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Addis Ababa Science and Technology University
Engineering Geology (CEng 2112) (Cr. Hr. 2)
Chapter 5: Engineering Geological Site Investigation and Mapping for different
Engineering Structures

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Chapter 5: Engineering Geological Site Investigation and Mapping
5.1 Objective and methods of site investigation
5.2 Dam site investigation
5.3 Reservoir site investigation
5.4 Tunneling route investigation
5.5 Bridge site investigation
5.6 Sub surface water and Influences of sub surface water
on different engineering works

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5.1 Site Investigation (Exploration)
5.1.1 Definition and Objective of Site Investigation
 Site investigation deals with collecting all the necessary information for
safe and economic design, construction and maintenance of civil
engineering structures, environmental management or
extraction/development of resources.
Applications
• Civil engineering:
 Buildings, industrial and offshore foundations.
 Reservoirs, fills and embankments.
 Slopes.
 Roads, airports and industrial pavements.
 Bridges.
 Retaining structures.
 Tunnels and underground space facilities.
• Mining and resource development.
• Environment: waste containment systems and site remediation.

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Objectives of site investigation:
The main objectives of site investigation include the following
 Determine the nature of soil/rock at the site and its stratification.
 Obtain disturbed and undisturbed soil and rock samples for visual
identification and appropriate laboratory tests.
 Determine the depth and nature of bedrock if and when encountered.
 Perform some in-situ tests: permeability, bearing capacity, shear
strength, compressibility/settlement behavior etc.
 Observe drainage conditions from and into the site.
 Identify potential geological hazards: landslides, earthquakes,
flooding, volcanoes etc.
 Assess the quantity, quality and proximity of construction materials for
the proposed project.

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Consequences of ignoring site investigation
Leaning tower
Failure of Transcona
grain elevator, 1914
Earthquake damage to
roads, Japan
Bridge collapse due to
foundation failure from
earthquake, Japan

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Collapse of highway due to retaining wall
failure – Singapore, 2004
Failure of Dam via Erosion of Abutment
After Heavy Rain
Natural Slope Failure in Tropical
Environment

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Another Shanghai Building
Reinforced soil failure

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When a project fails, who is often blamed?
When a project is successful, who is often recognized and awarded?

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5.1.2 Stages of Site Investigation
5.1.2.1 Project Conception Stage
5.1.2.2 Preliminary Investigation Stage
5.1.2.3 Main Investigation Stage
5.1.2.4 Construction Investigation Stage
5.1.2.5 Post-Construction Investigation Stage

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5.1.2.1 Project Conception Stage
 It is stage that exist after decision to initiate the project has been made.
 At this stage, a desk study will be undertaken on all available
geotechnical, geological and topographical data.
 The proposed site and its environs should be examined by an experienced
engineering geologist
 The objective of this stage is to try to identify potential problems that may
arise from site geotechnical conditions in relation to the proposed
engineering work.

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5.1.2.2 Preliminary Investigation Stage
 The evaluation of a project at its conception stage may reveal significant gaps
in basic knowledge of the site so that no recognition of the likely problems is
possible.
 In such a case some preliminary investigation may be required to
establish that basic knowledge.
 This would be undertaken using relatively simple and inexpensive techniques,
such as existing records (maps, photographs etc.), geological and engineering
geological mapping, geophysics and perhaps some boreholes.

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5.1.2.3 Main Investigation Stage
 In the main investigation stage, the work done should recover the
information required to design the engineering project.
 These information, obtained by whatever means, should be
appropriate to the ground conditions and the nature of the engineering
work.
5.1.2.4 Construction Investigation Stage
 One of the unfortunate facts of site investigation is that the prognosis made in
the investigation reports resulting from the main investigation are seldom
absolutely and totally correct.
 The ground conditions encountered must be monitored, recorded and assessed.
5.1.2.5 Post-Construction Investigation Stage
 Certain features like settlement may take many years to become complete
after construction of the project.
 Further investigations may be required to resolve this anomaly.

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5.1.3 Methods of Site Investigation
1. Field or in-situ investigation
Direct Methods
Indirect Methods
2. Laboratory investigation
Types of Field Investigation (Exploration)
I. Direct method
 It involves techniques such as geotechnical drilling and test pits methods
that are directly used to obtain data at the project site.
Advantage
 They provide in-situ data with definite results that best represents the field
condition(s).
Disadvantage
 They are destructive, time consuming and expensive.
 They provide data only at location of the test.

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II. Indirect method
 It involves techniques such as geophysical methods that are carried out on
the ground without sampling and boring.
Advantage
 They are less destructive, inexpensive, covers large area and less time
consuming.
Disadvantage
 It requires subjective interpretation of the ground data.
 Cannot provide definite results

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I. Geophysical Techniques: Indirect Methods
The different geophysical methods include:
 Ground Penetrating Radar (GPR).
 Electrical Resistivity (VES and Profiling).
 Seismic: Reflection and Refraction.
 Electromagnetic (EM).
 Magnetic.
 Gravity.
 Very Low Frequency (VLF).
Advantages of the different geophysical methods:
• Non-destructive.
• Cost effective.
• Provides preliminary or supplemental information.

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a) Resistivity method
 Current is injected into the ground through current electrodes (C1 &
C2) and measuring the resulting voltage difference at potential
electrodes (P1 & P2).
 Ground resistivity is related to different geological conditions:
mineral content, porosity, moisture content etc..

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Example: Resistivity
 Detecting seepage through embankment dam

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b) Seismic method
 Seismic impulse is generated at the surface by artificial means
e.g. explosive, hammer blows etc.
 Depending up on elastic property and density of material, elastic
wave travel through different materials at different velocity
 Generally the stronger /denser/the compact material; the greater
the velocity

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 Fracture zones of the rock (fault and shear zone) have lower velocity and
can be easily detected.
 The top soil cover records lower velocity thereby enabling the depth to
bed rock to be detected.
 wave velocity also increases with the increase in moisture content of the
formation thereby indicating position of the water table.

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II. Direct method of exploration
 It includes:
 Test-pits, trenches: for shallow depth 3-6m
 Bore holes or drill holes, CPT: for deeper depth >6m and below
ground water table
A) Test pit and trench
 Is dug either by hand or by a backhoe
 This is the best method of recording both
the vertical and lateral ground condition
 Limited to shallow depth (6m) and above
ground water table

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Fig. Drilling rig
B) Rotary core drilling
 Drill bit is pushed by weight of drilling equipment and rotated by a motor
 Drill through any type of soil or rock
 Can drill to depths of 7,500m
 Undisturbed samples can be easily recovered

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C) Cone penetration Test (CPT)
 A cylindrical probe with a base area 10 cm2 and cone angle 60o that is
pushed into the ground at a rate of 2cm/s
D) Deformability tests in rocks
The following in-situ tests are conducted for determination of modulus of
deformation:
• Plate loading test,
• Goodman jack test,
• Plate jacking test,

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Condition of rock mass discontinuities associated with different
Lugeon values.
LU = Lugeon Value (L/min*bar*m)
Q = average water intake in L/min
P = Total pressure in bar
L = Length of test section in meter
Lu = (10*Q)/(P*L)
E) Field permeability tests in rocks
There are different methods for determining the permeability of rocks in the field:
• Open-ended permeability tests,
• Packer tests,
• Flow through fissures.

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Houlsby (1976) classified the typical behaviors observed in practice into
five different groups as follows:
(i) Laminar Flow: The hydraulic conductivity of the rock mass is independent of the
water pressure employed. This behavior is characteristic of rock masses observing
low hydraulic conductivities, where seepage velocities are relatively small (i.e., less
than four Lugeons).
(ii) Turbulent Flow: The hydraulic conductivity of the rock mass decreases as the
water pressure increases. This behavior is characteristic of rock masses exhibiting
partly open to moderately wide cracks.
(iii) Dilation: Similar hydraulic conductivities are observed at low and medium
pressures; however, a much greater value is recorded at the maximum pressure.
This occurs when the water pressure applied is greater than the minimum principal
stress of the rock mass, thus causing a temporary dilatancy (hydro-jacking) of the
fissures within the rock mass. Dilatancy causes an increase in the cross sectional
area available for water to flow and thereby increases the hydraulic conductivity.

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(iv) Wash-Out: Hydraulic conductivities increase as the test proceeds,
regardless of the changes observed in water pressure.
 This behavior indicates that seepage induces permanent and
irrecoverable damage on the rock mass, usually due to infillings
wash out and/or permanent rock movements.
(v) Void Filling: Hydraulic conductivities decrease as the test proceeds,
regardless of the changes observed in water pressure.
 This behavior indicates that either:
 Water progressively fills isolated/non-persistent
discontinuities;
 Swelling occurs in the discontinuities; or
 Fines flow slowly into the discontinuities building up a cake
layer that clogs them.

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5.2. Site Investigation for different Civil Engineering
Structures
Dams and Reservoirs
Tunnels
Slopes
 Bridges
 Highways
Buildings
Waste disposal sites

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5.2.1 Dam and Reservoir Site Investigation
5.2.1.1 Dam Site investigation
 A dam is a hydraulic structure of fairly impervious material built across a
river to create a reservoir on its upstream side for impounding water for
various purposes.
 A dam and a reservoir are complements of each other.
 Dams are generally constructed in the mountainous reach of the river
where the valley is narrow and the foundation is good.
 Generally, a hydropower station is also constructed at or near the dam site
to develop hydropower.
 Dams are probably the most important hydraulic structure built on the
rivers.
 These are very huge structure and require huge money, manpower
and time to construct.

Classification of Dams
 Classification is based on its importance, structural form (design)
and materials used in construction.
 The selection of the dam type for a given site is determined by
both engineering and economic considerations.
 In economic sense;
The cost of various types of dams depends upon
availability of construction materials, purpose of the
project and transport facilities.
 In engineering sense;
Foundation conditions alone frequently dictate the type of
dam to be built at a particular site.
In addition, abutment condition also influence the selection
of dam types.
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a) Classification on the basis of Use/Purpose
Irrigation
Dams:
• These dams are primarily constructed for irrigation purpose by
storing large/small amount of water.
Hydro-
Electric
Power dam.
• These dams are mainly constructed for hydro Power generation.
Flood
Control
Dams
• These dams are mainly constructed to control the floods.
Ground
Water
Recharge
Dams:.
• These dams are build mainly to recharge the ground water.
Diversio
n Dams
• These dams are constructed mainly to store and divert the water
to the desired location.
Multipur
pose
Dams
• These dams are constructed for two or more purposes
like; irrigation, power generation, Flood control, diversion etc.
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(b) Based on Hydraulic Design:
 Overflow dams.
 Non-overflow dams.

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(c) Based on Materials
of Construction:
 Masonry dam
 Concrete dam
 Earth dam
 Rock-fill dam
 Timber dam
 Steel dam
 Combined concrete-
earth dam
 Composite dam.
(d) Based on rigidity:
Rigid dams:
 A rigid dam is quite stiff. It is constructed of
stiff materials such as concrete, masonry, steel
and timber.
 These dams deflect and deform very little when
subjected to water pressure and other forces
Non-rigid dams:
 A non-rigid dam is relatively less stiff
compared to a rigid dam.
 The dams constructed of earth fill dams. There
are relatively large settlements and
deformations in a non-rigid dam.
Rock-fill dams are actually neither fully rigid nor
fully non-rigid. These are sometimes classified as
semi-rigid dams.

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(e) Based on structural
action:
 Gravity dams
 Embankment dams
 Earth dams
 Rockfill dams
 Arch dams
 Buttress dams
 Others
 Steel dams
 Timber dams

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Dam Types, Based on structural action Advantages and
Disadvantages
A) Gravity Dams
A gravity dam resists the water
pressure and other forces due to its
weight (or gravitational forces).
 usually made of cement concrete
and straight in plan.
 are approx triangular in cross-
section, with apex at the top.
 In the past, the gravity dams were
made of stone masonry.
 Bhakra dam (structural
height of 226 m) was the
highest.
 The highest concrete gravity dam
is Grand Dixence Dam in
Switzerland (284 m high).

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Masonry Gravity Dam
(Non-overflow )
Concrete Gravity Dam with Overflow Section

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Advantages of Gravity Dams
 Gravity dams are quite strong, stable and durable.
 Are quite suitable across moderately wide valleys and gorges having steep
slopes where earth dams, if constructed, might slip.
 Can be constructed to very great heights, provided good rock foundations
are available.
 Are well adapted for use as an overflow spillway section.
 Are specially suited to such areas where there is very heavy downpour.
 Maintenance cost of a gravity dam is very low.
 Does not fail suddenly. There is enough warning of the imminent failure
and the valuable property and human life can be saved to some extent.
 Can be constructed during all types of climatic conditions.

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Disadvantages of Gravity Dams
 Gravity dams of great height can be constructed only on sound rock
foundations. These cannot be constructed on weak rocks
 Initial cost of a gravity dam is usually more than that of an earth dam. At
the sites where good earth is available for construction and funds are
limited, earth dams are better.
 Usually take a longer time in construction than earth dams, especially
when mechanized plants for batching, mixing and transporting concrete
are not available.
 Require more skilled labor than that in earth dams.
 Subsequent raising is not possible in a gravity dam.

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B) Earth Dams
 An earth dam is made of earth (or soil) and resists the forces exerted
upon it mainly due to shear strength of the soil.
 Are usually built in wide valleys having flat slopes at flanks (abutments).
 Can be homogeneous when the height of the dam is not great.
 Are of zoned sections, with an impervious zone (called core) in the
middle and relatively pervious zones (called shells or shoulders)
enclosing the impervious zone on both sides. Nowadays majority of dams
constructed are of this type.
 The highest dams of the world are earth dams (Rongunsky dam Russia,
325 m and Nurek dam, Russia, 317 m).

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Advantages of Earth Dams
 Are usually cheaper than gravity dams
if construction materials are available
near the site.
 Can be constructed on almost all types
of foundations, provided suitable
remedial measures are taken.
 Can be constructed in a relatively short
period.
 Skilled labors are not required in
construction of an earth dam.
 Can be raised subsequently.
 Are aesthetically more pleasing than
gravity dams.
 Are more earthquake-resistant than
gravity dams.
Disadvantages of Earth Dams
 Are not suitable for narrow
gorges with steep slopes.
 Cannot be designed as an
overflow section. A spillway
has to be located away from
the dam.
 Cannot be constructed in
regions with heavy
downpour.
 Maintenance cost of an
earth dam is quite high. It
requires constant
supervision.
 Fails suddenly without any
sign of imminent failure.

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C) Rock fill Dams
 A rock fill dam is built of rock
fragments and boulders of large size.
 An impervious membrane (cement
concrete or asphaltic concrete or
earth core) is placed on the rock fill
on the upstream side to reduce the
seepage through the dam.
 A dry rubble cushion is placed
between the rock fill and the
membrane for the distribution of
water load and for providing a
support to the membrane.
 Side slopes of rock fill are usually
kept equal to the angle of repose of
rock (1.4:1 or 1.3:1).
 Rock-fill dams are quite economical
when a large quantity of rock is
easily available near the site.

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Advantages of Rock fill Dams
Rock fill dams have almost the same
advantages and disadvantages over
gravity dams as discussed for earth
dams.
 Are quite inexpensive if rock
fragments are easily available.
 Can be constructed quite rapidly.
 Can better withstand the shocks
due to earthquake than earth
dams.
 Can be constructed even in
adverse climates
Disadvantages of Rock fill Dams
 Rock fill dams require more
strong foundations than earth
dams.
 Rock fill dams require heavy
machines for transporting,
dumping and compacting rocks.

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D) Arch Dams
An arch dam is curved in plan with its convexity towards the upstream side.
 Transfers the water pressure and other forces mainly to the abutments by
the arch action.
 Is quite suitable for narrow canyons with strong flanks which are capable
of resisting the thrust produced by the arch action.
 Section is triangular and is comparatively thinner.
 May have a single curvature or double curvature in the vertical plane.
 Are subjected to large stresses because of changes in temperature
shrinkage of concrete and yielding of abutments.
Exemples - Juguri dam (272 m), Russia, Vaiont dam (262 m), Italy, Manvoisin
dam (237 m) Switzerland.

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Advantages of Arch Dams
 Requires less concrete as compared
to a gravity dam.
 Are more suited to narrow, V-
shaped valley, having very steep
slopes.
 Uplift pressure is not an important
factor in the design of an arch dam
because the arch dam has less
width and the reduction in weight
due to uplift does not affect the
stability.
 Can be constructed on a relatively
less strong foundation because a
small part of load is transferred to
abutmentswhereas in a gravity dam
the full load is transferred to base.
Disadvantages of Arch Dams
 Requires good rock in the flanks
(abutments) to resist the thrust. If the
abutments yield, extra stresses
develop which may cause failure.
 Requires sophisticated formwork,
more skilled labour and richer
concrete.
 Cannot be constructed in very cold
climates because spalling of concrete
occurs due to alternate freezing and
thawing.
 Are more prone to sabotage.
 The speed of construction is relatively
slow.

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5 Buttress Dams
 Buttresses are triangular concrete walls
which transmit the water pressure from
the deck slab to the foundation.
 Buttress dams are of three types: (i) Deck
type, (ii) Multiple arch-type and (iii)
Massive-head type.
 The deck is usually a reinforced concrete
slab supported between the buttresses,
which are usually equally spaced.
 In a multiple-arch type buttress dam the
deck slab is replaced by horizontal arches
supported by buttresses.
 In a massive-head type buttress dam,
there is no deck slab. Instead of the deck,
the upstream edges of the buttresses are
flared to form massive heads which span
the distance between the buttresses.

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Advantages of Buttress Dams
 Buttress dams require less concrete
than gravity dams.
 Uplift/ice pressure is generally not
a major factor.
 can be constructed on relatively
weaker foundations.
 Vertical component of the water
pressure on deck prevents the dam
against overturning and sliding
failures.
 Can be designed to accommodate
moderate movements of
foundations without serious
damages.
 Can be easily raised subsequently
by extending buttresses and deck
slabs.
Disadvantages of Buttress Dams
 Buttress dams require costlier
formwork, reinforcement and more
skilled labour.
 Consequently, the overall cost
of construction may be more
than that of a gravity dam.
 Buttress dams are more susceptible
to damage and sabotage.
 Buttress dams cannot be constructed
in very cold climates because of
spalling of concrete.
 Because the upstream deck slab is
thin, its deterioration may have very
serious effect on the stability.

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E) Composite Dams
Composite dams are combinations of one or more dam types. Most often a large
section of a dam will be either an embankment or gravity dam, with the section
responsible for power generation being a buttress or arch.
The Bloemhof Dam on the Orange River of South Africa is an excellent example of a gravity/buttress dam.

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Dam site selection
The following factors govern the selection of an appropriate dam site
A) Foundation
 Suitable foundation site should be available at the site for a particular type
of dam. Example
 Gravity and rock fill dams requires sound foundation rock.
 Earth dams can be constructed on any type of foundation provided
that proper remedial measures are provided.
 Geological structures such as faults need to be avoided in the foundation.
 Organic and compressible materials should be avoided
 Joints and fissures that might lead to serious seepage have to be properly
treated.
B) Topography
 The river cross-section at the dam site should preferably have a narrow
gorge to reduce the length of the dam.
 However, the gorges should open out upstream to provide large basin for
the reservoir.

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C) Site for Spillway
 Good site for the location of a separate spillway is essential incase of earth or
rock fill dam. However, incase of gravity dam, spillway may be located at its
middle.
 Incase of separate spillway, the route need to be short and we need to look for
erosion resistance material to avoid necessity of lining materials and energy
dissipation.
D) Construction Material
 Materials required for dam construction (Soils, rocks, concrete and etc.) should
be available nearby without requiring much of transportation to achieve
economy and social impact.
E) Reservoir and Catchment Area
i. The cost of land property submerged in the water area should be minimum.
ii. The reservoir site should be such that quantity of leakage through its side and
bed is minimum.
iii. Preferably the catchments area should be such that it produces maximum
runoff and minimum silt.
F) Other Conditions
 The site selected should have a better communication, roadway, health
facilities, seismic risk, availability of labor, etc.

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5.2.1.2 Geological impacts on dam site selection
Most problems of dam are directly or indirectly related to the geological
setup of the area. these problems are;
I. Problems, which are related to incompetence and solubility of rocks.
II. Problems which are related to improper geological structures
III. Problems associated with abutments
I. Problems related to incompetence and solubility of rocks.
 Dams on shale
 Dams on soluble rocks
a) Dams on shale
 Shale is soft rock and when saturated with water under pressure
likely produces lubricating material making a slippery base.
 Shale’s bearing capacity is low and it becomes plastic when
wetted.

b) Dam on soluble rocks
 The soluble rocks include limestone, dolomite, and marble.
 These rocks are generally strong to support the weight of the dam.
 But they may contain underground openings.
II) Problems related to improper geological structures
• Dam on strata dipping upstream.
• Dam on strata dipping downstream.
• Dam built across strike of rocks.
• Dam on jointed and permeable rocks.
• Dam on faults
a) Dam on strata dipping upstream
 The resultant of the two forces acts nearly at right angles to the bedding planes
of the rocks.
 Also the upstream dip of the rock does not allow the water in the reservoir to
percolate below the dam.
 As a result, the leakage of water and the development of the up lift pressure will
be minimum. However it have an effects after a long time.
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b) Dam on strata dipping down stream
Dams built on rock beds dipping down stream are not safe due to the
following reasons:
The water, which enters the openings of the rocks below the dam,
causes uplift pressure, lubrication and dissolution that tend to
decrease the stability of the structure.
The resultant force R, which is due to the weight of the dam and
the horizontal water pressure acts nearly parallel to the bedding
planes and endangers the stability of the dam.
Dam on strata dipping down stream
Dam on strata dipping upstream
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c) Dam built across the strike of the rocks
 If a dam is aligned across the strike of the strata, then its foundation
will be on different rock types of varying properties.
 This situation leads to unequal settlements of the dam foundation.
d) Dam built on jointed and permeable rocks:
If the dam built on jointed and permeable rock the following problems will occur
Leakage and seepage
Pore pressure development. Hence consolidation by grouting has
to be done
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e) Dam built on faults zone
 Most trouble is faced if the dam is constructed across active, dipping downwards
fault and a fault that extend across the length of the dam.
 Faults (fault zone) cause the following problems
 Leakage at large scale-cost
 Great depth of weathering:- Needs digging and excavating out and refilling
 Decrease in the competence of the foundation rock:- Intensive grouting is
required
 Displacement of strata during movement
• Reopen the fault fissure
• Rapture of the dam
 Hence during the site investigation these all parameters should be studied
extensively and designing should be done accordingly.
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Selection of Type of Dam
 Selection of the most suitable type of dam for a particular site requires a
lot of judgment and experience.
 It is only in exceptional cases that the most suitable type is obvious.
Various factors govern the selection of type of dam
1. Topography and valley shape
2. Geology and foundation conditions
3. Availability of construction materials
4. Spillway size and location
5. Earthquake hazards
6. Climatic conditions
7. Diversion problems
8. Environmental considerations
9. Roadway
10. Length and height of dam
11. Life of dam
12. Overall cost
13. Miscellaneous considerations

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(a) Overtopping leading to washout; less cohesive silts,
sands, etc. at greatest short-term risk
(b) Internal erosion and piping with migration of fines
from core etc.: also in foundation (note regression of
‘pipe’ and formation of cavities; may initiate by internal
cracking or by preferential seepage paths in foundation or
along culvert perimeter etc.)
(c) Embankment and foundation settlement (deformation
and internal cracking); note also cross-valley deformation
modes.
(d) Instability (1): downstream slope too high and/or too
steep in relation to shear strength of the shoulder
material.
(e) Instability (2): upstream slope failure following rapid
drawdown of water level.
(f) Instability (3): failure of downstream foundation due to
overstress of soft, weak horizons.
Defect mechanisms, failure modes and
design principles: Embankment Dams
(a)
(b)
(c)
(d)
(e)
(f)

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5.2.1.3 Reservoir Site investigation
Reservoir
 Reservoir is a water body or lake which could be created when a barrier is
constructed across a river or a stream.
Advantages/uses of reservoirs
 Water supply.
 Irrigation.
 Hydroelectric power generation.
 Recreation.
 Flood control, Navigation etc.
Disadvantages of reservoirs
 Detract from natural settings, ruin nature's work.
 Inundate the spawning grounds of fish, and the potential for
archaeological findings.
 Inhibit the seasonal migration of fish, and even endanger some species
of fish.
 Water can evaporate significantly.
 Induce earthquakes.

Engineering geological considerations of reservoir site
The important
engineering
geological
considerations during
the reservoir site
investigation include:
• Water tightness of the
reservoir,
• Stability of the reservoir
slopes,
• Siltation of the reservoir
and
• Economic results of
impounding water.
60

61
5.2.1.4 Problems associated with reservoirs
The main geological problems associated with the reservoirs are:
♠ Ground water conditions
♠ Silting
♠ Permeable rocks
1) Influent rivers: Rivers which loses water
- Hence there is leakage under reservoirs
a) Ground water conditions

62
2) Effluent Rivers: which gain water from the ground water
- Hence there is no leakage

63
b) Silting of reservoirs
The amount of silt produced and supplied to the rivers depends mainly on:
♠ Lithological character and
♠ Topography of the catchment’s area
The measures that help to reduce silting of reservoirs are:
Generally, the softer the rock and the steeper the gradient = higher silting
♠ Vegetation ( bind loose material together)
♠ Covering with slabs on weak zones
♠ Diversion of sediment-loaded waters
♠ Terracing of the slope and construction of retaining walls
♠ Check dams

64
c) Permeable rocks
 The rocks, which are highly porous, are likely to cause serious
leakage from the reservoir
The following methods used to seal permeable zones:
♠ Natural silting
♠ Grouting
♠ Covering weak zones with concrete slabs

65
5.2.2 Investigation of tunnel site
 Tunnels are under ground passages or routes used for different purposes
 They are made by excavation of rocks below the surface or through the
hills or mountains, or sides of valley
Tunnels can be constructed for
various purposes like:
 Monitoring,
 Mining
 Transportation, and
 Storage
 Hydropower generation
 Irrigation.
5.2.2.1 Definition and Application of tunnel

Types of Tunnels
Tunnel can be
classified into four
types depending
on its purpose.
• Traffic tunnel: is a tunnel that is constructed
underground for the passage of roads and railways
• Hydropower tunnel: pass water under pressure
and produce power by colliding with generators.
• Public utility tunnels: are relatively small and
constructed for carrying utility lines for routing
power, pipeline and telecom cables.
• Diversion tunnels: are used for flood control or
supplying water for different purposes.
66

67
Shape Purpose
Circular Water and sewage
Elliptical Water and sewage mains
Horseshoe Roads and railways
Arched roof with vertical walls Roads and railways
Polycentric cross-section Roads and railways
Shape and Purpose of Tunnels

Methods of Tunnel Excavation
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.
68

The excavation
methods that are
commonly used
include
• Cut-and-cover
• Boring machine
• Drill and blasting
69

Cut-and-
cover
method
• In this construction method, the site is
fully excavated, the structure is 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.
70

Tunneling
by tunnel
boring
machine
(TBM)
• 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.
• 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/crushing 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.
72

Advantages of
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:
• More (better) geological information from the pre-investigation stage is
required.
• More sensitive to tunneling problems in poor rock mass conditions.
• Fixed circular geometry, limited flexibility in response to extremes of
geologic conditions, longer mobilization time, and higher capital costs.
• Only longer tunnel sections can be bored more economically (because of
larger investment and rigging costs)
• The TBM may get stuck under squeezing rock conditions.
• It is difficult to perform / install rock support at the tunnel face.
73

75
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.
Drill and
Blast Method
• 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.

Advantages of
Drill and Blast
Method:
• 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.
disadvantages
Drill and Blast
Method:
• Production of toxic gases and smoke from the explosives.
• Vibrations on nearby structures from the blasting;
• Rough surface gives head loss for water tunnels;
• The blasting creates new cracks in the rocks, which leads to increased need of
rock support;
• 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.
76

Tunneling under different ground condition
 The ground condition through which a tunnel are excavated can be soft
(soils) and hard (rocks).
 The excavation in rock can be done in one of the following conditions,
which can affect tunneling.
(I) Inclined strata (IV) Jointed rocks
(II) Folded rocks (V) Water bearing rocks
(III) Fault Zones (VI) Swelling Rocks
I. 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.
77

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. 78

II. Folded rocks
1.Tunnels along troughs: This encounters unfavorable conditions, because
rock masses along trough are harder and more resistant. There are 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
79

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 in all parts.
Fig. Tunnel aligned parallel to fold axis through limbs
80

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.
81

III. 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.
IV. 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.
V. Water bearing rocks
Excavation of a tunnel through the water bearing rock is difficult since ground
water rushes into the tunnel and causes flooding during excavation.
VI. Swelling rocks
Shale, unconsolidated tuff and anhydrite are examples of swelling rocks. They
absorb moisture and swell when they are exposed to water saturation.
82

To be suitable for tunneling, the geological condition:
 should be one type of rock
 should have 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 change its behavior under the
exposure to water (non-expandable)
 Not be highly weathered and shouldn’t result in collapse.
83

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.
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 in
the following table.
84

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, the
faster the excavation 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. Hence, the circular tunnels
will have longer stand-up time than the square or rectangle tunnels.
 If the tunnel is wider than its height, it will have a harder time
supporting itself, decreasing its stand-up time and if a tunnel has a
higher height than its width, the stand up time will increase making the
project easier.
85

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 the 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 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.
86

C) Support after excavation:
 Bolts, Anchor, Steel ribs, Shotcrete, 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.
87

v) Moisture in tunnel
Water is a governing factor in tunnel loads as well as in construction
possibilities and conditions.
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.
88

vi. Gasses in tunneling
 Carbon monoxide (CO), Carbon dioxide (CO2) and Methane (CH4)
are highly explosive with air (marsh gas)
 Hydrogene sulphide (H2S) and Sulphur dioxide (SO2) are highly
toxic and explosive
 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.
89

During the designing
and construction of
tunnel particular
attention should be
given to the following
types of information:
• Top of rock; depth of weathered rock.
• Water bearing zones, aquifers, fault zones, and caves.
• Karstic ground conditions.
• Presence of very strong (>250 MPa) and very
abrasive material that can affect TBM performance.
• Highly stressed material with potential for overstress.
• Potential for gases.
• Corrosive groundwater.
• Slake-susceptible material and material with potential
for swell.
• Materials that are affected by water (dissolution).
• Zones of weak rock (low intact strength, altered
materials, faulted and sheared materials).
90

91
Parameters to be investigated for rock slopes
In rock slope stability analysis, the following input parameters need to
be investigated:
I. Distribution of soil/rock masses and their associated
geological structures (laterally and vertically).
II. Discontinuity orientation in relation to the terrain/excavation.
III. Discontinuity condition: aperture, infill, continuity, roughness,
etc.
IV. Friction and cohesion values of the rocks, soils as well as the
discontinuities in these masses.
V. Unit weight of the rock material.
VI. Water pressure (magnitude and distribution) within the slope.
VII. Geometry and the likely mode of failure.
5.2.3 Site Investigation for Rock Slopes

92
5.2.4 Site investigations for Highways, Bridges, and
Railroads
5.2.4.1 Site investigation for Highways
The road system can be divided into the following:
• National highways.
• Provincial/regional highways.
• Major district roads and minor district roads.
• Village roads.
Major considerations in road engineering:
• Straight route
• Easy grades and curves
• Good sight distance
• Proper drainage
• Availability of building materials
• Availability of adequate road land
• Suitable bridge site
•Free from slides and snow condition.
• Good foundation condition:
 Settlement/consolidation
behavior.
 Shear strength.
 Swelling behaviour.
 Erodibility.
 Excavatability

93
5.2.4.2 Site Investigation for Bridge Foundations
Loads on Bridge
A) Permanent Loads:
 Dead Loads
 Superimposed Dead Loads
 Pressures (earth, water, ice, etc.)
B) Temporary Loads:
 Vehicle Live Loads
 Earthquake Forces
 Wind Forces
 Channel Forces
 Longitudinal Forces
 Centrifugal Forces
 Impact Forces
 Construction Loads
C) Deformation and Response Loads:
 Creep
 Shrinkage
 Settlement
 Uplift
 Thermal Forces
D) Group Loading Combinations.

94
General characteristics of bridge sites
Bridges are mostly build on valleys which are generally associated with:
• High degree of weathering.
• High surface and sub-surface water flow .
• Slope instability and associated ground failures.
• High degree of erosion.
• Highly variable soil-bedrock interface.
• Faults and/or other geological structures.
• Variable soil and rock geotechnical conditions.
• Buried channels and associated ground difficulties.
 All the above mentioned factors have an impact on the
bearing capacity and stability of piers and on the overall
cost, safety and stability of bridges.
Objective of site investigation
A complete geotechnical study of a site will:
1. determine the subsurface stratigraphy and stratigraphic relationships (and
their variability),
2. define the physical properties of the earth materials, and
3. evaluate the data generated and formulate solutions to the project-specific
and site-specific geotechnical issues.

5.2.4.3 Site Investigation for railroads
Rail Road Route Selection
Introduction
The location process begins by roughly defining potential routes or areas through
which a railroad might practically run. Additional and more detailed information is
then collected, and the route alternatives are gradually reduced until the final route
is chosen.
The ideal route is usually the one that:
 Is shortest in length.
 Has the lowest grades.
 Has the least curvature.
 Costs the least to build.
 Most conveniently serves the installation and all terminal areas, as
well as connecting carriers.
 Causes the least interference with other activities and modes of
transportation.
 Is environmentally compatible with adjacent land use.
 Provides reliability for use in all weather conditions.
Route selection involves several steps/processes: (a) defining control points and
potential corridors, (b) reconnaissance, (c) Initial survey, (d) Trial location, and (e)
final location. 95

Expected challenges in railroad constructions in
Ethiopia:
 Slope stability problems.
 Tunnelling challenges.
 Bridges and crossings.
 Foundation challenges: especially in geodynamically active
and tectonic areas.
 Construction materials: quality and quantity.
96
What are the forces on railways?
 Geological and geotechnical parameters for railways are similar as for highways
but parameters are more stringent for railways.

97
First phase railway
route:
• Awash-
Kombolcha-
Wodia-Mekelle;
• Woreta-Wodia-
Semera-Galafi;
• Addis Ababa-
Ejaji-Bedeke.
ERC (2010)
ix. Railroads

98
5.2.5 Site Investigation for Buildings

99
Parameters to be investigated
The purpose of the field exploration for building design and construction
include the following:
i. Knowledge of the general topography of the site as it affects foundation
design and construction, e.g., surface configuration, adjacent property, the
presence of watercourses, ponds, hedges, trees, rock outcrops, etc., and the
available access for construction vehicles and materials.
ii. The location of buried utilities such as electric power and telephone cables,
water mains, and sewers.
iii. The general geology of the area, with particular reference to the main
geologic formations underlying the site and the possibility of subsidence from
mineral extraction or other causes.
iv. The previous history and use of the site, including information on any defects
or failures of existing or former buildings attributable to foundation conditions.
v. Any special features such as the possibility of earthquakes or climate factors
such as flooding, seasonal swelling and shrinkage, permafrost, and soil
erosion.
vi. The availability and quality of local construction materials such as concrete
aggregates, building and road stone, and water for construction purposes.
vii. For maritime or river structures, information on tidal ranges and river levels,
velocity of tidal and river currents, and other hydrographic and meteorological
data.

100
viii. A detailed record of the soil and rock strata and groundwater conditions within the
zones affected by foundation bearing pressures and construction operations, or of
any deeper strata affecting the site conditions in any way.
ix. The depth of weathered/slightly weathered and the shape of bedrock surface.
x. Results of laboratory tests on soil and rock samples appropriate to the particular
foundation design or construction problems.
xi. Results of chemical analyses on soil or groundwater to determine possible
deleterious effects of foundation structures.
xii. Future plans with regard to the other structures.
xiii. The nature, depth and condition of the foundations of adjacent buildings and the
character of the strata in which the foundations were placed.
xiv. The geotechnical properties and distribution of soils and rocks including such
factors as: permeability, shear strength, and settlement/consolidation parameters.
xv. Groundwater levels and quality in various strata.
xvi. Landslides and landslide-related ground failures, etc.

102
5.2.6 Geological-geotechnical criteria for selecting waste disposal sites
(a) Composition and distribution of Superficial Deposits
In order to assess the subsoil for a disposal site it is necessary to know:
 the composition, the physical and the chemical properties as well as the sequence of a
strata,
 the lateral and vertical continuity and the distribution of the strata (facies changes),
 the porosity, the permeability (to water and leachate),
 the resistance to erosion and washing away of the particles and the stress deformation
behavior.
(b) Structure and Sequence of Solid Strata
Due to regional geological factors and morphological characteristics, superficial deposits
are often relatively thin and therefore the underlying solid rock strata may have to be
included in the survey. Here, the following factors need to be considered:
 the type of rock, mineralogical composition and stratigraphy,
 the state of weathering and weathering resistance,
 the solubility in water and leachate or other aggressive solutions,
 the type and position of geological boundaries,
 the extent, degree of separation and widths of individual joints,
 the tectonic and petrographical anisotropies in the rock mass,
 karstification and risk of subsidence,
 the deformation behaviour of the rock mass and
 the permeability to water, leachate, gases and other aggressive solutions.

103
(c) Determination of Hydrogeological Data
Disposal sites must be prevented from having unacceptable impacts on
groundwater, surface water, and particularly water abstraction sources.
Comprehensive knowledge of the groundwater regime is, therefore, required,
including the following detailed information:
 the groundwater regime, direction of flow, gradient and rate of flow,
including long-term and seasonal fluctuations,
 the permeability (horizontal and vertical) or transmissivity of the
outcropping strata, with maximum and minimum values,
 the distribution, thickness and depth of aquifers, aquicludes and aquitards,
including the locations of any spring,
 the groundwater levels, indicating hydraulic gradients and effective flow
velocity in the individual strata components if appropriate,
 the groundwater chemistry, including determination of naturally occurring
aggressive substances and groundwater quality,
 the groundwater protection zones,
 the influence of nearby open waters and their relationship with the
groundwater system,
 the effective rainfall, surface runoff, percolation rate, evaporation and
groundwater recharge.

104
(d) Consideration of Special Factors
Artificial interference with the subsoil may have significantly altered the
natural conditions. The existence of natural deposits worthy of protection or
archaeological factors, may preclude use of the site as a landfill. The
following points should be included in the survey:
 the stability of existing slopes if trenches are used,
 the potential for subsidence or uncontrolled emission of gas and
leachate caused by abandoned or existing mine workings and/or
gas/groundwater extraction wells (underground and surface
workings),
 the presence of workable natural materials in the subsoil,
 the presence of geological features or archaeological monuments
worthy of protection, and
 the background contamination of the subsoil and/or groundwater.

105
Site Characterization report
The site characterization report should include descriptions of:
1. Site topography and/or bathymetry,
2. Site geology,
3. Subsurface stratigraphy and stratigraphic relationships,
4. Continuity or lack of continuity of the various subsurface strata,
5. Groundwater depths and conditions, and
6. Assessment of the documented and possible undocumented variability
of the subsurface conditions.
In addition to the standard consideration of axial and lateral foundation
capacity, load–deflection characteristics, settlement, slope stability, and earth
pressures, there are a number of subsurface conditions that can affect
foundation design and performance:
• Liquefaction susceptibility of loose, granular soils;
• Expansive or collapsible soils;
• Mica-rich and carbonate soils;
• Corrosive soils;
• Permafrost or frozen soils;
• Perched or artesian groundwater.
When any of those conditions are present, they should be described and
evaluated.

5.3 Subsurface water & Engineering work
Runoff
Seasonal
spring
Saturated
Subsurface
water
106

I. Overview of subsurface water
Sub surface water is water that
is found at some depth
below the surface of the
earths.
The subsurface water can flow
in different direction
depending on its level and
subsurface structures. Thus
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.
Drainage The removal of excess water from
the land surface and/or from the soil profile.
Gaining Stream A stream that receives
ground-water discharge.
Loosing Stream- A stream that recharge the
groundwater.
Hydraulic Conductivity A measure of the
rate at which water will move through a
permeable soil or rock layer.
Leakage- the accidental admission or escape
of liquid or gas through a hole or crack or it is
the concentrated flow of water from reservoir
to down stream passing through geological
structures.
Seepage- the slow escape of a liquid or gas
through porous material or small holes or it is
the distributed flow of water from dam
reservoir to down stream passing through
porous medium.
107

II. Effects of subsurface water on engineering
Structures
Every engineering structure such as dam, building, highways, railways, roads
and other underground projects such as mining, tunnels could be affected
by the water (surface or subsurface) that is found in the site of construction
in different ways .
Subsurface water may pose problems during the construction stage, during its
performance stage and reduce the safe functioning of an engineering
project.
So it is an important aspect of any engineering geological investigation to
assess the possible effects of the subsurface water on the proposed
engineering projects.
In addition engineering project can also affects the subsurface water by
altering its quality and flow direction.
A detail and reliable investigation should focus on such effect before any
construction.
108

The main effects of subsurface water on engineering
structures are by:
Eroding the foundation of structures
Responsible in volume changing of soil or rocks of the
foundations which is the results of swelling up on
saturation and shrinkage during drying.
Facilitating the sliding of slope by reducing safety factors.
Affect excavation and construction methods by 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.
109

Effects of subsurface water on dam site
 Hydraulic structures such as dam are
mostly constructed on pervious
(permeable) soil through which seepage
flow occurs.
 The subsurface water is the most and
critical problems in the foundation and
abutments of dam project.
 Because some times, to reduce the
instability problems the dam foundation
are placed at great depth below
subsurface water.
 In such case there will always be an
inflow of water into the excavation,
which may block or retard the
construction activities.
 In another case during the excavation of
over burden materials different
discontinuities are intercepted, which
acts as a conduit for the flow of sub
surface water towards the structures.
Subsurface water lubricate the
discontinuity and facilitate the
failure of dam abutment.
Subsurface water fluctuations may
cause uplift problems in the dam
foundation area which in turn is
responsible for the settlement.
Sub surface water can bring different
dissolved chemical to the
foundation, which can react with
construction material and damage
the overall structures
Generally dam failures can be
grouped into four classifications
which may or may not be related to
the effect of subsurface water:
– Overtopping,
– Foundation failure
– Structural failure and
– Other unexpected failures.
110

Effects of subsurface water on tunnel
 The stability of tunnel is one of the
most important subjects in the
tunnel constructions, especially
when the groundwater table is
located above the tunnel.
 Ground water seepage occurs to
the tunnel when the tunnel
intersects with ground water table
at certain point on its extension.
 Tunneling beneath the
groundwater table causes
changes in the state of stress and
the pore water pressure
distribution.
 When the groundwater table is
above the tunnel, the water can flow
towards the tunnel.
 The water near the tunnel can
develop pore water pressure around
the tunnel and can result in tunnel
collapse.
 The water can saturate the roof of the
tunnel passage and results in ground
collapse by reducing the withstand
capacity of the soils.
111

Effects of subsurface water on Building Foundations
 Temporary or permanent rising
and lowering of the groundwater
table from man-made or natural
causes can affect buildings,
streets, underground utilities and
other structures.
 The foundation and base of every
engineering structure are on or in
the soils or rocks.
 When the rocks and soils are
exposed to subsurface water their
engineering properties can be
changed by saturation and pore
pressure effects.
This effect results in the reduction of
the 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.
112

Water chemistry- the chemistry of
subsurface water can vary 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 it acidity and total
dissolved solid (TDS).
Due to its chemistry, subsurface water
is the most dissolving agents on
engineering structure which responsible
for the formation of karst and solution
cavities.
This results in the collapse of structures
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.
Water quality and Engineering structures
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.
113

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
 In general, chemistry of ground water affects the stability of
engineering structures by:
In general the corrosivity of subsurface water can be determined from corrosivity ratio
coefficient (CR)
CR = (0.028Cl + 0.021SO4)/0.02(HCO3 +CO3)
If CR value is >1 the subsurface water is corrosive
114

5.4 Engineering geological mapping
115
Engineering geological mapping is the first step towards co-
operation between Geologists and Civil Engineers.
It provides a general representation of all components of
geological environment which has its significance in
 Land-use planning,
Design,
Construction and
Maintenance as applied to civil and mining engineering.
In building of the larger engineering works: Such as tunnels, dams,
railways, highways, etc

116
Engineering geological mapping is directed towards
understanding/determining the:
 Interrelationship between the geological environment and the
engineering situation.
 Nature and relationships of the individual geological
components.
 Active geodynamic processes.
 Processes likely to result from the changes being made as a
result of construction.
 Rock, soil, water, geomorphological conditions and
geodynamic processes are principal factors affecting the
engineering geological condition of a site.

117
Geological features represented on engineering geological maps include:
i. The character of the rocks and soils, including their distribution,
stratigraphical and structural arrangement, age, genesis, lithology, physical
state, and their physical and mechanical properties.
ii. Hydrogeological conditions, including the distribution of water-bearing
soils and rocks, zones of saturated open discontinuities, depth to water
table and its range of fluctuations, regions of confined water and piezometic
levels, storage coefficients, direction of flow, springs, rivers and lakes and
the limits and occurrence interval of flooding; pH, salinity, corrosiveness.
iii. Geomorphological conditions, including surface topography and
important elements of the landscape.
iv. Geodynamics phenomena, including erosion and deposition, aeolian
phenomena, permafrost, slope movements, formation of karst conditions,
suffusion, subsidence, volume changes in soil, data on seismic phenomena
including active faults, current regional tectonic movements, and volcanic
activity.
Aim of an engineering geological map:
 On engineering geological maps, of all types and scales, information
provided should be presented in such a way that not only the true nature but
also the engineering significance of the data can be understood and fully
appreciated.

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