Fundamental of Soil And Rock Mechanics
University of Gondar
Department of Geology
Course Title: Fundamental of soil and rock mechanics
Course Code: Geol. 3111
Credit hours: 3 credit or 5 ECTS
Course Category: Core course
Instructors: Azemeraw Wubalem (MSc)
email: alubelw@gmail.com
phone:0935452268
Office's 3rd floor room 28
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1
Chapter one: Introduction to soil mechanics
Chapter one outline
Objectives: students will able to
• Introduction
• Define soil and soil mechanics
• Soil mechanics
• Rock cycle and origin of soil
• Soil type and profile
• Evaluate how the soil is formed
• Determine the profile of soil
• Classify soils into different groups
based on the texture
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Introduction to soil mechanics
Introduction
What is soil?
▪ The earth’s crust is composed of soil and rock.
• In general sense of engineering, soil is defined as
▪ Rock can be defined as a natural aggregate of
the unconsolidated aggregate (or granular material)
of mineral grains and decayed organic matter along
minerals
that
are
connected
by
strong
bounding or attractive forces.
with the liquid and gas that occupy empty spaces
between the solid particles.
▪ Soil may be defined as the unconsolidated
• All man made structures, except those which float
sediments and deposits of solid particles that
or fly, are supported by natural soil or rock
have resulted from the disintegration and
deposited.
decomposition of rocks.
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3
Introduction to soil mechanics
What is soil mechanics?
• Is the branch of science that deals with the
physical properties of soil and behavior of soil
• When loads are applied, on what rate does soil
deform?
• How much load can we apply to soil before it
mass subjected to various type of forces.
fails?
• In other words, soil mechanics is the study of
both solid and fluid mechanical characteristics
of soils.
• How does soil fail?
❖Fluid mechanics issues:
❖Solid mechanics issues:
• How does water flow through soil? (how fast?)
• How much soil will deform when it is loaded?
• How can fluid flow through soil cause it to fail?
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4
Introduction to soil mechanics
Why we study soil mechanics?
• All branch of civil engineering require an
• Knowledge of soil mechanics is essential to
assure that the structures are properly
supported. This can avert structural damage
understanding of soil and how it behaves
and failure, loss of life, and financial loss
namely,
etc.
• Structural Engineering
Transportation Engineering
• Transportation Engineering
• Roadbeds are often built of soil, and the
• Environmental Engineering
roadways themselves can often pass through
• Hydraulic Engineering
mountains, cuts, fills, etc.
• Understanding SM can preclude problems
Structural Engineering
• All engineering structures are come in contact
with soil via their foundations e.g. bridge,
office, house etc.
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with pavement potholing and cracking, as
well embankment and slope failures that can
wipe out the entire roadways.
5
Introduction to soil mechanics
Environmental Engineering
Hydraulic Engineering
• Landfilling of solid wastes, or liquid toxins or
pollutants
often
spilled
or
• The
design
of
earthen
flow
retention
released
inadvertently onto or into soil.
• Therefore, important questions that need to be
addressed are:
structures such as dams, levees, dikes, storage
ponds, etc. require a knowledge of how water
is transported through soil.
• Will the pollutants remain in place, or possibly
• It also requires that to know how water
be transported through soil? If so, on what rate?
flowing through soil can cause failure by
• Can anything be done to cleanup the pollution?
mechanisms such as, boiling, piping, erosion,
(like providing barriers and other remediation
and scour.
measures)
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6
Introduction to soil mechanics
• In general,
Behavior of the structure depends
upon:
Properties of soils on which the
structure rests
Properties of the soils from which
they are derived (rocks)
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Properties of the rocks from which
they are composed.
7
Introduction to soil mechanics
• Engineering geologists or geotechnical • Before those, it is important to
engineers must study the properties of
revise about the classification of
soils, such as its:
rocks.
• Origin, grain size distribution
• Ability to drain water
• Strength
• Mechanical behavior of soil.
• When they are sheared or compressed or
when water flows through it.
• The rocks that form the earth’s
surface are classified as to origin
as:
• Igneous
• Sedimentary, and
• Metamorphic
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Introduction to soil mechanics
Igneous rocks
• Are formed when the molten state of magma
or lava crystalized or solidified.
• If the molten rock cools very slowly, the
• When the solution of minerals is
cooled more rapidly, tiny crystals
of the minerals are formed in a
vitreous matrix.
different materials segregate into large
crystals forming a coarse grained or • E.g.
granular structures. For e.g. Granite (quartz
and feldspar).
• Based on silica content rocks are classified
as acidic and mafic.
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rhyolite-extremely
fine
grained rocks
• Basalt-when
formed
with
ferromagnesian materials
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Introduction to soil mechanics
Sedimentary Rocks
• Are formed from accumulated deposits of
Metamorphic Rocks
• Resulted when any type of existing
soil particles or remains of certain
organisms that have become hardened by
pressure or cemented by minerals.
rock is subject to metamorphism, the
change brought about by combinations
• Due to abundant availability of cementing
of heat, pressure, and plastic flow so
minerals such as silica, carbonates, and
that the original rock structure and
iron oxides.
• For e.g., limestones, sandstone, shale,
mineral composition are changed. e.g.
Granite-Gnesis;sandstone-quartzite.
conglomerate, and breccia.
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Rock cycle and Origin of soils
• Origin of soil - is related to a complex
combination of conditions
and processes,
and it is the result of continuous processes
• It is associated to the rock cycle (weathering,
transportation, deposition, compaction, then
again disturbances and weathering etc.).
• A basic understanding of soil forming
relationships
will
aid
the
Engineering
Geologist in evaluating soils and their uses.
• Rock cycle
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11
Bowen’s Reaction Series and weathering for soil formation
Bowen’s Reaction Series
• The reaction series are similar to
the weathering stability series
• More stable higher weathering
resistance
• Weathering is more rapid for
parent material composed of less
stable minerals and it is faster at
higher temperatures
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Weathering and formation of soils
Climate- determines the amount of water and the
Rocks (IR, SR and MR)
•d
temperature.
•
Arid: Minimal leaching, slow dissolution
• Humid: Extensive leaching, rapid dissolution
• Cool: Active physical weathering, slow chemical
weathering.
Weathering
(physical/chemical/bio)
• Warm: Strong chemical weathering
Transported/in place
Soil type
Boulders, gravel, & sand, silt, & clay
Coarse soil
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Fine soil
13
Weathering and formation of soils
• Rocks whose chief mineral is quartz minerals
with
high
silica
content,
decomposes
to
predominantly sandy or gravely soil with little
clay
Soil type
• Soils can be grouped into two broad categories
(depending on the method of deposition):
1. Residual – formed from the weathering rock
• Basic rocks decomposed to the fine textured
and remain at the location of their origin.
Material which may possess little mineralogical
silt and clay soils
• The clays are not small fragments of the original
resemblance to the parent rock
materials that minerals that existed in the parent
2. Transported-those materials that have been
rock)
moved from their place of origin by gravity,
results
of
primary
rock
minerals
decomposing to form secondary minerals)
water, glaciers, or man either singularly or in
combination
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Origin of soil and type of soils
• Transported soils are classified
based on transported agency and
methods of deposition such as:
• Alluvial-transported by rivers
• Glacial-by
ice
(glaciation-massive
moving sheets of ice
• Colluvial-deposited through action of
landslide and slope wash.
• Lacustrine-deposited in quiet lakes
• Marine-deposited in sea water
• Aeolian-transported by wind
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Soil profiles
• A soil profile is a vertical cross-section of a soil
• B-Horizon
• The different soil layers are called soil horizons
Enriched in clay, iron and aluminum
(A-B-C horizons) and are differentiated on the
oxides and hence red-brown in
basis of color, structure, chemistry, texture,
color.
organic content, etc.
• A-Horizon
- Maximum biological activity with formation of
A
- Fine material leached from Ahorizon down reinforces the B-
B
Horizon to form a hardpan (or clay
pan)
humus
C-Horizon:
- Darker-colored than the lower layers
- Zone of weathered parent material
- Not suitable as a construction material or as a
foundation
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C
similar to the material from which
the soil developed
16
Provide
solutions, not
problems
I Will work
Hard ,I Will
Succeed
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Lecture 1
Come early,
stay late
you need to
learn to only
have fun
after all of
your work is
done
17
End of chapter 1
36
•This is all what I have to say.
•Thank you too much for your
eyeful attention!
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18
Chapter Two: Soil aggregate relationship
Chapter two outline
Objectives: students will able to
• Soil aggregate relationship
• Define soil state
• Soil state
• Phase relationship
• Relative density
• State the soil phase condition and
relationship
• Identify soil grain shape and structure
• Grain shape of soil
• Clays and their behavior and soil
structure
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• Differentiate
the
properties
different clay minerals
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of
Soil Aggregate Relationship
• Soil is a particulate material which
Constituents of Soil Mass
• The behavior of soil mass under stress is a
function of material properties such as:
means that a soil mass consists of
accumulation of individual particles
that
are
bonded
together
a) size and shape of grains, b) gradation,
mechanical
c)
through not strongly as for rock.
mineralogical
composition,
d)
or
attractive
by
means
arrangements of grains, e) inter particle • Spaces in between solid particles is
forces.
• Material properties- f(constituents of the
soil mass)
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called voids or pore spaces, which
may fill by water or gas.
• Therefore soil has soil particles,
water, and air.
20
Soil Aggregate Relationship
• Soil is inherently
•d
multiphase material
which are solid,
liquid and gaseous
phases.
• It can be two phases
when it is completely
dry or saturated
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Soil Aggregate Relationship
Solid phase consists of :
Liquid Phase
• Primary rock forming minerals (size
>2μm, poor reactivity, prone to
Water
disintegration)
• Clay minerals (basic materials that
form the soil mass, size< 2μm, high
reactivity).
• Cementing material (carbonates)
• Organic
matter
(high
Polluted
Water
Water
soluble
Water
insoluble
• Water soluble-chlorides, sulphates, bicarbonates (not
capable of binding solid grains), more corrosive and acidic
water • Water insoluble-carbonates (capable of binding solid
absorption, compressible, unstable)
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Pure
Water
Dissolved salts
grains)
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Soil Aggregate Relationship
Gaseous Phase
Air
Gasses
Air
Water
Solids
Solids
2-phase system:
dry soil
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•n
2-phase system;
saturated soil
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Soil Aggregate Relationship
Phase relationships
• Volume relationship
• Weight relationship
• Inter other relationship
1. Volumetric relations
• It is commonly used in soil and rock
mechanics such as:
• Void ratio e
• Porosity n
• Degree of saturation Sr
• Air content ac
• Air void ratio or percentage air voids na
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Soil Aggregate Relationship
• Void ratio (e) is the ratio of
volume of voids to the volume of
solids e
𝑉𝑣
=
𝑉𝑠
• Volume of voids (Vv) refers to
that portion of the volume of the
soil not occupied by solid grains
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Soil Aggregate Relationship
• In nature, even though the individual
void spaces are larger in coarse
grained soils, the void ratios of fine
grained soils are generally higher than
those of coarse grained soils.
• The ratio of volume of voids to total
volume V is defined as porosity (n).
• n=
• n=
𝑽𝒗
*100,
𝑽
𝒆
𝟏+𝒆
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but, V= 𝑽𝒗 + 𝑽𝒔
• n of soil cannot exceed 100%, which is
in the range of 0<n<100
• It is the f(shape of grains, uniformity of
grains size, and the condition of
sedimentation).
• n=25%-50% (natural sand)
• n=30-60% (soft natural clays)
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Soil Aggregate Relationship
• Out of porosity n, void ratio is used frequently
Natural water content of fine grained soils >
in soil engineering because: e=Vv/Vs and
coarse grained soils. No upper limit to w.
n=Vv/V
Degree of saturation Sr
❖Any change in V is a direct consequence of a
similar change in Vv and while Vs remains the
same.
For partially saturated soil mass.
(𝑉𝑣 −𝑉𝑎 )
𝑆𝑟 =
𝑉𝑣
∗ 100 = 𝑉𝑤 /𝑉𝑣
It is expressed in percentage, 0<Sr<100
Water content
Sr=0 for completely dry soil
• The water content w is given as Ww/Ws which
expressed in percentage. Ww is weight of
Sr=1 (100%) for completely saturated soil mass
0<Sr<100 for partially saturated soils.
water, and Ws is weight of solids (dry
conditions)
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Soil Aggregate Relationship
Degree of saturation of sand in various
states valid only sands
• Fine or silty sands are moist, wet or saturated.
• Clays are always completely or nearly saturated
Condition of sand
Sr %
except in the layer of soil subjected to seasonal
variation of temperature and moisture.
Dry
0
Humid
1-25
Damp
26-50
Moist
5-75
Wet
76-99
Saturated
100
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• Air content 𝑎𝑐 =
𝑉𝑎
𝑉𝑣
=(𝑉𝑣 −𝑉𝑤 )/𝑉𝑣 = 1 − 𝑆𝑟 .
• Ac=0 for saturated soil, but ac=1 for dry soil
mass.
• Air void ratio 𝑛𝑎 =
𝑉𝑎
𝑉
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Soil Aggregate Relationship
Weight Relationships
• Unit weight Ƴ =
𝑊
𝑉
is the ratio of weight of soil
to total volume of soil, which is the f(unit weight
of solid constituents, n, and Sr)
• Bulk unit weight (Ƴb) for a partially saturated
soil mass
• Ƴ𝑏 =
𝑊
𝑉
= (𝑊𝑤 + 𝑊𝑠 )/(𝑉𝑤 +𝑉𝑠 +𝑉𝑎 )
• Ƴ𝑠𝑎𝑡 =(𝑊𝑤 + 𝑊𝑠 )/(𝑉𝑤 +𝑉𝑠 )
• Dry unit weight
• Ƴ𝑑 =
𝑊𝑠
=𝑊𝑠 /V
𝑉𝑠 +𝑉𝑎
= (𝑊 − 𝑊𝑤 )/V
𝑊
=( -𝑤𝑊𝑠 /V)=Ƴ𝑏 -wƳ𝑑
𝑉
Ƴ𝑑 =Ƴ𝑏 /(1+w), for dry soil mass,
Vw=0.
• Where Ƴ𝑠𝑎𝑡 is saturated unit weight of the soil
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29
Soil Aggregate Relationship
Specific gravity
• Is the ratio of the unit weight of a substance to the
unit weight of water Yw at 4°c.
• In soil mechanics, specific gravity generally refers
to the specific gravity of solid particles Gs, and is
defined as the unit weight of solid particles to the
unit weight of water. 𝐺𝑠 =
Ƴ𝑠
Ƴ𝑤
• 𝐺𝑠 =weight of soil solids/weight of water volume
𝑊𝑠
.
𝑉𝑠 Ƴ𝑤
=
equivalent to that of water =(𝑊2 −𝑊1 )/ 𝑊4 − 𝑊1 −
𝑊𝑠
𝑉𝑠
• Unit weight of solid constituents Ƴ𝑠 =
• Specific gravity can be determined from laboratory
(𝑊3 − 𝑊2 ))
❖For most soils Gs ranges from 2.5-2.9.
• Gs=2.65 for sands.
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30
Soil Aggregate Relationship
Gs for a partially saturated soil:
• Mass specific gravity 𝐺𝑚 =
• 𝐺𝑚 (𝑑𝑟𝑦) =
Ƴ𝑑
Ƴ𝑤
Ƴ𝑏
Ƴ𝑤
𝑓𝑜𝑟 𝑑𝑟𝑦 𝑠𝑜𝑖𝑙
• 𝐺𝑚 (𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑) =
Ƴ𝑠𝑎𝑡
Ƴ𝑤
𝑓𝑜𝑟 𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑 𝑠𝑜𝑖𝑙
•=
• =
(𝑊𝑠 +𝑊𝑤 −𝑉Ƴ𝑤 ))
V
(𝑊𝑠 +𝑊𝑤
V
𝑉Ƴ𝑤 -treating whole
soil mass as one unit
−Ƴ𝑤 ))
• = Ƴ𝑠𝑎𝑡 −Ƴ𝑤
• Gm (dry)-mass specific gravity (dry state)
• Gm (sat)-mass specific gravity (saturated state)
Submerged (Buoyant) unit weight
Ƴ’=
𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑜𝑖𝑙 𝑖𝑛𝑠𝑖𝑑𝑒 𝑡ℎ𝑒 𝑤𝑎𝑡𝑒𝑟
𝑡𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒
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31
Soil Aggregate Relationship
Basic phase relationships
𝐺𝑠 =
Two approaches:
I.
Specific volume approach (Vs=1)
II.
Unit volume approach (V=1)
Ƴ𝑠
Ƴ𝑤
=
𝑊𝑠
,
𝑉𝑠 Ƴ𝑤
𝑊𝑠 =𝐺𝑠 Ƴ𝑤 ,
Ƴ𝑑 = 𝑊𝑠 /V=𝐺𝑠 Ƴ𝑤 /(1+e)
Ƴ𝑑 = 𝐺𝑠 Ƴ𝑤 /(1+e)
• Using specific volume approach, Vs is put as unit
volume.
• Specific volume V=1+e =
𝑉
𝑉𝑠
(which is nothing but
total volume per unit volume of solids)
Dry soil: from the definition of void ratio 𝑒 = 𝑉𝑣 /
𝑉𝑠 =𝑒 = 𝑉𝑣 , 𝑉𝑠 =1,
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32
Soil Aggregate Relationship
• For fully saturated soil:
• From the definition of water content
𝑊𝑤
𝑒Ƴ𝑤
w= =
𝑊𝑠
• From the definition of degree of saturation
𝐺𝑠 Ƴ𝑤
• e=𝑊𝐺𝑠
• Ƴ𝑠𝑎𝑡 =
For partially saturated soil:
𝑆𝑟 =
𝑊
=(𝐺𝑠 Ƴ𝑤 +𝑒Ƴ𝑤 )/(1+e)
𝑉
• Ƴ𝑠𝑎𝑡 = Ƴ𝑤 (𝐺𝑠 +e) /(1+e)
• Ƴ𝑑 =
𝑉𝑤
=w𝐺𝑠 /e,
𝑉𝑣
𝑊𝑠
𝑉
e=w𝐺𝑠 /𝑆𝑟 , 𝑆𝑟 =1, e=w𝐺𝑠
= 𝐺𝑠 Ƴ𝑤 /(1+w𝐺𝑠 /𝑆𝑟 )
33
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Soil Aggregate Relationship
Relationship between Yd, Gs, w, and na
Unit volume approach (V=1):
• 𝑉 = 𝑉𝑠 + 𝑉𝑤 + 𝑉𝑎
• In this approach total volume V is set as 1.
• 1 = 𝑉𝑠 /𝑉 + 𝑉𝑤 /𝑉 + 𝑉𝑎
• This approach is used when no weights or
• 1 − 𝑉𝑎 = 𝑉𝑠 /𝑉 + 𝑉𝑤 /𝑉
• Using
𝑊
Vs = 𝑠 , 𝑉𝑤
𝐺𝑠 Ƴ𝑤
=
𝑊𝑤
Ƴ𝑤
volumes are given in the problem statement.
• Porosity n is used,
• =Ƴ𝑑 /Ƴ𝑤 (w+1/𝐺𝑠 )
• Ƴ𝑑 =(1 − 𝑉𝑎 ) 𝐺𝑠 Ƴ𝑤 /(1+𝑤𝐺𝑠 )
• When soil becomes completely saturated 𝑉𝑎 =0
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34
Soil Aggregate Relationship
• Unit volume approach (V=1)
𝑉𝑣
, 𝑤𝑖𝑡ℎ
𝑉
• N=
𝑉𝑣
𝑉𝑠
=
𝑛
1
𝑉 = 1, 𝑛 = 𝑉𝑣 , 𝑒 =
= 𝑛, Ƴ𝑑 =
𝑊𝑠
𝑉
= 𝐺𝑠 Ƴ𝑤 (1 −
𝑛)
• Ƴ𝑏𝑢𝑙𝑘 =
𝑊
𝑉
= (1 − 𝑛)
𝐺𝑠 Ƴ𝑤 +w𝐺𝑠 (1 − 𝑛)Ƴ𝑤
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35
Soil Aggregate Relationship
For saturated soil:
Ƴ𝑠𝑎𝑡 = 𝐺𝑠 1 − 𝑛 Ƴ𝑤 + 𝑛Ƴ𝑤
𝑤 = 𝑛Ƴ𝑤 /𝐺𝑠 1 − 𝑛 Ƴ𝑤
𝑤 = 𝑒/𝐺𝑠 , e= 𝑤𝐺𝑠 (𝑓𝑜𝑟 𝑆𝑟 = 1)
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• For a dry soil:
• Ƴ𝑑 =
𝑀𝑠
𝑉
= (1 − 𝑛)𝐺𝑠 Ƴ𝑤
36
Soil Aggregate Relationship (Relative density)
• The term relative density is commonly
used to indicate the in situ denseness or
looseness of granular soil. It is defined
as
• 𝐷𝑟 =
𝑒𝑚𝑎𝑥 −𝑒
*100
𝑒𝑚𝑎𝑥 −𝑒𝑚𝑖𝑛
• Where Dr is relative density, e is in situ
• The relationships for relative density can also be
defined in terms of porosity, where nmax and nmin
porosity of the soil in the loosest and densest
conditions
void ratio of the soil, emax is void ratio
of the soil in the loosest state, emin is
void ratio of the soil in the densest state
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37
Soil Aggregate Relationship (Relative density)
• The maximum and minimum
void ratios for granular soils
described in depend on several
factors, such as
• Grain size
• Grain shape
• Nature
of
the
grain-size
distribution curve
• Fine contents, Fc (that is,
fraction smaller than 0.075 mm)
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38
Grain shape of soil
• The shape of soil grains is a useful
General classification of grain shape
soil grain property in the case of
• Bulky grins
coarse grained soils and it is
• Flaky grains
important
• Needle shaped grains
in
influencing
the
engineering behavior of soils.
• The shape of coarse grain soil can
be examined with naked eyes,
whereas fine grained soils require
• Bulky Grains: where all dimensions of a
grain are more or less the same.
• These are characteristics of sand and gravel
soils.
microscopic examination.
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39
Grain shape of soil
• Mechanical break down of parent rocks are source
of bulky grains.
• During their transportation by wind or water, the
sharp edges of the grains may get worn out and the
grains may become rounded.
• E.g. the shape of river gravels and wind blown
sands is rounded
• Alluvial sands-sub-angular to sub-rounded
• Soils containing particles with high angularity
tend to resist displacement and hence possess
higher shearing strength compared to those with
less angular particles.
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40
Grain shape of soil
• Flaky grains: plate shaped grains • Needle shaped grains: one dimension of
are
the
ones
in
which
one
dimension of grain, normally its
the grain is fully developed and is much
larger than the other two dimensions.
thickness bears no relationship with • Needle shaped grains are characteristic of
the other two lateral dimensions
the clay mineral Atapulgite.
which are much bigger.
• Submicroscopic crystals of clay
minerals usually exhibit flaky grain
shape.
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Clay minerals
• Clay minerals: are complex aluminum silicates
• n
composed of two basic units: (1) silica
tetrahedron and (2) alumina octahedron.
• Each tetrahedron unit consists of four oxygen
atoms surrounding a silicon atom.
• Tetrahedron units linkup in hexagonal pattern
and form tetrahedral layer which represented
by silica sheet.
• The other fundamental unit is octahedron
which represented by aluminum sheet or
gibbsa sheet.
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Clay minerals
• Octahedron units linkup to form • Clay minerals: is the chemical weathering product
octahedral layer.
• If the anions of octahedral sheet
are hydroxyls and 67% of the
resulted in groups of crystalline particles of
colloidal size less than 2μ.
• The combination of silica sheet and gibbsite sheet
can provide fundamental sheet structures.
cation positions are filled with
Al, then it is called Gibbsite
sheet
• Based on the structure, clay minerals are grouped
into Kaolinite, Illite, and Montmorillonite
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Clay minerals
I. Kaolinite: is common in sedimentary and
• n
Al
residual soils.
Si
• It is a repeating layer of one silicate sheet and
Al
➢ (OH)8Al4Si4O10
0.72 nm
Si
one gibbsite sheet or alumina sheet.
Al
• Forms strong hydrogen bonds between the
Si
hydroxyl of the gibbsite sheet and oxygen of
Al
the silicate sheet.
Si
joined by oxygen
sharing
• It has little tendency in the interlayer to allow
water and to swell.
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Clay minerals
II. Illite
•n
• It is repeated layers of gibbsite sheet sandwiched by
two silicate sheet
• Its similar with montmorillonite except its adjacent
silica layers are bounded by potassium ions instead
water
• It is common in stiff clays and shales, post glacial
marine and lacustrine soft clay and silt deposit
• Swelling potential of illite>kaolinite<montmorillonite
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Clay minerals
III. Montmorillonite
• Is also called smectite clay mineral
• It has bounded by weak bonds of weak
Vander Waals forces
• The amount of available water in the space
can control the spacing between S-G-S.
•n
Si
Al
Si
Si
Al
Si
0.96
nm
Si
Al
Si
• It is easily separated by water and has high
tendency to water absorption and swell
• Is dominate clay minerals in shales, residual
soils derived from volcanic ash
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Formation of Clay minerals
• Clay minerals are not stable, static
Clay minerals
occurrence
Kaolinite
Common in tropical and
subtropical area. Highly
weathered soils with good
drainage. In older soils.
Montmorillonite
Weathering products of
volcanic rocks or ash.
Common in sediments of arid
areas and often mixed with
clay mica. E.g. bentonite
Illite
Weathering of sedimentary
rock in arid regions. Found in
slate and shale.
Chlorite
common in marine sediments
and metamorphic rocks. Not
dominantly found.
entities in soils.
• Since
clay
minerals
are
the
products of chemical weathering of
rocks, both the climate and parent
rock, influence the type of minerals
found.
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Identification of Clay minerals
• No one method is satisfactory Three methods
for identification because of
• interference of minerals in a
mixture and
• Range
of
I. X-ray diffraction (XRD)
II. Differential thermal analysis (DTA)
III. Casagrande’s plasticity chart
composition
and
crystal structure of clays from
different sources.
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Clay Shapes and Surface Areas
• Water adsorption= f(SSA) and SSA is
Specific Surface Area (SSA)
f(particle size)
• Clay has flaky shape
• SSA is defined as the total
surface of the
Lambe and Whitm (1969)
Mineral
SSA
(m2/g)
Water absorbed
(%)
Quartzite
0.03
1.5x10-4
Kaolinite
10
0.5
which is simply the total surface of the
Illite
100
5
individual grains per dry mass of the grains,
Montmori 1000
llonite
individual grains per dry mass of the grains.
• 𝑆𝑆𝐴 =
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎
𝑣𝑜𝑙𝑢𝑚𝑒∗ρ
• SSA is inversely proportional to the particle size
• As the grain sizes decrease, the SSA of the soil
increases exponentially.
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49
Surface Charge of Clay Particles
• The surface of clays is generally
Isomorphous substitution
negatively charged, even though
• It is the replacement of a cation in the mineral
the resultant charge in a particle is
structure by another cation of lower valence,
neutral
but of the same physical size
• Due to:
• For example, replacements of silicon ion in a
• Isomorphous substitution
tetrahedral unit by aluminum ion ( which could
• Breakage of particles
happen when Al ions are more readily available
• Dissociation of hydroxyl (OH-)
in water)
radicals
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Clay Water Systems
• In dry clay, the negative charge is balanced by
•c
exchangeable cations like Ca+2, Mg+2, Na+1, and
K+1 surrounding the particles being held by
electrostatic attraction.
• When water is added to clay, these cations and a
few anions float around the clay particles.
• This configuration is referred to as a diffuse
double layer.
• The cation concentration decreases with the
distance from the surface of the particle
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Interaction of Clay Particles
• When many clay particles are
mixed together in water, particles
interact and their unit micelles
overlap each other.
• Several interactive forces (attractive
or repulsive) exist between particles
when those particles are brought
closer.
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Clay minerals and cation exchange capacity (CEC)
CEC
• The ability of the a clay particle to adsorb ions on
its surface or edges is called CEC
• CEC, measured in milliequivalents per 100g of dry
soil particles, is a measure of net –ve charge on the
soil particles, resulting from isomorphs substitution
and broken bonds at the boundary.
• CEC=f(mineral structure of clay and size of
particle)
• CEC of montmorillonite=10(CEC) kaolinite
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Mineral type CEC (meq/100g of dry
soil)
Quartzite
Very small (due to fine
particles and broken
bonds)
kaolinite
3-8
Illite
40
Montmorillo 80
nite
• Exchangeable
cations
are
the
+vely
charged ions from the soils in the pore
water which are attracted to the surface of
clay particles to balance the –ve charge.
53
Clay minerals and cation exchange capacity (CEC)
• The cations can be arranged in a series in
terms of their affinity for attraction as follows:
• Here Ca2+ ions replace Na1+ ions and
reduces
swelling
of
Na-
• Al3+>Ca2+>Mg2+>NH4+>K+>H+>Na+>Li+
montmorillonite, because the adsorbed
• This indicates that, Al3+ ion can replace Ca2+
water layer would become thinner and
ions and Ca2+ can replace Na+ ions.
undergoes a structural distortion.
• This process is called cation exchange.
Practical example for cation exchange
• Stabilization of sodium based clay soil using
lime,
Naclay
(montmorillonite
based)+CaCl2+Caclay+NaCl
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Particle Force and Soil Structure
Particle forces and behavior
• Particle surface forces are of an electrical nature
• The behavior of individual soil particles and their
• They are caused by unsatisfied electrical charges in
interaction with other particles is influenced by the
the particle crystalline structure (net-ve charges).
following forces:
• Weight of the particle Fg
• Surface forces Fs are directly proportional to the
surface area and hence for equidimensional particles,
• Particle surface forces Fs
Fs α D 2
• Weight force of the particle is the result of the
gravitational forces and is a function of the volume
•
of the particle
• Thus, for larger particle sizes, which include soil
• For equi-dimensional particles such as spheres of
diameter D, the weight Fg is directly proportional to
D3 or Fgα D3
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Fg/Fs α D, As D increase,
Fg/Fs increase
particles in the coarser fraction (>0.075mm) Fg is
predominant over Fs
55
Particle Force and Soil Structure
Particle forces and behavior
Soil Structure: the geometric arrangement of soil
• As the particle diameter decreases the ratio
particles with respect to one another
Fg/Fs decreases, that means surface forces
• Structures of Cohesive less Soils
• Single grained
predominate.
• Properties of soil mass = f(arrangement of
• Honeycomb
Structures of Cohesive Soils
grains)
• The system of discrete particles (grains), that
• Flocculent/dispersed
makeup soil are not strongly bonded together
• Size, shape of grains and minerals from which the
and hence are relatively free to move with
grains are formed determine the formation of a
respect to each other
particular soil structure
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Structures of Cohesive less Soils
Single grained structure
•n
• Each grain touches several of its neighbors in
such a way that the aggregate is stable even if
there are no forces of adhesion at the points of
contact between the grains.
• The arrangement may be dense or loose and the
properties of aggregate are greatly influenced by
the denseness or looseness
❖Loose state= high void ratio and low unit weight
❖Dense state= low void ratio and high unit weight
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Structures of Cohesive less Soils
• For granular soils (sand and gravel)
•n
the range of void ratios generally
encountered can be visualized by
considering an ideal situation in
which particles are spheres of equal
size
• The loosest and densest possible
arrangements that we can obtain from
these equal spheres are simple cubic
and the pyramidal type of packing
respectively.
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Structures of Cohesive less Soils
• In the case of natural granular soils,
• n
particles are neither of equal size nor
perfect spheres
a) The small size particles may occupy
void spaces between the larger ones,
which will tend to reduce the void ratio
of natural soils as compared to that for
equal spheres
b) On the other hand , the irregularity in
the shape of the particles generally
tends to increase void ratio of soil as
compared to ideal spheres
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Structures of Cohesive less Soils
Honeycomb structure-Single grained Soil
• It is found in soil contains particle of size 0.02mm
❖The attraction of particles is due to cohesion
between them, but this cohesion is just because of
their size but however, these soils are not plastic
to 0.002mm which are generally fine sands or silts.
in nature.
• When this type of soils is allowed to settle on the
ground, the particles will attract each other and
joins one with another and forms a bridge of
❖In fine sands, when water is added to dry fine
sand bulking of sand occurs which is nothing but
a structure of honeycomb.
particles.
• A large void is also formed between those bridges
which makes the soil very loose in nature.
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Structure of clay soils (fine grained soils)
• The
final
established
structures
from
the
of
clay
are
balance
of
interactive forces and external forces
applied to the clay assemblage
Final clay
structure with
particles’
interactive and
external forces
• If two particles (platelet shape) approach
each other in a suspension, the forces
acting on them are:
a) The Van der Waals force of attraction,
and
b) The repulsion between two +vely
charged ionized adsorbed water layers.
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Particle Force and Soil Structure
Dispersed clay structure
• If
the
final
interparticle
forces
are
repulsive, the particles want to separate from
each other when the boundary confinements
are removed.
• The net forces of repulsion are greatest in
the case of particles approaching face to face
• Lacustrine clays (deposited in fresh water
lakes) generally have a dispersed structure.
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Particle Force and Soil Structure
• In freshwater environments, more face-to-face
Flocculated clay Structure
• If the interparticle forces are attractive, then
particles
want
to
come
together,
making
flocculated structures are formed due to negative
charges at the edges
flocculated clay.
• In flocculated clays, surface and edge charges play
an important role.
• If the edge charges are positive, most likely the
edges are attracted to the flat surfaces of other clay
particles.
• This makes a card-house structure of flocculated
clay, most commonly in saltwater environments.
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Methods to identify soil structure
Ordinary microscope
SEM
• It is valid for coarse grained soils only
Scanning electron microscopy (SEM)
• uses electrons rather than light to form
an image.
• This is ideally suited for clayey soils, as
the resolution is sufficiently high and
hence it is possible to go for higher
magnifications (=1x105 times)
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End of chapter 2
36
•This is all what I have to say.
•Thank you too much for your
eyeful attention!
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