Minor Project Report 6i5r50

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MINOR PROJECT REPORT ON

“ SOIL STABILIZATION “

MEHR CHAND POLYTECHNIC COLLEGE

Department Of Civil Engineering

GUIDED BY

SUBMITTED BY

Er JAGROOP SINGH (PRINCIPAL) Mr Rajeev Bhatia (Lect)

GROUP – G

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INDEX s.No.

Topic

Page No.

1.0

Brief History Of - - “SOIL STABILIZATION

2.0

Introduction

3.0

Soil 3.1

Engineering Classification Of Soil

3.2

Constituents Of Soil

4.0

Major Soil Deposits Of India 4.1

Lake Deposits

4.2

Local Soils Found In Punjab

5.0

Names Of Organization Dealing With Soil Work In India

6.0

Necessity of Soil Stabilization

7.0

Deformation of soils

8.0

Bearing Capacity 8.1

9.0

Methods Of Improving Bearing Capacity Of Soil Methods Of Soil Stabilization

9.1

Mechanical Stabilization 9.1.1

Compaction

9.1.2

Consolidation

9.2

Cement Stabilization

9.3

Lime Stabilization

3

9.4

Bitumen Stabilization

9.5

Chemical Stabilization

9.6

Chemical injection

9.7

Grouting

9.8

Thermal Stabilization

9.9

Electrical Stabilization

9.10

Geo-textiles & Fabrics

9.11

Reinforcing Earth

9.12

Sand Drains

9.13

Fly Ash Stabilization

4

ACKNOWLEDGEMENT

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1.0 Brief History Of - - “SOIL STABILIZATION “- The stabilization of soils has been performed for millennia. It was recognized before Christian era began that certain geographic regions were plagued with surface materials and ambient condition that made the movement of armies and goods difficult, if not possible, over the paths between villages and towns. Are shown below fig-1.

Fig 1 :- Load Carrying Capacity By Chemical Stabilization The Mesopotamians and romans separately discovered that it was possible to improve the ability of pathways to carry traffic by mixing the weak soil with a stabilizing agent like pulverized limestone or calcium. This was the first chemical stabilization of weak soils to improve their load carrying capacity. Jump forward a few years to the war of Vietnam (1955 to 1975), the US military were looking for methods for rapid stabilization of weak soils for of its missions worldwide. Over the past 60 years they had used cement and lime these being the most effective stabilizers for road and airfield applications, but although with careful analysis of ground conditions and the make-up of the existing soils these traditional stabilizers did have a remedial effective. They urgently needed a stabilizer that could be used quickly without having to carry out extensive site tests

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that could increase the strength of the prevalent soft clay type local soils rapidly to the landing and take-off heavy C-17 and C-130 aircraft traffic on their temporary airfields.

2.0 Introduction Soil stabilization is the process of improving the engineering properties of the soil and thus making it more stable. It is required when the soil available for construction is not suitable for the intended purpose. In its broadest senses, stabilization includes compaction, pre-consolidation, drainage and many other such processes. However, the term stabilization is generally restricted to the process which alters the soil material itself for improvement of its properties. A cementing material or a chemical is adding to a natural soil for the purpose of stabilization.

Soil stabilization is used to reduce the permeability and compressibility of the soil mass in the earth structure and to increase its shear strength. Soil stabilization is required to increase the bearing capacity of foundation soil. However, the main use of stabilization is to improve the natural soil for the construction of highway and airfields. The principles of soil stabilization are used for controlling the gradient of soil and aggregate in the construction for base and sub-base for the highways and airfields.

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Soil stabilization is also used to make an area trafficable within a short period of time for military and other emergency purposes. Sometimes, soil stabilization is used for city and suburban streets to make them more noise-absorbing.

3.0 Soil Soil is composed of particles of broken rock that have been altered by chemical and mechanical processes that include weathering and erosion. It is a mixture of mineral and organic constituents that are in solid, gaseous and aqueous states.

3.1 Engineering Classification Of Soil Based on the engineering classification, soil may be grouped as: 1. Coarse grained soil or Granular soil 2. Fine grained soil or cohesive soil 3. Organic soil This classification is also known as general classification or broad classification of soil. 1. Coarse grained soil: The soil which consists of coarser size particles or coarse grains is termed as coarse grained soil. It is also known as cohesionless soil or granular soil as there is no cohesion or cohesive forces acting between the particles to bind them together. Particles of this type of soil are rounded, angular, bulky, and hard rock particles and 50% or more of total material (soil) by weight is retained on 75 micron IS sieve. Sand, gravel, cobbles are the common examples of coarse grained soil.

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Fig 3 :- Coarse Grained Soil The property of a coarse grained soil depends upon the uniformity of the size of the grains. Well graded sand is more stable as a foundation base as compared to a uniform or poorly graded gravel or sand. A granular soil has the following significant engineering properties; (a) It is an excellent foundation material. (b) It is the best embankment material. (c) It is the best backfill material for retaining walls. 2. Fine grained soil: The soil which consists of finer size particles or fine grains is termed as fine grained soil. It is also known as cohesive soil. The presence of cohesion or binding force in fine grained soil is due to the intermolecular forces of attraction between soil particles and binding of soil mass together by the capillary action of moisture present in the soil. This type of soil includes clays, silts, silty clays and clays mixed with sand or gravel.

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Fig 4 :- Fine Grained Soil In this type of soil, more than 50% of the total material es 75 micron IS sieve. The shear strength is largely .Clay is more cohesive than silt. Fine grained soils have following significant engineering properties; 1. It possesses low shear strength. 2. It is impervious practically. 3. It shrinks upon drying and expands upon wetting. 4. It is plastic and compressible. that cohesive property is due to presence of clay minerals in soil. So term cohesive soil is generally used for clayey soils only. 3. Organic Soil: The soil which contains a large percentage of organic matter and particles of decomposed vegetation matter is known as Organic soil. The soil with organic matter is weaker and more compressible than soils having the same mineral composition but lacking in fibrous nature and odour (smell) of decaying vegetation.

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Fig 5 :- Organic Soil The significant engineering properties of organic soils are: 1. It has low shear strength. 2. It is often permeable. 3. It is highly compressible. 4. It is poorest foundation material. These types of soils are present in the top layer of soil and are removed generally from a site prior to the start of construction.

3.2 Constituents Of Soil A soil mass consists of solid particles, water and air. The solid particles are called soil grains .The void space between the soil grains is partly filled with water and partly with air. Thus, Soil consists of three constituents viz. solid particles, air and water which are blended together to form a complex material.

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Fig 6 :- Three Face Diagram Of Soil

4.0 Major Soil Deposits Of India Based on the physiographic, climatic conditions and geological formation, the soils of India can be divided into following major groups – 1. Marine Deposits 2. Laterite Soil (Laterites) 3. Desert Soils 4. Black Cotton Soils 5. Alluvial Soil

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Fig 7 :- Major Soil Deposits Of India

1. Marine Deposits: The marine deposits have low shearing strength and are highly compressible in nature. The marine clays are very soft and sometimes contain a lot of organic matter. These deposits are confined mainly along a narrow belt near the coastal areas however in Rann of Kutch they are of wide extent. In the south west coast of India, there are thick layers of sand above deep deposit of soft and highly plastic marine clays. The problems in

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these areas relate foundation of structures, embankments on of low strength and high compressibility of these clays. 2. Black Cotton Soils: The black cotton soils extend over states of Maharashtra, Gujrat, Eastern Rajasthan, Madhya Pradesh, Karnataka and parts of Andhra Pradesh and Tamil Nadu .These are residual deposits formed from basalt or tap rocks. These are quiet suitable for growing cotton. These are expansive in nature and consist of clay mineral montmorillonite. On of high shrinkage and swelling of this soil with rapid moisture changes, the method of under reamed piling has been developed in these areas. These piles are taken to a depth where the moisture variation is negligible. 3. Lateritic Soils : Lateritic soils constitute an important soil group and extend over Kerela, South Maharashtra, Karnataka, Orissa and West Bengal. These are residual soils formed by decomposition of rock (basalt) . The presence of iron oxide gives these soils, the characteristic red colour. These laterites are soft and can be cut with a chisel when wet with natural moisture content but it will harden with time. In general, laterites are very satisfactory approach the parent rock), needs careful consideration. 4. Alluvial Soils: A large part of northern India is covered with alluvium (alluvial deposits). In the Indo-Gangetic and Brahmaputra flood plains, the alluvial deposits also occur at some places in the peninsular India. The distinct characteristics of these alluvial deposits are the presence of alternating layers of sand, silt and clay deposits. In West Bengal, organic strata are also encountered along with sand, silt and clay layers. Sand boils and liquefaction failures are the major problem associated with these areas. 5. Desert Soils: A large part of Rajasthan and ading areas is covered with windblown deposits of sand dunes. Dune sand is a wind deposited non plastic and uniformly graded fine

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sandy soil. As the sand is generally in loose condition, soil stabilization is necessary in these areas to increase strength, to reduce permeability and to reduce settlement.

4.1 Lake Deposits Lakes are different than marine environments. The sedimentation of lakes is ten times higher than in marine environment. Lakes are also smaller, nearly closed system and tides in lakes are less pronounced. Therefore energy levels in lakes are lower, coarser sediment (sand and gravel) is deposited in shallow water. Area of lakes, especially during summer, while finer-grained sediment (silt and clay) is deposited in deeper water areas of lakes, and more so during winter. Varves, alternating thin layers of light coloured coarser grained sediment and dark-coloured finer grained sediments, are one type of lacustrine deposit and form in both glacial and non-glacial lakes. Sedimentation is closed lake system consists of evaporate minerals, carbonate mud’s, sands and silts. Lacustrine deposits are often rich in organic shale’s which are important source rocks for petroleum.

4.2 Local Soils Found In Punjab Soil is the end product of the parent material resulting from the consistent influence of climate, topography are influenced to a very limited extent by the topography, vegetation and parent rock. The variation in soil profile characteristics are much more pronounced because of the regional climatic differences. Punjab can be divided into three distinct regions on the basis of soil types – 1. South-Western Punjab: This region covers the tehsils of fazilka, Muktsar, Bhatinda, Mansa and parts of ferozpur which border Haryana and Rajasthan states in the south-west .The soil is predominantly calcareous, developed under hot and arid to semi-arid conditions. The Ph value

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ranges from 7.8 to 8.5 which show that soil is normal in reaction. Grey and red desert, calsisol, regosol and alluvial soils are found in this zone. The soil of south-western Punjab can further be sub-divided into two categories. (a) Desert Soil: The soil covers fazilka tehsil of ferozpur district and south-western fringers of Muktsar teshil of faridkot district. The soil is deficient in nitrogen, phosphorus and potassium. Wind erosion is a serious problem especially during the hot summer. (b) Sierozem Soil: The soil is found in Bathinda district and faridkot and Muktsar tehsils of faridkot and most parts of Ferozepur tehsil. The texture of the soil is sandy loan to silt. The soil is deficient in nitrogen, phosphorus and potash. In some irrigated tracks, alkalinity and salinity pose a problem. Wind erosion is again a serious matter in areas where this soil group is predominant. 2. Central Punjab: The soil of this zone has developed under semi-arid condition. The soil is sandy loam to clayey with normal reaction (ph from 7.8 to 8.5) . The soil covers the districts of Sangrur, Patiala, Ludhiana, Jalandhar, Kapurthala, Amritsar, parts of Gurdaspur, Ferozepur and fringes of Kharar tehsil of Ropar district, Problem of alkalinity and Salinity is quite acute, especially in districts of Amritsar, Sangrur, Ferozepur, Gurdaspur and Patiala. The soil of the central zone generally recognized as alluvial, falls into two categories. (a) Arid and Brown Soil: This soil is found in Amritsar district( except in the north-eastern half of the Amritsar tehsil) in most of Sultanpur tehsil of Kapurthala, Zira and northern parts of Ferozpur, Moga, Rampur tehsil of Bathinda, Barnala, Sangrur and Sunam tehsils of Sangrur district and Samana tehsil of Patiala district. The texture is sandy-loam and the fertility is from medium to high. The soil is calcareous and lacks nitrogen but contains a fair amount of phosphorus and potash. (b) Tropical Arid Brown Soil: This soil covers parts of Amritsar, the south-western half of Gurdaspur tehsil, Batala tehsil, Kapurthala district expect Sultanpur, Jalndhar, Ludhiana,

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Patiala and the Malerkotla tehsil of Sangrur district. Some parts in the south-west of Ropar district also fall in the zone. The soil is deficient in nitrogen, potash and phosphorous. 3. Eastern Punjab: The soil had developed in the sub-humid foothill areas bordering Himachal Pradesh covering eastern parts of Gurdaspur, Hoshiarpur and fair amount of rainfall. Two soil types are as under. (a) Grey Brown Podzolic Soil: This soil is found in the Pathankot tehsil of Gurdaspur and northeastern parts of Ropar and Kharar tehsils. Because of surface run-off, the soil is not influenced by leaching, hence profile development is poor. (b) Reddish Chestnut Soil: The soil is found in a region covering Hosiarpur, Ropar and some parts of Gurdaspur. The carbonates are leached down to the lower layers. The soil Is moderately acidic and neutral in reaction (ph 6.5 to 7.5) and is deficient in nitrogen and phosphorus.

5.0 Names Of Organization Dealing With Soil Work In India There are various organization established by Government of India which are carrying out research activities on various aspects of Soil Mechanics and Foundation Engineering .Some of the important organization are listed below: 1. Central Soil Salinity Research Institute, Karnal (Haryana) 2. Central Soil and Material Research Station, New Delhi. 3. Central Soil and Water Conservation Research and Training Institute, Dehradun (Uttrakhand) 4. Central Road Research Institute, Okhla (New Delhi). 5. Indian Agricultural Research Institute, New Delhi.

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6.0 Necessity of Soil Stabilization 

Stabilized soil functions as a working platform for the project.



Stabilization waterproofs the soil.



Stabilization improves soil strength.



Stabilization helps reduce soil volume change due to temperature or moisture.



Stabilization improves soil workability.



Stabilization reduces dust in work environment.



Stabilization improves marginal materials.



Stabilization improves durability.



Stabilization dries wet soil.



Stabilization conserves aggregate materials.



Stabilization reduces cost.



Stabilization conserves energy.

7.0 Deformation of soils The change in shape or volume of soil mass accompanied by vertical or lateral movement under the effect of external loads is termed as deformation of soils.

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

Swelling – The increase in the volume of a soil mass, when water is added to it is called swelling.



Creep – The slow and gradual lateral advancement of the soil is termed as Creep.



Heaving – The upward movement of soil is termed as Heaving.



Plastic Flow – The continuous soil deformation in highly plastic soils subjected to constant shearing stress is termed as Plastic flow.



Lateral Movement – When soil is loaded, shearing stresses are induced in it. When shearing stresses reach a limiting value, shear deformation takes place in lateral direction causing shearing of soil.



Settlement – Settlement of a structure is its vertical downward displacement due to decrease in the volume of soil mass on which it is built. Settlement is of following types : 1. Uniform settlement: If the settlement of structure due to soil displacement is even, it is called uniform settlement. If the settlement of all the footings of a simple building is uniform, then there is no damage to the building. 2. Differential settlement: If the settlement of structure due to soil movement is uneven, it is called differential settlement or non uniform settlement. This settlement is more dangerous as it causes damage to the structure.

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3. Tilt: It is defined as angular distortion or rotation of a structure about its base. The leaning tower of Pisa in Italy built during 14th century is typical example of tilt. It is 179 feet in height and its top is out of plumb by about 20 feet. It occurs when the structure is unevenly loaded or when the soils are non uniform.

Fig 9 :- The Learning Tower Of Pisa

8.0 Bearing Capacity It may be defined as the largest intensity of pressure which may be applied by a structure or a structural member to the soil which s it without causing excessive settlement or danger of failure of the soil in shear. The load transmitted to the soil at the base of foundation compresses the

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soil even at greater depths. The depth upto which the effect of foundation load is felt is termed as significant depth.

8.1 Methods Of Improving Bearing Capacity Of Soil When the soil available near the ground level for construction is having low bearing capacity two alternatives can be adopted; 1. To adopt deep foundation i.e. construction of piles, well or piers etc. 2. To improve the bearing capacity of soil. As a construction of deep foundation is a costly affair, the best alternative is to improve the engineering properties of locally available soil to increase its load carry capacity The bearing capacity of soils can be improved by following methods; 1. By mechanical stabilization i.e. increasing the bearing capacity of the soil by changing its gradation. 2. The most common and important method of improving bearing capacity of soil is compaction (densifications with mechanical equipment, usually a roller). 3. Addition of cement, lime, fly ash, chemicals and other ixtures etc. to the soil improves its load carrying capacity. 4. Thermal stabilization and electrical stabilization is also used to improve the bearing capacity of soils especially loose silt, fine and clay. 5. The new soil strengthening technique involves mixing of randomly distributed discrete fibers to the soil.

9.0 Methods Of Soil Stabilization The different methods of soil stabilization are : -

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

Mechanical Stabilization 1. Compaction 2. Consolidation



Cement Stabilization



Lime Stabilization



Bitumen Stabilization



Chemical Stabilization



Chemical injection



Grouting



Thermal Stabilization



Electrical Stabilization



Geo-textiles & Fabrics



Reinforcing Earth



Sand Drains



Fly Ash Stabilization

9.1 Mechanical Stabilization Mechanical stabilization is the process of improving the properties of the soil by changing its gradation. Two or more types of natural soil are fixed to obtain a composite material which is superior to any of its components. To achieve the desired grading, sometimes of soil with coarse particles are added or the soil with fine particles are removed. Mechanical stabilization is also known as granular stabilization. Mechanical stabilization involves two operations:

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(1) Changing the composition of soil by addition or removal of certain constituents. (2) Densification or compaction. The particle sized distribution and composition are the important factors governing the engineering behaviour of a soil. Signification changes in the properties can be made by addition or removal of suitable soil fractions. for mechanical stabilization, where the primary purpose is to have a soil resistant to deformation and displacement under load, soil material can be divide under two fractions: the granular fraction retained on a 75micron IS sieve and a fine soil fraction ing a 75micron sieve. The granular fraction imparts strength and hardness. The fine fraction provides cohesion or binding property, water-retention capacity and also acts as filler for the voids of the coarse fraction. Mechanical stabilization has been largely used in the construction of cheap roads.

9.1.1 Soil Compaction Soil compaction is defined as the method of mechanically increasing the density of soil. In construction, this is a significant part of the building process. If performed improperly, settlement of the soil could occur and result in unnecessary maintenance costs or structure failure. Almost all types of building sites and

construction projects

utilize

mechanical compaction

techniques.

9.1.1.1 Necessity of Compaction – There are five principle reasons to compact soil:

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- Increases load-bearing capacity - Prevents soil settlement and frost damage - Provides stability - Reduces water seepage, swelling and contraction - Reduces settling of soil

9.1.1.2 Results of Poor Compaction

Both illustrations above show the result of improper compaction and how proper compaction can ensure a longer structural life.

9.1.1.3 Field Compaction The compaction of soil (filling or cutting) at the field site to achieve maximum dry density by rolling, ramming or vibration is termed as Field Compaction. The field compaction is done by one of the following compacting equipments : 1. Rollers – Different types of rollers used for compaction of soils are : -

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

Smooth-wheel Rollers



Pneumatic-tyred Rollers



Sheep-foot Rollers

Fig 10 :- Compaction By Rollings 2. Rammers – Rammers used for field compaction are : 

Hand operated Rammers



Mechanical Rammers

Fig 11 :- Compaction By Rammers 3. Vibrators

9.1.1.4 Soil types and conditions

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Every soil type behaves differently with respect to maximum density and optimum moisture. Therefore, each soil type has its own unique requirements and controls both in the field and for testing purposes. Soil types are commonly classified by grain size, determined by ing the soil through a series of sieves to screen or separate the different grain sizes. Soil classification is categorized into 15 groups, a system set up by AASHTO (American Association of State Highway and Transportation Officials). Soils found in nature are almost always a combination of soil types. A well-graded soil consists of a wide range of particle sizes with the smaller particles filling voids between larger particles. The result is a dense structure that lends itself well to compaction. A soil's makeup determines the best compaction method to use.

Fig 12 :- Sieve Analysis There are three basic soil groups: 

Cohesive



Granular

 Organic

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 Cohesive soils Cohesive soils have the smallest particles. As per IS : 1948 Silt and Clay have particle sizes < 0.075 mm. It is used in embankment fills and retaining pond beds. Characteristics Cohesive soils are dense and tightly bound together by molecular attraction. They are plastic when wet and can be molded, but become very hard when dry. Proper water content, evenly distributed, is critical for proper compaction. Cohesive soils usually require a force such as impact or pressure. Silt has a noticeably lower cohesion than clay. However, silt is still heavily reliant on water content.  Granular soils Granular soils range in particle size from 0.075 mm to 4.75 mm (sand) and 4.75 mm to 80 mm (gravel). Granular soils are known for their water-draining properties. Characteristics Sand and gravel obtain maximum density in either a fully dry or saturated state. Testing curves are relatively flat so density can be obtained regardless of water content. The table that follows give a basic indication of soils used in particular construction applications

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9.1.1.5 Effect of Moisture The response of soil to moisture is very important, as the soil must carry the load year-round. Rain,

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for example, may transform soil into a plastic state or even into a liquid. In this state, soil has very little or no load-bearing ability.

9.1.1.6 Moisture vs. Soil Density Moisture content of the soil is vital to proper compaction. Moisture acts as a lubricant within soil, sliding the particles together. Too little moisture means inadequate compaction - the particles cannot move past each other to achieve density. Too much moisture leaves water-filled voids and subsequently weakens the load-bearing ability. The highest density for most soils is at a certain water content for a given compaction effort. The drier the soil, the more resistant it is to compaction. In a water-saturated state the voids between particles are partially filled with water, creating an apparent cohesion that binds them together. This cohesion increases as the particle size decreases (as in claytype soils).

9.1.1.7 Soil Density Tests To determine if proper soil compaction is achieved for any specific construction application, several methods were developed. The most prominent by far is soil density. Tests to determine optimum moisture content are done in the laboratory. The most common is the

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Proctor Test, or Modified Proctor Test. A particular soil needs to have an ideal (or optimum) amount of moisture to achieve maximum density. This is important not only for durability, but will save money because less compaction effort is needed to achieve the desired results.

9.1.1.7.1 The Hand Test A quick method of determining moisture is known as the "Hand Test". Pick up a handful of soil. Squeeze it in your hand. Open your hand. If the soil is powdery and will not retain the shape made by your hand, it is too dry. If it shatters when dropped, it is too dry. If the soil is moldable and breaks into only a couple of pieces when dropped, it has the right amount of moisture for proper compaction. If the soil is plastic in your hand, leaves traces of moisture on your fingers and stays in one piece when dropped, it has too much moisture for compaction.

9.1.1.7.2 Proctor Test [IS: 2720 (Part VII)] The Proctor, or Modified Proctor Test, determines the maximum density of a soil needed for a specific job site. The test first determines the maximum density achievable for the materials and uses this figure as a reference. Secondly, it tests the effects of moisture on soil density. The soil reference value is expressed as a percentage of density. These values are determined before any compaction takes place to develop the compaction specifications. Modified Proctor values are higher because they take into higher densities needed for certain typed of construction projects. Test methods are similar for both tests. Proctor Test A small soil sample is taken from the jobsite. A standard weight is dropped several times on the soil.

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The material weighed and then oven dried for 12 hours in order to evaluate water content

Fig 13 :- Proctor Test . Modified Proctor Test This is similar to the Proctor Test except a hammer is used to compact material for greater impact, The test is normally preferred in testing materials for higher shearing strength.

9.1.2 Consolidation Consolidation is a process by which soils decrease in volume. According to Karl Terzaghi "consolidation is any process which involves decrease in water content of a saturated soil without replacement of water by air." In general it is the process in which reduction in volume takes place by expulsion of water under long term static loads. It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing its bulk volume. When this occurs in a soil that is saturated with water, water will be squeezed out of the soil.

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The process of consolidation is similar to the action of squeezing of water from a saturated sponge under pressure. As the consolidation of soils occurs, the water escapes. The solid particles shift from one position to the other by rolling and sliding which ultimately result in a closer packing. Thus it is worth noting that the decrease in volume of soil occurs not due t compression of solids or water, but due to relative arrangement of soil particles as the water escapes.

9.1.2.1 Consolidation Analysis (spring analogy) The process of consolidation is often explained with an idealized system composed of a spring, a container with a hole in its cover, and water. In this system, the spring represents the compressibility or the structure itself of the soil, and the water which fills the container represents the pore water in the soil.

Fig 14 :- Consolidation By Spring Method 1. The container is completely filled with water, and the hole is closed. (Fully saturated soil) 2. A load is applied onto the cover, while the hole is still unopened. At this stage, only the water resists the applied load. (Development of excess pore water pressure) 3. As soon as the hole is opened, water starts to drain out through the hole and the spring shortens. (Drainage of excess pore water pressure) 4. After some time, the drainage of water no longer occurs. Now, the spring alone resists the applied load. (Full dissipation of excess pore water pressure. End of consolidation)

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9.1.2.2 Time Dependency The time for consolidation to occur can be predicted. Sometimes consolidation can take years. This is especially true in saturated clays because their hydraulic conductivity is extremely low, and this causes the water to take an exceptionally long time to drain out of the soil. While drainage is occurring, the pore water pressure is greater than normal because it is carrying part of the applied stress (as opposed to the soil particles).

9.1.2.3 Consolidation of soil In case of coarse grained soils like sands and gravels, the removal of this pore water is easy since water freely moves from one region to another within these soil types. However, in case of fine grained soils like silty or clayey soils, consolidation is a time consuming process. As an analogy, consider soil mass to be like a sponge that is slightly wet. If we press the sponge, it will deform by compressing the air out of it. If we squeeze it further, water will be removed and the sponge will be compressed further. If the sponge (soil mass) is completely wet or soaked, it is termed as saturated. This is the condition when all voids are filled with water and no air voids exist.

Fig 15 :- Three Face Diagrams Of Consolidation Of Soil

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In case of fine grained soil on which a structure is to be built, high water content is not desired as the weight of the structure may cause sinking (consolidation settlement) of the structure in due time. Typically the permeability (ability of water to move through the soil voids) of fine grained soils is low, hence it takes a long time for consolidation process. So two aspects of consolidation settlement are important: 

The rate at which the consolidation is taking place



The total amount of consolidation.

It is very important to note that unlike settlement in sands and other coarse grained soil, consolidation settlement of fine grained soil does not occur immediately. Hence, it is common practice to ensure that the consolidation process is expedited and that most of the consolidation takes place during the various phases of construction. If the soil is such that it has never experienced pressure of the current magnitude in its entire history, it is called a normally loaded soil. The soil is called pre-consolidated (or over-consolidated) if at any time in history, it has been subjected to a pressure equal to or greater than the current pressure applied to it. In case of normally consolidated soils, the consolidation will be greater than that for a pre-consolidated soil. That is because the pre-consolidated soil has previously experienced greater or equal pressure and has undergone at least some consolidation under that pressure. So a preconsolidated soil is preferred over a normally consolidated soil. The rate, at which consolidation will take place, will depend on; 

The nature of soil,



The degree of saturation (how many percent voids are filled will water),



The amount and nature of the load on the soil, the soil history (normally or over consolidated), etc.

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9.2 Cement Stabilization Cement stabilization is done by mixing pulverized soil and Portland cement with water and compacting the mix to attain a strong material. The material obtained by mixing soil and cement is called soil-cement. The soil-cement becomes a hard and durable structural material as the cement hydrates and develops strength.

Fig 16 :- Cement Stabilization

9.2.1 Types of Soil-Cement 1. Normal Soil-Cement: - It consists of 5 to 14% of cement by volume. The quantity of cement mixed with soil is sufficient to produce a hard and durable construction material. The quantity of water used should be just sufficient to satisfy hydration requirements of the cement and to make the mixture workable. The normal soil-cement is quite weather-resistant and strong. It is commonly used for stabilizing sandy and other low plasticity soils. 2. Plastic Soil-Cement: - This type of soil-cement also contains cement 5 to 14% by volume, but it has more quantity of water to have wet consistency similar to that of plastering mortar at the time of placement.

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The plastic soil-cement can be placed, on steep or irregular slopes where it is difficult to use normal road-making equipment. It has also been successfully used for water-proof lining of canals and reservoirs. The plastic soil-cement can be used for protection of steep slopes against erosive action of water. 3. Cement-Modified soil: - It is a type of soil-cement that contains less than 5% of cement by volume. It is semi-hardened product of soil and cement. It is quite inferior to the two types As the quantity of cement used is small, it is not able to bind all the soil particles into a coherent mass. However, it interacts with the silt and clay fractions and reduces their affinity for water. It reduces the swelling characteristics of the soil. The use of cement-modified soils is limited.

9.2.2 Factors affecting cement stabilization - The soil stabilized with cement (Portland) is known as soil cement. The cementing action is believed to be the result of chemical reaction of cement with the siliceous soil during hydration. The binding action of individual particles through cement may be possible only in coarse-grained soils. In fine grained, cohesive soils, only some of the particles can be expected to have cement bonds, and the rest will be bonded through natural cohesion. The important factors affecting soil cement are : nature of soil, cement content, conditions of mixing, compaction and curing, and ixtures : 

Nature of soil Granular soils with sufficient fines are ideally suited for cement stabilization. Such soils can

be easily pulverized and mixed. They require the least amount of cement. Granular soils with deficient fines, such as beach sands and wind-blown sands, can also be stabilized but these soils require more cement, as it is difficult to move road-making equipment over such soils. When dry, it is desirable to keep them wet for better traction.

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Silty and clayey soils can produce satisfactory soil-cement but those with a high clay-content are difficult to pulverize. Moreover, the quantity of cement increases with an increase in clay content. The quality of soil-cement in this case is not good, as it may have high shrinkage properties. Organic matter, if present in colloidal form, interferes with the hydration of cement and causes a reduction in the strength of soil-cement. The trouble is more common in sandy soils than in clayey soils. The soil should be treated with calcium chloride to remedy the situation. Sodium hydroxide is also effective in correcting the ill effects due to organic matter. Sometimes, addition of a small quantity of silt or clay to sandy soils may aid in the cement reaction. 

Quantity of Cement The strength of soil cement increases with an increase in the amount of cement added to a

soil, and if such an increase in strength does not result, the soil may normally be considered unsuitable. The ordinary Portland cement is generally used for stabilization. High-early-strength cement can also be used, and is usually more effective than normal cement. The actual quantity of cement required for a particular soil is ascertained by laboratory tests. For base courses, samples are subjected to durability tests for determination of the quantity of cement required. It consists of 12 cycles of freezing and thawing or 12 cycles of wetting and drying. The maximum volume change (swelling + shrinkage) of 2% is generally permitted. Sometimes, the quantity of cement is determined according to minimum unconfined compressive strength. Generally, a minimum strength of about 15 kN/m2 for clayey soils and of about 5500 kN/m2 for sandy soils is specified. High strength is obtained by decreasing the water-cement ratio. This is done by increasing cement content for the same water content.

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The amount of cement required, expressed as a percentage by weight of dry soil, generally varies between 5 to 15%, finer soils requiring more cement. The amount of cement giving a compressive strength of 25 to 30 kg/cm2, should normally prove satisfactory for tropical climates. The approximate amounts needed for different types of soils may be as follows : gravels- 5 to 10% ; sands- 7 to 12% ; silts- 12 to 15% and clays- 12 to 20%. 

Quantity of Water The quantity of water used must be sufficient for hydration of cement and silt-clay cement and

for making the mix workable. Generally, the amount of water ascertained from compaction consideration is adequate for hydration as well. Water used should be clean and free from harmful salts, alkalis, acids or organic matter. In general, the water which is potable is also satisfactory for soil-cement. 

Mixing, compaction and curing A stronger and more durable soil-cement will be produced, if the soil-cement water mixture is

more intimately mixed. Mixing will, however, result in decreased strength if it is continued long after the cement hydration has begun. The amount of water to be added is decided from the considerations of good compaction and this amount is considered adequate enough for cement hydration also. It has been observed that better strength and stability develops, if the fined grained soils are compacted wet of optimum water content and the coarse grained soil on the dry side of the optimum water content. The greater is the compacted density, the stronger and the more durable will be the soil cement. Like concrete, the strength of soil-cement increases with age. Hence it should be moist cured for at least 7 days. Curing is rapid at higher temperatures.

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

ixtures Certain chemicals are sometimes added to soil-cement, with the purpose either to reduce the

cement consumption or to make a soil suitable for stabilization which is not responsive to cement alone in its natural state. Lime and calcium chloride are commonly used with clays and soil containing organic matter. Sodium carbonate and sodium sulphate have also been tried. Fly ash, as an additive, has been found very beneficial in the stabilization of dune sand. The fly ash acts as a pozzolona and also as a filler for increasing the density.

9.2.3 Methods of Mixing – Following methods are generally adopted for mixing soil and cement ----

1. Mix-in-place method: - In this method of construction, mixing of soil-cement is done at the place where it would be finally placed. It consists of the following steps : i.

The sub-grade is cleared of all undesirable materials such as boulders, debris, stumps. It is then leveled to the required formation level.

ii.

The leveled sub-grade is scarified to a depth equal to the proposed thickness of the soil cement.

iii.

The scarified soil is then pulverized till at least 80% of the soil es 4.75 mm IS sieve. It can be done either manually or with the help of a machine. Pulverization of highly plastic soil can be done easily if about 4% lime is added to it.

iv.

The pulverized soil is properly shaped to the required grade and the required quantity of cement is spread uniformly over the surface. It is then intimately mixed dry with rotary tillers or special soil mixers.

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v.

The required quantity of water is sprinkled over the surface and wet mixing is done till the mixture has a uniform colour. The operation should not last longer than 3 hours. The surface is then properly graded using towed graders.

vi.

Compaction is done using suitable methods. The thickness of the layer should not be more than 15 cm. Compaction should not take more than 2 hours.

vii.

The compacted soil-cement is moist cured for at least 7 days by providing a bituminous primary coat. Alternatively, it is kept damp by frequent application of a light spray of water. The mix-in-place method is quite simple, cheap and easily adaptable to different field

conditions. The main disadvantage is that the mixing is not uniform and high strength cannot be achieved.

2. Plant-mix method : There are two types of plants used in the plant-mix method ---a. Stationary Plant: - In this method, the excavated soil is transported to a stationary plant located at a suitable place. The required quantity of cement is added to the soil in the plant. Mixing is done after adding water. The time required to obtain a uniform mixture depends

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upon the type of soil. The mixed material is then discharged into dumper trucks and transported back to the sub-grade. It is spread and properly compacted. The stationary plant is useful for obtaining a uniform mix. In this method, the depth of treatment can be better controlled. However, the method is quite expensive as compared with mix-in-place method. The material has to be compacted as delivered and not as a complete section of the road. A further disadvantage is that the work may have to be stopped even after a minor breakdown in the plant. b. Travelling Plant: - A travelling plant can be move along the road under construction. The soil, after placement of cement over it, is lifted up by and elevator and discharged into the hopper of the mixer of the travelling plant. Water is added and proper mixing is done. The mix is then discharged and spread by a grader. It is then properly compacted.

The travelling plant method, like stationary plant, is useful for accurate proportioning and uniform mixing. The depth of treatment is also properly controlled and a uniform sub-grade surface as attained. However, the initial cost is very high.

9.3 Lime Stabilization

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Lime stabilization is done by adding lime to soil. When lime reacts with soil, there is exchange of cations in the absorbed water layer and a decrease in plasticity of the soil occurs. The resulting material is more friable than the original clay, and is, therefore, more suitable as sub-grade. Lime may be used alone, or in combination with cement, bitumen or fly ash. Sandy soils can also be stabilized with these combinations. Lime has been mainly used for stabilizing the road bases and subgrades.

Lime is produced by burning of lime stone in kilns. The quality of lime obtained depends upon the parent material and the production process. There are basically five types of lime ---i.

High calcium, quick lime (CaO)

ii.

Hydrated, high calcium lime [ Ca(OH)2 ]

iii.

Dolomite lime (CaO + MgO)

iv.

Normal, hydrated dolomite lime [Ca(OH) 2 + MgO]

v.

Pressure, hydrated dolomite lime [Ca(OH) 2 + MgO]

The quick lime is more effective as stabilizer than the hydrated lime; but the latter is more safe and convenient to handle. Generally, the hydrated lime is used.

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On addition of lime to soil, two main types of chemical reactions occur: (i)

Alteration in the nature of the absorbed layer through base exchange phenomenon and

(ii)

Cementing or pozzolonic action. Lime reduces the plasticity index of highly plastic soils making them more friable and easy to

be handled and pulverized. The plasticity index of soils of low plasticity generally increases. There is generally an increase in the optimum water content and a decrease in the maximum compacted density, but the strength and durability increases. The amount of lime required is dependent on the unconfined compressive strength on the CBR (California Bearing Ratio) test criteria. Normally 2 or 8% of lime may be required for coarse grained soils, and 5 to 10% for plastic soils. The amount of fly ash as ixture may vary from 8 to 20% of the soil weight.

9.3.1 Benefits of Lime Treatment 9.3.1.1 Drying With Lime; 

Minimizes weather-related construction delays



Extends construction season



Acts quickly–allowing return to work in hours

9.3.1.2 Lime Modification; 

Speeds up construction with stable working platform that resists subsequent rain.



Maximizes use of low cost, on-site materials.



Reduces plasticity.



Improves compactability.

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

Permits reworking.

9.3.1.3 Lime Stabilization; 

Chemically transforms clay soils



Permanently increases strength



Eliminates soil expansion



Creates excellent freeze-thaw resistance



Resists cracking



Reduces thickness of overlying pavement layers

9.3.1.4 Lime Dries Wet Soils Because quicklime chemically combines with water, it can be used very effectively to dry any type of wet soil. Heat from this reaction further dries wet soils. The reaction with water occurs even if the soils do not contain significant clay fractions. When clays are present, lime’s chemical reactions with clays increase the moisture holding capacity of the soil, which reduces free liquids and causes further drying. The net effect is that drying occurs quickly, within a matter of hours, enabling more rapid site access and soil compaction than by waiting for the soil to dry through natural evaporation. “Dry-up” of wet soil at construction sites is one of the widest uses of lime for soil treatment. Generally, between 1 to 4 percent of lime by mass of dry soil will improve a wet site sufficiently to allow construction activities to proceed.

9.3.1.5 Lime Modifies Clay Soils On many construction sites, there is a need for short-term soil modification to temporarily strengthen the working area. The benefits of modified soils include:

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•

Making clay soils friable and easier to handle.

•

Providing a working platform for subsequent construction.

•

Reducing plasticity to meet specifications.

•

Conditioning the soil for further treatment.

•

Spot treatment of spongy subsoil areas.

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.

9.3.1.6 Lime Permanently Stabilizes Clay Soils In contrast to lime modification, lime stabilization creates long-lasting changes in soil characteristics that provide structural benefits. Lime is used in stabilizing and strengthening subgrades (or sub-bases) and bases below pavements. Non-pavement applications for lime treatment include building foundations and embankment stabilization.

9.3.1.7 Hence, Ultimately Lime Stabilization;

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

Markedly reduces shrinkage and swell characteristics of clay soils.



Increases unconfined compressive strength by as much as 40 times.



Substantially increases load-bearing values as measured by such tests as CBR, R-value, Resilient Modulus, and the Texas Tri-axial tests.



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



Creates a water-resistant barrier. Impedes migration of surface water from above and capillary moisture from below; thus helping to maintain foundation strength.



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-silicate hydrates (CSH) and calcium-aluminateshydrates (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 pozzolonic 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. Lime addition of three to six percent by mass of the dry soil is the customary range for lime stabilization in road foundations. Precise amounts should be determined through mix design and testing protocols.

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9.3.2 Construction Procedures The goal of lime treatment for drying is to mix lime with the wet soil to create chemical reactions between the soil, water, and lime to remove standing water and transform unstable wet soils into workable materials. After spreading the lime, disc harrows alone may be adequate for mixing in extremely wet situations, but rotary mixers are still preferred for heavier soils. The construction steps in lime modification and lime stabilization are similar. In general, lime stabilization requires more thorough processing and job control than lime modification. Basic steps in both activities treat the soil to a prescribed depth. Sub-grade and sub-base stabilization measures include scarifying, partial pulverization, lime spreading, watering, mixing, and compaction to a specified density.

9.4 Bituminous Stabilization Bitumens are non-aqueous systems of hydrocarbons that are soluble in carbon disulphide (CS2). Tars are obtained by the destructive distillation of organic materials such as coal. Asphalts are materials in which the primary components are natural or refined petroleum bitumens. Bituminous stabilization is generally done with asphalt as binder. As asphalts are normally too viscous to be used directly, these are used as cut-back with some solvent, such as gasoline. These are also used as emulsions, but in this form they require a longer drying period.

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Any inorganic soil which can be mixed with asphalt is suitable for bituminous stabilization. In cohesionless soils, asphalt binds the soil particles together and thus serves as a bonding or cementing agent. In cohesive soils, asphalt protects the soil by plugging its voids and water proofing it. It helps the cohesive soil to maintain low moisture content and to increase the bearing capacity. The amount of bitumen required generally varies between 4 to 7% by weight. The actual amount is determined by trial and laboratory tests.

9.4.1 Type of Soil-Bitumen According to IS specifications, there is four type of soil-bitumen: 1. Soil-Bitumen (proper): - This is a water-proof, cohesive soil system. The best results are obtained if the soil satisfies the following criteria….. 

ing No. 4 (4.76 mm) Sieve 50%.



ing No. 40 (0.425 mm) Sieve 35 to 100%.



ing No. 200 (0.075 mm) Sieve 10 to 50%.



Plastic limit less than 18%.



Liquid limit less than 40%.



The maximum size of the particle should not be greater than one-third the compacted thickness of the soil-bitumen.

The quantity of bitumen varies from 4 to 7% of the dry weight. 2. Sand-Bitumen: - This is a bitumen stabilized cohesionless soil system. The sand should be free from vegetal matter or lumps of clay. The sand may require filler for its mechanical stability. However, it should not contain more than 25% minus No. 200 sieve material (i.e. the

48

material finer than No. 200 Sieve) for dune sands and not more than 12% in case in other types of sand. The quantity of bitumen required varies from 4 to 10% of the dry weight. 3. Water Proofed Clay Concrete: - A soil possessing a good gradation is water proofed by a uniform distribution of 1 to 3% of bitumen in this system. Soils of three different gradations have been recommended. For the three gradations, the percentage ing No. 200 Sieve varies between (i) 8 to 12; (ii) 10 to 16 and (iii) 13 to 30. 4. Oiled Earth: - In this system, a soil surface consisting of silt-clay material is made water proof by spraying bitumen in two or three applications. The amount of bitumen required is about 5 liters per square meter of soil surface. 9.4.2 Factors affecting bituminous stabilization 

Type of soil – Bituminous stabilization is very effective in stabilizing sandy soils having little or no fines. If a cohesive soil has the plastic limit less than about 20% and the liquid limit less than 40%, it can be effectively stabilized. However, plastic clays cannot be properly treated because of the mixing problems and large quantity of asphalt required. Fine-grained soils of the arid regions which have high ph value and contain dissolved salts do not respond well.



Amount of asphalt – The quality of the bitumen-stabilized soil improves with the amount of asphalt upto a certain limit. However, if the amount of the asphalt is excessive, it results in a highly fluid mixture that cannot be properly compacted.



Mixing – The quality of the product improves with more thorough mixing.



Compaction – The dry density of the bitumen-soil depends on the amount and type of compaction. It also depends upon the volatile content. In modified Proctor test, the maximum dry density occurs at a volatile content of about 8%. For samples cured and then immersed in

49

water, the maximum strength occurs at a moulding volatile content corresponding to the maximum compacted density.

9.4.3 Construction Methods Construction methods for bituminous stabilization are similar to those used for soilcement stabilization. However, the following points should be noted---1.

The optimum volatile content for compaction is generally much greater than that for stability.

The volatile content required for thorough mixing may be even greater, especially for clayey soils. It is, therefore, necessary to aerate the mix between mixing and compaction and between compaction and application. 2.

To obtain a high stability, the layer method of construction is preferred. Each layer is kept

about 5 cm. thick. When the lower layer has dried up, the subsequent layer is laid. The total thickness for bases is kept between 10 to 20 cm. 3.

In the mix-in-place method, the bitumen is sprayed in several es. Each layer is partially

mixed before the next . This method prevents the saturation of the surface of the sub-grade. 4.

Climatic conditions influence the amount of bitumen that can be applied, as the amount of

fluid (moisture) already present in the soil depends upon the climatic conditions.

9.5 Chemical Stabilization In chemical stabilization, soils are stabilized by adding different chemicals. The main advantage of chemical stabilization is that setting time and curing time can be controlled. Chemical stabilization is however generally more expensive than other types of stabilization. The following chemicals have been successfully used----

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9.5.1 Calcium Chloride – When calcium chloride is added to soil, it causes colloidal reaction and alters the characteristics of soil water. As calcium chloride is hydroscopic, it reduces the loss of moisture from the soil. It also reduces the chances of frost heave, as the freezing point of water is lowered. Calcium chloride is very effective as dust palliative. As the soils treated with calcium chloride do not easily pick up water, the method is effective for stabilization of silty and clayey soils which lose strength with an increase in water content. Calcium chloride causes a slight increase in the maximum dry density. However, the optimum water content is slightly lower than that of the untreated soil. It causes a small decrease in the strength of the soil. However, if the compacted soil is put to water inhibition, water pick up is reduced and the strength of the treated soil is greater than that of the untreated soil. It may be noted that most of the benefits of stabilization require the presence of the chemical in the pore fluid. As soon as the chemical is leached out, the benefits are lost. The performance of treated soils depends to a large extent on the ground-water movement. The construction methods are similar to those used for lime stabilization. The quantity of calcium chloride required is about 0.5% of the weight of the soil.

9.5.2 Sodium Chloride – The action of sodium chloride is similar to that of calcium chloride in many respects. However, the tendency for attraction of moisture is somewhat lesser than that of calcium chloride. When sodium chloride is added to the soil, crystallization occurs in the pores of the soil and it forms a dense hard mat with the stabilized surface. The pores in the soil get filled up and retard further evaporation of water. Sodium chloride also checks the tendency for the formation of shrinkage cracks. Sodium chloride is mixed with the soil either by the mix-in-place method or by the plant-mix method. It should not be applied directly to the surface.

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The quantity of sodium chloride required is about 1% of the soil weight.

9.5.3 Sodium Silicate – Sodium silicates, as well as other alkali silicates, have been successfully used for soil stabilization. The chemical is used as solution in water, known as water glass. The chemical is injected into the soil. Sodium silicate gives strength to soil when it reacts with it. It also makes the soil impervious. It also acts as a dispersing agent. The maximum compacted density is increased. The quantity of the chemical required varies between 0.1 to 0.2% of the weight of the soil. This method of stabilization is relatively inexpensive, but its long-term stability is doubtful. The treated soil may be losing strength when exposed to air or ground water.

9.5.4 Polymers – Polymers are long chained molecules formed by polymerizing of certain organic chemicals monomers. Polymers may be natural or synthetic. Resins are natural polymers. Calcium acrylate is a commonly used synthetic polymer. When a polymer is added to a soil, reaction takes place. Sometimes, the monomers are added with a catalyst to the soil. In that case, polymerization occurs along with the reaction.

9.5.5 Chrome Lignin – The chemical lignin is obtained as a by-product during the manufacture of paper from wood. Chrome lignin is formed from black liquor obtained during sulphite paper manufacture. Sodium bicarbonate or potassium bicarbonate is added to sulphite liquor to form chrome lignin. It slowly polymerizes into a brown gel. When the chemical is added to the soil, it slowly reacts to cause bonding of particles. The quantity of lignin required varies from 5 to 20% by weight. As lignin is soluble in water, its stabilizing effect is not permanent.

9.5.6 Other Chemicals –

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Some water proofers such as alkyl chloro silanes, siliconates amines and quaternary ammonium salts, have been used for water proofing of soils. Coagulating chemicals, such as calcium chloride and ferric chloride, have been used to increase the electrical attraction and to form flocculated structure in order to improve the permeability of the soil. Dispersant, such as sodium hexa-metaphosphate, are used to increase electrical repulsion and to cause dispersed structure. The compacted density of the soil is increased. Phosphoric acid combined with a writing agent can be used for stabilization of cohesive soils. It reacts with clay minerals and forms an insoluble aluminum phosphate.

9.6 Soil Stabilization By Injecting Chemicals Most of the recent work has been done on the altering of soil properties with chemicals has pertained to surface applications, with or without physical mixing of the chemical with the soil. There are, however, a number of problems in the field of civil engineering that could be solved effectively if chemicals could be injected into the soil to react within the soil to react within the soil mass and render the soil impervious and cohesive. Principal problems discussed include the prevention of seepage into underground structures, construction of caissons and tunnels, prevention of seepage beneath dam foundation, and sealing pervious strata in water and soil wells. Any soil mass consists of mineral grains, voids, organic matter, and amorphous matter. The voids can contain varying amounts of water from zero to saturation.

53

Introduction of chemicals into the soil mass can be effected by either of two methods – 

The first method, generally useful for surface applications, involves the mixing of the chemical with the soil by any of several means, wherein the soil mass is actually broken down and the chemical distributed mechanically throughout the soil mass.



Second method involves introduction of chemicals into the voids in the soils by injection, without significantly distributing the structure of the soil particles. Where water is present, the chemical displaces the water as it enters the mass.

9.6.1 Limitations There are several rather basic limitations of injection procedures, such as 

First, the soil mass must have a sufficiently high permeability so that the chemical can be introduced into the soil with reasonable pressures and in a reasonable length of time. For chemicals with the viscosity of water, injection into soils containing clay is to all practical purpose impossible. Injection into silts is possible only with high pressures. Thus only sands, gravels, and porous rock are suitable for chemical injection treatments.

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

A second limitation of injection techniques is that the chemicals as injected must have a low viscosity, and the chemical must have an extremely small particle size or be in solution. Even in sandy soils the diameters of voids in the soil is often small, and any particles in chemical which approach the size of the voids will tend to plug soil mass and prevent the penetrations of chemicals.



Of course, the greatest limitations of chemically injection techniques lies in the cost of the chemicals, and the number of problems in which it would be economical to use chemical grouts is limited, even though the application might be a certain success.

9.7 Thermal Stabilization Thermal change causes a marked improvement in the properties of the soil. Thermal stabilization is done either by heating the soil or by cooling it.

9.7.1 Heating – As the soil is heated, its water content decreases. Electric repulsion between clay particles is decreased and the strength of the soil is increased. When the temperature is increased to more than 100 ⁰C, the adsorbed water is driven off and the strength is further increased. When the soil is heated to temperature of 400 ⁰C to 600 ⁰C, some irreversible changes occur which make the soil non-plastic and non-expansive. The clay clods are converted into aggregates.

55

With further increase in temperature, there is some fusion and vitrification, and a brick-like material is obtained which can be used as an artificial aggregate for mechanical stabilization. This method is quite expensive because of large heat input. It is rarely used in practice.

9.7.2 Freezing – Cooling causes a small loss of strength of clayey soils due to an increase in interparticle repulsion. However, if the temperature is reduced to the freezing point, the pore water freezes and the soil is stabilized. Ice so formed acts as a cementing agent.

Water in cohesionless soils freezes at about 0 ⁰C . However, in cohesive soils, water may freeze at a much lower temperature. The strength of the soil increases as more and more water freezes. This method if stabilizing is very costly. This method is used only in some special cases. It has been successfully used to solidify soils beneath foundations. The method is commonly used when advancing tunnels or shafts through loose silt and fine sand. Freezing may cause serious trouble to adjacent structures if the freezing front penetrates these areas. It may cause excessive heaving. The method should be used after considering the above aspects.

9.8 Electrical Stabilization Electrical stabilization of clayey soils is done by a process known as electro-osmosis. As a direct current (D.C.) is ed through a clayey soil, pore water migrates to the negative electrode (cathode).

56

It occurs because of the attraction of positive ions (cations) that are present in water towards cathode. The strength of the soil is considerably increased due to removal of water.

9.8.1 Process Electro-osmosis is an expensive method, and is mainly used for drainage of cohesive soils. The cathode is a well point which collects the water (which is to drain off). Cations (+ve ions) are formed in pore water when the dissolved minerals go into solution. These cations move towards the negatively charged surface of clay minerals to satisfy electrical charge. As the water molecules act as dipoles, the cations also attract the negative end of dipoles. When the cations move to the cathode, they take with them the attached water molecules.

9.8.2 Significance of Elecro-Osmosis As considerable amount of water is removed from soil mass, the strength properties are increased. It is also found that a small reversing of the direction of flow helps in increasing the stability of the slope even if there is no significant decrease in the water content of soil. So this process also increases the slope stability substantially.

9.8.3 Limitations of Electro-Osmosis

57

The process requires specialized and sophisticated equipment as well as electrical consumption of high amount is associated with this process. Thus it is a highly expensive drainage process compared with other method

9.8.4 Suitability This method should be used only in exception cases where other method cannot be used. It is normally used to drain water in a cohesive soil of low permeability.

9.9 Stabilization By Grouting In this method of stabilization, stabilizers are introduced by injection into the soil. As the grouting is always done under pressure, the stabilizers with high viscosity are suitable only for soils with high permeability. This method is not suitable for stabilizing clays because of their very low permeability. The grouting method is costlier as compared with direct blending methods. This method is suitable for stabilizing buried zones of relatively limited extent, such as pervious stratum below a dam. The method is used to improve the soil that cannot be disturbed. An area close to an existing building can be stabilized by this method.

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9.9.1 Types of Grouting Depending upon the stabilizer used, grouting techniques can be classified as under: 1. Cement Grouting: - A cement grout consists of a mixture of cement and water. If the hole drilled in the soil is smooth, the water-cement ratio is kept low. Sometimes, chemicals are added to grout to increase its fluidity so that it can be injected into the soil. Cement grouting is quite effective for stabilizing rocks with fissures, gravel and coarse sand.

2. Clay Grouting: - In this method, the grout used is composed of a very fine-grained soil (bentonite clay) and water. The bentonite clay readily adsorbs water on its surface. The viscosity, strength and flow characteristics of the grout can be adjusted according to the site conditions. Clay grouting is suitable for stabilizing sandy soils. Sometimes, other chemicals are added to clay grout. Clay cement grout is a mixture of clay, bentonite and cement. Clay-chemical grout is a mixture of clay and sodium silicate. It is effective for medium and fine sand.

3. Chemical Grouting: - The grout used consists of a solution of sodium silicate in water, known as water glass. The solution contains both free sodium hydroxide and colloidal silicic acid. An insoluble silica gel is formed. As the reaction is slow, calcium chloride is generally added to accelerate the reaction. The method is suitable for medium and fine sands. However, the effect of chemical grouting is not permanent. 4. Chrome-lignin grouting : - The grout used is made of lignosulphates and a hexavalant chromium compound. When it is combined with an acid, the chromium ion changes valence and thereby oxidizes the lignosulphates into a gel.

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The method can be used to stabilize fine sand and coarse silt. 5. Polymer Grouting: - Various polymers have been successfully used in grouting of fine sands and silts. 6. Bituminous Grouting: - Sandy and silty soils have been grouted successfully using emulsified asphalt. Slow-setting emulsions are generally preferred, as these can travel a large distance into the stratum.

9.10 Stabilization By Geo-textiles And Fabrics The soil can be stabilized by introducing geo-textiles and fabrics which are made of synthetic materials, such as polyethylene, polyester and nylon. The geo-textiles sheets are manufactured in different thicknesses ranging from 10 to 300 mils ( 1 mil = 0.0254 mm = 25.4 µ). The width of the sheet can be up to 10 m. These are available in rolls of length upto about 600 m. Geo-textiles are manufacture in different patterns, such as woven, non-woven, grid and hybrid. The woven geotextiles are made by the use of thermal or chemical bonding of continuous fibers and then pressed through into relatively thin sheets. The grids of geo-textiles are made from a sheet of polymer by punching it and then non-woven and grid.

The geo-textiles are quite permeable. Their permeability is comparable to that of fine sand to coarse sand. These are quite strong and durable. These are not affected by even hostile soil

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environment. The use of geo-textiles in geotechnical and construction engineering has increased considerable in the last 2 decades. Geo-textiles are being increasingly used for the site improvement, soil stabilization and various other related works. While selecting geo-textiles for a particular job, due importance should be given to the major function that the geo-textile has to perform, as explained below ---1. Geo-textiles as separators: - Geo-textiles are commonly used as separators between two layers of soils having a large difference in particle sizes to prevent migration of small-size particle into the voids of large-size particles. The main use as separators is in the construction of highways on clayey soils.

As the particle size of granular base course of the highway is much larger than that of the sub-grade ( clayey soil in this case ), it is the usual practice to provide an intervening soil layer of a soil containing grain-sizes intermediate between that of the sub-grade and the base course to prevent migration of clay particles into the base course. Instead of the intervening soil layer, geo-textile can be provided to serve the same purpose. The size of perforations should be according to the requirement. Thus a geo-textile sheet is used between the sub-grade and the base course.

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2. Geo-textile as Filter: - It is the usual practice to provide a properly graded filter to prevent the movement of soil particles due to seepage forces. The filter is so designed that the particle size of the filter is small enough to hold the protected material in place. If the filter material is not properly selected, the particles of the soil move into the pores of the filter and may prevent proper functioning of the drainage. It may also lead to piping.

Geo-textiles can be used as filters instead of conventional filter. When the silt-laden turbid water es through the geo-textile, the silt particles are prevented from movement by the geo-textile. The modification in the soil and void of the geo-textile occurs and after sometime an equilibrium stage is attached. For relatively thin geo-textile sheet, most of the filtration occurs within the soil just upstream of the geo-textile fabric. 3. Geo-textile as Drain: - A drain is used to convey water safely from one place to the other. As the geo-textiles are pervious, they themselves function as a drain. They have a relatively higher water-carrying capacity as compared to that of surrounding soil. Drainage occurs either perpendicular to the plane of the sheet or in-plane of the sheet. In the first case, it functions primarily as a filter. In the latter case, it acts as a water carrier and thus relatively bulky geo-textile or a composite system of geo-textile is required. In the above applications of the geo-textile, the following advantages are generally achieved----

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

The installation is generally easier and faster.

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The system has greater stability.

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The quantity of soil to be excavated and disposed of is less.

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The load on the structure is less.

4. Geo-textile as Reinforcement for strengthening soil : - Geo-textiles have a high tensile strength. These can be used to increase the load-carrying capacity of the soil. Geo-textiles are used as reinforcement in the soil, which is poor in tension but good in compression. The action is somewhat similar to that of steel bars in a reinforced concrete slab. Geo-textiles when used as reinforcement for soils have solved many construction problems on soft and compressible soils. The geo-textiles have been used in the construction of unpaved roads over soft soils. These are laid over the soil and the base course of the road is placed directly over it. When vehicles over the road, the geo-textile deforms and its strength is mobilized. 5. Geo-textiles used as reinforcement in retaining walls: - Geo-textiles can be used as reinforcement in the construction of earth-retaining structures. Geo-textiles are used to form the facing of the retaining wall as well as reinforcement. Such retaining walls are also called fabric reinforced retaining walls. The following procedure is used for the construction of the fabric-reinforced wall---

First the ground surface is leveled and the first geo-textile sheet of the required width is laid over the surface such that about 1.5 m. to 2 m. of the sheet at the wall surface is draped over temporary wooden form.

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Granular material is placed over the geo-textile sheet and compacted with a roller of suitable weight.

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After compaction, the sheet is folded.

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The second geo-textile sheet is placed over the compacted layer over the granular material and draped over the wooden form and the process is repeated.

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The front face of the wall is protected by the use of shotcrete of gunite. Shotcrete is the cement concrete with low water content. It is sprayed over the soil surface at a high pressure.

9.11 Reinforced Earth The soil can be stabilized by introducing thin strips in it. In reinforced earth, thin metal strips or strips of wire or geo-synthetics are used as reinforcement to reinforce the soil. The essential feature of the reinforced earth is that friction develops between the reinforcement and soil. By means of friction, the soil transfers the forces built up in the earth mass to the reinforcement. Thus tension develops in the reinforcement when the soil mass is subjected to shear stresses under loads.

The main application of the reinforced earth is in the reinforced earth wall. The wall consists of a facing element, reinforcement and the back fill. At the exposed vertical surface of the earth mass, facing elements are used to provide a sort of barrier so that the soil is contained. The facing units are

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generally prefabricated from units which are small and light so that they can be easily transported and placed in position. These are usually made of steel, aluminium, reinforced concrete or plastic. These should be strong enough to hold back fill. Moreover, these should be such that the reinforcement can be easily fastened to them. The facing units generally require a small plain concrete footing at the bottom so that they can be easily built. The reinforcement is connected to the facing element and extended back into the backfill zone. The friction developed in the reinforcement restrains the facing element. First a layer of reinforcement strips is placed at the level ground surface and the backfilling is done with a granular soil. The soil with less than 15% ing No. 200 sieve is used. The entire process of laying strips and backfilling is continued till the required height of the reinforced earth wall is attained. Galvanized steel strips are commonly used as reinforcement. Each strip is about 50-100 mm wide and several meters in length. The thickness is upto 9 mm. Sometimes metal rods, wires and geotextiles are used as reinforcement.

9.12 Sand Drain Method Of Compaction “Sand drains “ are vertical bore holes constructed in earth embankments, filled with suitably graded sand to increase the rate of drainage and accelerating the process of compaction .The basic function of a sand drain is to provide an easier and shortest path for water to travel when it is squeezed out of soil layers during the construction of road embankments. Sand drains are constructed by driving a steel casing into the embankment and back filling it with graded sand.

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The casing is withdrawn after the sand has been filled. The sand drains are generally laid in a square pattern or a triangular pattern .The spacing of sand drains is kept smaller then the thickness of embankment in order to reduce the length of radial drainage path. The layout of sand drains installation shows the number of vertical sand drains embedded in the embankment. The sand drains are also knows as sand columns. They are used to stabilize the weak or compressible clay soils, which are unable to bear the load of overlying structures. A sand blanket or sand cover is placed over to top of the sand drains to connect them and accelerate the process of drainage. A surcharge load can also be placed. Thus, we see that sand drains act as weak piles and reduce the stresses in the clay strata. By horizontal and vertical drainage, they accelerate the process of dissipation of excess pore water resulting in consolidation of soil.

9.13 Stabilization By Fly Ash Fly ash is one of the residues generated in combustion and comprises the fine particles that rise with the flue gases. In an industrial context, fly ash usually refers to ash produced during combustion of coal. It is generally captured by electrostatic precipitators or other particle filtration equipments before the flue gases reach the chimneys of coal-fired power plants.

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9.13.1 Benefits : 

Light weight, lesser pressure on sub-soil.

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High shear strength.

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Coarser ashes have high CBR value.

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Pozzolanic nature, additional strength due to self-hardening.

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Amenable to stabilization.

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Ease of compaction.

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High permeability.

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Non plastic.

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Faster rate of consolidation and low compressibility.

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Can be compacted using vibratory or static roller.

Earth Cover Bottom ash or Pond ash

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Typical cross section of fly ash road embankment

Guidelines for use of fly ash in road embankments published recently by Indian Roads Congress (SP- 58:2001) –  Loose layer thickness of 400 mm can be adopted if vibratory rollers are used  Moisture content - OMC + 2 per cent  Use of vibratory rollers advocated  Minimum dry density to be achieved - 95 per cent of modified Proctor density  Ash layer and side soil cover to be constructed simultaneously BIBLIOGRAPHY

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www.google.com

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www.googlebooks.com

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www.wikipedia.com

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www.dictionary.com



www.engineeringcivil.com

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www.globalspec.com

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Soil and foundation engineering By S. Jagroop Singh

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Soil engineering by BC Punmia

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Soil engineering by KR Arora

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civil.blogspot.com



www.ask.com

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www.answer.com