Download QUS 112 Intro Engineering Geology - Unesco

UNESCO
UNESCO-NIGERIA TECHNICAL &
VOCATIONAL EDUCATION
REVITALISATION PROJECT
PROJECT-PHASE II
NATIONA DIPLOMA IN
QUANTITY SURVEYING
INTRODUCTION TO ENGINEERING GEOLOGY
COURSE CODE: QUS 112
THEORY
YEAR 1 – SEMESTER II
1
Version 1: February 2009.
INTRODUCTION TO ENGINEERING GEOLOGY
TABLE OF CONTENTS
WEEK 1
GEOLOGICAL FORMATION OF THE EARTH
1.1
Evolution of the Universe.
1.2
The Big Bang Theory.
1.3
Formation of the solar system.
1.3.1
Facts of the solar system.
1.3.2
Current theory on the formation of the solar system.
1.3.2.1 Nebular Hypothesis.
1.3.2.2 Protoplanet Hypothesis.
SELF ASSESMENT TEST
WEEK 2
ROCKS
1.1 What is Rocks?
I.2 The Rock Cycle.
1.3 The Engineering Definition of Rock.
1.4 Basic Mineralogy of Rocks.
1.5 Mineralogy Identification for Engineering Purposes.
1.6 Rock Identification.
1.7 Rock Properties for Engineering.
1.8 Format for Descriptions of Rock.
SELF ASSESMENT TEST
2
WEEK 3
MINERALS
1.1 Mineral
1.1.1 Mineral Definition and Classification.
1.1.2 Mineralogy Identification.
WEEK 4:
EROSION
1.1 Definition.
1.2 Rate of Erosion.
SELF ASSESMENT TEST
WEEK 5
EARTHQUAKES
1.1 Focus.
1.2 Epicentre.
1.3 Intensity.
1.4 Magnitude.
SELF ASSESMENT TEST
WEEK 6
PLATE TECTONICS
1.1 Definition
1.2. Plate boundaries.
1.3 Earthquakes and Plate Tectonics.
SELF ASSESMENT TEST
WEEK 7
BASIC CONCEPT OF STRUCTURAL GEOLOGY
1.1 Strike & Dip.
1.2 Joints.
1.3 Cleavage.
3
1.4 Fold.
1.5 Fault, in Geology.
SELF ASSESMENT TEST
WEEK 8
GEOLOGICAL MAP INTERPRETATIONS
1.1 Definition
1.2 Responsibilities of the Engineering Geologist
SELF ASSESMENT TEST
WEEK 9
WEATHERING AND EROSION
1.0 Definition.
1.1 Primary Controls on Weathering.
1.2 Secondary Controls.
1.3 Types of weathering.
1.4 Rates of weathering.
1.5 Products of weathering.
2.0 Erosion.
SELF ASSESMENT TEST
WEEK 10
GEOLOGICAL FACTORS AFFECTING STABILITY OF SLOPES.
1.1 Conditions Which Affect Stability of the Rock.
SELF ASSESMENT TEST
WEEK 11
GEOLOGICAL FACTORS AFFECTING CUTTING &
EMBANKMENTS.
SELF ASSESMENT TEST
4
WEEK 12
GEOLOGICAL CONDITIONS AFFECTING IMPOUNDED
SURFACE WATER (RESEVOIR & DAMS).
1.1 Geological Characters for Investigation.
1.2 Geology of the Site.
1.3 Engineering Properties of Rocks.
1.4 The Geological Condition That Affect Dams
SELF ASSESMENT TEST
WEEK 13
GEOLOGICAL CONSIDERATION IN TUNNELING AND
DRILLING
1.1 Geological Considerations in Tunnelling.
SELF ASSESMENT TEST
WEEK 14
SOIL
1.1 Definition
1.2 Soil Formation.
1.3 Terminologies for soil.
SELF ASSESMENT TEST
WEEK 15
GEOLOGICAL CONSIDERATION IN FOUNDATIONS
1.1 Foundations.
1.2 Geological Conditions.
SELF ASSESMENT TEST
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WEEK 1:
1.1
GEOLOGICAL FORMATION OF THE EARTH
Evolution of the Universe
About 11 to 15 billion years ago all of the matter and energy in the Universe was
concentrated into an area the size of an atom. At this moment, matter, energy, space and
time did not exist. Then suddenly, the Universe began to expand at an incredible rate and
matter, energy, space and time came into being (the Big Bang). As the Universe
expanded, matter began to coalesce into gas clouds, and then stars and planets. Our solar
system formed about 5 billion years ago when the Universe was about 65 % of its present
size today, the Universe continues to expand.
1.2
The Big Bang Theory
About 15 billion years ago a tremendous explosion started the expansion of the universe.
This explosion is known as the Big Bang. At the point of this event all of the matter and
energy of space was contained at one point. What exhibited prior to this event is
completely unknown and is a matter of pure speculation. This occurrence was not a
conventional explosion but rather an event filling all of space with all of the particles of
the embryonic universe rushing away from each other. The Big Bang actually consisted
of an explosion of space within itself unlike an explosion of a bomb were fragments are
thrown outward. The galaxies were not all clumped together, but rather the Big Bang lay
the foundations for the universe.
The origin of the Big Bang theory can be credited to Edwin Hubble.
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1.3
Formation of the solar system
1.3.1 Current theory on the formation of the solar system
1.3.1.1 Nebular Hypothesis:
In this theory, the whole Solar System starts as a large cloud of gas that contracts under
self-gravity. Conservation of angular momentum requires that a rotating disk form with a
large concentration at the centre (the proto-Sun). Within the disk, planets form.
7
The Nebular Hypothesis in its original form was proposed by Kant and Laplace in the
18th century.
8
Summary
Here is a brief outline of the current theory of the events in the early history of the solar
system:
1. A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed and
collapses under its own gravity. The disturbance could be, for example, the shock
wave from a nearby supernova.
2. As the cloud collapses, it heats up and compresses in the center. It heats enough
for the dust to vaporize. The initial collapse is supposed to take less than 100,000
years.
3. The center compresses enough to become a protostar and the rest of the gas
orbits/flows around it. Most of that gas flows inward and adds to the mass of the
forming star, but the gas is rotating. The centrifugal force from that prevents some
of the gas from reaching the forming star. Instead, it forms an "accretion disk"
around the star. The disk radiates away its energy and cools off.
4. First brake point. Depending on the details, the gas orbiting star/protostar may be
unstable and start to compress under its own gravity. That produces a double star.
If it doesn't ...
5. The gas cools off enough for the metal, rock and (far enough from the forming
star) ice to condense out into tiny particles. (i.e. some of the gas turns back into
dust). The metals condense almost as soon as the accretion disk forms (4.55-4.56
billion years ago according to isotope measurements of certain meteors); the rock
condenses a bit later (between 4.4 and 4.55 billion years ago).
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6. The dust particles collide with each other and form into larger particles. This goes
on until the particles get to the size of boulders or small asteroids.
7. Run away growth. Once the larger of these particles get big enough to have a
nontrivial gravity, their growth accelerates. Their gravity (even if it's very small)
gives them an edge over smaller particles; it pulls in more, smaller particles, and
very quickly, the large objects have accumulated all of the solid matter close to
their own orbit. How big they get depends on their distance from the star and the
density and composition of the protoplanetary nebula. In the solar system, the
theories say that this is large asteroid to lunar size in the inner solar system, and
one to fifteen times the Earth's size in the outer solar system. There would have
been a big jump in size somewhere between the current orbits of Mars and
Jupiter: the energy from the Sun would have kept ice a vapor at closer distances,
so the solid, accretable matter would become much more common beyond a
critical distance from the Sun. The accretion of these "planetesimals" is believed
to take a few hundred thousand to about twenty million years, with the outermost
taking the longest to form.
8. Two things and the second brake point. How big were those protoplanets and how
quickly did they form? At about this time, about 1 million years after the nebula
cooled, the star would generate a very strong solar wind, which would sweep
away all of the gas left in the protoplanetary nebula. If a protoplanet was large
enough, soon enough, its gravity would pull in the nebular gas, and it would
become a gas giant. If not, it would remain a rocky or icy body.
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9. At this point, the solar system is composed only of solid, protoplanetary bodies
and gas giants. The "planetesimals" would slowly collide with each other and
become more massive.
10. Eventually, after ten to a hundred million years, you end up with ten or so planets,
in stable orbits, and that's a solar system. These planets and their surfaces may be
heavily modified by the last, big collision they experience (e.g. the largely metal
composition of Mercury or the Moon).
1.3.1.2 Protoplanet Hypothesis:
The current working model for the formation of the Solar System is called the protoplanet
hypothesis. It incorporates many of the components of the nebular hypothesis, but adds
some new aspects from modern knowledge of fluids and states of matter.
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SELF ASSESMENT QUESTION
1. Explain the concept of the Big Bang theory.
2. Outline the events that occurred in the early history of the solar system according
to the Nebular hypothesis
12
WEEK 2
1.0
ROCKS
What Is A ‘Rock’?
In Geology, ‘Rock is defined as the solid material forming the outer rocky shell or crust
of the earth. There are three major groups of rocks by its origin:
(1)
Igneous rocks: cooled from a molten state; e.g., granite, basalt …;
(2)
Sedimentary rocks: deposited from fluid medium; the products of weathering of
other rocks in water; e.g., sandstone, mudstone…;
(3)
Metamorphic rocks: formed from pre-existing rocks by the action of heat and
pressure.e.g. Dolomite, marble …;
I.2
The Rock Cycle
Although we may consider rocks as "solid" and "permanent", in reality they are
constantly changing and transforming over geologic time. Volcanic (igneous) rocks at
the earth's surface are weathered, and the particles resulting from that weathering are
carried by streams into oceans, where they form sedimentary rocks such as sandstone and
shale. Those sedimentary rocks, in turn, may be subducted into the mantle, where they
are melted into magma, once again to rise towards the earth's surface to crystallize as
igneous rocks.
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The rock cycle summarizes the processes involved in the formation of rocks and the
modification of pre-existing rocks and is a good summary of Physical Geology. First
proposed
1.3
by
James
Hutton
(Father
of
Geology)
in
the
late
1700s.
The Engineering Definition of Rocks
Rock is the hard and durable material. By an excavation point of view, Rocks are the
earth materials that cannot be excavated without blasting. This definition clearly excludes
other kinds of earth materials such as soils, and glacial tills, etc. Here is another
engineering definition of rocks: The earth materials that do not slake when soaked into
water. For example, a thick loess deposit is regarded as rock geologically and regarded as
soil in engineering.
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1.4
Basic Mineralogy of Rocks
Rocks are formed with minerals. What is a mineral?
1.
A naturally occurring chemical element or compound.
2.
Formed by inorganic processes.
With an ordered arrangement or pattern for its atoms – crystalline structure. Possesses a
definite chemical composition or range of compositions.
1.5 Mineralogy Identification for Engineering Purposes
From an engineering point of view, certain properties of minerals, especially when they
are introduced into or encountered with another mineral, are of special concern to
engineers. For example, gypsum in a limestone can become swelling when water
presents; pyrite (the fool’s gold) in shale can be deteriorated by acid water; swelling clays
in shale can become wetting and cause instability problem of a slope. Thus, fundamental
mineralogical acknowledge is needed when identifying engineering material is needed.
1.6 Rock Identification
Rocks are identified mostly by its;
-
Texture.
-
Mineral composition.
-
Field relationships.
-
Color.
-
Hardness.
-
Specific weight.
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-
Crystal form.
-
Magnetism.
Apparently, some techniques used in identifying minerals can also be used to classify the
rock type.
1.7 Rock Properties for Engineering
Rocks are significant for two major reasons in engineering:
1.
As building materials for constructions.
2.
As foundations on which the constructions are setting;
For the consideration of rocks as construction material the engineers concern about:
a. Density to some extent (for calculating the weight, load to the foundation, etc.).
b. Strength.
c. Durability;
For the consideration of rocks as the construction foundation the geological engineers
concern about the rocks:
a.
Density.
b.
Strength.
c.
Compressibility;
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1.8 Format for Descriptions of Rock
Engineering geology rock descriptions should include generalized lithologic and physical
characteristics using qualitative and quantitative descriptors. A general format for
describing rock in exploration logs and legends on general note drawings is:
•
Rock unit (member or formation) name.
•
Lithology with lithologic descriptors composition (mineralogy) grain/particle size.
texture color.
•
Bedding/foliation/flow texture.
•
Weathering.
•
Hardness/strength.
•
Contacts.
•
Discontinuities (includes fracture indexes).
•
Permeability data (as available from testing).
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SELF ASSESMENT QUESTIONS
1. What is the major significance of rocks to engineering?
2. In considering a rock for construction and foundation, what should be
the major concern of an engineer?
3. Classify the major groups of rocks by their origin.
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WEEK 3:
MINERAL
1.1 Definition
A mineral is a naturally occurring solid formed through geological processes that has
a characteristic chemical composition, a highly ordered atomic structure, and specific
physical properties. A rock, by comparison, is an aggregate of minerals and need not
have a specific chemical composition. Minerals range in composition from pure
elements and simple salts to very complex silicates with thousands of known forms.
1.2 Mineral Definition and Classification
To be classified as a true mineral, a substance must be a solid and have a crystalline
structure. It must also be a naturally occurring, homogeneous substance with a defined
chemical composition.
‘’A mineral is an element or chemical compound that is normally crystalline
and that has been formed as a result of geological processes’’.
1.3 Mineralogy Identification
Minerals can be identified by its;
1.
Color.
2.
Streak (strip).
3.
Luster.
4.
Hardness.
5.
Specific weight.
6.
Cleavage.
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7.
Fracture.
8.
Crystal form.
9.
magnetism
10.
Tenacity.
11.
Diaphaneity.
12.
Striation.
13.
chemical reaction
Color
Minerals are colored because certain wave lengths of light are absorbed, and the color
results from a combination of those wave lengths that reach the eye. Some minerals
show different colors along different crystallographic axes. Mineral identification by
colors can be deceptive!!
21
Streak
The streak of a mineral is the color of the powder left on a streak plate (piece of
unglazed porcelain) when the mineral is scraped across it
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Luster
Luster refers to how light is reflected from the surface of a mineral. The two main
types of luster are metallic and non-metallic.
Types of non-metallic luster; adamantine, vitreous, pearly, greasy, silky, earthy
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Hardness
The hardness of a mineral is its “scratchability”, determined by Moh’s hardness scale.
The hardest mineral known, diamond, was assigned the number 10.
Specific Gravity
Specific gravity is the "heaviness" of a mineral. It is defined as a number that
expresses the ratio of the weight of a mineral and the weight of an equal volume of
water.
The specific gravity depends on:
-
The kind of atoms that comprise the mineral.
-
How the atoms are packed together.
-
Common rock-forming minerals (quartz, feldspar, calcite, etc.) have
specific gravity near 2.7
Cleavage
Cleavage is the ability of a mineral to break along preferred planes. Minerals tend to
break along certain planes where atomic bonds are weak. Minerals can have one, two
plane or three plane cleavages. Copper –none
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Crystal forms
Crystal forms are displays of well-formed crystal faces by a mineral. Crystal faces
formed during crystallization process vs. cleavage faces formed when mineral breaks.
Beryl -hexagonal-Diamond-octahedron
27
SELF ASSESMENT QUESTION
1. How are minerals classified?
2. List and explain the characteristics used in identifying minerals.
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WEEK 4:
EROSION
1.1 Definition
Erosion, removal of sediment, rock, and soil from the landscape, resulting in the
formation of new landforms and the lowering of the land surface, a process known as
denudation.
This is a rather broader term signifying the breakdown of rocks of the crust by such
natural agencies that are characterised with some motion as wind, running water and
moving ice. By virtue of their velocity, these agents not only gradually disintegrate
the rock but also remove the broken particles and fragments to far off distances
ranging from few metres to hundred and thousand of kilometres
During transportation the sediment further erodes the landscape by battering and
rubbing against the surfaces over which it passes. The fragments also knock against
each other, and break into smaller pieces.
Water is probably the most significant agent of erosion today, and rivers carry more
sediment from the land to the oceans every year than either ice or wind.
Wind is the most important agent of erosion in arid, desert climates, where
precipitation is low and vegetation sparse.
1.2 Rate of Erosion
The rate of erosion is highest where the action of water removes debris from the land.
However, there are other factors that must be considered in measuring rates of
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erosion, including climatic variations, wind speeds, vegetation cover, relief, and
geology.
Erosion is generally a gradual process. However, extreme events can remove large
amounts of weathered material, and reshape the landscape suddenly.
SELF ASSESMENT QUESTION
1. What are the factors to be considered in measuring the rate of erosion?
2. Define erosion and list the agents reasonable for causing erosion.
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WEEK 5:
EARTHQUAKES
Earthquakes are caused by the rupture and sudden movement of the Earth’s crust
along pre-existing weak zones. They are usually associated with the boundaries of
crustal plates. A quake's energy may be absorbed into the rock to deform its structure;
it may be released if the rock is brittle enough or if the movement is severe enough.
The breakage and subsequent motion is an earthquake.
Frequency and Location of Earthquakes: 90% of all earthquake activity is associated
with subduction zone activity. The Pacific basin is so prone to quake activity since the
Pacific Plate is being subducted along most of its margin. Sub ducted material will
produce quakes to a depth of 450 miles after which depth, temperatures and pressure
let the rocks respond to stress as plastics and tend not to be brittle enough to produce a
quake.
1.1 Focus: initial rupture point of the earthquake;
-
75% are shallow focus: within 40 miles of the surface.
-
19% are intermediate focus; between 40 and 185 miles of surface.
-
5% are deep focus: 185 to 450 miles of surface.
1.2 Epicentre: position on surface directly above the focus.
Measuring Earthquake Intensity and Magnitude: magnitude and intensity are scales
that are used to measure the degree of damage and energy that are produced during an
earthquake.
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1.3 Intensity: a qualitative assessment of the effects of the earthquake. Measure of the
degree of observable effects of the movement as well as damage that has occurred.
The amount of damage is determined by how much the ground shakes, how long the
quake lasts, how well the buildings are constructed and whether or not the building is
built deep into the ground or just on the top of the soil. Buildings that are built deep
into the ground down to the bedrock will probably not be as badly damaged.
Buildings with lots of metal, cement and glass will probably break. Buildings with
lots
of
wood
will
probably
shake
but
not
fall
down.
Magnitude: a quantitative measurement of the amount of energy released by the
quake. Each step on the Richter scale represents an increase of 10 times the shaking or
rock movement (amplitude) and an increase of 30 times the amount of energy
released. The difference between an earthquake that measures 6.5 on the Richter scale
and a quake that measures 7.5 is that the 7.5 releases 30 times more energy. It would
take 30 magnitude 6.5 quakes to equal the energy output of one 7.5 quake. That's a lot
of shaking! Even scarier that that, it would take 30 x 20, or 900 magnitude 6.5 quakes
to equal just one 8.5 quake.
The map below shows location of earthquakes around the globe. They are not evenly
distributed; the boundaries between the plates grind against each other, producing
most earthquakes. So the lines of earthquakes help define the plates
32
Fig. 1 – World map showing tectonically active zones (Earthquake prone zones).
SELF ASSESMENT QUESTION
1. What is responsible for an Earthquake?
2. Explain ‘intensity and magnitude’ of an Earthquake.
Eart
WEEK 6: PLATE TECTONICS
1.1 Definition
Plate tectonics is the study of the lithosphere, the outer portion of the earth consisting
of the crust and part of the upper mantle. The lithosphere is divided into about a dozen
large plates which move and interact with one another to create earthquakes,
mountain ranges,
s, volcanic activity, ocean trenches and many other features.
Continents and ocean basis are moved and changed in shape as a result of these plate
movements.
The sequence of maps below show how a large supercontinent, known as Pangaea
was fragmented into several pieces, each is part of a mobile plate of the lithosphere.
These pieces were to become Earth's current continents. The time sequence show
through the maps traces the paths of the continents to their current positions.
In the early 1960's, the related concepts of "sea-floor spreading" and "plate
tectonics" emerged as powerful new hypotheses that geologists used to interpret the
features and movements of the Earth's surface layer. According to the plate tectonics
theory, the Earth's surface consists of about a dozen rigid slabs or plates, each
averaging at least 50 miles thick. These plates move relative to one another at average
speeds of a few inches per year -- about as fast as human fingernails grow. Scientists
recognize three common types of boundaries between these moving plates:
1. Divergent or spreading -- adjacent plates pull apart, such as at the MidAtlantic Ridge, which separates the North and South American Plates from
the Eurasian and African Plates. This pulling apart causes "sea-floor
spreading" as new material is added to the oceanic plates.
2. Convergent -- plates moving in opposite directions meet and one is dragged
down (or sub ducted) beneath the other. Convergent plate boundaries are
also called subduction zones and are typified by the Aleutian Trench, where
the Pacific Plate is being sub ducted under the North American Plate.
3. Transform fault -- one plate slides horizontally past another. The best known
example is the earthquake-prone San Andreas Fault zone of California,
which marks the boundary between the Pacific and North American Plates.
35
1.2.1 Plate Boundaries
Plate boundaries are found at the edge of the lithospheric plates and are of three types,
convergent, divergent and conservative. Wide zones of deformation are usually
characteristic of plate boundaries because of the interaction between two plates. The
three
boundaries
are
characterized
by
their
distinct
motions.
The first sort of plate boundary is called a divergent boundary, or spreading centre. At
these boundaries, two plates move away from one another. As the two move apart,
mid-ocean ridges are created as magma from the mantle upwells through a crack in
the oceanic crust and cools. This, in turn, causes the growth of oceanic crust on either
side of the vents. As the plates continue to move, and more crust is formed, the ocean
basin expands and a ridge system is created. Divergent boundaries are responsible in
part for driving the motion of the plates.
1.3 Earthquakes and Plate Tectonics
Earth scientists believe that most earthquakes are caused by slow movements inside
the Earth that push against the Earth's brittle, relatively thin outer layer, causing the
rocks to break suddenly. This outer layer is fragmented into a number of pieces, called
plates. Most earthquakes occur at the boundaries of these plates
Earthquakes occur only in the outer, brittle portions of these plates, where
temperatures in the rock are relatively low. Deep in the Earth's interior, convection of
the rocks, caused by temperature variations in the Earth, induces stresses that result in
movement of the overlying plates. The rates of plate movements range from about 2
to 12 centimetres per year and can now be measured by precise surveying techniques.
36
The stresses from convection can also deform the brittle portions of overlying plates,
thereby storing tremendous energy within the plates. If the accumulating stress
exceeds the strength of the rocks comprising these brittle zones, the rocks can break
suddenly, releasing the stored elastic energy as an earthquake.
SELF ASSESMENT QUESTION
1. List and briefly explain the three common boundaries between plates.
2. Discuss the relationship between plate tectonics and earthquakes.
37
WEEK 7:
BASIC CONCEPT OF STRUCTURAL GEOLOGY
1.1 Strike & Dip
A plane can be defined by two lines. For geology, we use either;
-
Strike and Dip; the compass direction of the line of intersection created
by a dipping bed or fault and a horizontal surface.
Or
-
Dip and Dip direction; the angle at which a rock layer is inclined from
the horizontal. The direction of dip is at right angle to the strike.
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39
40
41
1.2 Joints
A joint in the Earth's crust along which no section of the crust has been displaced
relative to another section, in response to forces of tension or compression as a result
of tectonic movement.
1.3 Cleavage
Cleavage is the tendency minerals have to break or split along the planes of their
crystal structure. This sample of fluorite shows a smooth cleavage face on the left
where the mineral broke along the plane of its cubic crystal structure.
42
1.4 Fold.
Folds and Folding, in geology, bends in layered, or stratified rocks.
A fold can be defined as a bend in rock that is the response to compressional forces.
Folds are most visible in rocks that contain layering.
Most stratified rocks exposed in rivers, quarries, or around shorelines were originally
sediments
laid
down
as
horizontal
or
near-horizontal
layers,
or
beds.
However, where we see them now, not only have they solidified, but they are usually
inclined, or dip, in one direction or another, and they have been tilted. If the exposure
is large enough, the inclined beds can be traced up over an arch or down into a trough.
Folds usually occur in sedimentary rocks, which were originally deposited in flat
layers. Pressure from crustal movements deep within the planet causes the buried
strata to bend. Later, erosion and uplifting of the rocks caused by plate movements
bring the rock layers back to the surface. An upward bend in a fold is called an
anticline, and a downward bend is a syncline.
43
44
1.5 Fault in Geology
A fracture in the Earth's crust along which a section of the crust has been displaced
relative to another section, in response to forces of tension or compression as a result
of tectonic movement.
This movement may be in a vertical or horizontal direction, or a combination of the
two. The fracture may range from centimetres to hundreds of kilometres long
The Picture above shows three faults. The leftmost fault is a normal fault (match the
black strata)
In other words, Faults are surfaces along which rocks have fractured and been
displaced. There are three major types of faults: strike-slip, normal, and reverse. The
tectonic stresses caused by plate motions (see previous section) build up over time
45
and eventually cause breaks in the crust of the Earth along which the rocks
sporadically grind past one another. When this happens, earthquakes occur.
The outer part of the Earth is relatively cold. So when it is stressed it tends to break,
particularly if pushed quickly!
These breaks, across which slip has occurred, are called faults. The most obvious
manifestations of active faulting are earthquakes.
The faulting patterns can have enormous economic importance.
Faults can;
- control the movement of groundwater,
- exert a strong influence on the distribution of mineralization and the subsurface
accumulations of hydrocarbons.
- Have a major influence on the shaping of the landscape.
Movement on faults, with earthquakes, shatters rocks. In some places these new
materials are economically important as ready-made aggregate. In other places they
can be a problem for engineers, making hillsides unstable.
SELF ASSESMENT QUESTIONS
1. Show diagrammatically how strike and dip are measures.
2. What is the major economic importance of fault patterns?
46
WEEK 8: GEOLOGICAL MAP INTERPRETATIONS.
1.1 Definition
A geologic map or geological map is a special-purpose map made to show geological
features.
Geologic mapping is defined as the examination of natural and manmade exposures of
rock or unconsolidated materials, the systematic recording of geologic data from these
exposures, and the analysis and interpretation of these data in two or threedimensional format (maps, cross sections, and perspective [block] diagrams). The
maps and cross sections generated from these data:
(1)
Serve as a record of the location of factual data.
(2)
Present a graphic picture of the conceptual model of the study area based on
the available factual data.
(3)
Serve as tools for solving three-dimensional problems related to the design,
construction,
and/or
maintenance
of
engineered
structures
or
site
characterization.
This chapter presents guidelines for the collection and documentation of surface and
subsurface geologic field data for use in the design, specifications, construction, or
maintenance of engineered structures and site characterization studies.
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1.2 Responsibilities of the Engineering Geologist
An engineering geologist defines, evaluates, and documents site-specific geologic
conditions relating to the design, construction, maintenance, and remediation of
engineered structures or other sites. This responsibility also may include more
regionally based geologic studies, such as materials investigations or regional
reconnaissance mapping. An engineering geologist engaged in geologic mapping is
responsible for:
•
Recognizing the key geologic conditions in a study area that will or could
significantly affect hazardous and toxic waste sites or a proposed or existing
structure;
•
Integrating all the available, pertinent geologic data into a rational,
interpretive, three-dimensional conceptual model of the study area and
presenting this conceptual model to design and construction engineers, other
geologists, hydrologists, site managers, and contractors in a form that can be
understood.
The process and responsibilities of engineering geology mapping are illustrated in the
figure below
The engineering geologist needs to realize that geologic mapping for site
characterization is a dynamic process of gathering, evaluating, and revising geologic
data and that the significance of these data, both to the structure and to further
exploration, must be continually assessed. The initial exploration program for a
structure is always based on incomplete data and must be modified continuously as
the site geology becomes better understood. The key to understanding the site geology
48
is through interpretive geologic drawings such as geologic maps, cross sections,
isopachs, and contour maps of surfaces. These working drawings periodically revised
and re-interpreted as new data become available, are continuously used to assess the
effects of the site geology and to delineate areas where additional exploration is
needed. These drawings are used in designs, specifications, and modelling and
maintained in the technical record of the project.
SELF ASSESMENT QUESTIONS
1. What are the responsibilities of an engineering geologist engaged in geological
mapping?
49
50
WEEK 9:
WEATHERING AND EROSION
1.0 Definition.
Weathering is the disintegration and decomposition of rocks and minerals at or near
the earth's surface as a result of physical, chemical, and biological processes. No
transport or entrainment is considered.
All rocks at the surface will weather and erode whether they are igneous,
metamorphic, or sedimentary.
We divide weathering into two principal process types, although they do not work
independently:
Physical (or mechanical) weathering - disaggregation with no change in chemistry:
creates surface area
Chemical weathering - alteration to cause chemical or mineralogical changes:
weakens rocks
1.1
Primary Controls on Weathering
1.
Climate (temperature and precipitation).
2.
Geology (rock type and distribution).
51
1.2
Secondary Controls
1.
Topography (relief and aspect).
2.
Vegetation
1.3
Types of Weathering
(changes
chemistry
and
is
vigorous
physically).
Mechanical Weathering - physical disintegration of rock
frost wedging
unloading
thermal expansion
biological activity
Chemical Weathering - rock chemically decomposes
Agents are water, CO2, and oxygen (solution, oxidation, and hydrolysis
1.4
Rates of Weathering
Depends on:
1.5
o
Rock type - granite or limestone
o
Climate - humid vs. arid
Products of Weathering
Clastic particles (sediment) - solid particles from mechanical weathering
Classification depends on size: boulder, cobble, pebble, sand, silt, clay.
Accumulations of these products called clastic or detrital sediments.
52
o
Dissolved particles - ions or molecules dissolved in water. These
dissolved materials can be precipitated from water and accumulate as
chemical and biochemical sediments)
o
Soil - loose, unconsolidated, un-eroded sediment material
Product of centuries of mechanical and chemical weathering of rock plus addition of
organic material
Weathering effects generally decrease with depth, although zones of differential
weathering can occur and may modify a simple layered sequence of weathering.
Examples are:
(1)
Differential weathering within a single rock unit, apparently due to
relatively higher permeability along fractures.
(2)
Differential weathering due to compositional or textural differences.
(3)
Differential weathering of contact zones associated with thermal
effects such as interflow zones within volcanics.
(4)
Directional weathering along permeable joints, faults, shears, or
contacts which act as conduits along which weathering agents
penetrate more deeply into the rock mass.
(5)
Topographic effects.
SELF ASSESMENT QUESTION
1. Enumerate the primary and secondary controls of weathering.
2. What are the factors that affect rate of weathering?
53
WEEK 10:
GEOLOGICAL
FACTORS
AFFECTING
STABILITY
OF
SLOPES.
1.0 Conditions Which Affect Stability of slopes.
•
Joints or joint systems — describe spacing, continuity, length, whether open
or tight, slickenside, planarity, waviness, cementation, fillings, dip and strike,
and water.
•
Shear zones — describe severity of shearing and physical condition of rock in
and adjacent to the zone, whether material is crushed or composed of breccia,
gouge, or mylonite; describe gouge thickness, physical properties, mineralogy
and alteration; dip and strike, and water.
•
Faults — give dimensions of fault breccia and/or gouge and adjacent
disturbed or fractured zones, amount of displacement, if determinable, and the
fault's effect on stability of rock.
SELF ASSESMENT QUESTIONS
1. List and explain the factors that affect the stability of slopes.
54
WEEK 11:
GEOLOGICAL
FACTORS
AFFECTING
CUTTING
&
EMBANKMENTS.
Embankment dams may consist almost entirely of impermeable material, such as clay,
or may have a core of impermeable material bounded both upstream and downstream
by zones of more permeable material, such as sandy gravel or rock fill.
The geological factors that affect embankments are as listed below:
1.
Topography.
2.
Subsurface conditions.
3.
Surface and ground water conditions.
4.
External loads such as structures.
5.
The removal of support at the toe of the slope by erosion or by excavation.
6.
Loading the head of the slope with debris from another landside or with
manmade fills or loads from structures. Modern fills, which are constructed
and de4signed under the direction of a geo-technical engineer, are generally
stable.
SELF ASSESMENT QUESTIONS
1. Explain the effect of sub – surface conditions as it affects construction of an
embankments.
55
WEEK 12:
GEOLOGICAL CONDITIONS AFFECTING IMPOUNDED
SURFACE WATER (RESEVOIR & DAMS).
A DAM may be defined as a solid barrier constructed at a suitable location across a
river valley with a view of impounding water flowing through that river.
The main object of placing a dam across a river is to impound its water behind the
dam.
Naturally, this would require that;
(a) Topographically; a place which is most suitable for the purpose is selected.
Ideally, it would be a narrow gorge or a small valley with enough catchment area
available behind so that when a dam is placed there it would easily store a calculated
volume of water in the reservoir created upstream.
(b) Technically, the site should be a sound as possible: strong, impermeable and
stable. Strong rocks at the site make the job of the designer much easy: he can evolve
best designs. Impermeable site ensure better storage inventories. Stability with
reference to seismic shocks and slope failure around the dam, epically, upstream, are a
great relief to the public in general and the engineer in particular. The slides, and
slope failures around and under the dam susceptibility to shocks during an earthquake
could prove highly hazardous.
1.1 Geological Characters for Investigation
For achieving the above surveys of the entire catchment area followed by detailed
geological mapping of the reservoir area have to be conducted. These should reveal
56
1.
Main topographical features.
2.
Natural drainage patterns.
3.
General characters and structures of rock formations such as their
stratification, folding and faulting and igneous intrusions.
4.
The trend and rate of weathering and erosion in the area.
Such a study when interpreted properly would rule out some areas for the dam
placement and help in identifying the locations that are most suitable topographically
and economically, where further detailed geological and geophysical surveys need to
be conducted.
1.2 Geology of the Site
Lithologies: The single most important feature that must be known thoroughly at the
site and all around and below the valley up to a reasonable depth are the lithologies,
i.e. types of the rocks that make the area.
Structure: Along with lithologies, the structural features of rocks of the site are also
thoroughly investigated. This involves detailed mapping of planes of weakness like
bedding planes, schistosity, foliation, cleavages, joints, shears zones, faults and fault
zones, folding and the associated features.
Dip and Strike: the strength of sound, unfractured stratified rock is always greater
when the stresses are acting normal to the bedding planes than if applied in other
directions. This being so, horizontal beds should offer best support for the weight of
the dam.
57
Consequently, the most UNFAVOURABLE strike direction is the one in which the
beds strike parallel to the axis of the dam and the dip is downstream beds with
upstream dips are quite favourable sites for dam foundations.
Faults; These structures can be source of danger to the dam in a number of ways.
Thus, the faulted rocks are generally shattered along the rapture surfaces; Dams
founded on beds traversed by fault zones and on major fault planes are more liable to
shocks during an earthquake compared to dams on non-faulted rocks. This single
factor in itself is of great importance, especially when the area in which dam is
proposed happens to be seismically active
Folds: The most notable effects of folds on rocks are: shattering and jointing along
the axial planes and stressing limbs. Consequently, dams aligned along axial regions
of folds would be resting on most unsound rocks in terms of strength.
Joints: No sites are totally free from jointing. Hence, sites cannot be abandoned, even
if profusely jointed. However, the detailed mapping of all the aspects and characters
of joiting as developed in the rocks of proposed site has to be taken up with greatest
caution.
1.3
Engineering Properties of Rocks
Only lithological and structural studies are not fully sufficient for the selection of a
site for a dam.
58
1. Strength Parameters: The determination of strength of the foundation rocks in
the case of gravity dam and also of abutment rocks in the case of arch dams
are considered starting points in working out designs of these dams.
2. Porosity and Permeability It is essential that investigations are carried out to
establish fully the magnitude of permeability of the rocks at the site.
1.4 The Geological Condition That Affect Dams
I.
The nature and condition of the rock on which the dam is to be founded should
be known. The strength of the rocks, which must be to carry the weight of the
load (dam) without crushing or shearing, should also be know.
ii.
The structural features such as the dip of strata, spacing of bedding planes,
presence of folds, faults, joint and zones of crushed rock should be
determined.
iii.
The permeability of the rock and the nature of the matter circulation through it
should be known as it governs the degree of leakage below the dam.
iv.
The dam must be built on one uniform formation of rock because different
bearing strength may arise if there is more than one kind of rock in the
foundation and this may lead to unequal settlement of structure.
v.
The presence of open joints in the rock should be checked. If may open joints
are present, a grouting programme should be carried out in order to secure the
foundation against excess leakage, and to reduce uplift pressure on the dam.
vi.
The presence of sharp local folding may present weak points in a formation
likewise broad and gentle fold structures.
Therefore, this should also be
considered
vii.
The presence of faults does not really condemn a dam site but the geological
history of the faulting should be investigated. A dam should not be built across
59
a fault know to have been active in recent times except under special
circumstances.
viii.
Unscheduled rapid draw- down of impounded water.
ix.
Significant slides or settlement of materials in area adequate to reservoirs.
x.
New seepage or leakage or significant gradual increase in pre-existing seepage
or leakage.
xi.
Natural disasters such as floods or earthquakes
xii.
Failure or unusual movement, subsidence or settlement of any part of the dam.
xiii.
Significant damage to slope protection.
SELF ASSESMENT QUESTIONS
1. List the geological conditions that affect dams.
2. What are the geological factors to be considered that may tend to affect
impounded surface water.
60
WEEK 13:
GEOLOGICAL CONSIDERATION IN TUNNELING
1.1 Tunnelling
Tunnel, passage, gallery, or roadway beneath the ground or underwater. Tunnels are
used for highway traffic, railways, and subways; to transport water, sewage, oil, and
gas; to divert rivers around dam sites while the dam is being built; and for military
and civil-defence purposes.
.Geological speaking, only two classes of tunnels are recognized: tunnels driven
through rocks (rock tunnelling) and tunnels driven through soil, loose sediments or
saturated ground (soft-ground tunnelling).
Geological investigations are very essential in tunnelling projects. These determine to
a large extent solutions to following engineering problems connected with tunnelling.
The following geological characters are broadly established for the entire area in
which the tunnels project is to be located as a result of preliminary surveys:
i.
The general topography of the area marking the highest and the lowest points,
occurrence of valleys, depressions, bare and covered slopes, slide areas, and in
hilly regions and cold climates, the snow-line.
ii.
The lithology of the area, meaning thereby, the composition, attitude and
thickness of rock formations which constitute the area.
iii.
The hydrological conditions in the area, such as depth of water table,
possibility if occurrence of major and minor aquifers of simple type and of
61
artesian type and the likely hydrostatic heads along different possible routes or
alignments.
iv.
The structural conditions of the rock, that is extent and attitude of major
structural features such as folding faulting, unconformities, jointing and
shearing planes, if developed. Existences of buried valleys are also established
during the preliminary surveys.
1.2 Geological Considerations in Tunnelling
Rocks may be broadly divided into two categories in relation to tunnelling:
consolidated and unconsolidated or soft ground. Geological characters that have a
direct bearing on a tunnel project will differ almost in all details in these categories.
Only a brief accounts is given below.
1. A consolidated Rocks
This group includes the massive igneous, sedimentary and metamorphic rocks that
very often form major mountain ranges and sub-mountain regions. Most tunnels in
the mountains pass through these rocks. Tunnels design, method of its excavation and
stability are greatly influenced by following geological conditions: lithology,
geological structures and ground water conditions.
2. Lithology
It has already been mentioned that information regarding mineralogical composition,
textures and structures of the rocks through which the proposed tunnels is to pass is of
great importance in deciding;
i.
the method of tunnelling,
ii.
The strength and extent of lining and, thus
iii
the cost of the project
62
Hard and Crystalline Rocks are the favourites with the tunnel. Rocks failing in this
group include granties, diorites, syenites, gabbros, basalts and all the related igneous
rocks, sandstones, limestone’s, dolomites, quartzite’s, arkoses, greywacke and the
metamorphic groups. When any one of these rocks is stresses, such as during folding
or fractured as during faulting, tunnelling in these rocks proves greatly hazardous.
3. Geological Structure
The design, stability and cost of tunnel depend not only on the rock but also on the
structures developed in these rocks. Following main structural features of rocks have
to be fully determined along the proposed tunnel route: dip and strike, folding,
faulting, shear zones and joint systems.
4. Dip and Strike
These two quantitative properties of rocks determine the attitude (disposition in
space) of the rock and hence influence the design of excavation (tunnel) to a great
extent. Three general cases may be considered.
5. Horizontal strata
Such a situation is rare in occurrence for longer tunnels. When encountered for small
tunnels or for short lengths of long tunnels horizontally layered rocks might be
considered quite favourable. In massive rocks, that is, when individual layers are very
thick, and the tunnel diameter not very large, the situation is especially favourable
because the layer would then over bridge flat excavations by acting as natural beams.
63
2.1 Drilling
Drilling is an act of boring a hole on the ground with the aim of investigating the
nature of sub – surface material.
SELF ASSESMENT QUESTIONS
1. List the geological character that has to be established in any tunnel project.
2. What is the effect of lithology and structure in a tunnel project?
64
WEEK 14:
SOIL
1.1 Definition
In engineering applications, soil may be defined as generally un-indurated
accumulations of solid particles produced by the physical and/or chemical
disintegration of bedrock and which may or may not contain organic matter.
1.2 Soil Formation
Why does one soil look different than another? What geological forces act upon soils
and what are the effects of the action? The famous geologist, Hans Jenny, answered
these and other questions in his description of five factors responsible for soil
formation.
1. Rocks and minerals change by the processes of weathering. Typically
weathering causes mineral materials to disintegrate into smaller parts. The
elements released as products of weathering may form new, secondary
minerals. Weathering products that are loose or unconsolidated are called soil.
Weathering can be accomplished by one or a combination of physical and
chemical
processes.
Physical weathering is most pronounced in cold and dry climates. Physical
processes include effects of temperature. Most notable is the force exerted by
the expansion of water as it freezes. Another physical process is abrasion
caused by bombardment of minerals by materials suspended in wind and
flowing water. Finally, plant roots established in the crack of rocks often exert
65
a
force
strong
enough
to
cleave
the
rock.
Chemical weathering dominates in warm or moist climates. Worldwide,
chemical weathering processes tend to be more important in soil formation
than physical forces. The chemical processes are described in the textbook.
The processes include: oxidation and reduction (of great importance for ironcontaining minerals), carbonation (dissolution of minerals in water that has
been made acidic by carbon dioxide), hydrolysis (when water splits into
hydrogen and hydroxide, and one or both components participate directly in
the chemical process), and hydration (when water is incorporated into the
crystal structure of a mineral, changing mineral properties). Minerals differ
greatly in their resistance to weathering (see Table 2-1 in the textbook).
2. Soil Formation means the development of a particular soil in a particular
place. Weathering produces unconsolidated mineral material that one would
call soil. However the formation of any given unique soil includes process that
continue to operate long after this unconsolidated material is formed. The
products of weathering are the materials in which soils form. Soil materials are
often transported vertically within the soil profile (see Figure 2-2 in the
textbook). Materials can be added (such as humus from plants or sediments
added by wind or water) or can be lost (such as by erosion or by leaching
through the soil profile). The unconsolidated materials in which soils form is
called parent material. The term, soil formation, implies the formation of soil
horizons and other features in the parent material.
66
3. Parent material arrives on location through various routes and mechanisms.
The parent material in a landscape may have been transported to the location
(see Figure 2-9 in the textbook). Residual soils formed in place from
weathering of the underlying bedrock. Residual soils are common in the great
plains of the United States. Residual landforms include mesas, plateaus and
plains.
Stream deposits are very common worldwide. Such deposits are called
alluvium or alluvial. Examples of alluvial soils are: flood plains (as along river
Kaduna), alluvial fans (as in the piedmont region of the south-eastern states),
lacustrine deposits (dispersed material deposited as a stream entered a lake),
and marine deposits (flocculated or aggregated material deposited as a stream
entered the sea). In some instances marine deposits form deltas.
Wind deposits are called eolian materials. Only small particles are readily
carried by wind. Large sands are too heavy to be carried by most winds. Clays
also rarely blow because they form aggregates with other clay particles
producing large clods or peds. Silts and fine sands are examples of material
readily carried by wind. Silts are small mineral fragments, barely visible to the
naked eye. (A typical silt particle is about as wide as the thickness of a sheet
of paper.) Wild-blown silt deposits are important in agriculture for the ease
with which they are cultivated. Such deposits are called loess. Sand dunes and
landscapes
made
of
ash
are
also
eolian.
Gravity deposits are colluvial material or colluvium. Such material results
67
from mass-wasting, mud flows, or the gradual movement of individual
particles down a slope. These deposits are found on a relatively small-scale
such as mountain valleys, and are usually coarse materials.
4. According to the classic work by Hans Jenny, five soil-forming factors
account for the differences in soils. The five factors are: climate, relief,
organisms, parent material, and time. The acronym "CROPT" may help you
remember this.
5. Parent Material is the only factor that can be considered inherited as opposed
to acquired. The effects of parent material on a soil include such feature as soil
texture, pH and mineral constituents.
6. Climate is often considered the most powerful soil-forming factor. Climate is
expressed as both temperature effects and rainfall effects. Temperature
controls rates of chemical reactions. Many reactions proceed more quickly as
temperature increases. Warm-region soils are therefore normally more
developed or more mature than are cool-region soils. Rainfall affects leaching,
pH and soil aeration. In addition to direct effects of climate, climate also
profoundly affects vegetation which in turn also affects soil formation.
7. Organisms (biota) affect and are affected by soil formation. Man is perhaps
now the most influential of all organisms. He affects the soil by such activities
as: ploughing, irrigating, mining, clearing, disposing and levelling. The effects
of large animals other than man on the land are minor. The effects of
vegetation on soil formation are very profound. Different soils form in a
grassland than under a forest. Much of this difference is due to the rapid
nutrient cycling in grasslands. Vegetation effects extent of cover, thereby
68
influencing runoff and erosion. Vegetation type and amount directly
influences the type and amount of organic matter accumulation on the soil,
and thereby influences such soil chemical properties as pH and nutrient
supply. Finally, vegetation is the food source for most micro organisms so the
vegetation exerts a strong influence on soil microbial populations.
8. Relief (topography) modifies the effects of other factors. Relief modifies
climate by affecting the smoothness of the surface and also the angle at which
the soil surface orients towards the sun. A convoluted surface dilutes solar
energy over more surface area than does a smooth surface. In the northern
hemisphere a north-facing slope will be cooler than a south-facing slope.
Relief also affects the amount of rainfall that infiltrates a given parcel of soil.
A steep slope will encourage runoff. A soil in a sloping location will
experience less effective rainfall than that which one would measure in a rain
gauge. Likewise, a low area may receive run-on water beyond the actual
rainfall. Also, relief influences erosion. Soil horizons form from the top
downwards. If the topsoil readily erodes away as it forms, the soil formation
processes appear to have halted.
9. Soil formation is a function of time. Soil development is a process, not an
event. Soils change over time. Clays are secondary minerals. They form in the
soil, then change forms, and all the while they are moving downward with
leaching rainwater. Similarly, organic matter forms on the surface as it moves
downward with rainwater. It declines after reaching a maximum as old soils
loose their ability to produce vegetation fast enough to keep up with
decomposition. These are highly dynamic processes. Soils as viewed today are
69
just snapshots in time. Soils looked different in the past and will look different
in the future.
1.3 Terminology for Soils
Cobbles and boulders—particles retained on a 3-inch (75-mm) U.S. Standard sieve.
The following terminology distinguishes between cobbles and boulders:
• Cobbles—particles of rock that will pass a 12-in (300-mm) square opening
and be retained on a 3-in (75-mm) sieve.
• Boulders—particles of rock that will not pass a 12-in (300-mm) square
opening.
Gravel—particles of rock that will pass a 3-in (75-mm) sieve and is retained on a No.
4 (4.75-mm) sieve. Gravel is further subdivided as follows:
• Coarse gravel—passes a 3-in (75-mm) sieve and is retained on 3/4-in (19mm) sieve.
• Fine gravel—passes a ¾-in (19-mm) sieve and is retained on No. 4 (4.75mm) sieve.
Sand—particles of rock that will pass a No. 4 (4.75-mm) sieve and is retained on a
No. 200 (0.075-mm or 75-micrometer [µm]) sieve. Sand is further subdivided as
follows:
• Coarse sand—passes No. 4 (4.75-mm) sieve and is retained on No. 10
(2.00-mm) sieve.
• Medium sand—passes No. 10 (2.00-mm) sieve and is retained on No. 40
(425-µm) sieve.
• Fine sand—passes No. 40 (425-µm) sieve and is retained on No. 200 (0.075mm or 75-µm) sieve.
70
Clay—passes a No. 200 (0.075-mm or 75-µm) sieve. Soil has plasticity within a
range of water contents and has considerable strength when air-dry. For classification,
clay is a fine-grained soil, or the fine-grained portion of a soil, with a plasticity index
greater than 4 and the plot of plasticity index versus liquid limit falls on or above the
"A"-line.
Silt—passes a No. 200 (0.075-mm or 75-µm) sieve. Soil is non plastic or very slightly
plastic and that exhibits little or no strength when air-dry is a silt. For classification, a
silt is a fine-grained soil, or the fine-grained portion of a soil, with a plasticity index
less than 4 or the plot of plasticity index versus liquid limit falls below the "A"-line.
Organic clay—clay with sufficient organic content to influence the soil properties is
an organic clay. For classification, organic clay is a soil that would be classified as
a clay except that its liquid limit value after oven-drying is less than 75 percent of its
liquid limit value before oven-drying.
Organic silt—silt with sufficient organic content to influence the soil properties. For
classification, an organic silt is a soil that would be classified as a silt except that
its liquid limit value after oven-drying is less than 75 percent of its liquid limit value
before oven-drying.
Peat—material composed primarily of vegetable tissues in various stages of
decomposition, usually with an organic odour, a dark brown to black color, a spongy
consistency, and a texture ranging from fibrous to amorphous.
SELF ASSESMENT QUESTIONS
1. In any engineering application, how are soils defined.
2. What are the factors responsible for soil formation?
71
WEEK 15:
GEOLOGICAL CONSIDERATION IN FOUNDATIONS
1.1 Foundations
Structures or other constructed works are supported on the earth by foundations. The
word “foundation” may mean the earth itself, something placed in or on the earth to
provide support, or a combination of the earth and the elements placed on it. The
foundation for a multi-story office building could be a combination of concrete
footings and the soil or rock on which the footings are supported. The foundation for
an earth-fill dam would be the natural soil or rock on which the dam is placed.
Concrete footings or piles and pile caps are often referred to as foundations without
including the soil or rock on which or in which they are placed. The installed elements
and the natural soil or rock of the earth form a foundation system; the soil and rock
provide the ultimate support of the system. Foundations that are installed may be
either soil-bearing or rock-bearing. The reactions of the soil or rock to the imposed
loads generally determine how well the foundation system functions. In designing the
installed portions, the designer must determine the safe pressure which can be used on
the soil or rock and the amount of total settlement and differential settlement which
the structure can withstand.
72
For the purpose of site investigation for foundation engineering operation, the general
geology of the area with particular reference to the main geological formation
underlying the site is expected to be considered.
1.2 Geological Conditions
The geological conditions in any foundation have to do with whether the rocks in the
area of the proposed structure are decayed or weathered and if their strength is such
that can safely support the proposed structure. Some of there conditions are;
1. Ground movement.
-
Subsidence.
-
Seismicity.
-
Mass wasting.
2. Compressible weak material.
73
3. Irregular rock head.
4. High permeability associated with high water table.
5. Soil type.
6. Topography.
7. Rock type.
8. Depth of weathering.
9. Type of building.
Other factors to be considered are; spacing of bedding planes, faults, folds, strata dip,
joints, permeability of the rock, nature of water circulating through it, bedding
discontinuities, state of weathering, geological formation, age and type of deposit and
possibility of subsidence from mineral extraction.
The foundation engineer is mainly interested in the basement (fresh rock) and only
knowledge of geological formation of the area under investigation will enable him to
estimate the depth to the basement.
Similarly, knowledge of geology enables the engineers to delineate active zone of
earth that are prone to natural disasters such as earthquake, landslide etc. This will
guide him or her against post construction failure.
Investigation into rock formations for foundation engineering purpose are concerned
first with the allowable bearing pressures for spread foundation or working loads on
piles and second with the conditions which are likely to be met if excavations have to
be taken into the rock strata (Geological formation on bedding) for deep foundations.
74
The engineer must therefore have information of the depth of any weathering of the
rock, the presence of any shattered zones or
The possibility of the occurrence of deep drift filled clefts shallow holes or concealed
cavities and the quantity of water likely to be pumped from excavations. Much of this
information can be obtained in a general way by advice from a geologist from his
knowledge of local conditions and the study of published maps. Indeed the advice of
a geologist in connection with the sitting of any important project on rock formation is
very necessary to avoid post construction failure.
Good foundations are provided by metamorphic rocks like quartzite. The presence of
any open joint in a foundation poses a threat to the structure and may also require a lot
of grouting to prevent any leakage in case of dams; the grouting reduces pressure on
the dam.
Lime stones are very unreliable in foundation because of their solubility, thus many
solution cavities are usually present. Very deep cavities may sometimes be formed.
Where a foundation is to be constructed on soft soil like clay or shale, which are
known to have relatively low bearing strength, under any load, its strength will
decrease when a constant with water.
Another important consideration is the presence of open joint in a foundation. This is
mostly common where sandstones are present. The solution is grouting in large or
small scales depending on how deep and wide the joints are.
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The topography of a site which to do with the nature of the land surface. The
engineer must consider whether the land is sloppy and if so the slopes are to the
advantage of the proposed building. If the land is not sloppy, has good drainage and
can suitably carry the structures load without uneven settlement or expansion, the
construction can commence.
The state of weathering of rocks, which are located on the site, is also to be
considered and whether grouting alone will sufficiently dispel any danger. This can
be obtained through geological tests depending on the type of structure. If it is a dam,
then the most suitable is to certify that there is only one type of rock present on the
site.
Another geological consideration is whether the area is prone to erosion. Though
erosion may not pose much of a threat to the structure if well taken car of, it is
advisable to construct proper drainages for any run-off water in the area before
commencing work.
SELF ASSESMENT QUESTION
1. What are the geological conditions in any foundation?
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