1

Introduction

Before proceeding with construction in a particular area it is important to determine the type of

soil in that area as it is the type of soil that determines the construction methods or practices to be

used like soil compaction, type of concrete being used, stabilizing agent, etc. So, knowing about

the methods used to determine the soil type and the standard soil classification system being used

all over the world is a necessity.

One of the challenges SKG Sangha faces is the construction of biogas plants in semi-arid and

arid areas where there is huge presence of expansive or collapsible soils. In this report more

focus is held on expansive soils than collapsible soils as India has large tracks of expansive soil

known as Black Cotton soil, covering about 20% of total area. Since expansive and collapsible

soils have a tendency to change their volume to a large extent, they cause heavy distress to

engineering constructions. So, it is very important to find economical ways that can counter this

problem.

This report mainly focuses on these two aspects that are determining the soil type and

suggestions for construction of biogas plant on expansive and collapsible soils.

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1. Characterization of Soil Type

1.1 Sieve Analysis [1]

Sieve analysis is the procedure used to assess the particle size distribution of a granular material.

A sieve analysis can be performed on any type of non-organic or organic granular materials.

Being a simple technique, it is the most common method used.

Sieve analysis involves a nested column of sieves with each lower sieve in the column having

smaller openings than the one above as shown in the fig 1.1. A weighed sample is poured into

the top sieve which has the largest screen openings. At the base is a round pan, called the

receiver. The column is placed in a mechanical shaker. The shaker shakes the column. After the

shaking is complete the material on each sieve is weighed. The weight of the sample passing

through each sieve is divided by the total weight of the sample to give the percentage passing

through each sieve. The results of this test are provided on a semi log graph. The vertical scale

(arithmetic) of the graph measures the percentage passing and the horizontal scale (logarithmic)

measures the sieve size.

Figure 1.1 Particle size analysis of soils using sieves. This figure gives the grain sizes of different types of soils.

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Figure 1.2 Particle size grading coefficients. This figure shows how to find the values of D10 and D60.

From the figure 1.2,

D10 – a horizontal line is drawn from percentage passing 10% and from the point where it

intersects the graph a vertical line is drawn downwards. The value at which this vertical line

intersects the horizontal axis is taken as D10.

In the same way the values of D30 and D60 are found.

Coefficient of curvature (Cc) =

Coefficient of uniformity (Cu) =

These coefficients are later used for classifying soils on the basis of Unified soil classification

system (USCS).

Standard Sieves to be used are 3inches, No.4, No.10 and No.200.

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1.2 Different Methods for Carrying Out Sieve Analysis [1]

There are different methods for carrying out sieve analyses, depending on the material to be

measured.

1.2.1 Throw Action Sieving

Figure 1.3 Throw Action Sieving (adapted from en.wikepedia)

Here the vertical throwing motion is combined with a slight circular motion as shown in the

figure 1.3 which results in distribution of the sample amount over the whole sieving surface. If

the particles are smaller than the openings, they pass through the sieve. If they are larger, they

are thrown upwards again. The rotating motion increases the probability that the particles present

a different orientation to the mesh when they fall back again and thus might eventually pass

through the mesh.

1.2.2 Horizontal Sieving

Figure 1.4 Horizontal Sieving (adapted from en.wikepedia)

In a horizontal sieve shaker the sieve stack moves in horizontal circles in a plane as shown in the

figure 1.4. Horizontal sieve shakers are preferably used for needle-shaped, flat, long or fibrous

samples, as their horizontal orientation means that only a few disoriented particles enter the mesh

and the sieve is not blocked so quickly.

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1.2.3 Tapping Sieving

Figure 1.5 Tapping Sieving (adapted from en.wikepedia)

A horizontal circular motion overlies a vertical motion which is created by a tapping impulse as

shown in the figure 1.5. These motional processes are characteristic of hand sieving and produce

a higher degree of sieving for denser particles than throw-action sieve shakers.

1.2.4 Wet Sieving

Most sieve analyses are carried out dry. But there are some applications which can only be

carried out by wet sieving. This is the case when the sample must not be dried; or when the

sample is a very fine powder which tends to agglomerate. A wet sieving process is set up like a

dry process. Above the top sieve a water-spray nozzle is placed which supports the sieving

process additionally to the sieving motion. The rinsing is carried out until the liquid which is

discharged through the receiver is clear. Sample residues on the sieves have to be dried and

weighed. When it comes to wet sieving it is very important not to change to sample in its volume

(no swelling, dissolving or reaction with the liquid).

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1.3 Determining the Liquid and Plastic Limit

Plastic limit: The plastic limit of a soil is the lowest water content at which the soil remains

plastic.

Liquid limit: The limiting moisture content at which the cohesive soil passes from liquid state to

plastic state.

Apparatus required:

Figure 1.6 liquid and plastic limit tests apparatus (Courtesy of University of Texas at Arlington)

Dish: preferably unglazed porcelain or similar mixing dish, about 115 mm (4.5 in.) in diameter.

Spatula: having a blade 75 to 100 mm (3 to 4 in.) long and about 20 mm (3/4 in.) wide.

Rolling Surface: a ground glass plate or a piece of smooth, unglazed paper

Containers: corrosion resistant, suitable for repeated heating and cooling, having close fitting lids

to prevent the loss of moisture.

Balance: conforming to AASHTO M 231, class G1, sensitive to 0.01 g with a 1200 g capacity.

Oven: thermostatically controlled, capable of maintaining temperatures of 110 ±5°C (230 ±9°F).

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Grooving tool of both standard and ASTM types

Casagrande’s liquid limit device

1.3.1 Liquid Limit Test [2]

Procedure:

Step 1: Preparation of Soil (purpose: create small grains of soil, which absorb water readily)

A. Grind the soil with pestle and mortar (try to maintain the natural moisture content of the soil).

B. Place soil in a #40 sieve and shake the sieve over a mixing bowl. Some soil may not pass

through the sieve. Take the non-passing soil and repeat part A and B, until approximately 200

grams pass the sieve.

C. Weigh three tares on a balance and record the weight.

A B

Figure 1.7 A. Step 2.Mixing of Soil, B. Step 3.Place Soil in Liquid Limit Device

Step 2: Mixing of Soil (purpose: mix soil with distilled water to form a paste).

A. Add small amount of distilled water to soil sample in the mixing bowl.

B. Mix soil with spatula.

C. Repeat A and B until soil is consistent and pasty.

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Step 3: Place Soil in Liquid Limit Device (purpose: begin testing soil)

A. Use a spatula to place the soil in the cup of the liquid limit device.

B. Fill the cup evenly to a depth of 10 mm. Smooth the soil with the spatula to remove

entrapped air and form a smooth surface.

Step 4: Make a Groove in the Soil (purpose: separate soil in two equal parts).

A. Clean grooving tool with a paper towel.

B. With the grooving tool, make a groove in the soil from back to front. Be careful not to

disturb the soil next to the groove. The groove should be through the center of the cup.

C. For normal fine grained soil: The Casagrande’s tool is used to cut a groove 2mm wide at the

bottom, 11mm wide at the top and 8mm deep.

D. For sandy soil: The ASTM tool is used to cut a groove 2mm wide at the bottom, 13.6mm

wide at the top and 10mm deep.

A B

Figure 1.8 A. Step 4.Make a Groove in the Soil, B. Step 5.Testing

Step 5: Testing (purpose: to find how many blows it takes to close gap).

A. Turn the crank of the liquid limit device at a rate of two drops per second.

B. Record the number of drops required to close a 20 mm. length of the groove.

C. With a clean spatula, immediately remove a slice of soil from across the closed gap.

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D. Place the slice in a pre-weighed labeled tare. Then weigh the tare and slice on a balance and

record the weight.

E. Place the tare and slice in the drying oven for 1-2 days.

Step 6: Additional Sampling (purpose: record more data to find accurate liquid limit).

A. Perform several more tests by repeating steps 2-5, but add more distilled water or air dry soil

sample.

B. Perform tests until least one sample is obtained in the ranges: 15-25, 20-30, 25-35, and 30-40

blows. It is best to try to obtain a sample in the 15-25 blow range first, 20-30 blow range second,

25-35 blow range third, and 30-40 blow range last. This is because it is easier to add moisture to

the soil than to take it away.

Step 7: Wrapping Up (purpose: finishing the tests and doing calculations).

A. After 1-2 days remove the tares and soil from the drying oven.

B. Weigh the tares and dry soil on balance recording the weight.

C. Calculate the moisture content in each sample.

Calculations:

Moisture content % =

Plot the number of blows vs. water content % on a semi log graph. The number of drops should

be on the log-scale and water content % on the arithmetic scale.

Then draw the best fit straight line through your data points as shown in the figure 1.9. The

liquid limit is where this line intersects with 25 drops as shown in the figure 1.9.

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Figure 1.9 liquid limit test graph (adapted from engineeringcivil.com).In this graph the line

intersects with the 25 drops at water content of 46.4%.So,liquid limit is 46.4%.

1.3.2 Plastic Limit Test [3]

Materials Required:

The plastic limit sample should be obtained from the soil prepared for the liquid limit test at any

point in the process at which the soil is plastic enough to be easily shaped into a ball without

sticking to the fingers excessively when squeezed. Obtain approximately 8 g of soil for the

plastic limit test.

Procedure:

1. From the sample pull a 1.5 to 2 g mass. Squeeze and form the test sample into an ellipsoidal-

shape mass.

2. Roll this mass between the palm and the rolling surface with just sufficient pressure to roll the

mass into a thread of uniform diameter along its length.

3. Roll out between 80 and 90 strokes per minute, counting a stroke as one back and forth

motion. The sample must be rolled into the 3 mm (1/8 in.) thread in no longer than 2 minutes.

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4. Break the thread into six or eight pieces when the diameter of the thread reaches 3 mm (1/8

in.).

5. Squeeze the pieces together into an ellipsoidal-shape mass and reroll.

6. Continue this process of alternately rolling to a thread 3 mm (1/8 in.) in diameter, cutting into

pieces, gathering together and rerolling until the thread crumbles under the pressure required for

rolling and the soil can no longer be rolled into a thread.

7. Gather the portions of the crumbled soil together and place in a suitable, tared container &

cover.

8. Repeat steps one to seven until 8 g of sample have been tested and placed in the covered

container.

9. Determine the moisture content of the sample as done for the liquid limit test.

Calculations:

The moisture content, as determined in Step 9 above, is the Plastic Limit. It is better to run

several trials on the same material to ensure a proper determination of the Plastic Limit of the

soil.

Plasticity Index:

The Plasticity Index (PI) of the soil is equal to the difference between the Liquid Limit (LL) and

the Plastic Limit (PL).

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1.4 Unified Soil Classification System (USCS) [7] [4]

It is used in the U.S. and much of the world for geotechnical work other than roads and

highways.

In the unified system soils are designated by a two-letter symbol: the first identifies the primary

component of the soil, and the second describes its grain size or plasticity characteristics. For

example, poorly graded sand is designated SP and low plasticity clay is CL.

Five first-letter symbols are used:

G for gravel

S for sand

M for silt

C for clay

O for organic soil

Particles larger than 76 mm (3 inches) should be excluded from the Unified Soil Classification System by using a 3 inch sieve.

The specific rules for classification are summarized as follows

Organic soils are distinguished by a dark-brown to black color, an organic odor, and

visible fibrous matter.

For soils that are not notably organic the first step in classification is to consider the

percentage passing the No. 200 sieve.

If less than 50% of the soil passes the No. 200 sieve, the soil is coarse grained, and the

first letter will be G or S;

If more than 50% passes the No. 200 sieve, the soil is fine grained and the first letter will

be M or C.

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1.4.1 For fine-grained soils and organic soils,

Classification of fine grained soils is based on Atterberg limits (plastic and liquid limits)

determined by the fraction passing the No. 40 sieve. The liquid limit and plasticity index are

determined and plotted on the plasticity chart. The vertical line at LL = 50 separates high-

plasticity soils from low-plasticity soils. The A-line separates clay from silt. The equation of the

A-line is PI=0.73(LL-20).

Inorganic soils with liquid limits below 50 that plot above the A-line and have PI values greater

than 7 are lean clays and are designated CL (clays with low plasticity) and those with liquid

limits above 50 that plot above the A-line are fat clays and are designated CH (clays with high

plasticity).

Inorganic soils with liquid limits below 50 that plot below the A-line are silt and are designated

ML (low plasticity silt); those with liquid limits above 50 that plot below the A-line are elastic

silts and are designated MH (high plasticity silt).

Soils with sufficient organic contents to influence properties that have liquid limits below 50

are classified as OL; those with liquid limits above 50 are classified as OH. Soils that are

predominantly organic, with visible vegetable tissue, are termed peat and given the designation

Pt.

Figure 1.10 Plasticity chart

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1.4.2 For coarse-grained soils,

The coarse fraction is that portion of the total sample retained on a No. 200 sieve. If more than

half of the coarse fraction is gravel (retained on the No. 4 sieve), the soil is gravel and the first

letter symbol is G. If more than half of the coarse fraction is sand, the soil is sand and the first

letter symbol is S.

Clean sands and gravels (having less than 5% passing the No. 200 sieve) are given a second

letter P if poorly graded or W if well graded. For clean sands (less than 5% passing the No. 200

sieve), the classification is well-graded sand (SW) if Cu ≥ 6 and 1 £ Cc £ 3.Otherwise the

classification is poorly graded sand (SP). Clean gravels (less than 5% passing the No. 200 sieve)

are classified as well-graded gravel (GW) if Cu ≥ 4 and 1 £ Cc £ 3. If not met, the soil is poorly

graded gravel (GP).

Sands and gravels with more than 12% by weight passing the No. 200 sieve are given a second

letter M if the fines are silty or C if fines are clayey.

Sands and gravels having between 5 and 12% are given dual classifications such as SP-SM.

Silts, clays, and organic soils are given the second letter H or L to designate high or low

plasticity.

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1.5 Benefits of Unified Soil Classification System [5]

Soil Classification system helps determining the candidate additive to be used in soil

stabilization. Stabilization is the process of blending and mixing of materials with a soil to

improve certain properties of the soil.

1.5.1 Selection of candidate additives:

Figure 1.11 Gradation triangle for aid in selecting a commercial stabilizing agent (Courtesy of U.S Army, Navy and Air Force)

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The selection of stabilizers is made using figure 1.11 and table 1.1. The soil gradation triangle in

figure 1.11 is based upon the soil grain size characteristics and the triangle is divided into areas

of soils with similar grain size. The selection process is continued with table 1.1 which indicates

for each area shown in figure candidate stabilizers and restrictions based on grain size or

plasticity index (PI). In the second column of table 1.1 is a listing of soil classification symbols

applicable to the area determined from figure. This is an added check to insure that the proper

area was selected. Thus, information on grain size distribution and Atterberg limits must be

known to initiate the selection process. Data required to enter figure are: percent material passing

the No. 200 sieve and percent material passing the No. 4 sieve but retained on the No. 200 (i.e.,

total percent material between the No. 4 and the No. 200 sieves). The triangle is entered with

these two values and the applicable area (1A, 2A, 3, etc.) is found at their intersection. The area

determined from figure 1.11 is then found in the first column of table 1.1 and the soil

classification is checked in the second column. Candidate stabilizers for each area are indicated

in third column and restrictions for the use of each material are presented in the following

columns. These restrictions are used to prevent use of stabilizing agents not applicable for the

particular soil type under consideration. For example, assume a soil classified as a SC, with 93

percent passing the No. 4 and 25 percent passing the No. 200 with a liquid limit of 20 and plastic

limit of 11. Thus 68 percent of the material is between the No. 4 and No. 200 and the plasticity

index is 9. Entering figure 1.11 with the values of 25 percent passing the No. 200 and 68 percent

between the No. 4 and No. 200, the intersection of these values is found in area 1-C. Then going

to the first column of table 1.1, we find area 1-C and verify the soil classification, SC, in the

second column. From the third column all four stabilizing materials are found to be potential

candidates. The restrictions in the following columns are now examined. Bituminous

stabilization is acceptable since the PI does not exceed 10 and the amount of material passing the

No. 200 does not exceed 30 percent. However it should be noted that the soil only barely

qualifies under these criteria and bituminous stabilization probably would not be the first choice.

The restrictions under Portland cement indicate that the PI must be less that the equation

indicated in footnote b. Since the PI, 9, is less than that value, Portland cement would be a

candidate material. The restrictions under lime indicate that the PI not be less than 12 therefore

lime is not a candidate material for stabilization, The restrictions under LCF stabilization indicate

that the PI must not exceed 25, thus LCF is also a candidate stabilizing material. At this point,

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the designer must make the final selection based on other factors such as availability of material,

economics, etc. Once the type of stabilizing agent to be used is determined, samples must be

prepared and tested in the laboratory to develop a design mix meeting minimum engineering

criteria.

Table 1.1 Guide for selecting a stabilizing addictive (Courtesy of U.S Army, Navy, Air Force)

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1.6 Cost of the Equipments used for Classifying Soils

1) Digital balance 1000 x 0.01 g - Rs.8,000.00 each(required one is 1200 x .01 g)

2) Evaporating dish- Rs.250.00 each

3) Spatula - Rs. 90.00 each

4) Moisture tins - Rs.30.00 each

5) Oven thermostatically controlled 45 x 45 x 45 cm - Rs. 14,000 each

6) 20 cm diameter brass sieves 4.75mm, 2.00mm, .425mm, .150mm, .075mm and lid & pan

Rs.600 each.(A 3 inch sieve is required).

7) Sieve shaker, electrically operated for 20cm diameter - Rs.22,000.00.

8) Cassagrand’s liquid limit device (hand operated) - Rs 3,000.00.

Contact,

V.K. Instruments,

#26, Saleem House,

2nd cross, Nagappa road,

Srirampuram,

Bangalore-560 021

Phone no: 808 23424288

E-Mail- vktestinst@yahoo.co.in

Mobile- 9845787850

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2. Expansive and Collapsible Soils

2.1 Expansive Soils [5] [6]

Figure 2.1 Cracks due to volume changes in expansive clay soils

Soils that exhibit volume change from change in soil moisture (as shown in the figure 2.1) are

referred to as expansive or swelling clay soils (like black soils). The lightweight structures are

severely affected due to high swelling pressure exerted by these soils. Such type of large scale

distress, due to expansive nature of expansive soil, can be prevented by either obstructing the soil

movement and reducing the swelling pressure of soil or making the structure resistant to damage

from soil movement. The swelling pressure of the soil can be considerably reduced by adding

stabilizing agents like lime, flyash, etc.

2.1.1 Stabilization with lime

If it has been determined that a soil has potential for excessive swell, lime treatment may be the

most appropriate. Lime reduces the swell in an expansive soil to greater or lesser degrees

depending on the activity of the clay minerals present. The depth to which lime should be

incorporated into the soil is generally limited by the construction equipment used. However, 2 to

3 feet generally is the maximum depth. In general, all lime treated fine-grained soils exhibit

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decreased plasticity, improved workability and reduced volume change characteristics. However,

not all soils exhibit improved strength characteristics. It should be emphasized that the properties

of soil-lime mixtures are dependent on many variables. Soil type, lime type, lime percentage and

curing conditions (time, temperature, moisture ) are the most important.

2.1.1.1 Types of lime:

The most commonly used products are hydrated high-calcium lime, monohydrated dolomitic

lime, calcitic quicklime, and dolomitic quicklime. Hydrated lime is used most often because it is

much less caustic than quick-lime, however, the use of quicklime for soil stabilization has

increased in recent years mainly with slurry-type applications. The design lime contents

determined from the criteria presented herein are for hydrated lime. If quicklime is used the

design lime contents determined herein for hydrated lime should be reduced by 25 percent.

2.1.1.2 Soil Modification using lime:

After initial mixing, the calcium ions (Ca++) from the lime migrate to the surface of the clay

particles and displace water and other ions. The soil becomes friable and granular, making it

easier to work and compact. At this stage the Plasticity Index of the soil decreases dramatically,

as does its tendency to swell and shrink. The process, which is called “flocculation and

agglomeration," generally occurs in a matter of hours. Small amounts of lime, such as 1 to 4

percent by mass of dry soil, can upgrade many unstable fine-grained soils. With heavy clay soils,

additional lime may be necessary for these purposes. Modification improvements are generally

temporary and will not produce permanent strength in clay soils.

2.1.1.3 Lime content for lime-modified soils: The amount of lime required to improve the

quality of a soil is determined through the same trial and error process used for cement-modified

soils.

2.1.1.4 Soil Stabilization using lime:

In contrast to lime modification, lime stabilization creates long-lasting changes in soil

characteristics that provides structural benefits. Lime stabilization chemically changes most clay

soils:

1. Markedly reduces shrinkage and swell characteristics of clay soils.

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2. Increases unconfined compressive strength by as much as 40 times.

3. Substantially increases load-bearing values as measured by such tests as CBR, R-value,

Resilient Modulus, and the Texas Triaxial tests.

4. Develops beam strength in the stabilized layer and greatly increases the tensile or flexural

strength.

5. Creates a water-resistant barrier.Restricts migration of surface water from above and

capillary moisture from below, thus helping to maintain foundation strength.

6. In addition to lowering the plasticity in most cases and initially strengthening the improved

soil, the strengthening effect increases over time.

When adequate quantities of lime and water are added, the pH of the soil quickly increases to

above 10.5, which enables the clay particles to break down. Silica and alumina are released and

react with calcium from the lime to form calcium-silicatehydrates (CSH) and calcium-aluminate-

hydrates (CAH). These compounds form the matrix that contributes to the strength of lime-

stabilized soil layers. As this matrix forms, the soil is transformed from its highly expansive,

undesirable natural state to a more granular, relatively impermeable material that can be

compacted into a layer with significant load bearing capacity. In a properly designed system,

days of mellowing and curing produce years of performance. The controlled pozzolanic reaction

creates a new material that is permanent, durable, resistant to cracking, and significantly

impermeable. The structural layer that forms is both strong and flexible.

2.1.1.5 Lime content for lime-stabilized soils: The following procedures are recommended for

determining the lime content of lime stabilized soils.

Step 1: The preferred method for determining initial design lime content is the pH test. In this

method several lime-soil slurries are prepared at different lime treatment level such as 2, 4, 6,

and 8 percent lime and the pH of each slurry is determined. The lowest lime content at which a

pH of about 12.4 (the pH of free lime) is obtained is the initial design lime content. Procedures

for conducting the pH test are indicated in appendix A.

Step 2: Using the initial design lime content, conduct moisture-density tests to determine

the maximum dry density and the optimum water content of the soil lime mixture.

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Step 3: Prepare triplicate samples of the soil lime mixture for unconfined compression and

durability tests at the initial design lime content and at lime contents 2 and 4 percent above

design content.

Step 4: The lowest lime content which meets the unconfined compressive strength requirement

and demonstrates the required durability is the design lime content.

2.1.2 Stabilization with Bitumen

Stabilization of soils and aggregates with asphalt differs greatly from cement and lime

stabilization. The basic mechanism involved in asphalt stabilization of fine-grained soils is a

waterproofing phenomenon. Soil particles are coated with asphalt that prevents or slows the

penetration of water which could normally result in swelling of expansive soils. In addition,

asphalt stabilization can improve durability characteristics by making the soil resistant to the

detrimental effects of water such as volume changes.

2.1.2.1 Types of bituminous stabilized soils:

(1) Sand bitumen: A mixture of sand and bitumen in which the sand particles are cemented

together to provide a material of increased stability.

(2) Gravel or crushed aggregate bitumen: A mixture of bitumen and a well-graded gravel or

crushed aggregate that, after compaction, provides a highly stable waterproof mass of sub-base

or base-course.

(3) Bitumen lime: A mixture of soil, lime, and bitumen that, after compaction, may exhibit the

characteristics of any of the bitumen-treated materials indicated above. Lime is used with

materials that have a high Plasticity Index (PI), i.e. above 10.

2.1.3 Stabilization with Lime-Cement and Lime-Bitumen

The advantage in using combination stabilizers is that one of the stabilizers in the combination

compensates for the lack of effectiveness of the other in treating a particular aspect of a given

soil.

2.1.3.1 Lime-cement: Lime can be used as an initial additive with Portland cement. The main

purpose of lime is to improve workability characteristics mainly by reducing the plasticity of the

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soil. The design approach is to add enough lime to improve workability and to reduce the

plasticity index to acceptable levels.

2.1.3.2 Lime-asphalt: Lime can be used as an initial additive with asphalt as the primary

stabilizer. The main purpose of lime is to improve workability characteristics and to act as an

anti-stripping agent. In the latter capacity, the lime acts to neutralize acidic chemicals in the soil

or aggregate which tend to interfere with bonding of the asphalt. Generally, about 1-2 percent

lime is all that is needed for this objective.

2.1.4 Stabilization with flyash

As fly ash is freely available, if the construction site is in the vicinity of a Thermal Power Plants,

it can be used for stabilization of expansive soils. Flyash consists of often hollow spheres of

silicon, aluminium and iron oxides and unoxidized carbon. There are two major classes of flyash,

class C and class F. The former is produced from burning anthracite or bituminous coal and the

latter is produced from burning lignite and sub bituminous coal. Both the classes of fly ash are

puzzolans, which are defined as siliceous and aluminous materials. Thus Fly ash can provide an

array of divalent and trivalent cations (Ca2+,Al3+,Fe3+etc) under ionized conditions that can

promote flocculation of dispersed clay particles. Thus, expansive soils can be potentially

stabilized effectively by cation exchange using flyash.

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2.2 Collapsible Soils [6]

Figure 2.2 Cracks in the structure due to compaction of collapsible soils (Image is a courtesy of New Mexico bureau of geology and mineral resources)

Collapsible soils are soils that compact and collapse after they get wet. The soil particles are

originally loosely packed and barely touch each other before moisture soaks into the ground. As

water is added to the soil in quantity and moves downward, the water wets the contacts between

soil particles and allows them to slip past each other to become more tightly packed.

Another term for collapsible soils is "hydrocompactive soils" because they compact after water

is added. The amount of collapse depends on how loosely the particles are packed originally and

the thickness of the soil that becomes wetted.

Collapsible soils consist primarily of silt sized particles loosely arranged in a cemented

honeycombed structure. The loose structure is held together by small amounts of water soluble

cementing agents such as clay minerals and CaCO3. The introduction of water dissolves or

softens the bonds between the silt particles and allows them to take a denser packing under any

type of compressive loading.

When dealing with collapsible soils that are subject to wetting depths of ≤ 2 meters, common

measures are to:

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1. Pre wet the soil.

2. Compact the soil using heavy rollers and heavy tamping.

3. Treat the soil with sodium silicate and/or calcium chloride solutions to provide cementing

that is not water soluble.

When dealing with collapsible soils subject to large wetting depths, then deep foundations

through the collapsible soils are commonly used.

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Conclusion

Classification of various types of soils and how this classification is used to determine the

stabilizing agent used for stabilizing the soils has been discussed. This soil classification will be

very useful for the organization as this classification system is used as a base to decide the future

course of action. This soil classification system helps in knowing the approximate bearing

capacity, California Bearing Ratio, etc without performing the tests required for finding these

values.

SKG Sangha faces a major problem for constructing biogas plants on black cotton soils

(expansive soils). The best stabilizing agent for stabilizing these soils is lime which decreases the

swelling and the plasticity of these soils. When it comes to areas nearby thermal power plants

using flyash will be very economical as flyash is waste product of these plants. Other stabilizing

agents which can serve the purpose like bitumen which prevents water from penetrating and

hence preventing swelling can also be very useful for stabilization. When it comes to collapsible

soils, best way is treating the soil wth sodium or calcium chloride and hence cementing the soil.

Finally soil classification can be very useful and use of lime for stabilization can solve the

problem of expansive soils.

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Appendix A

pH TEST TO DETERMINE LIME REQUIREMENTS FOR LIME STABILIZATION

Materials: Lime to be used for soil stabilization.

Apparatus: Apparatus include pH meter (the pH meter must be equipped with an electrode

having a pH range of 14), 150-milliliter (or larger) plastic bottles with screw-top lids, 50-

milliliter plastic beakers, distilled water that is free of CO2, balance, oven, and moisture cans.

Procedure:

1. Standardize the pH meter with a buffer solution having a pH of 12.45.

2. Weigh to the nearest 0.01 gram samples of air-dried soil, passing the No. 40 sieve and

equal to 20.0 grams of oven-dried soil.

3. Pour the soil samples into 150-milliliter plastic bottles with screw-top lids.

4. Add varying percentages of lime, weighed to the nearest 0.01 gram, to the soils. (Lime

percentages of 0, 2, 3, 4, 5, 6, 8, and 10, based on the dry soil weight, may be used.)

5. Thoroughly mix soil and dry lime.

6. Add 100 milliliters of distilled water that is CO2 free to the soil-lime mixtures.

7. Shake the soil-lime and water for a minimum of 30 seconds or until there is no evidence

of dry material on the bottom of the bottle.

8. Shake the bottles for 30 seconds every 10 minutes.

9. After 1 hour transfer part of the slurry to a plastic beaker and measure the pH.

10. Record the pH for each of the soil-lime mixtures. The lowest percent of lime giving a pH

of 12.40 is the percent required to stabilize the soil. If the pH does not reach 12.40, the

minimum lime content giving the highest pH is required to stabilize the soil.

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References

1. En.wikepedia.com

2. Engineeringcivil.com

3. Geotechnical laboratory, University of Texas at Arlington

4. AASHTHO

5. Soil Stabilization of Pavements- U.S Army, Navy and Air Force

6. 53:139 Foundation Engineering (The University of Iowa)-C.C. Swan, Instructor

7. Civilengineeringportal.com

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Glossary

Agglomerate (5) - Form into one cluster

Asphalt (22) - A dark bituminous substance found in natural beds and as residue from petroleum

Bitumen (22) - Any of various naturally occurring impure mixtures of hydrocarbons

Cementing (25) - bind or join with or as if with cement

Coarse (14) - Of textures that are rough to the touch or substances consisting of relatively large

particles

Flocculation (20) - forming woolly cloudlike aggregations

Friable (20) - Easily broken into small fragments or reduced to powder

Groove (8) - A long narrow furrow cut either by a natural process or by a tool

Mortar (7) - A bowl-shaped vessel in which substances can be ground and mixed with a pestle

Pestle (7) - A heavy tool of stone or iron (usually with a flat base and a handle) that is used to

grind and mix material

Plasticity (13) - The property of being physically malleable; the property of something that can

be worked, hammered or shaped without breaking

Pozzolanic (21) - need something else for its functioning

Sieve (2) - A strainer for separating lumps from powdered material or grading particles

Spatula (7) - A hand tool with a thin flexible blade used to mix or spread soft substances

Tamping (25) - Press down tightly

Tare (9) - An adjustment made for the weight of the packaging in order to determine the net

weight of the goods

30