushing the excavated material ahead of the cutterhead
against the surrounding ground.
B- Compressed-Air TBM
 A compressed-air TBM can have either a full-face cutterhead or
excavating arms. Confinement is achieved by pressurizing the air
in the cutter chamber.
C- Slurry Shield TBM
 It has a full-face cutterhead. Confinement is achieved by
pressurizing boring fluid inside the cutterheadchamber.
 most suited for tunnels through unstable material subjected to
high groundwater pressure.

D - Earth Pressure Balance Machine
 It has a full-face cutterhead. Confinement is achieved by pressurizing
the excavated material in the cutterhead chamber.
 in which spoil is admitted into the TBM via a screw conveyor
arrangement which allows the pressure at the face of theTBM.
262
 The process for bored tunneling involves all or some of the
following operations:
Probe drilling (when needed)
Grouting (when needed)
Excavation (or blasting)
Supporting
Transportation of muck
Lining or coating/sealing
Draining
Ventilation Wollo University, Ethiopia Elias A.

263
CHAPTER SIX
ROADS, BRIDGES AND RAILWAYS

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