Engineering Geology Lecture 3

Chapter 3
Hazardous earth processes and Engineering works
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Chapter Outline
Introduction
Landslide and its impact on the ground
Earth quake hazard
Volcanic hazards
Land Subsidence and Collapse
3

At the end of this chapter
Students able to understand different geohazard
Students able to identify and mapping geohazard
Students able to perform hazard/susceptibility
mapping and seeking appropriate engineering
solutions
4

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

7
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.

8
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.

10
(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.
 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.

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(d) Volcanic activities is the upcoming of materials from the
interior part of the earth.
 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.

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Slope Failures/ Landslides
 What are landslides ?
 What causes landslides ?
 How do we recognize landslides ?: active, dormant,
potential?
 How do we analyze landslides?
 How do we remedy a landslide ?

Definition of a Landslide
Landslides are rock, earth, and/or debris flowing or
sliding down slopes due to gravity
The slope could be natural
or man-made, in excavation or fill
in soil or rock

Natural Hill Slopes
Cut Hill Slopes
Man-made Embankments
Excavations/Trenches In
Natural Ground

The Failure Plane (Surface of Rupture)
The failure plane may or may not be circular
Slope failures in rock and soil are generally different
– in soils the failure plane generally passes through the
soil mass
– in rocks the shear plane is generally along the
discontinuities (joints)

Types of Landslide
16
b) Translational slides occurs
when the
failure surface is
approximately flat or slightly
undulated
a) Rotational slides move along
a surface
of rupture that is curved and
concave.

D) Toppling occurs when one
or more
rock units rotate about their
base and
Collapse.
C) Fall:
Free falling of detached
bodies of bedrock
(boulders) from a cliff or
steep slope
Types of Slope Movements
C. FALL
D. TOPPLE

Slope Movements…
E. Various Flows
Mudflow in Ineget area ,
Ethiopia
Debris flow

Types of Slope Movements
F. Lateral Spread
Lateral spreading occurs when the
soil mass spreads laterally and this
spreading comes with tensional cracks in
the soil mass.

20
Creep is extremely slow
downward movement of
dry surfacial matter.
Movement of the soil occurs
in regions which are
subjected to freeze-thaw
conditions. The freeze lifts
the particles of soil and
rocks and when there is a
thaw, the particles are set
back down, but not in the
same place as before.
It is very important for CEs
to know the rate of
movement
Soil Creep

Mechanism of slope failures
• There are main types of slope failure mechanism which are:
• Rotational failure
• Wedge failure
• Toppling failure
• Plane failure
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States of Landslide
An active landslide is currently moving
A suspended landslide has moved within the last twelve months
but is not active at present
A reactivated landslide is an active landslide that has been inactive
An inactive landslide has not moved within the last twelve months
Inactive landslides can be subdivided into these states:
A dormant landslide is an inactive landslide that can be reactivated
by its original causes or other causes
An abandoned landslide is an inactive landslide that is no longer
affected by its original causes
A stabilized landslide is an inactive landslide that has been
protected from its original causes by artificial remedial measures
A relict landslide is an inactive landslide that developed under
geomorphological or climatic conditions considerably different
from those at present 24

Sliding Block Experiment
Increasing the normal force increases the resistance to
sliding and vice versa
N
S
T
N
S
T
N
R
T = N tan ø + C T/N = tan ø
ø

T = N tan Ø + C Mohr- Coulomb Equation
T = RESISTANCE
N = NORMAL FORCE / STRESS
Ø = ANGLE OF INTERNAL FRICTION
C = COHESION INERCEPT
FACTOR OF SAFETY (FOS) =
RESISTING FORCE
DISTURBING FORCE
N tan Ø + C
S
FOS =
FOS > 1; SLOPE IS STABLE
FOS < 1; SLOPE IS UNSTABLE
FOS = 1, ??

Sliding Block on Slope
N
S
T
N = W cos θ
θ
W
S = W sin θ
T = N tan Ø + C = (W cos θ) tan Ø + C
N tan Ø + C
S
FOS =
FOS =
(W cos θ) tan Ø + C
W sin θ

Effect of Water on Slope Stability
Water induces an uplift pressure (buoyancy)
U = Pore water pressure
partially saturated soils
fully saturated soils
water flow in soils (seepage)
Water in soil: is it beneficial or a detriment ? experiment
with sand
FOS =
(W cos θ-U) tan Ø + C
W sin θ

What Causes Landslides
Factor of Safety =
Decrease in Resisting Force
Increase in Destabilizing Force
Resisting Force
Destabilizing Force

Increase in Destabilizing Force
Earthquakes
Loading at the Top
Seepage Pressure
EARTHQUAKE
FORCE

Toe Cutting
Toe Erosion
`
Steepening of Slope

Increase in Pore Water Pressure
Decrease in Negative Pore Water Pressure
U
U
FOS =
W sin θ
N N
FOS =
(W cos θ-(-U) tan Ø + C
W sin θ

Draw Down
Changes in Soil Properties With Time
Deforestation
C
C

Recognizing Landslides
Active landslides
– most easily recognized
– tell-tale signs
Inactive (dormant) landslides
– more difficult to recognize
– some old tell-tale signs exist
Potential landslides
– most difficult to recognize
– presence / absence of contributing factors
– require detailed study
– experience of region

Indicative Signs of Landslides
Tension cracks
Depression at top (water ponding)
Bulging at toe

Water Seepage (Generally at Toe)
Tilted and crooked trees
Change in vegetation
roots sheared

Change in topography
Change in drainage pattern

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F.C. Dai et al.
(2002)
landslide Hazard Mapping Methods and Procedures

Data Needs for Landslide hazard Mapping
Data which required for LHZ is
different for different methods, for
instance, Statistical analysis methods
will need:
– Topographic map
– Geologic map
– Data of seismic history
– Satellite / google earth image
– Hydrologic record data
– Landslide inventory map
– Land use /land cover map
– Slope map
– Slope aspect map
– Curvature map
– Soil map
– Elevation map
– Geomorphology
Geotechnical approach will
need : soil thickness, soil
strength, unit weight,
groundwater pressures, slope
geometry etc.
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landslide Hazard Zonation Procedure
Three key assumptions are used to
LHZ preparation such as
– Identified landslides and factors
of LS
– Estimate relative significance of
the factors
– Estimate future slope instability
based on factors of past LS
Soft wares for landslide hazard
mapping
GIS, Excel, SPSS, GeoStudio, and
Google Earth
To prepare LHZ the following
skills are important :
RS & GIS analysis techniques
Data collection & RS data type
selection
LS inventory map preparation
Image processing &
interpretation
Data interpretation &
verification11/6/2019 40

1. Qualitative(a heuristic) assessment general
steps:
– Preparing casual factor data layer (GIS
– Layer)
– Normalization using internal
weighting of the causal factors
– Generation of weighted normalized
data layers
– External weighting of each criteria-
data layer
– Generation of a hazard index,
– Classification of the area
2. Quantitative (a statistical)
assessment general steps:
– Recognizing & collecting of
the causal factors & landslides
– Calculating the statistical
parameters
– Determining the weights for
causal factors
– Sum all weighted causal
factors in GIS.
– Classifying the area
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landslide Hazard Zonation Procedure cont..

Information value & Logistic regression methods
Z = b0 + b1x1+b2x2 + b3x3…bnxn
N
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Flow Charts for Slope Stability Analysis using Limit equilibrium method
n
Limit equilibrium
method

Remediation of Landslides
DISTRESS
SITE RECONNAISSANCE
UNDERSTAND MECHANISM
UNDERSTAND CAUSE
GEOTECHNICAL INVESTIGATIONS
ANALYSIS
STUDY POSSIBLE ALTERNATIVE SOLUTIONS
CHOOSE BEST OPTION
THERE IS NO TYPICAL SOLUTION

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Stabilization/mitigation methods
What are the remedies or measures specific to reducing
the risks associated with:
 Slope failures?
 Earthquakes?
 Ground subsidence, heave, etc?
 Volcanic activity?

Factor of safety =
To increase the factor of safety
– increase the resisting force
– decrease the destabilizing force
resisting force
destabilizing force

Increase the Resisting Forces
Load the toe
Provide resisting support
– retaining walls
Buttress
concrete or masonry
piled

Afforestation
Increase soil resistance
– additives
– grouting
– chemical treatment
C

Reinforcement

Decrease the destabilizing forces
Flatten the slope
Reduce surface and sub-surface water in the slopes
U
U
FOS =
W sin θ
•interceptor drains
•berm drains
•toe drains
•horizontal drains
•sub-surface/toe drains

Decrease the Destabilizing Forces
Reduce load at the top
Toe protection
Surface protection

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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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Surface protection of slope
• Slopes in soft rock or soil are prone to serious erosion during heavy
rain and some rock slopes suffer form deterioration due to
weathering when exposed.
• Local conditions and the types of materials will generally determine
the measures which are taken on any particulars site.
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 Vegetation cover is one of the most common methods of surface
protection in shallow slope stability. This can be by;
 By reducing the pore water pressure through evapotranspiration,
 Intercept direct impact of precipitation and reducing the effective
surface area to reduce percolation.
 The plant roots tightly strengthen the underlying soils.

5411/6/2019
Slope reinforcement method
• Rock bolt in rock slopes; rock bolt is a long anchor bolt, for
stabilizing rock excavations, which may be used in tunnels or rock
cuts. It transfers load from the unstable exterior, to the confined (and
much stronger) interior of the rock mass.
• Shotcrete : For the slopes having conditions prone to rapid
weathering and break down upon exposure, use of pneumatically
applied mortar or shotcrete is very effective.
• Gabions: The use of gabions can be considered for slope protection.
Gabions are rock filled wire baskets which are strong, heavy, flexible
and permanent.

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Shot
Crete
Rock bolt
Gabion

Landslide Hazard in Ethiopia
4
• landslide has been a frequent
problem in Ethiopia spatially in the
high land parts of north, south,
western and rift escarpment valley
(Ayele et al.,2014).
• Over 700 landslide sites recorded in
Ethiopia; mostly affecting rural
communities, infrastructures, farm
lands, dwelling houses
(KifleWoldearegay,2013).
Earth slide along Jimma-Agaro road
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Landslide type, factors ,distribution and effects in Ethiopia
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
rock fall& toppling have little
association with rain fall
Kifle Woldearegay(2013)
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Along Shire-May Tsebri
road
Tarmaber area,
Feresmay area
Jimma
area
Mush area
(Debreberhan)
Adishu
area

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

Landslide distribution in high land of Ethiopia
The following areas are subjected for landslide
hazard because of its complex
geomorphological, hydrological, and
geological setting.
Desie town, norther highland of
Ethiopia
Abay Gorge centeral highland of
Ethiopia
Jima basin central highland of
Ethiopia
Goffa, gilgel gibe II & sodo shone
area(southern Ethiopia )
Tekeze basin northern Ethiopia
Paleozoic glacial & post glacial
sediments in Tigray , northern
Ethiopia
Tarmaber & surrounding area in central
highland of Ethiopia
In different Welo area northern highland
of Ethiopia
Wondogenet area(southern Ethiopia )
Figure 7. Locations
of landslide-
affected areas in
relation to the
rainfall distribution
(mean
annual rainfall in
mm) in Ethiopia.
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Landslide effects in the high land of Ethiopia
landslides in the highlands of Ethiopia indicated that high hazards to economic, social and environmental
significance. E.g.
Researcher Magnitude of
damage
Year of
occurrence
Damaged Elements
• L. Ayalew (1999) >100km
>200
>500ha
300
1993 - 1998 • Asphalt road
• Dwelling House demolished
• Lands
• Human lives climbed
• Woldearegay (2005) 135
3500
$ 1.5M US Dollar
1998 - 2003 • human lives have been lost,
• people were displaced and
• an estimated worth property has been damaged
• Ayenew and Barbieri (2005) • landslides in Dessie town have been affecting
roads, buildings, pipe-lines, and other
infrastructures in the town.
• (Woldeaegay, 2008; Atsbeha,
2008; Schmeider et al., 2008)
• >3000
• 1250
• 4
• 4
• over 1500 ha
Landslide in tarmaber
• people were displaced;
• dwelling houses were demolished;
• Churches, Mills, and one elementary school were
destroyed;
• farm land was devastated.
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Bonga-Mizan road
Landslide effects in the high land of Ethiopia

2. Landslide damage
A) in Tora Meda
B) in Getesemane
C) in Yewaye
D) in Angot
E) in Ineget
F) in Sekela and G) in Debre
Yakob
n
Landslide Effects in the high land of Ethiopia

10. Some examples of of landslide hard zonation in different part of Ethiopia
Researcher LHZ/LSZ methods location Identified Factors
Tenalem Ayenew a, Giulio
Barbieri
b(2004)
Semi quantitative
approach
VES
in the city of
Dessie
• Geology ,topography,geomorphology,geothechnical
properties of soil & rocks
• Hydro metrological conditions
• Bekele et al (2009) Review & field
survey
Desie,Tigray,
wondogenet
& Blue Nile
gorge
• geological and hydrological conditions, human
interference , springs and rain fall
Birhanu Erimias 2014 slope stability
susceptibility
evaluation
parameter (SSEP)
route from
Alem ketema
– Ambat
village, north
Shewa
• intrinsic parameters: slope geometry, slope
material, structural discontinuities, land use/ land
cover & groundwater
• external triggering factors : seismicity, rainfall &
manmade activities
• B. Temesgen I**, M. U.
Mohammed’ and T. Korme’
(2001)
GIS environmental
modelling and
statistical approach
Wondogenet Lithology,structure, slope aspect,watercourse,vegetation
biomass
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Earth Quake
An earthquake (also
known as a quake,
tremor or temblor) is
the perceptible shaking
of the surface of the
Earth, resulting from
the sudden release of
energy in the Earth’s
crust that creates
seismic waves.
64

Seismology-science
Seismograph-instrument
Seismogram-recorded data
Epicenter- point on the surface
Hypocenter (focus)- source of
earthquake
Focal depth- vertical distance from
epicenter to focus
Earthquake magnitude- amount of
energy
Earthquake intensity- amount of
damages
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Earth Quake
Earthquake Terminologies

Why Earthquakes are concentrated in plate
boundary?
67
 Usually, the rock is moving along large cracks in Earth's
crust called faults.
 Most earthquakes happen at or near the boundaries
between Earth's tectonic plates because that's where
there is usually a large concentration of faults.
 Movement along those faults can cause earthquakes
too

Earthquake Hazard
Factors that influence the seismic response at a site are:
Type and lithological composition of materials, especially
superficial deposits with a geotechnical behavior corresponding to that
of soils.
Thickness of sediments and the depth of substratum
Dynamic soil properties
Depth of water table
Surface and substratum morphology
Presence of faults, their situation and characteristics.
The building designs and construction materials
Magnitude of the quake,
The distance from the epicentre
Depth of focus
Density of the population11/6/2019 68

1. Ground Shaking or Ground Motion
The earth shakes with the passage of earthquake waves,
which radiate energy that had been “stored” in stressed
rocks, and were released when a fault broke and the rocks
slipped to relieve the pent – up stress
If an earthquake generates a large enough shaking
intensity, structures like buildings, bridges, and dams can
be severely damaged, and cliffs and sloping ground
destabilized. Perched or stacked object may fall and
injure or bury anyone close by.
Ground shaking will vary over an area due to such
factors as topography, bedrock type, and the location
and orientation of the fault rupture.11/6/2019 69
Earthquake Hazard

1. Soft first story/inadequate shear strength
Column failure/ broken joints/too
much weight for building design or
construction
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Mid-story collapse, Kobe earthquake
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2. Ground or Surface Rupture
 Surface rupture is an offset of the ground surface
when fault rupture extends to the Earth’s surface.
 Any structure built across the fault is at risk of
being torn apart as the two sides of the fault slip
past each other.
 Normal – and reverse – (collectively called dip –
slip) faulting surface ruptures feature vertical
offsets while strike – slip faulting produces lateral
offsets.
 Many earthquake surface ruptures are combinations of
both. Structures that span a surface fault are likely to
suffer great damage surface ruptures
Earthquake Hazard

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3. Liquefaction
 Soil liquefaction is a phenomenon in which the strength
and stiffness of a soil is reduced by earthquake shaking or
other rapid loading.
 It normally occur in saturated soils, that is, soils in which
the space between the individual particles is completely
filled with water.
 Prior to an earthquake, the water pressure is relatively
low – the weight of the buried soil rests on the framework of
grain contacts that comprise it.
 However, earthquake shaking can disrupt the
structure, the soil particles no longer support and all
the weight, and the groundwater pressure begins to
rise.
Earthquake Hazard

75
4. Earthquake – induced ground subsidence and lateral
spreading
 Subsidence, or lowering of the ground surface, often
occurs during earthquakes. This may be due to
downward vertical displacement on one side of a fault,
and can sometimes affect a huge area of land.
 Coastal areas can become permanently flooded as a
result.
 Subsidence can also occur as ground shaking causes
loose sediments to settle and to lose their load bearing
strength or to slump down sloping grounds.
 Lateral spreading occurs where sloping ground starts
to move downhill, causing cracks to open up, that are
often seen along hill crest and river banks.
Earthquake Hazard

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5. Tsunami
 A tsunami also known as a seismic sea wave,
is a series of waves in a water body caused by the
displacement of a large volume of water, generally in an
ocean or a large lake.
 Earthquakes, volcanic eruptions and other
underwater explosions, landslides, glacier
caving, meteorite impacts and other
disturbances above or below water all have
the potential to generate a tsunami.
Earthquake Hazard

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Earthquake Hazard
6. Earthquake – induced landslides
 Landslides are frequently triggered by strong ground
motions.
 They are important secondary earthquake
hazards.
 The term landslide includes a wide range of ground
movement, such as rock falls, deep failure of slopes, and
shallow debris flows. However, gravity acting on a steep
slope is the primary reason for all landslides.
 Strong earthquake- induced ground shaking greatly
increases the likelihood of landslides where landscape is
susceptible to these types of ground failure.
 If the ground is saturated with water, particularly following
heavy rainfall, the shaking will result in more landslides
than normal

Earthquake hazards in Ethiopia
Ethiopia is situated along main African rift system along which active
volcanic and earthquake have been taking place.
Due to its location right on one of the major tectonic plates in the
world, earthquakes have been a fact of life in Ethiopia for a very long
time.
Urbanization is increasing in Ethiopia and infrastructures are
constructing without consideration of earthquake for a long period.
The effects of earthquake depends on the soil thickness, soil wetness,
construction materials and distance from the source.
Based on these factors, buildings in Ethiopia are susceptible to
earthquake destruction.
Some record data shows that, Ethiopia was face difference earthquake
magnitude in the past history.
These earthquake are distributed along the rift valley.
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Location Year Magn
Distance of
Epicenter from
Addis (kms) **
Damage
• Langano 1906 6.8 110 Felt as far as Addis.
• Kara Kore 1961 6.7 150
Town of Majete destroyed.
Kara Kore seriously
damaged.
• Central Afar
Area
1969 6.6 Town of Serdo destroyed.
• Wendo Genet 1983 300
• Langano 1985 6.2 110
• Rift Valley
Area
1987 6.2 200
Widely felt and widely-
spread damage.
• Dobi [Central
Afar]
1989 6.3 200 Several bridges damaged.
• Adama 1993 6.0 (?) < 100
Injuries and damage in
Adama Also felt in Bishoftu
and Addis Ababa.
• Lake Shala -
Adamitulu
1999 250
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To reduce this effect seismic building code is prepared in 1983.
The Ethiopian building code standard (EBCS) has a series of ten
standard to design building under different construction materials.
– EBCS-1-95- Wind analysis and design of buildings
– EBCS-2-95 - Structural Use of Concrete.
– EBCS-3-95 - Design of Steel Structures.
– EBCS-4-95 - Design of Composite Steel and Concrete
Structures.
– EBCS-5-95 - Utilization of Timber.
– EBCS-6-95 - Design of Masonry Structures.
– EBCS-7-95 - Foundations.
– EBCS-8-95 Earthquake analysis and design of buildings.
– EBCS-9-95 - Plumbing Services of Buildings.
– EBCS-10-95 - Electrical Installation of Buildings.
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Based on the susceptibility of the area to damage when earth quake
takes place, Ethiopia is zoned into five seismic zones.
This zoning is used to develop seismic building code standard;
which is a guide to design a safe building which can withstand a
seismic effects.
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What to do?
Much of the risk may be mitigated by:
Implementation of Early Warning Systems.
Improvement and implementation of building
codes.
Fault mapping.
Hazard mapping used to selecting less susceptible
area using hazard zonation map
82

Early Warning System
In many cases, the magnitude of an impending
earthquake may be estimated a few tens of
seconds prior to the arrival of the destructive
ground motion to the populated area. In such
cases, many lives may be saved simply by:
• Shutting down power supplies.
• Shutting down of nuclear reactors.
• Stopping or reducing the speed of fast trains.
30

Building codes
• Earthquake don’t kill - buildings do (but
also tsunami).
• Earthquake resistant construction costs
only 10% more than nonresistant
construction.
• Structures in CA built after 1976, when a
new building code was implemented,
suffered very little damage in the 1989 Loma
Prieta and the 1994 Northridge.
• The implementation gap.
31

Fault mapping
* Earthquakes occur on faults.
Yet, the location of some major faults is unknown.
32

How would you mitigate damages from….
•
•
•
•
Ground Rupture
Liquefaction
Ground Shaking
Tsunami
48

87
Ground Rupture
 Avoid construction
 Relocate sensitive facilities
 Implement low use facilities
–Playing fields
–Green space
Liquefaction
• Recognize liquefaction potential
• In-situ remediation
• Avoid construction in liquefaction prone areas

Tsunami Mitigation
Early warning system
– Broadcast signal to beaches after a major
earthquake anywhere in the ocean basin
Safety guidelines
–Go to high ground
–Climb a tree
Ground Shaking
Recognize the degree of probable ground shaking in
the area
Improve construction methods to accommodate
shaking without collapse
88

Earthquake Hazards
Ground Shaking
55

Volcanic hazards
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What type of hazards can you think of associated with a
volcanic eruption?

Volcanic hazards
Primary Effects
 lava flows
 pyroclastic eruptions
 poisonous gas emissions
 Collapse/ subsidence
Secondary Effects
– mudflows and debris avalanches
– flooding (glacial outburst floods)
– Tsunamis
– Seismicity
– atmospheric effects and climate
change
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Lava flow
Lava is molten rock that flows out of a volcano or volcanic vent.
Depending on its composition and temperature, lava can be very fluid or
very sticky (viscous).
Hazards of most lava that flow with slow speed can easily avoided, but a
lava flow usually cannot be stopped or diverted. Because lava flows are
extremely hot - between 1,000-2,000°C (1,800 - 3,600° F) - they can
cause severe burns and often burn down vegetation and structures.
Lava flowing from a vent also creates enormous amounts of pressure,
which can crush or bury whatever survives being burned.
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Pyroclastic fall/ flow
Pyroclastic falls occur when tephra (
fragmented rock) of variable size is
ejected from a volcanic vent during an
eruption and falls to the ground some
distance away from the vent.
The fall can destruct human and
infrastructures by burying.
Pyroclastic material injected into the
atmosphere may have global as well as
local consequences.
When the volume of an eruption cloud is
large enough, and the cloud is spread far
enough by wind, pyroclastic material may
actually block sunlight and cause
temporary cooling of the Earth's surface
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Lahars
Lahars are a specific kind of
mudflow made up of volcanic
debris.
Lahars flow like liquids, because
they contain suspended material
Lahars can travel at speeds of
over 80 km/h.
Lahars are not as fast or hot as
other volcanic hazards, but they
are extremely destructive.
They will either bulldoze or bury
anything in their path
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Poisonous volcanic gas
Most of the gas released in an eruption is water vapor (H2O), and
relatively harmless, but volcanoes also produce carbon dioxide
(CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), fluorine gas
(F2), hydrogen fluoride (HF), and other gases.
All of these gases can be hazardous - even deadly - in the right
conditions.
Secondary Effects
– mudflows and debris avalanches
– flooding (glacial outburst floods)
– Tsunamis
– Seismicity
– atmospheric effects
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How Can We Get Information about Going Volcanic
eruption?
1. Volcanos give signs like
Very small earth quake beneath the
volcano
Inflation or swelling of the volcano
Increased emission of heat and gas from
the vents on the volcano
2. Using deformation monitoring
Tilt meters are used to measure the
deformation of the volcano
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Volcanic Hazards- Management
 Volcanic Management falls in to :- P r e d i c t i o n a n d
P r e v e n t i o n
 The vast power and scale makes prevention an almost
impossible task.
 However prediction and disaster preparedness can reduce
the risk and consequence of eruptions
 Volcanologist use:-
 Remote sensing (heat anomalies)
 Seismic Monitoring (Harmonic tremors)
 Gas analysis for example changes in S02 levels

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Volcanic Hazard in Ethiopia

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Volcanic Hazard in Ethiopia

Collapse and Subsidence
These processes refer to vertical movements of material, in general
terms sudden movements or collapse and slower movements or
subsidence.
Collapse of underground cavities in rock, which may or may not be
reflected at ground level.
Shallow collapse in rock or soil that reaches ground level.
Subsidence or slow, gradual lowering of the ground level.
In the first case, movements tend to occur as a result of the collapse
of the ceiling of underground cavities, when the strength of the
overlying rocks is not sufficient to bear the stresses upon them.
The behavior of the material is brittle and their failure is violent.
Whether the collapse affects ground level or not depends on the
strength and geotechnical characteristics of the overlying material
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Collapse
Collapse depends on the following factors:
Volume and shape of the cavities.
Thickness of the overburden(depth of the cavity itself).
Strength and mechanical behavior of the overlying materials.
Natural cavities or caves are associated with karstic or soluble
materials, such as carbonates and evaporate rocks, where
dissolution process creates caves.
When certain dimensions are reached, these generate a
disequilibrium or instability causing the failure of the roof or
ceiling of the cavity.
If the strength is low, then the ground surface will collapse.
Caves are also formed in volcanic materials.
The surface results of karstic collapse are sinkholes, although these
may also be generated by the gradual solution of surface rocks or
by subsidence in the soft soils covering karstic materials.
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Subsidence
Subsidence is generally a very slow process although it is often accelerated by
human activity.
It can affect all ground types, generally soils and is due to ground stress changes
caused by the following:
– Lowering of the water table
– Underground mining and tunnels
– Extraction of petroleum or gas
– Intensive aquifer exploitation
– Slow process of material dissolution
– Morph tectonic and sedimentation processes
– Consolidation in soft and organic soils.
– A drawdown of the water table, in periods of drought or because of pumping from
aquifers, affects unconsolidated materials, which undergo changes in their state of
stress as a result of decreasing water pressures.
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Investigation of Collapse or Subsidence
Surveys to evaluate the likelihood of movement in a given zone
should be done to identify:
 Lithology liable to suffer settlement or collapse from natural
processes: carbonate rocks, gypsum, salts etc.(karstic and
saline materials in general) and areas with underground
cavities and collapse that reach ground level.
 Soft and deformable lithology.
 Areas with natural or man made processes which may trigger
subsidence.
• Detecting underground cavities may be difficult depending on their size and
depth. Geophysical and borehole methods are the most effective means of
detection. 113

Corrective Measures
As it is impossible to avoid large scale collapse and
subsidence , measures to mitigate their effect should be based
on their prevention.
Cavities up to a certain volume should be filled once their
volume and depth is known, ensuring that subsidence is no
longer active.
Subsidence may be prevented and controlled by acting on the
processes which cause it.
Ground can recover and return to the initial equilibrium
conditions if it has not exceeded its elastic deformation, as in
the case of ground affected by drawdown of the water table.
Subsidence due to underground excavations can be countered
by jet grouting and ground consolidation treatment.
114

115
Ground shearing from
unequal subsidence
of recent sediments
over an uneven
bedrock surface
Development of
sinkholes by the
collapse of soil
arches in pinnacle
weathered
limestone as a
consequence of
the reduction of
the groundwater
table (from A to B)

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Collapse of limestone cavities can result from:
 Increase in arch span from cavity growth until the strength is
insufficient to support the overburden weight.
 Increase in overburden weight over the arch by increased
saturation from rainfall or other sources, or from groundwater
lowering, which removes the buoyant force of water.
 Entry of granular soils by raveling into a cavity near the rock
surface.
 Applications of load to the surface from structures, fills, etc.

117
Recent sink, not shown in aerial photos dated 1969 (Figure 10.19). Photo taken in
1972. (From Hunt, R.E., Bull. Assoc. Eng. Geol., 10, 1973. With permission.)

Shrinkage, settlement and expansive hazards
The Soil contracts due to
its clay minerals and the
structure of the clay
allowing water to be
imbedded in-between the
clay layers
Process is reversible, and
causes contraction of the
soil.
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• This processes is takes place due to change in moisture content.
• The process develop cracking and failure on engineering
structures built on them.

What can be done to reduce the effect?
Test soil before building
If expansion is greater then 10 %, it is critical
Remove soil
Mix soil with material that does not expand
Keep consistent soil moisture
Have strong foundations in buildings that can handle
the changes in volume.
Read in detail
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Engineering Geology Lecture 3

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Engineering Geology Lecture 3