CN 301- GEOENVIRONMENTAL ENGINEERING

LECTURE 3

LEARNING OUTCOMES

1. Explain the soil characteristics related to geo-

environmental

2. Explain basic soil test

SOIL FORMATION

Soils are formed by the disintegration (or more precisely,

evolution) of rock material of the earth’s relatively deeper crust,

which itself is formed by the cooling of volcanic magma.

The stability of crystalline structure governs the rock formation.

As the temperature falls, new and often more stable minerals

are formed. For instance, one of the most abundant minerals in

soils known as quartz acquires a stable crystalline structure

when the temperature drops below 573°C.

The intermediate and less stable minerals (from which quartz

has evolved) lend themselves to easy disintegration during the

formation of soils.

The disintegration process of rocks leading to the formation of soils is called

weathering.

It is caused by natural agents; primarily wind and water (note that these are

the same agents that aid the evolution and life in other kingdoms).

The specific processes responsible for weathering of rocks are:

i. Erosion by the forces of wind, water, or glaciers, and alternate freezing and

thawing of the rock material.

ii. Chemical processes, often triggered by the presence of water. These

include:

Hydrolysis (reaction between H- and OH- ions of water and the ions of

the rock minerals),

Chelation (complexation and removal of metal ions),

Cation exchange between the rock mineral surface and the

surrounding medium

Oxidation and reduction reactions,

Carbonation of the mineral surface because of the presence of

atmospheric CO2.

iii. Biological processes which, through the presence of organic compounds,

affect the weathering process either directly or indirectly.

Once the rock material is weathered, the resultant soil

may either remain in place or may be transported by

the natural agencies of water, air, and glaciers.

In the former case, the soils are called residual soils.

Depending on the natural agent involved, the

transported soils are called alluvial or fluvial (water-

laid), aeolian (wind-laid), or glacial (ice-transported)

soils.

Several subdivisions are often made based on the

transportation and deposition conditions.

THE PROPERTIES OF THE SOIL DEPOSITS FORMED

DEPEND ON THE SOIL-FORMING FACTORS

Five independent variables may be viewed as governing soil formation:

Climate - Amount of moisture available, temperature,

chemical reaction speed and rate of plant growth

Organisms present - Organisms influence the soil's physical and

chemical properties and furnish organic matter to

soil

Topography - Angle: like Steep is poorly developed soils but flat

to undulating surface is the best. Orientation

(direction the slope is facing) - soil temperature and

Moisture

The nature of the

parent material

- Original mineral makeup and important in young

soils. Residual soil–from bedrock. Transported soil–

carried from elsewhere

Time - varies for soils in different climates, locations

MINERALOGY COMPOSITION IN SOIL

Five mains groups of mineral composition in soil (regular structure

elements and atomic elements) are :

i. CARBONATES - calcite and dolomite usually use in

cement

ii. OXIDES

iii. HYDROUS

OXIDES

– gibbsite and brucite minus OH’s sheet in

clay minerals

iv. PHOSPHATE – using for fertilizer

v. SILICATE – 90% of all soil

FIGURE :SOIL COMPOSITIONS SCHEMATIC DIAGRAM

SILICATES

Silicates constitute well over 90% of the earth's

crust.

The fundamental unit of all silicate structures is

the SiO4 tetrahedron.

It consists of four O2- ions at the apices of a

regular tetrahedron coordinated to one Si4+ at

the center.

The individual tetrahedral are linked together

by sharing O2- ions to form more complex

structures.

Silica tetrahedron: The silica tetrahedron consists of four oxygen ions

and one silica ion.

The molecular arrangement is such that the four oxygen ions are

spaced at what would be the corners and tip of a three-dimensional,

three-sided pyramid, with the silicon located within the pyramid.

Oxygen ions at the base are shared by adjacent tetrahedrons, thus

combining and forming a sheet.

QUARTZ

Commonly found in soil and the mineral

composition SiO2.

The Quartz shape are in three dimensions and

each of quartz cannot absorb in acid and

cannot break easily.

There is no isomorphous substitution in

quartz, and each silica tetrahedronis firmly

and equally braced in all directions.

As a result, quartz has no planes of weakness

and is very hard and highly resistant to

mechanical and chemical weathering.

Quartz is not only the most common mineral

in sand and silt-sized particles of soils, but

quartz or amorphous silica is frequently

present in colloidal (1 to 100 nm) and

molecular (< 1 nm) dimensions.

FELDSPAR some of the silicon atoms are replaced by aluminum. This

results in a negative charge and in distortion of the crystal structure, because Al atoms are larger than Si atoms.

The negative charge is balanced by taking in cations such as K+, Na+, and Ca+ in orthoclase, albite, and anorthite feldspars, respectively. The distortion of the lattice and the inclusion of the cations cause cleavage planes that reduce the resistance of feldspars to mechanical and chemical weathering.

For these reasons, feldspars are not as common as quartz in the sand-, silt-, and claysized fractions of soils, even though feldspars are the most common constituent of the earth's crust.

MICACommon micas such as muscovite and biotite

are often present in the silt- and sand-sized

fractions of soils.

In a unit sheet of mica, which is 1 nm thick, two

tetrahedral layers are linked together with one

octahedral layer.

In muscovite, only two of every three octahedral

sites are occupied by aluminum cations,

whereas in biotite all sites are occupied by

magnesium.

In well-crystallized micas one fourth of the

tetrahedral Si+4 are replaced by A1+3.

The resulting negative charge in common micas

is balanced by intersheet potassiums. In a face-

to-face stacking of sheets to form mica plates,

the hexagonal holes on opposing tetrahedral

surfaces are matched to enclose the intersheet

potassiums.

ALUMINA OCTAHEDRONS:

The alumina octahedron consists of six-oxygen and one-aluminum.3

oxygen is in the top place of the octahedrons, and three are in the

bottom plane. The aluminum is within the oxygen grouping. It is possible

that the aluminum ion may be replaced with magnesium, iron, or other

neutral ions. The aluminum sheet is 5 x 10-7mm thick. Oxygen from the

tip of a silica tetrahedron can share an alumina sheet, thus layering

sheets. Different sheet arrangements are then combined to form the

different clay minerals. The composition and typical properties of the

more commonly occurring clay minerals are Kaolinite, Illite and

Montmorillonite

KAOLINITE

is a common mineral in soils and is the most common member of this

subgroup. A Kaolinite is the most prevalent clay mineral and is very stable,

with little tendencies for volume change when exposed to water. Kaolinite

layers are stack together to form relatively thick particle. Particles are plate

shaped. The composition is one-silica, one alumina sheet that is very

strongly bonded together. Kaolinites have very little isomorphous

substitution in either the tetrahedral or octahedral sheets and most

kaolinites are close to the ideal formula Al2Si2O5 (OH) 4.

ILLITE

Illite - has irregular plate shape, more plastic than kaolinites.

Its does not expand when exposed to water unless potassium deficiency

exists. This clay is most prevalent in marine deposits.

The composition is an alumina sheet sandwiched between two silica

sheets to form a layer. Potassium provides the bonds between the

layers.

MONTMORILLONITE

has irregular plate shapes or is fibrous because of the weak bond

between layers this clay readily absorbs water between layers.

This mineral has a great tendency for large volume change. The

composition is an alumina sheet sandwiched between two silica

sheets to form a layer.

Iron or magnesium may replace the alumina in the aluminum

sheet.

FUNDAMENTAL PROPERTIES OF SOILS

The soil type or category is based on particle size, however, where

the soil particle size is too small to be observed, an additional

physical property, known as plasticity is utilized as a criterion for

evaluation

Soil is all the material located above bedrock and can be grouped

into four major categories or types including gravel, sand, clay and

silt.

These four categories can be reduced to two groups termed coarse-

grained soil and fine-grained soil.

PARTICLE SIZE AND SHAPE

Particle size and shape affects the mechanical behavior of soils,

however, the effect of varies for coarse-grained and fine-grained soils.

The size and shape of the granular soil particles can increase or

decrease the tendency of particles to fracture, crush and degrade.

The grading of gravels and sands may be qualified in the field as well

graded (good representation of all particle sizes from largest to

smallest).

Poorly graded materials may be further divided into uniformly graded

(most particles about the same size) and gap graded (absence of one or

more intermediate sizes).

SOIL STRUCTURE

Soil structure is the shape that the soil takes based on its

physical and chemical properties; it is the geometric

arrangement of soil particles with respect to one another.

The process of sedimentation or rock weathering creates the

initial soil structure.

Among the many factors that effect soil structure is the shape,

size, and mineral composition of the soil particles, and the

nature and composition of soil water.

The basic terminology used to define the soil structure are

single-grained, honeycombed, flocculated and dispersed with

variations dependent upon the composition of the soil.

COHESIONLESS SOILS

The particle arrangement of cohesionless soils (gravel, sand

and silt) has been likened to arrangements attained by

stacking marbles, or “single-grained”.

In single grained structures soil particles are in a stable

position, with each particle in contact with the surrounding

ones.

For similar sized particles large variations in the void ratio are

related to the relative position of the particles.

COHESIVE SOILS

The term cohesive is used for clay soils, which have an inherent

strength, based on their particle structure which provides

considerable strength in an unconfined state.

The cohesiveness of a clay is due to its’ mineralogy and is a

controlling factor determining the shapes, sizes, and surface

characteristics of a particle in a soil.

It determines interaction with fluids.

Together, these factors determine plasticity, swelling,

compression, strength, and fluid conductivity behavior

IDENTIFICATION OF FUNDAMENTAL

CHARACTERISTIC SOILS

Fine grained soils are identified on the basis of some simple tests for :

i. Dry strength Dry strength is a qualitative measure of how hard it is to crush a dry

mass of fine grained soil between the fingers. Clays have very high dry

strength and silts have very low dry strength.

ii. Dilatancy Dilatancy is an indication of how quickly the moisture from a wet soil

can be brought to the surface by vibration. In silty soils, within a few

strikes water rises to the surface making it shine. In clays, it may

require considerable effort to make the surface shiny. In other words,

dilatancy is quick in silts and slow in clays.

iii. Toughness Toughness is a qualitative measure of how tough the soil is near its

plastic limit (where the soil crumbles when rolled to a 3 mm diameter

thread). Toughness increases with plasticity. Silty soils are soft and

friable (crumble easily) at Plastic Limits (PL), and clays are hard at PL.

The fines can also be identified by feeling a moist pat; clays feel sticky

and silts feel gritty. The stickiness is due to the cohesive properties of

the fines, which is also associated with the plasticity, and therefore

clays are called cohesive soils. Gravels, sands and silts are called

granular soils.

PARTICLE SIZE DISTRIBUTION OF SOILS

The grain size distribution of a coarse grained soil is generally determined

through sieve analysis, where the soil sample is passed through a stack of

sieves and the percentages passing different sizes of sieves are noted.

The grain size distribution of the fines are determined through hydrometer

analysis, where the fines are mixed with distilled water to make 1000 ml of

suspension and a hydrometer is used to measure the density of the soil-

water suspension at different times.

Three Limits Are in General Used to Characterize

The Clayey Soils:

Limit Description

i. Shrinkage limit which is the water content at which the

soil passes from solid to semisolid state

ii. Plastic limit which is the water content at which

transition from semisolid to plastic state

takes place

iii. Liquid limit which indicates the water content

required in order for the clayey soil to

begin exhibiting flow characteristics like

liquids

SOIL TEST

The BS 5930:1999 (Code of Practice for Site Investigations)

summarizes the purposes of laboratory testing to be to describe

and classify the samples, to investigate the fundamental behavior

of the soils in order to determine the most appropriate method to

be used in the analysis, and to obtain soil parameters relevant to

the technical objectives of the investigation.

The laboratory tests for soils commonly carried out include:

• Moisture content, which read in conjunction with liquid and

plastic limits gives an

• indication of undrained strength;

• Liquid and plastic limits to classify fine grained soil and the

fine fraction of mixed

• soils;

SOIL TEST

Particle size distribution to give the relative proportions of gravel, sand, silt

and clay;

Organic matter which may interfere with the hydration of Portland cement;

Mass loss of ignition which measures the organic content in soil, particularly

peat;

Sulfate content which assesses the aggressiveness of the soil or

groundwater to buried concrete;

pH value which is usually carried out in conjunction with sulfate contents

tests;

California bearing ration (CBR) used for the design of flexible pavements;

Soil strength tests such as Triaxial compression, unconfined compression

and vane shear;

Soil deformation tests;

Soil permeability tests.