chapter-1

Assessment of Soil Stabilization Using Fly Ash and Rice Husk Ash

INTRODUCTION

The term ‘soil’ has different meanings in different scientific fields. It has

originated from the Latin word solum. To an agricultural scientist, it means ‘the loose

material on the earth’s crust consisting of disintegrated rock with an admixture of organic

matter, which supports plant life.’ To a geologist, it is a mixture of organic matter, which

has not been transported from the place of origin. But to a civil engineer the term ‘soil’

means, the loose unconsolidated inorganic material on the earth’s crust produced by the

disintegration or rocks overlying hard rock with or without organic matter. Foundations of

all structures have to be placed on or in such soil, which is the primary reason for our

interest as civil engineers in its engineering behavior.

Soil may remain at the place of its origin or it may be transported by various

natural agencies. It is said to be ‘residual’ in the earlier situation and ‘transported’ in the

latter.

1.1 ORIGIN OF SOILS:-

Soils are formed by weathering of rocks due to mechanical disintegration or

chemical decomposition. When a rock surface gets exposed to atmosphere for an

appreciable time, it disintegrates or decomposes into small particle and thus the soils are

formed.

Soil may be considered as an incidental material obtained from the geologic cycle

which goes on continuously in nature. The geologic cycle consists of erosion,

transportation, deposition and upheaval of soil. Exposed rocks are eroded and degraded

by various physical and chemical processes. The products of erosion are picked up by

agencies of transportation such as water and wind, and are carried to new locations where

they are deposited. This shifting of the material disturbs the equilibrium of forces on the

earth and causes large scale earth movements and upheavals. This process results in

further exposure of rocks and the geologic cycle gets repeated.

If the soil stays at the place of its formation just above the parent rock, it is known

as residual soil or sedentary soil. When the soil has been deposited at a place of its origin,

DEPARTMENT OF CIVIL ENGINEERING SVIST,KADAPA(YSR)


it is called a transported soil. The engineering properties of residual soils vary

considerably from the top layer to the bottom layer. Residual soils have a gradual

transition from relatively fie material near the surface to large fragments of stones at

greater depth. The residual soil formation is generally limited to a few meters.

The engineering properties of transported soils ate entirely different from the

properties of the rock at the place of deposition. Deposits of transported soils ate quite

thick and ate usually uniform. Most of the soil deposits with which a geotechnical

engineer has to deal ate transported soils.



1.2 FORMATION OF SOIL :-

Soil is formed by the process of ‘wearing’ of rocks, that is disintegration and

decomposition of rocks and minerals at or near the earth’s surface through the actions of

natural or mechanical and chemical agents into smaller and smaller grains.

The factors of wearing may be atmospheric, such as changes in temperature and

pressure; erosion and transportation by wind, water and glaciers; chemical action such as

crystal growth, oxidation, hydration, carbonation and leaching by water, especially rain

water, with time.

Obviously, soils formed by mechanical weathering (that is disintegration of rocks

by the action of wind, water and glaciers) bear a simplicity in certain properties to the

minerals in the parent rock. Since chemical changes which could destroy their identity

does not take place.

It is to be noted that 95% of the earth’s crust consists of igneous rocks and only

the remaining 5% consists of sedimentary and metamorphic rocks. However sedimentary

rocks are present on 80% of the earth’s surface area. Feldspars are the abundantly present

(60%) in igneous rocks. Amphiboles and pyroxenes, quartz and micas come next in that

order.

Rocks are altered more by the process or chemical weathering than by mechanical

weathering. In chemical wearing some minerals disappear partially or fully, and new

compounds are formed. The intensity of weathering depends upon the presence of water

and temperature and dissolved materials found in water which cause the weathering of

rocks. Chemical weathering has the maximum intensity in humid and tropical climates.

‘Leaching’ is the process of where by water- soluble parts in the soil such as

calcium carbonate are dissolved and washed out from the soil by rain fall or percolating

subsurface water. ‘Leaching’ soil, in which certain areas of Kerala abound, is formed by

leaching.

Harder minerals will be more resistant to the weathering action, for example,

Quartz, present in igneous rocks. But, prolonged chemical action may affect even such as



clay minerals illite, kaolite, and montmorillonite. ‘Clay’ mineralogy has grown into a

very complicated and broad subject.

1.3 EXPANSIVE SOILS

The stability and bearing power of the soil is considerably improved by soil

stabilization through controlled compaction, proportioning and the addition of suitable

admixtures. Swelling soil is not suitable for the construction work on account of its

volumetric changes.

It swells and shrinks excessively with change of water content. Such tendency of

soil is due to the presence of fine clay particles which swell, when they come in contact

with water, resulting in alternate swelling and shrinking of soil due to which differential

settlement of structure takes place. Stabilization of black cotton soil has been done in this

project work by using stone dust as admixture.

1.4 BLACK COTTON SOIL

Expansive soils are soils or soft bedrock that increases in volume or expand as

they get wet and shrink as they dry out. In India this Expansive soil is called ‘black cotton

soil’. Colour of this oil reddish brown to black and this helps for cultivation of cotton, so

is called black cotton swelling soil covers about 20% of the land area in India. They are

also commonly known as bentonite, expansive, or Black Cotton soil. In India Black

cotton soil also known as ‘regurs’ are found in extensive regions of Deccan Trap. They

have variable thickness and are underlain by sticky material locally known as “Kali

MittiIn terms of geotechnical Engineering, Black Cotton Soil is one which when

associated with as engineering structure and in presence of water will show a tendency to

swell or shrink causing the structure to experience moments which are largely unrelated

to the direct effect of loading by the structure.



Fig1.1 Black Cotton Soil

These clays are characterized by

1. Having a particle size, below 2 micron.

2. A large specific surface area (SSA)and

3. A high Cation Exchange Capacity (CEC).

4. High liquid limit and plasticity index.

Black cotton soils are one of the most prevalent causes of damage to buildings

and construction. This in turn can be immense loss to a nation’s economy. The damages

that can be result from construction on swelling soil can include.

1. Severe structural damage,

2. Cracked driveways, sidewalks and basement floors,

3. Heaving of roads and highway structures,

4. Condemnation of buildings,

5. Disruption of pipelines and sewer lines.



1.5 DISTRIBUTION IN INDIA

In India, an area about one-fifth is occupied by black cotton soil. The area covers

mostly the Deccan Trap plateau, between 73o80’ East longitude and 15o to 24o north

latitude. Thus, most of soil in and around Mumbai, Madras, Gwalior, bundelkh and

Khandwa, Indore, Nagpur and even some on the river banks is Black cotton. That means

these soils are predominant in Deccan trap plateau region,

1.6 TYPICAL CHARACTERISTICS:-

Black cotton soils are generally reddish brown to black in colour and occur from

0.5m to 10m deep and have high compressibility. The generally observed characteristics

of black cotton soils are recorded in table below:-

S.No PROPERTY VALUE

1 Specific Gravity 2.69

2 Atterberg Limits

Liquid Limit 44%

Plastic Limit 20%

Plastic Index 24%

Shrinkage Limit 24.34%

3 Standard Proctor test

Maximum Dry Density 1.4g/cc

Optimum Moisture Content 16.28%

4 Grain Size Analysis

Sand 25%

Silt 30%

Clay 45%

5 Shear Parameters

Cohesion 0.6kg/cm2

Angle of Internal Friction 110

6 Free Swell Index 75%



1.7 CHEMICAL COMPOSITION

Black cotton Soil are made of varying properties of clay minerals like

Montmorillonite, Illite and Kaolinite, chemicals like iron oxide and calcium carbonate (in

the form of canker nodules), and organic matter like humus. Montmorillonite is the

predominant mineral of Black cotton soils. The swelling and shrinkage behavior of black

cotton soil originate mainly from this mineral are hydrous silicates of aluminum and

magnesium. They are made of sheets of silica (tetrahedral) and alumina (octahedral)

stacked on above the other forming sheet like of flaky particle. Montmorillonite has a

three-sheeted structure with expanding lattices. The structure carries negative charge, due

to isomorphic substitution of some aluminum ions by magnesium ions and minerals

become chemically active.

Table 1.7 Chemical Composition of Black Cotton Soil

S.No. Property Range

1. pH value >7(Alkaline)

2. Organic content 0.4 to 204%

3. CaCO3 5 to 15%

4. SiO2 50 to 55%

5. SiO2, Al2O3 3 to 5%

6. Montmorillinite mineral 30 to 50%

Problems associated with black cotton soil :

Black Cotton soils are problematic for engineers everywhere in the world, and

more so in tropical countries like India because of wide temperature variations and

because of distinct dry and wet seasons, leading to wide variations in moisture content of

soils. The following problems generally occur in black cotton soil.

High compressibility

Black Cotton soils are highly plastic and compressible, when they are saturated.

Footing, resting on such soils under goes consolidation settlements of high magnitude.



Swelling

A structure built in a dry season, when the natural water content is low shows

differential movement as result of soils during subsequent wet season. This causes

structures supported by such swelling soils to lift up and crack. Restriction on having

developed swelling pressures making the structure suitable.

Shrinkage

A structure built at the end of the wet season when the natural water content is

high shows settlement and shrinkage cracks during subsequent dry season.

Effects of black cotton soil on structures

Because of affinity of black cotton soil to the moisture, seasonal movement of

moisture to the soil below the structure during wet and dry season respectively, creates a

cumulative increase of moisture below the structure and produces a dome shaped heave

at the central proportions of the structures thus leading to differential of level. Differential

causes inclined cracks in structure flexible road pavements shows waviness and rigid

pavement produces cracks. The pavement cracks in may even reach the de seated sub

grade, which during subsequent wet season may be filled water and make the road

unserviceable. Canals in the black cotton soil areas develop large transverse cracks,

which during flooding open out, and cause heavy leakage and even failure of

embankment. Cracks in any number do not really pose a threat to the safety of the

structures, except when they are too wide or they continue to increase in width. Starting

as hair cracks they go on widening.

Season, the piers are still supported by friction with the soil. When it begins to

rain, water enters deep into the soil through the cracks. After 5 to 10 large storms, the soil

swells, lifting the house and piers. In the dry season, the groundwater table falls and the

soil dries and contracts. As tension cracks grow around the pier, the skin friction is

reduced and the effective stress of the soil increases (due to drying). When the building

load exceeds the remaining skin friction or the effective stress of the soil increases to an

all-time high, adhesion is broken by this straining, and the pier sinks.


chapter-1

Documents