Soil Compaction and Stabilization Lecture Notes PDF

NNAMDI AZIKIWE UNIVERSITY, AWKA FACULTY OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING CVE 441 – SOIL MECHANICS II LECTURE NOTE ON SOIL COMPACTION AND SOIL STABILISATION INTRODUCTI...

NNAMDI AZIKIWE UNIVERSITY, AWKA FACULTY OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING CVE 441 – SOIL MECHANICS II LECTURE NOTE ON SOIL COMPACTION AND SOIL STABILISATION INTRODUCTION TO COMPACTION Shallow and deep compaction methods have been commonly used to improve geomaterial properties near surface and at depth through a densification process by vibration, pressure, kneading, and/or impact on ground surface. This technology is effective to improve cohesionless geomaterial or cohesive geomaterial with low plasticity. Due to the difference in soil structure, cohesionless and cohesive geomaterials can be densified by different means. Cohesionless geomaterials, consisting of large particles, can be effectively densified by vibration to rearrange particle packing patterns. The soil fabrics of cohesive geomaterials can be effectively changed by high pressure. Vibration is not effective to change soil fabrics of cohesive geomaterials. It is also true that high pressure within a small area is not effective to compress cohesionless geomaterials because they will fail under high pressure due to the low confining stress. Conventional plate or roller compaction has been used for many years, and it densifies geomaterial to a shallow depth by repeated passing of a vibratory plate or a roller on a relatively thin lift. Intelligent compaction is a new technology and has evolved in the past few years through research and implementation. In addition to providing the same compaction capabilities as conventional compaction, intelligent compaction includes sensors, which provide feedback on the location, stiffness, and machine driving power on geomaterial on a color-coded map. With this map, an engineer or operator can identify areas that require more or less compaction to create a uniform foundation. In the construction of highway embankments, earth dams, and many other engineering structures, loose soils must be compacted to increase their unit weights. Compaction increases the strength characteristics of soils, which increase the bearing capacity of foundations constructed over them. Compaction also decreases the amount of undesirable settlement of structures and increases the stability of slopes of embankments. Smooth-wheel rollers, sheepsfoot rollers, rubber-tired rollers, and vibratory rollers are generally used in the field for soil compaction. Vibratory rollers are used mostly for the densification of granular soils. Vibroflot devices are also used for compacting granular soil deposits to a considerable depth. Compaction of soil in this manner is known as vibroflotation. Benefits of Soil Compaction The following benefits are obtained when soil is properly compacted:  Increase in unit weight  Increase in shear strength  Increase in bearing capacity  Increase in stability of slopes of embankments  Decrease on settlement of structures and foundations  Reduces the permeability of the soil 1 General Principles of Compaction Compaction, in general, is the densification of soil by removal of air, which requires mechanical energy. The degree of compaction of a soil is measured in terms of its dry unit weight. When water is added to the soil during compaction, it acts as a softening agent on the soil particles. The soil particles slip over each other and move into a densely packed position. The dry unit weight after compaction first increases as the moisture content increases. (See Figure 1.) Figure 1: General principle of compaction Note that at a moisture content w0, the moist unit weight (γ) is equal to the dry unit weight (γd), or γ = γd(w = 0) = γ1 When the moisture content is gradually increased and the same compactive effort is used for compaction, the weight of the soil solids in a unit volume gradually increases. For example, at w = w1, γ = γ2 However, the dry unit weight at this moisture content is given by: γ = γd(w = w1) = γd(w = 0) + Δ γd Beyond a certain moisture content w = w2 (Figure 1), any increase in the moisture content tends to reduce the dry unit weight. This phenomenon occurs because the water takes up the spaces that would have been occupied by the solid particles. The moisture content at which the maximum dry unit weight is attained is generally referred to as the optimum moisture content. The laboratory test generally used to obtain the maximum dry unit weight of compaction and the optimum moisture content is called the Proctor compaction test (Proctor, 1933). 2 Standard Proctor Compaction Test In the Proctor test, the soil is compacted in a mould that has a volume of 944 cm3. The diameter of the mould is 101.6 mm. During the laboratory test, the mold is attached to a baseplate at the bottom and to an extension at the top (Figure 2a). The soil is mixed with varying amounts of water and then compacted in three equal layers by a hammer (Figure 2b) that delivers 25 blows to each layer. The hammer has a mass of 2.5 kg and has a drop of 30.5 mm. Figure 2: (a) Standard Proctor Mould (b) Standard Proctor rammer There are other types of laboratory compaction and their details are as shown in Table 1. Table 1: Details of common compaction methods Type of Size of mould Weight of Height of Number of Number of Compaction (cm3) rammer (kg) fall (mm) blows layers Standard Proctor 944 2.5 304.8 25 3 Modified 944 4.5 457 25 5 Proctor British Standard 1000 2.5 304.8 27 3 Light (BSL) British Standard 1000 4.5 457 27 5 Heavy (BSH) West African 1000 4.5 457 10 5 Standard (WAS) 3 For each test, the moist unit weight of compaction, γ, can be calculated as: γ= (1) where: W = weight of the compacted soil in the mold Vm = volume of the mold For each test, the moisture content of the compacted soil is determined in the laboratory. With the known moisture content, the dry unit weight γd can be calculated as: (2) Where: w (%) = percentage of moisture content. The values of γd determined from Eq. (2) can be plotted against the corresponding moisture contents to obtain the maximum dry unit weight and the optimum moisture content for the soil. Figure 3: Moisture content versus dry density at a particular compactive effort The addition of water to a dry soil helps in bringing the solid particles together by coating them with thin films of water. At low water content, the soil is stiff and it is difficult to pack it together. As the water content is increased, water starts acting as a lubricant, the particles start coming closer due to increased workability and under a given amount of compactive effort, the soil-water-air mixture starts occupying less volume, thus effecting gradual increase in dry density. As more and more water is added, a stage is reached when the air content of the soil attains a minimum volume, thus making the dry density a maximum. For a given moisture content w and degree of saturation S, the dry unit weight of compaction can be calculated as follows. From the phase relationship of soils; 4  (3) Where: Gs = specific gravity of soil solids γw = unit weight of water e = void ratio But, Se = Gsw (4) Thus: (5) For a given moisture content, the theoretical maximum dry unit weight is obtained when no air is in the void spaces—that is, when the degree of saturation equals 100%. Hence, the maximum dry unit weight at a given moisture content with zero air voids can be obtained by substituting S = 1 into Eq. (5), or (6) where γzav = zero-air-void unit weight. To obtain the variation of γzav with moisture content, use the following procedure: 1. Determine the specific gravity of soil solids. 2. Know the unit weight of water (γw). 3. Assume several values of w, such as 5%, 10%, 15%, and so on. 4. Use Eq. (6) to calculate γzav for various values of w. Figure 4 also shows the variation of γzav with moisture content and its relative location with respect to the compaction curve. Under no circumstances should any part of the compaction curve lie to the right of the zero-air-void curve. 5  Figure 4. Saturation lines superimposed on compaction curves FACTORS AFFECTING SOIL COMPACTION The preceding section showed that moisture content has a strong influence on the degree of compaction achieved by a given soil. Besides moisture content, other important factors that affect compaction are soil type and compaction effort (energy per unit volume). The relative compaction in the field depends on the following factors: Geomaterial type Moisture content Compaction method, such as static pressure, kneading, vibration, and impact Compactive effort including applied energy, compactor size, lift thickness, and number of passes Relative layer stiffness (i.e., upper layer over lower layer) Effect of Soil Type The soil type—that is, grain-size distribution, shape of the soil grains, specific gravity of soil solids, and amount and type of clay minerals present—has a great influence on the maximum dry unit weight and optimum moisture content. Figure 5 shows typical compaction curves obtained from four soils. 6  Figure 5: Typical compaction curves for four soils Effect of Compaction Effort The compaction energy per unit volume used for the standard Proctor test can be given as: For the standard proctor compaction, the compactive effort can be calculated as follows: If the compaction effort per unit volume of soil is changed, the moisture–unit weight curve also changes. This fact can be demonstrated with the aid of Figure 6, which shows four compaction curves for a sandy clay. The standard Proctor mold and hammer were used to obtain these compaction curves. The number of layers of soil used for compaction was three for all cases. However, the number of hammer blows per each layer varied from 20 to 50, which varied the energy per unit volume. 7  Figure 6. Effect of compaction energy on the compaction of a sandy clay From the preceding observation and Figure 6, we can see that: 1. As the compaction effort is increased, the maximum dry unit weight of compaction is also increased. 2. As the compaction effort is increased, the optimum moisture content is decreased to some extent. The preceding statements are true for all soils. Note, however, that the degree of compaction is not directly proportional to the compaction effort. FIELD COMPACTION Most of the compaction in the field is done with rollers. The four most common types of rollers are: 1. Smooth-wheel rollers (or smooth-drum rollers) 2. Pneumatic rubber-tired rollers 3. Sheepsfoot rollers 4. Vibratory rollers Smooth-wheel rollers (Figure 7) are suitable for proof rolling subgrades and for finishing operation of fills with sandy and clayey soils. These rollers provide 100% coverage under the wheels, with ground contact pressures as high as 310 to 380 kN/m2. They are not suitable for producing high unit weights of compaction when used on thicker layers. 8  Figure 7. Smooth-wheel roller Pneumatic rubber-tired rollers (Figure 8) are better in many respects than the smooth-wheel rollers. The former are heavily loaded with several rows of tires. These tires are closely spaced—four to six in a row. The contact pressure under the tires can range from 600 to 700 kN/m2, and they produce about 70 to 80% coverage. Figure 8. Pneumatic rubber-tired roller Pneumatic rollers can be used for sandy and clayey soil compaction. Compaction is achieved by a combination of pressure and kneading action. Sheepsfoot rollers (Figure 9) are drums with a large number of projections. The area of each projection may range from 25 to 85 cm2. These rollers are most effective in compacting clayey soils. The contact 9 pressure under the projections can range from 1400 to 7000 kN/m2. During compaction in the field, the initial passes compact the lower portion of a lift. Compaction at the top and middle of a lift is done at a later stage. Figure 9: Sheepsfoot roller Vibratory rollers are extremely efficient in compacting granular soils. Vibrators can be attached to smooth-wheel, pneumatic rubber-tired, or sheepsfoot rollers to provide vibratory effects to the soil. The vibration is produced by rotating off-center weights. Handheld vibrating plates (Figure 10) can be used for effective compaction of granular soils over a limited area. Vibrating plates are also gang-mounted on machines. These plates can be used in less restricted areas. Figure 10: Handheld vibrating plate compactor SELECTION OF COMPACTION EQUIPMENT Different compaction equipment can produce different action on fill. Smooth drum rollers can apply uniform pressure on fill while sheepsfoot rollers can apply highly concentrated pressure on ill. Pneumatic rubber tire rollers can apply pressure and kneading action on fill, while vibratory rollers can apply vibration on fill. 10 Since granular fills have high friction between particles, they can be more effectively densified by vibration. On other hand, cohesive fills have cohesion and capillary force between particles. These forces can be more effectively broken by high pressure; therefore, rollers with high contact pressure should be used for cohesive fills. Table 2 provides the guidance for this selection. Vibratory rollers are more suitable for sands and gravels, while sheepsfoot or pad foot rollers are more suitable for clays and silts. Table 2. Recommended Type of Compaction Equipment Lift Thickness and Number of Passes Table 3 provides the recommended lift thickness and number of passes for different compaction equipment. Table 3. Lift thickness and number of passes for different compaction equipment. 11 Design Parameters and Procedure Design Parameters The design parameters for conventional compaction may include the following parameters: Project requirement(s) Relative compaction Area and thickness of compacted ill Type and gradation of ill Type of equipment Optimum moisture content and maximum dry unit weight or minimum and maximum void ratios Borrow volume Thickness and number of lifts Number of passes Design Procedure The following procedure may be adopted to design a compaction project: 1. Collect fill material. 2. Conduct laboratory tests (including sieve analysis test, Atterberg limit tests, and specific gravity test) to classify soil type. 3. Run standard and/or modified Proctor compaction tests in laboratory to obtain the compaction curve, including the zero air void line and determine optimum moisture content and maximum unit weight. For a preliminary design, the optimum moisture content and the maximum dry unit weight may be estimated using empirical equations. For cohesionless fill, minimum and maximum void ratio tests should be performed. 4. Based on the project requirements (e.g., required shear strength, modulus, permeability, shrinkage potential, and/or swell potential), run laboratory tests at a trial dry unit weight (typically 95% of the maximum dry unit weight) or different compaction levels to determine the required dry unit weight by meeting the project requirements so that the required relative compaction can be calculated. 5. Based on fill type and gradation, select compaction equipment. 6. Based on the selected compaction equipment and fill type, select lift thickness and number of passes. 7. Based on the area and thickness of compacted fill, calculate fill volume and borrow volume. 8. For a large and important project, a field trial is recommended to verify/adjust the above design parameters. Determination of Field Unit Weight of Compaction When the compaction work is progressing in the field, knowing whether the specified unit weight has been achieved is useful. The standard procedures for determining the field unit weight of compaction include: 1. Sand cone method 2. Rubber balloon method 3. Nuclear method Following is a brief description of each of these methods. Sand Cone Method (ASTM Designation D-1556) The sand cone device consists of a glass or plastic jar with a metal cone attached at its top (Figure 11). The jar is filled with uniform dry Ottawa sand. The combined weight of the jar, the cone, and the sand filling the jar is determined (W1). In the field, a small hole is excavated in the area where the soil has 12 been compacted. If the weight of the moist soil excavated from the hole (W2) is determined and the moisture content of the excavated soil is known, the dry weight of the soil can be obtained as: where w = moisture content. Figure 11. Field unit weight determined by sand cone method After excavation of the hole, the cone with the sand-filled jar attached to it is inverted and placed over the hole (Figure 11). Sand is allowed to flow out of the jar to fill the hole and the cone. After that, the combined weight of the jar, the cone, and the remaining sand in the jar is determined (W4), so; W5 = w1 – w4 where W5 = weight of sand to fill the hole and cone The volume of the excavated hole can then be determined as: where Wc = weight of sand to fill the cone only γd(sand) = dry unit weight of Ottawa sand used The values of Wc and γd(sand) are determined from the calibration done in the laboratory. The dry unit weight of compaction made in the field then can be determined as follows: 13 Rubber Balloon Method (ASTM Designation D-2167) The procedure for the rubber balloon method is similar to that for the sand cone method; a test hole is made and the moist weight of soil removed from the hole and its moisture content are determined. However, the volume of the hole is determined by introducing into it a rubber balloon filled with water from a calibrated vessel, from which the volume can be read directly. The dry unit weight of the compacted soil can be determined as above. Nuclear Method Nuclear density meters are often used for determining the compacted dry unit weight of soil. The density meters operate either in drilled holes or from the ground surface. It uses a radioactive isotope source. The isotope gives off Gamma rays that radiate back to the meter’s detector. Dense soil absorbs more radiation than loose soil. The instrument measures the weight of wet soil per unit volume and the weight of water present in a unit volume of soil. The dry unit weight of compacted soil can be determined by subtracting the weight of water from the moist unit weight of soil. 14  SOIL STABILISATION ‘Soil Stabilisation’, in the broadest sense, refers to the procedures employed with a view to altering one or more properties of a soil so as to improve its engineering performance. Soil Stabilisation is only one of several techniques available to the geotechnical engineer and its choice for any situation should be made only after a comparison with other techniques indicates it to be the best solution to the problem. It is a well known fact that, every structure must rest upon soil or be made of soil. It would be ideal to find a soil at a particular site to be satisfactory for the intended use as it exists in nature, but unfortunately, such a thing is of rare occurrence. The alternatives available to a geotechnical engineer, when an unsatisfactory soil is met with, are (i) to bypass the bad soil (e.g., use of piles), (ii) to remove bad soil and replace with good one (e.g., removal of peat at a site and replacement with selected material), (iii) (redesign the structure (e.g., floating foundation on a compressible layer), and (iv) to treat the soil to improve its properties. The last alternative is termed soil stabilisation. Although certain techniques of stabilisation are of a relatively recent origin, the art itself is very old. The original objective of soil stabilisation, was, as the name implies, to increase the strength or stability of soil. However, techniques have now been developed to alter almost every engineering property of soil. The primary aim may be to alter the strength and/or to reduce its sensitivity to moisture changes. The most common application of soil stabilisation is the strengthening of the soil components of highway and airfield pavements. CLASSIFICATION OF THE METHODS OF STABILISATION A completely consistent classification of soil stabilisation techniques is difficult. Classifications may be based on the treatment given to soil, on additives used, or on the process involved. Broadly speaking, soil stabilisation procedures may be brought under the following two heads: Stabilisation without additives Stabilisation with additives Stabilisation without additives may be ‘mechanical’—rearrangement of particles through compaction or addition or removal of soil particles. It may be by ‘drainage’—drainage may be achieved by the addition of external load, by pumping, by electro—osmosis, or by application of a thermal gradient— heating or cooling. Stabilisation with additives may be cement stabilisation (that is, soil cement), bitumen stabilisation, or chemical stabilisation (with fly ash, lime, calcium or sodium chloride, sodium silicate, dispersants, physico-chemical alteration involving ion-exchange in clay-minerals or injection stabilisation by grouting with soil, cement or chemicals). The appropriate method for a given situation must be chosen by the geotechincal engineer based on his experience and knowledge. Comparative laboratory tests followed by limited field tests, should be used to select the most economical method that will serve the particular problem on hand. Field-performance data may help in solving similar problems which arise in future. It must be remembered, however, that soil stabilisation is not always the best solution to a problem. 15 SOIL STABILISATION BY ADDITIVES Soil stabilization is the alteration of one or more soil properties, by mechanical or chemical means, to create an improved soil material possessing the desired engineering properties. Soils may be stabilized to increase strength and durability or to prevent erosion and dust generation. Regardless of the purpose for stabilization, the desired result is the creation of a soil material or soil system that will remain in place under the design use conditions for the design life of the project. Soils vary throughout the world, and the engineering properties of soils are equally variable. The key to success in soil stabilization is soil testing. The method of soil stabilization selected should be verified in the laboratory before construction and preferably before specifying or ordering materials. NEED FOR SOIL STABILIZATION FOR ROADS Long term performance of pavement structures often depends on the stability of the underlying soils. Engineering design of these constructed facilities relies on the assumption that each layer in the pavement has the minimum specified structural quality to support and distribute the super imposed loads. These layers must resist excessive permanent deformation, resist shear and avoid excessive deflection that may result in fatigue cracking in overlying layers. Available earth materials do not always meet these requirements and may require improvements to their engineering properties in order to transform these inexpensive earth materials into effective construction materials. This is often accomplished by physical or chemical stabilization or modification of these problematic soils. In situ sub grades often do not provide the support required to achieve acceptable performance under traffic loading and environmental demands. Although stabilization is an effective alternative for improving soil properties, the engineering properties derived from stabilization vary widely due to heterogeneity in soil composition, difference in micro and macro structure of soils, heterogeneity of geologic deposits, and due to differences in physical and chemical interactions between the soil and candidate stabilizers. These differences necessitate the use of site specific treatment options for stabilization. SOIL STABILIZATION VERSUS SOIL MODIFICATION The terms modification and stabilization can sometimes be very ambiguous. Modification refers to the stabilization process that results in improvement in some property of the soil but does not by design result in a significant increase in soil strength and durability. Modification also refers to soil improvement that occurs in the short term, during or shortly after mixing, i.e., within hours. This modification reduces the plasticity of the soil and thus improves the consistency to the desired level and improves short-term strength to the desired level. Short-term is defined as strength derived immediately within about 7 days of compaction. Even if no significant pozzolanic or cementitious reaction occurs, the textural changes that accompany consistency improvements normally result in measurable strength improvement. On the other hand, Stabilization is the process of blending and mixing materials with a soil to improve certain properties of the soil. The process may include the blending of soils to achieve a desired gradation or the mixing of commercially available additives that may alter the gradation, texture or plasticity, or act as a binder for cementation of the soil. Stabilization occurs when a significant, longer-term reaction takes place. This longer-term reaction can be due to hydration of calcium-silicates and/or calcium aluminates in Portland cement or class C fly ash or due to pozzolanic reactivity between free lime and soil pozzolans or added pozzolans. A strength increase of 350 kPa or greater of the stabilized soil strength compared to the untreated soil strength under the same conditions of compaction and cure, is a reasonable criterion for stabilization. 16 ADVANTAGES OF SOIL STABILIZATION Pavement design is based on the premise that minimum specified structural quality will be achieved for each layer of material in the pavement system. Each layer must resist shearing, avoid excessive deflections that cause fatigue cracking within the layer or in overlying layers, and prevent excessive permanent deformation through densification. As the quality of a soil layer is increased, the ability of that layer to distribute the load over a greater area is generally increased so that a reduction in the required thickness of the soil and surface layers may be permitted. The stabilization of soil for road pavements offers the following advantages: Improved engineering characteristics: Soil stabilization improves the engineering properties of the soil, e.g.: (i) strength - to increase the strength and bearing capacity, (ii) volume stability – to control the swell-shrink characteristics caused by moisture changes, and (iii) durability - to increase the resistance to erosion, weathering or traffic loading. Quality improvement: The most common improvements achieved through stabilization include better soil gradation, reduction of plasticity index or swelling potential, and increases in durability and strength. In wet weather, stabilization may also be used to provide a working platform for construction operations. These types of soil quality improvement are referred to as soil modification. Thickness reduction: The strength and stiffness of a soil layer can be improved through the use of additives to permit a reduction in design thickness of the stabilized material compared with an unstabilized or unbound material. The design thickness strength, stability, and durability requirements of a base or subbase course can be reduced if the particular stabilized material meets the specified gradation. Reduced maintenance requirements: Stabilization can reduce maintenance, improve soil properties and provide an all-weather surface. Stabilization can provide an improved surface condition through less dust, rutting, potholes and corrugating. Mixture designs should include strength criteria for evaluation of the optimum binder content. When a paving material is unsurfaced (i.e. no wearing course), it should have resistance to abrasion and raveling caused by vehicular traffic. Stabilization may be used to reduce raveling, increase skid resistance, or reduce dust. However, pavements that are stabilized by a cementing action cannot be maintained by routine grading and periodic reshaping. Where maintenance of the wearing surface is to be accomplished in this manner, the soil should be modified and not stabilized. METHODS OF SOIL STABILIZATION Over the years engineers have tried different methods to stabilize soils that are subject to fluctuations in strength and stiffness properties as a function of fluctuation in moisture content. Stabilization can be derived from thermal, electrical, mechanical or chemical means. The first two options are rarely used. Mechanical stabilization, or compaction, is the densification of soil by application of mechanical energy. Densification occurs as air is expelled from soil voids without much change in water content. This method is particularly effective for cohesion less soils where compaction energy can cause particle rearrangement and particle interlocking. But, the technique may not be effective if these soils are subjected to significant moisture fluctuations. The efficacy of compaction may also diminish with an increase of the fine content, fraction smaller than about 75μm, of the soil. This is because cohesion and inter particle bonding interferes with particle rearrangement during compaction. Altering the physio-chemical properties of fine-grained soils by means of chemical stabilizers/modifiers is a more effective form of durable stabilization than densification in these fine-grained soils. Chemical 17 stabilization of non-cohesive, coarse grained soils, soils with greater than 50 percent by weight coarser than 75 μm is also beneficial if a substantial stabilization reaction can be achieved in these soils. In this case the strength improvement can be much higher, greater than tenfold, when compared to the strength of the untreated material. The most common methods of soil stabilization for roads include:  Mechanical stabilization.  Lime stabilization.  Cement stabilization.  Lime-Fly Ash (with or without cement) stabilization.  Bituminous stabilization.  Chemical stabilization.  Geotextiles, fibers, prefabricated materials, etc. FACTORS FOR SELECTION OF STABILIZER In the selection of a stabilizer, the factors that must be considered are the type of soil to be stabilized, the purpose for which the stabilized layer will be used, the type of soil improvement desired, required strength and durability of the stabilized layer, cost, and environmental conditions. There may be more than one candidate stabilizer applicable for one soil type, however, there are some general guidelines that make specific stabilizers more desirable based on soil granularity, plasticity, or texture. Portland cement for example is used with a variety of soil types; however, since it is imperative that the cement be mixed intimately with the fines fraction (< 0.075 mm), the more plastic materials should be avoided. Generally, well-graded granular materials that possess sufficient fines to produce a floating aggregate matrix (homogenous mixture) are best suited for Portland cement stabilization. Lime will react with soils of medium to high plasticity to produce decreased plasticity, increased workability, reduced swell, and increased strength. Lime is used to stabilize a variety of materials including weak subgrade soils, transforming them into a ―working table‖ or subbase; and with marginal granular base materials, i.e., clay-gravels, to form a strong, high quality base course. Fly ash is a pozzolanic material, i.e. it reacts with lime and is therefore almost always used in combination with lime in soils that have little or no plastic fines. It has often been found desirable to use a small amount of Portland cement with lime and fly ash for added strength. This combination of lime-cement-flyash (LCF) has been used successfully in base course stabilization. Asphalt or bituminous materials both are used for waterproofing and for strength gain. Generally, soils suitable for asphalt stabilization are the silty sandy and granular materials since it is desired to thoroughly coat all the soil particles. Extreme climatic conditions can also have an influence on the correct choice of stabilizer, inhibiting the use of some, and encouraging the use of other, irrespective of cost. In general, the hot arid and cold wet regions require special consideration. MECHANISMS OF SOIL STABILIZATION For successful soil stabilizer applications it is imperative to understand the mechanism of stabilization of each additive. A basic understanding of stabilization mechanisms assists the user agency in selecting the stabilizer or additive best suited for a specific soil not only from the standpoint of developing the engineering properties desired for the pavement sub-layers but also to minimize the risk of long-term deleterious reactions that might compromise pavement structural capacity or even induce disruptive volumetric changes such as sulfate-induced heave. 18 In order to determine an appropriate soil-additive combination and to reduce the risk of deleterious reactions for a specific soil-stabilizer combination field exploration is required. For soil stabilization operations, the exploration process is less complex than for structural foundations as the depth of the influence zone is less. The stabilization mechanism may vary widely from the formation of new compounds binding the finer soil particles to coating particle surfaces by the additive to limit the moisture sensitivity. Therefore, a basic understanding of the stabilization mechanisms involved with each additive is required before selecting an effective stabilizer suited for a specific application. Chemical stabilization involves mixing or injecting the soil with chemically active compounds such as Portland cement, lime, fly ash, calcium or sodium chloride or with viscoelastic materials such as bitumen. Chemical stabilizers can be broadly divided in to three groups: Traditional stabilizers such as hydrated lime, Portland cement and Fly ash; Non-traditional stabilizers comprised of sulfonated oils, ammonium chloride, enzymes, polymers, and potassium compounds; and By-product stabilizers which include cement kiln dust, lime kiln dust etc. Among these, the most widely used chemical additives are lime, Portland cement and fly ash. Although stabilization with fly ash may be more economical when compared to the other two, the composition of fly ash can be highly variable. MECHANICAL STABILIZATION Mechanical stabilization is accomplished by mixing or blending soils of two or more gradations to obtain a material meeting the required specification. The soil blending may take place at the construction site, a central plant, or a borrow area. The blended material is then spread and compacted to required densities by conventional means. Mechanical stabilization is the development of natural forces of cohesion and internal friction within the existing soil. In some instances, the soil can be stabilized sufficiently by compaction alone. Usually, the local soil can be stabilized only with the addition of a reasonable amount of soil or gravel materials. Mechanical stabilization is used when soil or gravel materials with suitable grading and plasticity are unavailable locally. Mechanical stabilization involves mixing or blending two or more selected materials in the proportions required to modify particle size distribution and plasticity. Mixing can be carried out on site prior to final shaping and compaction. The alternative is to use grid rollers or rock crushers on site to arrive at an appropriate mix. Generally, mechanical stabilization requires the blending of soil and aggregates together in a well-graded (i.e., a complete range of particle sizes) mixture and compacting these to a high density. A common application of mechanical stabilization is the blending of a granular material lacking in fines with a sand-clay. This blending of the materials has the potential to improve strength, abrasion resistance, imperviousness, and compatibility. The following points should be observed in proportioning and blending mixtures:  Avoid complicated ratios, which are difficult for field control  Impossible to correct for grading below 75 micron sieve  Assure adequacy of pulverization and mixing operations LIME STABILIZATION Lime stabilization is more suitable for hot wet regions. Stabilization with lime will reduce the plasticity of the soil, increase its workability, reduce swell, and modify the material to provide optimum strength. For each type of soil there is an optimum lime content and the addition of further quantities in excess of the amount will adversely affect the properties of the mixture. The amount of lime necessary (percent by mass) to stabilize a material depends on the amount and type of clay mineral in the soil. 19 The use of small amounts of lime (1 to 3 percent) may reduce the plasticity index and be sufficient to stabilize some soils such as clayey gravel with good grading but moderately high plasticity. The use of lime contents of 3 to 6 percent may result in considerable change in the material constitution. Lime, generally, reacts well with most plastic materials such as clayey sands (SC) and silty clays (ML). However, materials with plasticity indices lower than 10 may not react readily. Testing is necessary to determine the reactivity of the material to lime. The stabilized soil should retain some cohesion poorly graded clayey sand and gravels, when treated with small percentages of lime. If too much is added they can become friable (easily crumbled or pulverized) and become completely non-cohesive, causing failures. Consequently, base material treated with lime should conform to the grading requirements normally specified for untreated material. In general, all lime treated fine grained soils exhibit 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, and moisture) are the most important. Various forms of lime have been successfully used as soil stabilizing agents for many years. However, 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 quicklime, 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. However, the following points must also be taken into account before contemplating using lime stabilization to improve soil properties:  Stabilization of sub grades with lime is not recommended for the reduction of heaving where freeze-thaw cycles occur.  Organic matter will decrease Lime‘s effectiveness.  Normal range of lime is 1-1/2 to 8 percent.  Construction should be completed before winter, to allow for complete strength development.  Drainage must be adequate before stabilizing commences.  Compact and shape to a tight finish. CEMENT STABILIZATION Portland cement can be used either to modify and improve the quality of the soil or to transform the soil into a cemented mass with increased strength and durability. The amount of cement used will depend upon whether the soil is to be modified or stabilized. Several different types of cement have been used successfully for stabilization of soils. Ordinary Portland cement and air-entraining cements have been used extensively in the past and gave about the same results. At the present time, sulfate resistant cement has largely replaced OPC as greater sulfate resistance is obtained while the cost is often the same. High early strength cement has been found to give a higher strength in some soils, because this type of cement has a finer particle size and a different compound composition than do the other cement types. Cement stabilization has been widely used in pavement construction. However, cement is usually not an appropriate stabilizing agent for the wearing course of a pavement. The cementitious bonds formed are not strong enough to resist the action of traffic without being protected by some kind of wearing surface. Also, because of the cementation, cement, unlike lime, cannot be reworked following initial 20 mixing and subsequent setting and are not amenable to being reworked with maintenance equipment, such as graders. It can, however, be used as a sub-base stabilizing agent. Cement may be used to stabilize a wide range of soils, from fine-grained clays and silts to sandy materials. Generally, for fine grained materials, cement is used with clays or silts when the plasticity index (PI) is relatively low. Cement stabilization should be avoided in soils where the sulfate exceeds about 1%. The amount of cement required to improve the quality of the soil through modification is determined by the trial-and-error approach. If it is desired to reduce the PI of the soil, successive samples of soil- cement mixtures must be prepared at different treatment levels and the PI of each mixture determined. The minimum cement content that yields the desired PI is selected, but since it was determined based upon the minus 40 fraction of the material, this value must be adjusted to find the design cement content based upon total sample weight expressed in equation (1). A = 100BC (1) Where: A = design cement content, percent total weight of soil B = percent passing 400 micron sieve size, expressed as a decimal C = percent cement required to obtain the desired PI of minus 400 micron material, expressed as a decimal However, if the objective of modification is to improve the gradation of a granular soil through the addition of fines then particle-size analysis should be conducted on samples at various treatment levels to determine the minimum acceptable cement content. STABILIZATION WITH LIME-FLYASH (LF) & LIME-CEMENT-FLYASH (LCF) Stabilization of coarse-grained soils having little or no fines can often be accomplished by the use of LF or LCF combinations. Fly ash, also termed coal ash, is a mineral residual from the combustion of pulverized coal. It contains silicon and aluminum compounds that, when mixed with lime and water, forms a hardened cementitious mass capable of obtaining high compressive strengths. Lime and fly ash in combination can often be used successfully in stabilizing granular materials since the fly ash provides an agent, with which the lime can react. All sand, gravel, and combination sand/gravel soils can be stabilized with lime-fly ash or lime-cement-fly ash combinations. The amount of fines in these soils should not exceed 12 percent and the Plasticity Index should not exceed 25. Thus LF or LCF stabilization is often appropriate for base and subbase course materials. Fly ash is classified according to the type of coal from which the ash was derived. Class C fly ash is derived from the burning of lignite or subbituminous coal and is often referred to as ―high lime‖ ash because it contains a high percentage of lime. Class C fly ash is self-reactive or cementitious in the presence of water, in addition to being pozzolanic. Class F fly ash is derived from the burning of anthracite or bituminous coal and is sometimes referred to as ―low lime‖ ash. It requires the addition of lime to form a pozzolanic reaction. Design with LF is somewhat different from stabilization with lime or cement. For a given combination of materials (aggregate, fly ash, and lime), a number of factors can be varied in the mix design process such as percentage of lime-flyash, the moisture content, and the ratio of lime to fly ash. It is generally recognized that engineering characteristics such as strength and durability are directly related to the quality of the matrix material. The matrix material is that part consisting of fly ash, lime, and minus aggregate fines. Basically, higher strength and improved durability are achievable when the matrix material is able to ―float‖ the coarse aggregate particles. In effect, the fine size particles overfill the void spaces between the coarse aggregate particles. For each coarse aggregate material, there is a quantity of matrix required to effectively fill the available void spaces and to ―float‖ the coarse aggregate particles. The quantity of matrix required for maximum dry density of the total mixture is 21 referred to as the optimum fines content. In LF mixtures it is recommended that the quantity of matrix be approximately 2 percent above the optimum fines content. At the recommended fines content, the strength development is also influenced by the ratio of lime to fly ash. Adjustment of the lime-fly ash ratio will yield different values of strength and durability properties. BITUMINOUS STABILIZATION 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 or soil agglomerates are coated with asphalt that prevents or slows the penetration of water which could normally result in a decrease in soil strength. In addition, asphalt stabilization can improve durability characteristics by making the soil resistant to the detrimental effects of water such as volume. In non-cohesive materials, such as sands and gravel, crushed gravel, and crushed stone, two basic mechanisms are active: waterproofing and adhesion. The asphalt coating on the cohesionless materials provides a membrane which prevents or hinders the penetration of water and thereby reduces the tendency of the material to lose strength in the presence of water. The second mechanism has been identified as adhesion. The aggregate particles adhere to the asphalt and the asphalt acts as a binder or cement. The cementing effect thus increases shear strength by increasing cohesion. Criteria for design of bituminous stabilized soils and aggregates are based almost entirely on stability and gradation requirements. Freeze-thaw and wet-dry durability tests are not applicable for asphalt stabilized mixtures. Bituminous stabilization is more suitable for hot dry areas. The addition of bituminous binder is intended to provide cohesion for non-plastic materials and to reduce water penetration through the soil. Bituminous stabilization is best suited to granular materials, and material that can be readily granulated. Bitumen stabilized material has limitations when used as the wearing course for pavement. Unless substantial quantities of bitumen are added, its binding action will be insufficient to prevent ravelling from traffic and weathering. Such higher bitumen content will usually be uneconomical. Gravels, sandy loam (SM), sand-clays (SC) and crushed rock have been successfully treated using bituminous stabilization. Fine-grained soils with increasing amounts of material passing a 75 micron sieve can be stabilized with bitumen; however, they will require increasing quantities of asphalt material and increasing costs accordingly. Materials with a PI of less than 10 are most suited to this form of stabilization. The bituminous stabilized soils are of following three types: Sand bitumen: A mixture of sand and bitumen in which the sand particles are cemented together to provide a material of increased stability. 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 subbase or base course quality. Lime bitumen: 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 PI, i.e. above 10. Bituminous stabilization is generally accomplished using asphalt cement, cutback asphalt, or asphalt emulsions. The type of bitumen to be used depends upon the type of soil to be stabilized, method of construction, and weather conditions. In frost areas, the use of tar as a binder should be avoided because of its high temperature susceptibility. Asphalts are affected to a lesser extent by temperature changes, but a grade of asphalt suitable to the prevailing climate should be selected. As a general rule, the most satisfactory results are obtained when the most viscous liquid asphalt that can be readily mixed into the soil is used. For higher quality mixes in which a central plant is used, viscosity-grade asphalt cements should be used. Much bituminous 22 stabilization is performed in place with the bitumen being applied directly on the soil or soil-aggregate system and the mixing and compaction operations being conducted immediately thereafter. For this type of construction, liquid asphalts, i.e., cutbacks and emulsions are used. Emulsions are preferred over cutbacks because of energy constraints and pollution control efforts. The specific type and grade of bitumen will depend on the characteristics of the aggregate, the type of construction equipment, and climatic conditions. STABILIZATION WITH LIME-CEMENT AND LIME-ASPHALT 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 or characteristics of a given soil. For instance, in clay areas devoid of base material, lime has been used jointly with other stabilizers, notably Portland cement or asphalt, to provide acceptable base courses. Since Portland cement or asphalt cannot be mixed successfully with plastic clays, the lime is incorporated into the soil to make it friable, thereby permitting the cement or asphalt to be adequately mixed. While such stabilization practice might be more costly than the conventional single stabilizer methods, it may still prove to be economical in areas where base aggregate costs are high. Two combination stabilizers are considered: lime-cement and lime-asphalt. Lime-cement: Lime can be used as an initial additive with Portland cement or the primary stabilizer. The main purpose of lime is to improve workability characteristics mainly by reducing the plasticity of the soil. The design approach is to add enough lime to improve workability and to reduce the plasticity index to acceptable levels. The design lime content is the minimum that achieves desired results. 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. LIME TREATMENT OF EXPANSIVE SOILS Expansive soils as defined for pavement purposes are those that exhibit swell in excess of three percent. Expansion is characterized by heaving of a pavement or road when water is imbibed in the clay minerals. The plasticity characteristics of a soil often are a good indicator of the swell potential. If it has been determined that a soil has potential for excessive swell, lime treatment may be appropriate. Lime will reduce swell in an expansive soil to greater or lesser degrees depending on the activity of the clay minerals present. The amount of lime to be added is the minimum amount that will reduce swell to acceptable limits. The depth to which lime should be incorporated into the soil is generally limited by the construction equipment used. However, 60 to 90 cm generally is the maximum depth that can be treated directly without removal of the soil. STABILIZATION WITH GROUND GRANULATED BLAST FURNACE SLAG Ground granulated blast furnace slag (GGBS) is a by-product from the blast-furnaces used to make iron. These operate at a temperature of about 1500 ͦ C and are fed with a carefully controlled mixture of iron-ore, coke and limestone. The iron ore is reduced to iron and the remaining materials form a slag that floats on top of the iron. This slag is periodically tapped off as a molten liquid and if it is to be used for the manufacture of ggbs it has to be rapidly quenched in large volumes of water. The quenching optimises the cementitious properties and produces granules similar to coarse sand. This granulated‘ slag is then dried and ground to a fine powder in sophisticated production facilities, capable of processing up to half a million tonnes annually, to tightly controlled fineness. The ggbs powder is very-slow setting cement in its own right but, for most practical purposes, it needs to be activated and accelerated by alkali. On its own, GGBS has only slow cementitious properties and Portland cement normally provides the alkalinity to activate and accelerate these properties. Lime can also be used to provide the necessary alkali for activation. Laboratory research and field trials have 23 confirmed that sulfides, as well as sulfates, are liable to cause disruptive expansion in stabilised soils. It has been shown that GGBS + lime combinations are practical and effective options for soil stabilization, and provide technical benefits. In particular the incorporation of ggbs, is very effective at combating the expansion associated with the presence of sulfate or sulfide in soil. Lime + GGBS stabilization offers other advantages for soil stabilization:  A slower early-rate of strength development gives considerably more time for construction operations.  There is also extra ability to self-heal, in the case of early-life damage caused by overloading  In the long-term, there is an increased strength that will improve the structural performance CHEMICAL STABILIZATION Various chemical compounds, including calcium chloride and magnesium chloride have often been used as stabilizers on an experimental basis, as stabilizers. The use of chemicals may possibly provide short-term advantages by acting as a dust suppressant and providing binding action to form a hard running surface. However, the long-term advantages appear to be limited as a result of leaching of the stabilizer from the pavement. Because of the life expectancy, and possible adverse effects from using some chemicals, their use in the treatment of pavement materials is not recommended as a long-term solution to materials instability. The benefits and costs of using chemicals compared to a bituminous seal or stabilization by mechanical means or using lime or cement needs to be analyzed in the light of relevant experience. This experience to date appears to indicate that the benefits from using chemical stabilization may be short-lived due to the effects of leaching of the chemical from the treated material. In addition to the above chemicals, several proprietary brands of stabilizers are available. These products are generally byproducts of manufacturing processes such as pulp and wood processing, and to date appear to perform similarly to the chemical discussed above, particularly with respect to their life span and effectiveness. Nevertheless, authorities have used such products with varying degrees of success depending on the function to be performed or the problem to be overcome. STABILIZATION WITH GEOTEXTILES Geotextiles can be used over very soft soils to help spread loads through their tensile strength properties and thereby increase the locations load bearing capacity. Geotextiles can also act as a separator to prevent excess fines from penetrating a granular material placed over it or as a water barrier to prevent moisture from entering the pavement, whenever any cover aggregate is to be added to a soil containing more than 10 percent fines; a geotextile is required as a separation layer. Geotextiles can be used to construct various drainage layers within and next to the pavement to control and remove excess moisture. The use of geotextiles, especially for expedient applications, will facilitate trafficking over low bearing capacity soils. Geotextiles can reduce or eliminate the need for more conventional stabilization materials. When used for separation the geotextile should meet drainage or filter requirements for the local soil conditions. The geotextile openings should be sized to prevent soil particle movement. The geotextile must have the strength to meet survivability requirements related to the sub grade conditions and cover arterials. Design procedures exist where the geotextile is to be used as either as a water barrier or for reinforcement. To operate as a water barrier the geotextile must normally be coated with a bitumen material. Seams between geotextile sheets may be field seamed together by different methods; however, in the field, they are usually just overlapped a given distance to eliminate fastening problems. 24 STABILIZATION USING FIBERS, AND PREFABRICATED MATERIALS Stabilization using fibers involves mixing hair-like fibers into the moist soil using a pulver-mixer. Fiber stabilization is most applicable for sands and silty sands that are classified as SW, SP, SM, and some SM-SC types of soils. The use of fibers in high-plasticity soils has had inconsistent results; therefore, their use should normally be limited to the coarse grained soil types as mentioned above. The fabricated materials referred to for soil stabilization include Uni-Mat, Hex-Mats, and any other fabricated material that can be used as a trafficking surface to support loads on a soft soil. STABILIZATION WITH RICE HUSK ASH AND LIME SLUDGE The substantial amounts of waste materials like rice husk ash and lime sludge are being produced by various industries throughout the country as a by-product. These materials are causing hazardous effects to the lands and surroundings and a great problem for their disposal. Use of this waste material in road construction can alleviate the problem of their disposal to great extent. In India, studies were conducted at IIT Roorkee for its use in stabilizing the soil mass, the results indicated that its usage having great impact on the improvement of soil properties. The study suggested that it is very useful for stabilizing the clayey soils. The results of the study are given below:  lt increases the liquid limit and plastic limit thereby decreasing the PI value of soil  It increases the unconfined compressive strength of soil.  It increases the soaked CBR of the soil.  The optimum proportioning of lime sludge and rice husk ash for maximum unconfined compressive strength and lowest plasticity index is 16% and 10% respectively.  The soaked CBR however kept on increasing at 15% and 20% rice husk ash. CONCLUDING REMARKS Soil stabilization is the alteration of one or more soil properties, by mechanical or chemical means to create an improved soil material possessing the desired engineering properties. Soils may be stabilized to increase strength and durability or to prevent erosion and dust generation. Regardless of the purpose for stabilization, the desired result is the creation of a soil material or soil system that will remain in place under the design use conditions for the design life of the project. Engineers are responsible for selecting or specifying the correct stabilizing method, technique, and quantity of material required. In Nigeria, soils vary from place to place, and the engineering properties of soils are equally variable. The key to success in soil stabilization is soil testing. The method of soil stabilization selected should be verified in the laboratory before construction and preferably before specifying or ordering materials. 25