final engneering geology

8/10/2019 Final Engneering Geology

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ENGINNERING GEOLOGY


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INTRODUCTION

When the St. Francis dam in southern

California failed in 1928 with a loss of many

lives and damages in millions of dollars ; the

civil engineering profession a woke to the

idea that the careful design of a structure

itself isnt all that is required for its safety.

This is followed by the appearance of newspecialist engineering geology.


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..When designing a structure, it is necessary todetermine and take into account theenvironmental conditions under which it will beconstructed. These notably should include soils

and rocks that will support the structure, theircomposition, state and properties and also themode of their occurrence, thickness of individuallayers, jointing of the hard rock mass etc..

The mode of occurrence of subsurface water andits regime in the soil mass being explored are animportant component of the problem in hand..


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In addition, a structure to be built may be

under deleterious effect of one or more

natural geological processes or phenomena

known as geodynamic. This include; frost and

foremost, seismic phenomena (earth quakes),

landslide and rock fall, collapse induced by

caving of the roofs of substenaneous voidsetc..


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.. The goal of this course is to give the student

of the geology department, the basic

geotechnical principles that can provide a

solid basis for the solution of problems

connected with natural environment of an

engineering structure, particularly the

surrounding ground..


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Definitions

Soil:In general sense of engineering, soil is defined as the uncemented aggregateof mineral grains and decayed organic matter (solid particles; along with theliquid and gas that occupy the empty spaces between the soil particles)..Soil is used as a construction materials in various civil engineering projects, and itsupport structural foundations..

Soil mechanics:

It is the branch of science that deals with the study of physical properties of soiland the behavior of soil masses subjected to various types of forces..

Soil engineering:

It is the application of the principles of soil mechanics to particle problems..


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Engineering geology:

It is defined as a branch of human knowledge that uses geological informationcombined with practices and experience to assist the engineer in the solution ofproblems in which such knowledge may be applicable.

Or its the branch of geology which studies the dynamics of the upper horizon of theearth crust due to the human engineering activity.

Recently it is the science which studies the upper part of the earth crust as a mediumfor human activity Also the engineering geology is the science which concerns withthe exploitation and protection of geological environment..

Geotechnics:

It means the application of earth science (geology, soil mechanics, pedology,hydrology, geophysics etc.)


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Engineering geology can be subdivided into twomain branches:

The soil engineering & Engineering geodynamics which include the

engineering geological processes e.g.Landslides, carstification, earthquakes, action ofsurface and subsurface waters, action of marinewaters and etc..


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Chapter one

Soils & Rocks


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Soils :

The mineral grains that form the solid phases of a soilaggregate are the product of rock weathering, the main

physical properties of soil are dictated by the size, shapeand chemical composition of the grains.

Rocks:

Igneous- Metamorphic- Sedimentary

Rock forming minerals :


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Soil Horizons

Layer of Soil Parallel toSurface

Properties a functionof climate, landscapesetting, parentmaterial, biologicalactivity, and other soil

forming processes. Horizons (A, E, B, C, R,

etc)

Image Source: University of Texas, 2002


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Residual Rocks and Minerals

Residual minerals weather in place to form soils.

Bedrock

Soil

http://www.gly.uga.edu/railsback/FieldImages.html


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Products of subaerial weathering

1- Source rock2- Secondary minerals3- Soluble materials

Source rock : becomes either,

Young soil +rock fragments + immature minerals e.g. biotite, Pyroxene,Amphibole, Calcic feldspar

or

Mature soil + Q z+ K feldspar + Muscovite.

Secondary minerals:Formed by chemical reactions: clay minerals, ferric oxides, hydroxides, Al

hydroxides.

Soluble materials:

CO32+, SiO42+, K, Na, Ca, Mg.


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Clay minerals

The term clay is used as mineral name or textural name (-1 or -2 m). Clays are small crystalline particles of one or more members of a

small group of minerals. They are primarily hydrous aluminumsilicates with magnesium or iron occupying all or part of the Alpositions in some minerals, and with alkalis (e.g. Na+, K+) or alkalineearth (e.g. Ca++, Mg++) also present as essential constituents in some

of them.. The clay minerals have unit cells with a residual negative charge. Most clay minerals exhibit plasticity when mixed with limited amount

of water. They have relatively high resistance to weathering (end product).

They found as residual clay, form the main constituents of shale andmuds, found in the argillaceous limestones and marles.


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Clay minerals







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Structural unites of the clay minerals

The clay minerals are one member of the family whichcalled Phyllosillicates .

The different clay minerals groups are characterized bystacking arrangements of sheets (sometimes chains) andmanner in which two or three sheet layers are holdtogether.

The two basic units in clay minerals structures are the Silicatetrahedron and the Aluminum or Magnesium octahedron.Silica sheet composition (Si4O10)

4.Octahedron sheetcomposition either,

Gibbsite sheet, Gibbsite mineral [Al2(OH)6], Di-octahedral..

Or Brucite sheet, Brucite mineral [Mg3(OH)6], Tri-octahedral.


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Synthesis

Noncrystalline clay -

allophane

Mitchell, 1993


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Group 1:1

Is made of a silica and octahedral sheet.Structural formula is[ (OH)8Si4Al4O10]. And thebonding between the successive layers is bothby Vander Waals forces and hydrogen bonds,

No swelling. The members of Kaolinite subgroup are nacrite-

dickite and hallysite. Nactite and dickite are rareand appear to form as a result of hydrothermal

processes. Two form of hallysite are present thenonhydrated [(OH)8Si4Al4O10], and hydrated one[(OH)8Si4Al4O10.4H2O)].


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1:1 Minerals-Kaolinite

Basal spacing is 7.2

Si4Al4O10(OH)8. Platy shape

The bonding between layers are van derWaals forces and hydrogen bonds(strong bonding).

There is no interlayer swelling

Width: 0.1~ 4m, Thickness: 0.05~2 m

layer

Trovey, 1971 ( fromMitchell, 1993)

17 m


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Clay minerals







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Structural unites of the clay minerals

The clay minerals are one member of the family whichcalled Phyllosillicates .

The different clay minerals groups are characterized bystacking arrangements of sheets (sometimes chains) andmanner in which two or three sheet layers are holdtogether.

The two basic units in clay minerals structures are the Silicatetrahedron and the Aluminum or Magnesium octahedron.Silica sheet composition (Si4O10)

4.Octahedron sheetcomposition either,

Gibbsite sheet, Gibbsite mineral [Al2(OH)6], Di-octahedral..

Or Brucite sheet, Brucite mineral [Mg3(OH)6], Tri-octahedral.


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23

Synthesis

Noncrystalline clay -

allophane

Mitchell, 1993


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Group 1:1

Is made of a silica and octahedral sheet.Structural formula is[ (OH)8Si4Al4O10]. And thebonding between the successive layers is bothby Vander Waals forces and hydrogen bonds,

No swelling. The members of Kaolinite subgroup are nacrite-

dickite and hallysite. Nactite and dickite are rareand appear to form as a result of hydrothermal

processes. Two form of hallysite are present thenonhydrated [(OH)8Si4Al4O10], and hydrated one[(OH)8Si4Al4O10.4H2O)].


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1:1 Minerals-Kaolinite

Basal spacing is 7.2

Si4Al4O10(OH)8. Platy shape

The bonding between layers are van derWaals forces and hydrogen bonds(strong bonding).

There is no interlayer swelling

Width: 0.1~ 4m, Thickness: 0.05~2 m

layer

Trovey, 1971 ( fromMitchell, 1993)

17 m


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Group 2:1

Structuraly this group formed from one octahedralsheet sandwidshed between two tetrahedral sheet.

The Smectite minerals : The bonding between successive layers is by Van der

Waals forces and by cations that may present tobalance charge deficiencies in the structure.Thesebonds are weak and easily separated by cleavage oradsorption of water or other polar liquids.

The theoretical composition in the absence of latticesubstitutions is [(OH)4Si8Al4O20.n(interlayer)H2o].


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2:1 Minerals-Montmorillonite

nH2O+cations

5 m

Si8Al4O20(OH)4nH2O (Theoretical

unsubstituted). Film-like shape.

There is extensive isomorphoussubstitution for silicon and aluminumby other cations, which results incharge deficiencies of clay particles.

nH2O and cations exist between unitlayers, and the basal spacing is from9.6 to (after swelling).

The interlayer bonding is by van derWaals forces and by cations whichbalance charge deficiencies (weak

bonding).There exists interlayer swelling,

which is very important toengineering practice (expansiveclay).

Width: 1 or 2

m, Thickness: 10

~1/100 width(Holtz and Kovacs, 1981)


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Origins of Charge Deficiencies

1. Imperfections in the crystal lattice -

2. Isomorphous substitution. The cations in the octahedral or tetrahedral sheet can be replaced by

different kinds of cations without change in crystal structure (similarphysical size of cations).

Al in the octahedral sheet may be replaced by Mg, Fe, Zn, Ni, Li or othercations.

Al may replace up to 15% silicon in the tetrahedral sheet. The charge deficiency resulting from these substitutions is balanced by the

exchangeable cations that take up positions between the unit cell layers andon the structure particles.

This is the main source of charge deficiencies for montmorillonite.Only minor isomorphous substitution takes place in kaolinite.


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Isomorphous substitution

Some minerals formulas are listed below:

-Di octahedralsmectite (Montmorillonite):

Monmorillonite (OH)4Si8(Al Mg)4O20 Beidellite (OH)

4

(Si Al )Al4

O20

Nontronite (OH)4(Si Al) Fe4O20

-Tri octahedral smectite (Sponite):

Hectorite (OH)4 Si8(Mg Li)6 O20 Saponite (OH)4 (Si Al) Mg6O20

Sauconite (OH)4(SiAl)8(Zn Mg)6O20


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2:1 Minerals-Illite (mica-like minerals)

potassium

Si8(Al,Mg, Fe)4~6O20(OH)4(K,H2O)2. Flaky shape.

The basic structure is very similar to the mica,so it is sometimes referred to as hydrous mica.Illite is the chief constituent in many shales.

Some of the Si4+ in the tetrahedral sheet arereplaced by the Al3+, and some of the Al3+ in

the octahedral sheet are substituted by theMg2+ or Fe3+. Those are the origins of chargedeficiencies.

The charge deficiency is balanced by thepotassium ion between layers. Note that thepotassium atom can exactly fit into the

hexagonal hole in the tetrahedral sheet andform a strong interlayer bonding.

The basal spacing is fixed at 10 in thepresence of polar liquids (no interlayerswelling).

Width: 0.1~ several m, Thickness: ~ 30 7.5 m Trovey, 1971 ( fromMitchell, 1993)

K


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Illites differ from the mica in the following:

Fewer of Si++position are occupied by Al3+in

illite.

There are some randomness in stacking of

layers in illite.

There is some relatively less K in illilte. Well

organized illite contain 9 to 10% of K2O.

The size of illite particles occurring naturally

is very small


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Glauconite: it is an Fe-rich illite contain Mgand Fe in octahedral sheet as well as Al.usually occurs as earthy green pellets.

The vermiculite consist of a regularinterstratification of biotite mica layers anddouble molecular layers of water. Generalformula is[(OH)4(Mg Ca)x(Si Al)8-x(Mg

Fe)6O20yH2O].(x~1:1.4, y~8)


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2:1 Minerals-Vermiculite (micalike minerals)

The octahedral sheet is brucite.The basal spacing is from 10 to 14

.

It contains exchangeable cationssuch as Ca2+and Mg2+and twolayers of water within interlayers.

It can be an excellent insulationmaterial after dehydrated.

Illite Vermiculite

Mitchell, 1993


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Group 2:1:1 (The chlorite minerals)

The basal spacing is fixed at 14 .

Gibbsite or

brucite

The structure is similar to

vermeculite except that an

organized octahedral sheet

replaces the double water layer

between mica layers.


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Chain Structure Clay Minerals

Some clay minerals are formedfrom bonds (double chains) of silicatetrahedron, The minerals

palygorskite (attapulgite) andsepiolite differ primarily in thereplacements within the structure.

.They have lathlike or threadlikemorphologies.

The particle diameters are from 50to 100 and the length is up to 4 to5 m.

Attapulgite is useful as a drilling mudin saline environment due to its highstability.

4.7 m

Trovey, 1971 ( from Mitchell, 1993)

Attapulgite


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Mixed layer Clay

Due to the greatest similarity in the crystal structureamong the different minerals, the interstratification oftwo or more different layer type often occur within asingle particles.

Interstratification may be, regular, with a definiterepitation of the different layers in sequence, or it may berandom. The most abundant mixed layer mineral iscomposed of expanded, water bearing and contractednonbearing layers.

Montmorillonite-illite is the most common, althoughchlorite-vermiculite and chlorite-montmorillonite are oftenencountered.

Rectorite is a regular interstratification clay with a mica-like layer alternating in a regular manner withmontmorillonite like layer.


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Interaction of Water and Clay

Minerals.


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Charged Clay Particles

External or interlayer surfaces arenegatively charged in general.

The edges can be positively ornegatively charged.

Different cations balance chargedeficiencies.

Dry condition

- or +

- or +

Cation

Kaolinite and negative gold sol

(van Olphen, 1991)


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The dipolar water is attracted by: Negatively charged surface of the clay particles. The cations in the double layer.

Hydrogen bonding where hydrogen atoms in thewater molecules are shared with oxygen atomson the surface of the clay.

The partially hydrated cations in the pore water.

The force of attraction between water and claydecreases with distance from the surface of theparticles.


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Clay particle charge and the double

layer Clay particles carry a net negative charge on their surfaces

and some positively charged sites at the edges. This is theresult of both of isomorphous substitution and of a breakin continuity of the structure at its surface.

In dry clay, the negative charge is balanced byexchangeable cations like Ca2+, Mg2+, Na+and K+surrounding the particles being held by electrostaticattraction.

When water is added to clay, these cations and smallnumber of anions float around the clay particles. This isreferred to a diffuse double layer (fig. 1.8a). The cationconcentration decreases with distance from the surface ofthe particles (fig. 1.8b).


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Diffuse Double Layer

Clay particle with

negatively chargedsurface

x Distance xConcentrat

ion

Exponential decay

Cations

Anions

-

-

++ -

-

-

--


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All water held to the clay particles by forces

of attraction is known as double layer water.

The intermost layer of double layer water,

which is held very strongly by clay, is knownas adsorbed water.

Figure 1.11 shows the absorbed and double-

layer water for typical montmorillonite andkaolinite particles.


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43

Clay-Water Interaction (Cont.)

Relative sizes of adsorbed water layers on sodium montmorillonite andsodium kaolinite

Holtz and Kovacs, 1981


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44

Interaction of Clay Particles(or Layers)

Interlayer

Interparticle

Layer

Particle


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45

Interaction Forces

Net force between clay particles (or interlayers)

= van der Waals attraction +

Double layer repulsion (overlapping of the double layer)+

Coulombian attraction (between the positive edge and negativeface)

VO forces


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Thickness of Double Layer

Thickness ofdouble layer K

Valence:eargchElectron:e

ionconcentratCation:n

eTemperatur:T

ttanconsBoltzman:k

typermittivilativeRe:

vacuumintyPermittivi:

en2

kTK

0

0

2/1

22

0

0

K repulsion force

n0 K repulsion force v K repulsion force

T K repulsion force (?)

decreases with increasing

temperature


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47

Interaction of Clay ParticlesDispersed fabric

The net interparticle forcebetween surfaces isrepulsive

Increasing

Electrolyte concentration n0Ion valence vTemperature T (?)

Decreasing

Permittivity Size of hydration ionpH

Anion adsorption

Reduce the doublelayer repulsion (onlyapplicable to some cases)

Flocculated or

aggregated fabric

Flocculated fabric

Edge-to-face (EF):positivelycharged edges and negativelycharged surfaces (more common)

Edge-to-edge (EE)

Aggregated fabric

Face-to-Face (FF)Shifted FF


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Interaction of Clay Particles (Cont.)

(1) Decrease pH

(2) Decrease anion adsorption

(3)Size of hydrationClay

Particle

The total requirednumber of cationsis 10

+

+


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49

Atterberg Limit of Clay Minerals

Lambe and Whitman, 1979

Na-montmorilloniteThicker double layer

LL=710

Ca-montmorillonite

Thinner double layer

LL=510

The thickness of double

layer increases with

decreasing cation valence.


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50

Cation Replaceability

Different types and quantities of cations are adsorbed to balance chargedeficiencies in clay particles.

The types of adsorbed cations depend on the depositional environment.For example, sodium and magnesium are dominant cations in marineclays since they are common in sea water. In general, calcium and

magnesium are the predominant cations.

The adsorbed cations are exchangeable (replaceable). For example,

Na Na Na Na

Na Na Na Na

+4CaCl2 +8NaClCa

Ca

Ca

Ca

(Lambe and Whitman, 1979)


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Cation Exchange Capacity (cec)

The quantity of exchangeable cations is termed the cationexchangeable capacity (cec) and is usually expressed asmilliequivalents (meq) per 100 gram of dry clay ( from Mitchell, 1993).

One equivalent = 6.021023electron charges or 96500 Coulombs, which is 1Faraday.


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52

Swelling PotentialPractically speaking, the three ingredients generally necessary for

potentially damaging swelling to occur are (1) presence ofmontmorillonite in the soil, (2) the natural water content must bearound the PL, and (3) there must be a source of water for the

potentially swelling clay (Gromko, 1974, from Holtz and Kovacs, 1981)


U.S. Bureau ofReclamation


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53

Interaction of Water and Clay

Minerals.


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54

Charged Clay Particles

External or interlayer surfaces arenegatively charged in general.

The edges can be positively ornegatively charged.

Different cations balance chargedeficiencies.

Dry condition

- or +

- or +

Cation

Kaolinite and negative gold sol

(van Olphen, 1991)


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The dipolar water is attracted by: Negatively charged surface of the clay particles. The cations in the double layer.

Hydrogen bonding where hydrogen atoms in thewater molecules are shared with oxygen atomson the surface of the clay.

The partially hydrated cations in the pore water.The force of attraction between water and claydecreases with distance from the surface of the

particles.

l i l h d h d bl


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Clay particle charge and the double

layer Clay particles carry a net negative charge on their surfaces

and some positively charged sites at the edges. This is theresult of both of isomorphous substitution and of a breakin continuity of the structure at its surface.

In dry clay, the negative charge is balanced by

exchangeable cations like Ca2+, Mg2+, Na+and K+surrounding the particles being held by electrostaticattraction.

When water is added to clay, these cations and smallnumber of anions float around the clay particles. This is

referred to a diffuse double layer (fig. 1.8a). The cationconcentration decreases with distance from the surface ofthe particles (fig. 1.8b).


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Diffuse Double Layer

Clay particle withnegatively charged

surface

x Distance xConcentration

Exponential decay

Cations

Anions

-

-

++ -

-

-

--


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All water held to the clay particles by forces

of attraction is known as double layer water.

The intermost layer of double layer water,

which is held very strongly by clay, is knownas adsorbed water.

Figure 1.11 shows the absorbed and double-

layer water for typical montmorillonite andkaolinite particles.


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59

Clay-Water Interaction (Cont.)

Relative sizes of adsorbed water layers on sodium montmorillonite andsodium kaolinite



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Interaction of Clay Particles(or Layers)

Interlayer

Interparticle

Layer

Particle


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61

Interaction Forces

Net force between clay particles (or interlayers)

= van der Waals attraction +

Double layer repulsion (overlapping of the double layer)+

Coulombian attraction (between the positive edge and negativeface)

VO forces


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Thickness of Double Layer

Thickness ofdouble layer K

Valence:

eargchElectron:e

ionconcentratCation:n

eTemperatur:T

ttanconsBoltzman:k

typermittivilativeRe:

vacuumintyPermittivi:

en2

kTK

0

0

2/1

22

0

0

K repulsion force

n0

K

repulsion force

v K repulsion force

T K repulsion force (?)

decreases with increasingtemperature


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Interaction of Clay ParticlesDispersed fabric

The net interparticle forcebetween surfaces isrepulsive

Increasing

Electrolyte concentration n0Ion valence vTemperature T (?)

Decreasing

Permittivity Size of hydration ionpH

Anion adsorption

Reduce the doublelayer repulsion (onlyapplicable to some cases)

Flocculated or

aggregated fabric

Flocculated fabric

Edge-to-face (EF):positivelycharged edges and negativelycharged surfaces (more common)

Edge-to-edge (EE)

Aggregated fabric

Face-to-Face (FF)Shifted FF


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Interaction of Clay Particles (Cont.)

(1) Decrease pH

(2) Decrease anion adsorption

(3)Size of hydrationClay

Particle

The total requirednumber of cationsis 10

+

+


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65

Atterberg Limit of Clay Minerals

Lambe and Whitman, 1979

Na-montmorilloniteThicker double layer

LL=710

Ca-montmorillonite

Thinner double layer

LL=510

The thickness of double

layer increases with

decreasing cation valence.


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66

Cation Replaceability

Different types and quantities of cations are adsorbed to balance chargedeficiencies in clay particles.

The types of adsorbed cations depend on the depositional environment.For example, sodium and magnesium are dominant cations in marineclays since they are common in sea water. In general, calcium and

magnesium are the predominant cations.

The adsorbed cations are exchangeable (replaceable). For example,

Na Na Na Na

Na Na Na Na+4CaCl2 +8NaCl

Ca

Ca

Ca

Ca

(Lambe and Whitman, 1979)


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67

Cation Exchange Capacity (cec)

The quantity of exchangeable cations is termed the cationexchangeable capacity (cec) and is usually expressed asmilliequivalents (meq) per 100 gram of dry clay ( from Mitchell, 1993).

One equivalent = 6.021023electron charges or 96500 Coulombs, which is 1Faraday.


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68

Swelling PotentialPractically speaking, the three ingredients generally necessary for

potentially damaging swelling to occur are (1) presence ofmontmorillonite in the soil, (2) the natural water content must bearound the PL, and (3) there must be a source of water for the

potentially swelling clay (Gromko, 1974, from Holtz and Kovacs, 1981)


U.S. Bureau ofReclamation


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Engineering ApplicationsDispersion agents (drilling mud; hydrometer analysis)

Sodium hexa-metaphosphate (NaPO3) and sodium silicate (Na2SiO3) are used asthe dispersion agent in the hydrometer analysis. How does this dispersion agentwork?

Three hypotheses:

(1) Edge-charge reversal

The anions adsorption onto the edge of the clay particle may neutralize the positive edge-charge or further reverse the edge-charge from positive to negative. The edge-chargereversal can form a negative double layer on the edge surfaces to break down flocculatedstructure, and assist in forming a dispersed structure.

(2) Ion exchange

The sodium cations can replace the divalent cations existing in the clay particles such as

Ca2+

and Mg2+

. The decrease of cation valence can increase the thickness of the doublelayer and interparticle repulsion, which can assist in forming a dispersed structure.

(3) pH

The higher pH may make the edge-charge tend to be negative, which can break down theflocculated structure and assist in forming a dispersed structure. The adding of dispersingagent such as sodium carbonate may slightly increase the pH.

The effect of the clay minerals in


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The effect of the clay minerals in

Engineering Geology

The presence of clay minerals in soils and rocks have advantageand disadvantage effects on the Engineering Geological behaviorof soils and rock mass.

The advantage Effects:

Linening of the irrigation canals. Used in drilling mud. Injection of some porous sediments to decrease porosity and

increase shearing strength. Used in adsorbing of some toxic heavy metals.

The presence of thin layer on the top of silty sediments preventsthe penetration of the water to the slope, prevent its sliding slope.


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Disadvantages Effects: The presence of clays in a slope sediments increase the

probability of its sliding. The construction engineering structures on the clay lenses

produce their tipping. The swelling of clay under the foundation of structure

produce the warping of their floors and sometimes to thefailler.

The consolidation of clays under the load of engineering

structure may produce a great settlement of this structure. The presence of clayey sediments slope produces a high

pore pressure, which decrease the stability of this slopeand its sliding.


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Valley and Rivers


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Valley formation

From the engineering point of view, valley is characterized by its,longitudinal profile and its cross sections.

The longitudinal profile of a valley is the profile along its thalweg.The average of the thalweg is the average gradient of the valley.The cross sections of the valley for engineering purposes are taken frombank perpendicular the thalweg.

The lengthening of the valley is accomplished mostly by head erosion, orgradual destruction of rocks and soil masses in the upstream direction[i.e.] the backward of the valley.

While cutting backward the stream flowing in the valley also wear its beddownward, at least locally. In the course of time, the floor of the valleywill cut down to the ground-water level [water table].


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Types of streams

1- The permanent stream:Characterized by the flow in both dry and wet seasons [watersupply from the ground water].

2- Intermitent streams:Periodically dry out but have permanent water table below the

stream bed.3- Ephemeral streams:

Contain water only after rainfall or snow melting and have noconnection with water table.

It should be noted that if the valley of a small stream is filled withearth material and its dry surface is used for engineering purpose,for instance for a housing project the water flow


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Generally, the valley is much wider than the

stream flowing in it, except during flood

periods the width of the valley varies

according to the erodability of the materialon the valley slopes.

The engineering significance of river


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The engineering significance of river

terraces

The engineering significance of river terraces isconsiderable, particularity in localization of highways andrailroads. If two cites or regions that have to be connectedby railroads have considerable difference in elevations, itis often convenient to flow the valleys of the streams or

their tributaries flowing between those cities or regions. Ifthe walls of a valley are steep the construction of a routealong that valley may be difficult and expensive. Thepresence of terraces, however avoids (or at leastdecreases) excavation in the wall of the valley and thus

reduces the construction costs. In addition, rivers terracesgenerally are good sources of sand and gravel forconstruction.


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Flood Plains and Deltas

The stream sediments can be deposited due to

the decreasing of water carrying these

sediments. This occurs (e.g.) when a stream

reaches a flood plain, a wide flat part of thevalley subjected to floods when the stream

carries an excess of water.

The delta or delta deposits when the running

water of the stream reaches the standing water

of the ocean or other basin into which it flows.


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Aeolian Soils and Aeolian Deposits

Aeolian, or wind-blown, soils may be

subdivided into two groups, namely,

1-Loess soils (collapssable soil) such as

primary and secondary loess.2- Sands forming sand dunes.

f l


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Some properties of loess

1- In nature, loess deposits are variable thickness, and layers 200 ft.thickness have been found.

2- It is very porous.

3- The material is slightly or moderately plastic.

4- Its permeability in the vertical direction is greater than the horizontalpermeability because of stratification.

5- If the loaded loess deposit is wetted, it rapidly consolidated and thestructure constructed on it settles. This property of loess is sometimestermed hydroconsolidation. There are two explanations of this property:Most of loess in the United States has clay hulls of films around siltgrains. Added water lubricates the clay and makes the silt grain slidewith respect to one another, which causes the structure built on the loess

deposit to settle. According to the other explanation, the settlementsexhibited by loess in other part of the world can be clay lubrication andalso to the removal by water of weak CaCO3cement from loess


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Engineering problems in loess areas

Because of the hydroconsolidation properties, loess may be a dangerousfoundation material if brought in contact with water.

** In the case of dames and especially their reservoirs, wetting of underlyingmaterial by the water from the reservoir means considerable settlement of thedam.

**Failer of smaller structures on loess that became saturated are also known. In aspectacular case of an overnight settling and cracking of a house, the accidentwas caused by the discharge of water from a house forgotten on the lawn.

**Another difficulty with loess is its ready ability to [pipe] under water action. Ifwater start to leak from an excavation or a canal, it forms a path inside the loess

mass which gradually progresses and widens, often irregular, until failer occurs.Similar accidents also may take place in the case of steel pipe placed in loess.Water finds its way around the pipe, and cavities as large as [9ft.] in diameterhave been known to develop.


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** The loess settlement problem doesnt appear to seriousif an concrete structure is built on a foundation that is notcontact with excessive amount of water ,[e.g.]the footingof towers transmission line. Such tours, however shouldntbe placed in local depression which would permit

accumulate around the tower.-Remolding of several upper feet of loess at the surfaceand careful recompaction of the remolded material maycreate a reliable platform for building footings.

-In order to control settlement of earth dams on loess,

grouting procedures have been devised, when slurry ofloess and bentonite or loess only is pumping into a poreholes made in the foundation.


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**Embankments made of compacted loessproved to be entirely satisfactory.

**Excavation in loess usually isnt difficultbecause of the capacity of the loess material tostand on almost vertical slopes. Steep loessbanks, however, are subjected to the formationof gullies and accumulation of the spalled

material at the toes of the bank. As a gullyadvances toward the top of the slop, the slopmay slide.

Engineering problems with sand-


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Engineering problems with sand

dunes areas Stabilization movable sand is major problem in construction and

maintence of high ways and railroads crossing dune zone indeserts. This can be done, for example, by seading the dune withsuch grass varieties as many thrive, planting young trees (mostlyconifers, e.g. pine) and treating the sand with crude road oil.

The dune on which the transmission-line towers is placed may

move away, leaving the foundation of the tower completelyexposed and hence in an unstable condition. For the foundation ofvery expensive structure, however, piles should be driven to adepth unaffected by any shifting of the dunes.

Because the water-sorbing capacity of the dune sand the building

and maintence of hydraulic structure in sand-dune terrain are veryexpensive. Irrigation canals in sand-dune terrain should be lined

All i l d W f d S il


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Alluvial and Water formed Soils

**Alluvial Soil:Eroded soil carried by water and deposited is alluvialsoil or alluvium. Immediately adjacent to the steepportion of the valley and headwaters of the stream,boulders and coarser gravel might be expected. At adistance of several miles from the place of erosionsmall sizes (fines) predominate.

Besides forming terraces and benches in the valley

itself, deposition of alluvium also may occur on plains,thus forming relatively flat deposits. Alluvial fan canbe found when the gradient abruptly decrease.


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Alluvial deposits are stratified, but they areshow some heterogeneity, e.g., it is notunusual to find a bed of alluvial clay severalfeet long, although it might be fairly narrowand only a few feet thick. Rather uniformsand and gravel beds of considerabledimension may be found, and although there

may be lens-like inclusion of sand in gravelbeds and vice versa, these deposits as awhole are fairly continuous.

Engineering Significance of alluvial


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g g g f f

deposits:

Alluvial deposits generally provide an

excellent source for coarse construction

materials, such as concrete aggregates, or

previous material for highway subgrade orhigh embankments.


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Open work gravel, the material of open work gravel is poorlygraded, and fines are completely lacking. Thus the voids areusually open.

Open work gravel usually occurs as lenticular bodies.

It has very high permeability, thus, excessive seepage lossesthrough these gravels could be expected if they passedunderneath a dam. The injection of grout or a silt or clay into theopen work gravel to decrease its permeability. Sometimes sealingthe deposits tacks placing a layer of silt over their surface.

Bed slides in highway cuts have occurred in open work gravel andassociated material in several cases. On the credit side for open

work gravel is that it often furnishes very prolific supplies of waterwhen penetrated by water wells.


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**Swamps and Peat bogs:

Swamps or marshes are area of wet, soggy

ground, saturated or almost saturated, but

generally not entively covered with water. Theyare filled with decaying vegetable matter. Some

swamps have been formed by lakes and sluggish

streams. Swamps often are developed at sea or

lake coasts. In the former case, salty tidal flats

are often intermittent with swampy land.


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Peat in mass of plant remnants in which the

process of humification is under way (gives

fibrous dark brown peat) or already

completed (give black peat). The naturalmoisture content of peat material is several

hundred percent (may reach 1000%) by dry

weight.

Engineering problems in Swamps andP t d it


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Peat deposits:

-The bearing values of the material that may be classified asorganic terrain are very low, and only very light structure such assecondary roads built on them.

-Drainage prior to construction is highly recommended in order todecrease the moisture content and increasing the shearingstrength of material.

-In building important highways across organic terrains,particularly peat areas, the practice of American engineers is toremove the peat and replace it with adequate fill material, e.g.,

sand, gravel etc.

-Heavy structures in peat areas should be built on piles.

Residual Soils


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Residual Soils

Residual soils develop easily on sedimentary rocksparticularly on limestone (especially in wet climateformation area of Terra Rosa Soil) because of theircomparatively weakresistance to solution andweathering. Signs to destruction in limestone are

sinkholes and caves. The contact between residualsoil and underlying limestone usually is highlyirregular. Subsurface investigation in limestone andoverlying soils should be carried out in considerablewhereas the rock between them may be filled with

caverns. The residual soil on S.S greatly depends onthe type of cement and climate.

Swelling Soil


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Swelling Soil

Appears in many areas in Egypt, Nile valley,Western Desert.

The swelling pressure in the vertical directionmakes failer of the walls and raising of the floor

of the Houses. The swelling pressure in thehorizontal direction effects on the retainingwalls.

The most famous clays which appear these

properties are the bentonite which is rich insmectite clay minerals content. This clay is usedas grounting material and in drilling mud.

Detection of free swelling


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Detection of free swelling

Dry soil- gridding-sieve No.40-10 cm3of soilin 100 cm3cylinder with distilled water- 24

hours- measuring of the volume of

swelling soil (v) . Free swelling (F.S.) =

Determination of free swelling


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Determination of free swelling

Stra

in

Swelling

Subsidence (Settlement)

h

Compression

Swell ing pressure (0.025-0.3 MPa)

Swelli ng of cement

Free swelling(h)

Factors affecting on swelling


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ff g g

Mineral composition Chemical composition and concentration of

dissolved salts External pressure

Solutions of swelling problems: Replacement of swelling soul by sand and gravel

for a depth ranging between 0.5 to 2.5 msdepending on the type of the soil. This method is

successfully applied in many places in Egypt, e.g.Nacer City, Ismailia, Dakhla&Kharga Oasis, Alex-Cairo desert road.

Solutions of swelling problems:


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1- Replacement of swelling soul by sand and gravel for a depthranging between 0.5 to 2.5 ms depending on the type of the soil.This method is successfully applied in many places in Egypt, e.g.Nacer City, Ismailia, Dakhla&Kharga Oasis, Alex-Cairo desert road.

2- Addition of lime with ratio 3-5 %.3- Remolding.4- Constructions of barrier to prevent the reaching of water to the

foundation.5- Immersion of soil with water before construction (but it is difficult

to know the swelling reaches to the maximum or no.)6- The foundations of the constructions must be strong and rain

forced.

Summery of Collapsible Soil


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Summery of Collapsible Soil

Primary loess: wind-blown material which still in the same location where it was originallydeposited by wind and has undergo little, if any, chemical decomposition. secondary loess: Constituents of loess

Sorted mixture of silt, fine sand and clay particles. Open cohesive fabrics. High percent of silt size particles, poorly graded and contains high carbonate content. *Properties: Variable thickness Permeability Hydro consolidation and collapse. *Engineering Problems: Considerable settlement of dams. Failer of smaller structure. Its ready ability to pipe especially around steel pipes. Power-transmission line must be no constructed on the depression in which water may be come. The best solution for they problem is the remolding. Excavation is usually not difficult due to the presence of gully and the ability of loess to stand at

high slope angle.

Two methods to study the hydro-


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consolidation of loess

final engneering geology

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