Group 1 Chemistry of Soils & Rocks

8/8/2019 Group 1 Chemistry of Soils & Rocks

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THE

CHEMISTRYOF

SOILS / ROCK


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PRESENTED BY

Ms. Uiras

Ms. Uiras

Mr. Nangolo

Mr. Nangolo

Ms Taapopi

Ms Taapopi


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1. Introductory Concepts

2.

Soil as a Construction Material3. Types of Soils in Namibia and their use in the

construction industry

4. Environmental chemistry of soils

SCOPE


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Soil and regolith

Soil-Forming Factors

Soil Components

Soil Properties and Chemistry

Chemical composition

Soil Profiles

Soil Classification

2. INTRODUCTION


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Soils are the main,

naturally occurringmaterials

Soil is found in the toplayer s of regolith

Regolith = soil +

portions of bedrock

In engineering, soil isreferred to as regolith :loose rock material or

layer coveringsolid/bedrock and iscreated by the action of

wind and water

Soil and Regolith


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y Geologic parent rock factor:Rocks are said to be the parent of soil because all soils originatedirectly or indirectly from them and are classified according totheir mode of formation. By a combination of physical andchemical processes rock masses are reduced to particles ranging

in sizes down to 0.001 mm.Soils result when collections of these particles are re-deposited,often in bodies of water, and are compressed and consolidated byfurther depositions

y Climatic - factor comprises of the meteorological conditionswhich surround the soil system such as temperature, rainfall,humidity ( the agents of weathering)

Soil Forming Factors


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y Topographic factor :

The general shape of the land surface in terms of its slopeand position relative to its geographic coordinates, affect the

moisture and temperature of the soil

Example Steep soils facing the sun are warmer, compared to thethose that dont. Steep soils erode and lose their topsoil as theyform and display different soils characters on both top and

bottom layers of the slope.

Precipitation has Significant impact on soil formation as itdetermines runoff water. Water erosion is more effective onsteeper, unvegetative slopes



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y Topographic factor :



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y Biological factor :

Decomposition of

Plants, animals with the

aid micro organisms

have an effect on the

formation of soil by

speeding up the rate of

dissolution of parent

material



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y Time factor :

Soil formation is an ongoing process. Over time soils shows

features that reflect the various forming factors.



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Soil forms from a

complex interaction

between earth

materials, climate andorganisms acting over

time , by the process of

weathering:

physical/mechanical

break down and

chemical alteration of

earth material

Soil developmemt


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y During the early stage of soildevelopment, soils are dominated bycharacteristics inherited from parentmaterial. Later on they show a changein their chemical characteristics.


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y During the early stage

of soil development,soils are dominated bycharacteristicsinherited from parentmaterial. Later on they

show a change in theirchemicalcharacteristics.


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Soils are a complex mixture of:

Inorganic Materials

Organic Matter

Soil Air

Soil Water

Soil Components


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Organic matter:

y 0.5 - 5% of soil volume

y made up of different substances that are

gradually broken down by microorganisms.y Includes carbohydrates, proteins, lignins, fats,

waxes.

Many of these compounds do not decomposecompletely and are transformed to humus,


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Soil Components

Soil Air

Air makes up approximately 25% of total soil volume.


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Soil Components

Soil Air

Soil is made up ofapproximately 40-60% ofpores. The pore sizeinfluences water holdingcapacity and aeration.

Porosity :Ratio of the volume of voids(containing air and/or water)in a soil to the total volumeof the soil expressed as a

percentage

Void ratio: is the ratio of thevolume of voids to thevolume of solid particles in

a soil


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Soil Components

Soil Water

Percolation:

Refers to the

downward f low ormovement of waterthrough the soilprofile

Capillary action :

The movement of aliquid along thesurface of a solid

caused by theattraction ofmolecules of theliquid to that of thesolids


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Soil Components

Soil Water

Eluviation: processof removingminerals from soil

with water

Illuviation: processof depositing theremoved minerals inunderlying layers


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Soil properties

Color

Red color results from leaching removal ofmaterials by dissolving them away from solids


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Soil properties

T

exture

clay

Silt

Sand


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Soil properties

T

exture

Unweathered primary materials:

have little capacity to hold water and nutrients and are

relatively chemically unreactive.

Gravel: 2 - 4mm

Sand: 0.05 - 2.0 mm

Silt: 0.002 - 0.05 mm

Weathered secondary materials:

Clay: is a secondary mineral less than 0.002 mm indiameter.


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Soil properties

T

exture

Textural classification:

y

Infiltration of water: rapid in sands, slow in clays.y Drainage: rapid in sands, slow in clays.

y Aeration: sand has rapid gas exchange; clay slow.

y Fertility: sand has low fertility, clay high, depending on type.


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Soil properties

Structure


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Soil properties

Moisture

Porous and Non Porous Soil: the rate of permeability


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Colloids:

y less than 0.1 micrometer(10-7 meters)

y clays + organic particles

y when mixed with waterform a suspension

y absorb water

Soil properties and Chemistry


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Acidity/Alkalinity:


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Clays: high cation exchange capacity (CEC) because they are

negatively charged and can attract, retain and exchange cations.

Their water holding capacity is very high because of their large

surface area per unit mass:

Ion Exchange:


y Three-layer clays: (Si-Al-Silattice) montmorillonite andillite have high CEC.

y

Two-layer clays: (Si-Al lattice)like kaolinite have low CEC.

y Amorphous clays: arecomposed of oxides of Fe and

Al and have very low CEC


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y oxygen (46.7%), silicon (27%), aluminium (8.1 %) and iron

(5.0%).y Ca, Mg, K, Na, P and S are present in the minerals and in the soil

solution.y Si, and Al occur as constituents of minerals and as oxides.y Fe occurs mainly in the form of oxides and ferromagnesium

minerals.y Ca occurs mainly in calcite, gypsum, apatite and dolomite.y Mg is present mainly in dolomite and hornblendey K occurs mainly in microcline and micay P occurs as aluminium phosphate and calcium phosphate and in

the organic form as phospholipids, inositol, choline, etc.y N occurs mainly in the organic form as proteins, amino acids,

etc.y All micronutrients like Mo, Fe, Mn, Zn, Cu, B occur in the

inorganic form.

Chemical composition of Soils


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Horizons specifically refer to

variations in soil profiles.

Soil Horizons (horizontal

layers)that exist within a vertical

column of soil

A coherent layer of soil, similar

in characteristics such as

composition, texture and color

that define the horizon fromother soil types, unique to

different geologic and climatic

areas

Soil Profile


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In order to understand relationships between different soils andto determine the usefulness of a soil for a particular use soils areclassified into categories

y Orders are the highest category of soil classification. Ordertypes end in the letters sol. In Namibia we use the WorldReference Base for soil Resources ( WRB)

Soil Classification


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2. SOIL AS A CONSTRUCTION MATERIAL


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History

Soil is the oldest construction and engineering material

& was used as the main element during construction

y Sumerian civilization : Ur ( 50th century B.C)

y Western civilizations: Rome ( 5th century B.C) and Greek (28th century B.C)

y African civilization: Nubian ( 2nd century B.C ) andEgyptians ( 3rd century B.C)

y Namibia : San people (7th century B.C)


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In engineering soil is mostly used in the following areas:

y As a Bearing capacity for the foundation of structures

y As a mixing agent to form concrete etc

y

As a stability slope

soil is one of the most complex fields in civil engineering tothe point that when it comes to the factor of safety in

design whatever has direct contact with soils, e.g.foundations, or soil based constructions, e.g.embankments, it has the highest number, i.e. theuncertainty in soil analysis and design is high. These mostlikely come from the way soil originates.

Soil as a Construction Material


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Effect of soil chemistry on structures: Concrete and Steel

y Ideally steels & concrete should be in the passive state in soils,but presence of water and aggressive chemicals like chloride

ions, sulphates, as well as types of bacteria, can cause localizedcorrosion.

Steel:The risk of corrosion, expressed as low, moderate, or high, is

based on soil drainage class, total acidity, electrical resistivitynear field capacity, and electrical conductivity of the saturationextract.. Steel installations that intersect soil boundaries or soillayers is more susceptible to corrosion than steel in installationsthat are entirely within one kind of soil or within one soil layer.



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Concrete:

The risk of corrosion also is expressed as low, moderate, or high.It is based on soil texture, acidity, and amount of sulfates in thesaturation extract.

The factors contributing to Corrosion:

Soil Resistivity

y Indicator of soil corrosivity

y Corrosivity is associated with electrochemical reaction in soils,

y highly resistive soils slow down corrosion reactionsy Resistivity decreases with increase in moisture & chemical

concentration

y Well drained soils less corrosive



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Acidity of the Soil

y Normal pH range of 4.5 8.0.

y As acidity increases so does the risk of corrosion

y Soil acidity is due to: mineral leaching, decomposition of acidicplants, industrial wastes, acid rain and certain forms of micro-

biological

Presence of Chlorides, Sulphates and Other Dissolved Species:

Most Important Factors

y

Cl-

ion concentrationCl- ion participates in pitting iniation of steel

presence decreases soil resitivity

presence due to: brackish groundwater, historical geologicalseabeds



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y Soil moisture

contains sulphate ions, H+, HCO3-

sulphates less corrosive than Cl-

Risk: sulphates can be converted to highly corrosive sulphides by

reactions in soil

Microbiologically Induced Corrosion (MIC)

y

Corrosion induced by activities of microorganismsy Presence Sulphate Reducing Bacteria increases risk of corrosion

y By product of SRB metabolism = sulphide ions

y Sulphide ions react with metal surface



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Oxygen concentration cellsy O2 takes part in cathodic reaction

y Supply of O2 changes with type of soil = creation of O2concentration cells

y

Increase in O2 concentration = increase in redox potential



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y Acid chemically reacts with lime in concrete wearing awayat the structure, and exposing the aggregate.y Acid attacks steel leading to structural weakness and

failurey When acidic soils are exposed they will shrink and crack.This can result in subsidence and cracking of foundations

y As concrete surfaces deteriorates this leads to the corrosionof steel reinforcements

y Acidic soil => lack of vegetation => more rapid soil loss =>significant long term destruction of road structureShiftingor sinking of foundations may result in structural

cracking, damage or collapse.y Salt damage to roads and highways includes the

breakdown of concrete, bitumen and asphalt withassociated pot holing, cracking and crumbling of theroad base.


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y The major concern is to not allow excessive wateragainst the foundation. Wet alkaline soil willdamage concrete foundations and stucco siding.


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Mechanical properties:

y shear strength :

The internal resistance per unit area that the soil mass can offerto resist sliding failure along any plane inside it. Stresses inducedalong any plane in a soil mass can be converted to

- Normal stress : stresses perpendicular to the plane

- Shear stress : stresses along the plane



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A

B

SOIL MASS



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y Consolidation:

The ongoing removal, of water form the pores of saturatedcohesive materials with subsequent change in volume, bycontinued pressure.

When a load is applied to saturated clay the load is initiallycarried by the pore water, this load is gradually transferred to the

soil particles as the water is removed under pressure. Duringthese process settlement occurs, known as consolidationsettlement because of the reduction in volume associated withthe removal of water.

Settlement will continue to take place until excess pore pressure

has been dissipated. Depending on the permeability of clay andadjacent subsoil this process may occur over years. Before thefoundation is constructed the layer carries the normaloverburden load. After completion there is still some overburdenload plus the effect of pressure exerted by the foundation



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y Permeability:

The capacity of a soil to allow water to pass through

y Effective stress :

The intergranular stress which exists between soil particles atany particular level in a soil mass

y Slope stability :

The assessment of the stability of earth slope against

failure, normally takes the form of a slip along one or moreruptured surfaces somewhere within the soil mass.



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y Atterberg Limits

Liquid Limit, plastic limit, and shrinkage limit, related to

plasticity of a soil.

(Holtz and Kovacs, 1981)

LL

PI

HL


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3. TYPES OF SOILS IN NAMIBIA

CONSTRUCTION INDUSTRY

The most common soil types in Namibia are:

sandy soils (arenosols)young soil on solid rocks (leptosols)Fertile fluvisols


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y The most common soil types used inconstruction are shown in table below:


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y Dune sands

Coastal sand dune systems are fragile, dynamic resources that

comprise only about two percent of Maines overall coastline.


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River sand


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Arenosols


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y A knowledge of environmental soil chemistry is paramountto predicting the fate, mobility and potential toxicity ofcontaminants in the environment. The vast majority ofenvironmental contaminants are initially released to thesoil. Once a chemical is exposed to the soil environment a

myriad of chemical reactions can occur that mayincrease/decrease a contaminants toxicity. These reactionsinclude adsorption/desorption, precipitation,polymerization, dissolution, complexation, andoxidation/reduction. These reactions are often disregardedby scientists and engineers involved with environmental

remediation. Understanding these processes will enable usto better predict the fate and toxicity of contaminants andwill ultimately provide us with the knowledge to developsound and cost-effective remediation strategies.

ENVIRONMENTAL SOIL CHEMISTRY


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END

Group 1 Chemistry of Soils & Rocks

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