10 CHAPTER 2: LITERATURE REVIEW 2.1. General A pavement is a relatively stable crust constructed over the natural soil for the purpose of supporting and distributing wheel loads and for providing an adequate wearing course. These pavements are damaged at a shorter period due to change in the soil properties and due to the repeated application of wheel loads which may result in excessive settlement. Further moisture variation, frost action, Increase (or) decrease of water content in the clay soil causes further deterioration of the pavement which necessitates repair process at a higher cost. The strength of the soil is improved by stabilization. 2.2 Mechanics of Soil Stabilization The term soil stabilization means the improvement of the stability or bearing power of soil by the use of controlled compaction, proportioning and or the addition of suitable admixture or stabilizer. Soil stabilization deals with physical, physico-chemical and chemical methods to make the stabilized soil serve its purpose as pavement component materials. The basic principles in soil stabilization may be stated as follows: Evaluating the properties of given soil Deciding the method of supplementing the lacking property by the effective and economical method of stabilization Designing the stabilized soil mix for desired stability values. Considering the construction procedure by adequate compaction of stabilized layers. Soil stabilization may result in any one or more of the following changes: Increase the drain ability of the soil Increase stability
11 Reduce volume changes Control the undesirable effects associated with clay. Reduce settlement Increases sharing resistance Increases the bearing capacity of soil. Improving the local soil This can be achieved by mechanical (or) chemical methods to make the soil stabilized for fulfilling its purpose as pavement component material. Soil type is one of the key features used to determine which method and material should be used for achieving best compaction. 2.3 Mechanical stabilization The Objective of mechanical stabilization is to blend different available soils so that when compacted, they give the desired stability. In some areas the natural soil at an existing location may have weak in nature (poor CBR). It may due to clay, silt or fine sand. Suitable soil may be selected (contains granular material) and this is to be blended with the available soils to improve the soil properties at a lesser cost in manpower and materials to achieve best results (Increase of CBR). The mechanical stability of soil-aggregate mixtures depends upon the mechanical strength of aggregate, the mineral composition of the materials, the gradation of the mixture, the plasticity characteristics of the binder soil and the compaction. With respect to mineral composition, any material which is resistance to weathering can be used. Sodium sulphate and sodium carbonate have a detrimental effect on the stability because of their high volume changes caused by hydration and dehydration. Presence of chlorides and carbonates are beneficial.
12 Factors affecting mechanical stability: The stability of mechanical stabilized soil mixes depends on the following factors: Mechanical strength of aggregates, gradation, properties of soil, presence of salts, mica etc and compaction. Limitation of mechanical stabilization: Original soils contains fines, use of coarser fractions for blending may be expensive If clay has been added to Stabilize soils, it should be susceptible to frost action. Creation of dust clouds results in nuisance to traffic Reduction of soil cohesion (or) binding forces ultimately leads to material disintegration. 2.4 Chemical stabilization These are chemical substances that can enter in the natural reactions of the soil and control the moisture getting to the clay particles, therefore converting the clay fraction to permanent cement that holds the mass of aggregate together. The chemical stabilizer in order to perform well must provide strong and soluble cations that can exchange with the weaker clay cations to remove the water from the clay lattice, resulting in a soil mass with higher density and permanent structural change (Raul velas quez et al, 2005) It uses the following Cement Lime Fly ash Bituminous materials Other stabilizing chemical admixtures
13 Limitation of chemical stabilization: In the case of cement stabilization increase in cement content causes increase in strength and durability [Higher cost]. Also surface chemical factors, organic matter and sulphate content alter the properties and durability of soil-cement. The presence of Magnesium sulphate reacts with hydrated cement and cause reduction in strength. In the case of lime stabilization, it creates a dust (Hydrated lime) and this is not suitable for thickly populated areas. Water is a critical component in compaction. Less water results in adequate compaction and more water makes compaction ineffective. Lime requires certain quantum of water for slacking process and it poses a problem in compaction. More mixing time is required when lime is used and it makes the process uneconomical. 2.5 Bio-enzymatic Soil Stabilization 2.5.1 About bio-enzyme Bio-enzyme is a natural, non-toxic, non-flammable, non-corrosive liquid enzyme formulation fermented from vegetable extracts that improves the engineering qualities of soil, facilitates higher soil compaction densities, and increases stability. Enzymes catalyze the reactions between the clay and the organic cat-ions and accelerate the cat-ionic exchange process to reduce adsorbed layer thickness. For other types of chemical stabilization, chemicals are mixed with soil, which is difficult to mix thoroughly, but Bio-enzyme is easy to use as it can be mixed with water at optimum moisture content and then it is sprayed over soil and compacted. 2.5.2 Mechanism of soil stabilization by bio-enzyme In clay water mixture positively charged ions (cat-ions) are present around the clay particles, creating a film of water around the clay particle that remains attached or adsorbed on the clay
14 surface. The adsorbed water or double layer gives clay particles their plasticity. In some cases the clay can swell and the size of double layer increases, but it can be reduced by drying. Therefore to truly improve the soil properties, it is necessary to permanently reduce the thickness of double layer. Cat-ion exchange processes can accomplish this. By utilizing fermentation processes specific micro-organisms can produce stabilizing enzyme in large quantity. These soilstabilizing enzymes catalyze the reactions between the clay and the organic cat-ions and accelerate the cat-ionic exchange without becoming part of the end product. In the present study, one type of bio-enzyme has been used for stabilization of five types of soil with varying index properties. Detailed laboratory tests were carried out to ascertain the benefits in terms of reduction in design thickness. 2.5.3 Bio-enzyme as soil stabilizer in road construction Cost effective roads are very vital for economical growth in any country. There is an urgent need to identify new materials, improve road construction techniques to expand the road network. Commonly used materials are fast depleting and this has led to an increase in the cost of construction. Hence, the search for new materials and improved techniques to process the local materials has received an increased impetus. When poor quality soil is available at the construction site, the best option is to modify the properties of the soil so that it meets the pavement design requirements. This has led to the development of soil stabilization techniques. Since the nature and properties of natural soil vary widely, a suitable stabilization technique has to be adopted for a particular situation after considering the soil properties. Soil improvement by mechanical or chemical means is widely adopted. In order to stabilize soils for improving strength and durability, a number of chemical additives, both inorganic and organic, have also been used. Recently bio-enzymes have emerged as a new chemical for soil stabilization. Bio-
15 enzymes are chemical, organic, and liquid concentrated substances which are used to improve the stability of soil sub-grade for pavement structures. Bio-Enzyme is convenient to use, safe, effective and dramatically improves road quality. 2.5.4 Enzyme stabilization effects Organic cations generated by the growth of vegetation and micro organisms also have the capability to exchange position with other ions attracted to the clay platelet in the soil. In contrast with metal cations, the organic cations have large flat structures that approach the size of small clay particles. These organic cations can blanket the clay particle and effectively neutralize its negative charge in a short distance, thus greatly reducing the double layer thickness. Certain soil micro organisms make use of this chemistry to stabilize their environment. They produce specific enzymes that catalyze the reactions between the clays and the organic cations, producing clods of stable soil among the roots of the vegetation. These soil-stabilizing enzymes accelerate the cationic exchange without becoming part of the end product. By utilizing fermentation processes, specific microorganisms can produce stabilizing enzymes in large quantities. Nature Plus manufactures and formulations the Terrazyme Soil Stabilizer Product in this manner. These fermentation, formulated products are non-toxic and environmentally harmless. When exposed to the air, the microorganisms multiply rapidly and produce large organic molecules, which the enzyme attaches to the clay platelets. The negative charges on the clay platelets are neutralized through this process and the size of the electrical double layer shrinks. This limits further adsorption of water or the resultant swelling with loss of density. The enzyme is regenerated by the reaction and goes on to perform again. Reactions, which might otherwise take years to occur, can be carried out in weeks. While some soil strength gain will become apparent within days, field research indicates that strength will continue to
16 increase over a period of several weeks. As the soil is exposed to air and the reaction proceeds, water, which was initially adsorbed on, the clay will be released from the clay structure and can evaporate. As this occurs, clay particles are drawn closer together, soil density increases and the road bearing structure strengthens. Terrazyme soil stabilization products are also designed to improve soil strength by increasing the density of initial compaction and facilitating the removal of pore water, which minimizes the destructive impact of water under conditions of loading. Combined with the impact of Terrazyme on the electrical double layer of clay particles, these factors can promote the formation of a stabilized soil structure of superior density and load bearing strength even in soils lacking a significant plastic fines fraction. Field results indicate that soil treated with Terrazyme can reach 95% Modified Proctor with significantly reduced compaction effort. Immediately following usual compacting procedures on road materials treated with Terrazyme, nuclear densitometer readings have indicated densities in excess of 100%. These superior initial densities correlate with lower void percentages in the treated and compacted road layer. Higher densities and reduced percentages of voids indicate that realignment of soil particles may have occurred during compacting, thus allowing treated soil particles to achieve closer proximity and higher strength. Compacting of the soil at the optimum moisture content using proper construction methods and equipment is essential to produce the high density necessary for enzymatic stabilization. The reduction in voids resulting from this realignment would decrease pore water within the treated road layer and inhibit water penetration. This impact of the Terrazyme Treatment Process on the soil structure correlates with observed reductions in permeability. All of the factors
17 discussed can combine to minimize the destructive impact of soil water on the load bearing structure of the treated layer. Because the organic ions are very large, little migration takes place within the pore water. Therefore, to achieve the desired results, intimate mixing is required to distribute the enzyme solution throughout the soil. In addition, compaction of the soil at the optimum moisture content using proper construction methods and equipment is essential to produce the high density necessary for enzymatic stabilization. Close adherence to all aspects of the TerraZyme Treatment Process is important to soil stabilization success. 2.6 Review of Literature In Brazil, Bio-Enzymes are now being used for projects in private sectors as well as in municipal, state and some federal projects. Based on laboratory experience and field tests, programme standards for the selection of materials for road construction have been developed, and construction materials are now specified using these standards. Bio-enzyme from Australia is a natural, non-toxic bio-degradable liquid concentrate that mixes easily in water for application with standard water spraying equipment. Bio-Enzyme is a low cost additive with long lasting effects. By altering the physical and chemical characteristics of soil, materials treated with Bio- Enzyme retain higher performance levels and extended life span. Bio-Enzyme may be used to increase the Maximum Dry Density (MDD) and Unconfirmed Compressive Strength (UCS) values of a marginal material to achieve specified standards for a base course. Bio-Enzyme manufactured in USA and The Netherlands also increases the unconfined compressive strength (UCS) and California Bearing Ratio of sub-grade soil. Among the soil materials stabilized by the bio-enzymes in the trials are sandy clay, silty clay, sandy silt, plastic and non-plastic clay, sandy loam, fine loam, and loam mixed with clay.
18 The dosage levels of the bio-enzymes vary from 1 litre to 5 litres for 5 m3 of soil depending on the soil type, soil characteristics, and product concentration. The amount of dilution water depends on soil dryness. In the field, measurements are made to determine the difference between the actual field soil moisture content (%) and the optimum moisture content (%) to add water to obtain OMC for maximum compaction. With the test experiments, it was found that Bio-Enzyme is suitable for all soil having clay content more than 5 percent with the exception of pure sandy and slerite soil. Information on the performance of the treated soil structural layers of the roads comes from periodic measurements in the field with DCP (Dynamic Cone Penetrometer) equipment. Based on previous laboratory test results and field trials in Brazil, it was found that CBR of stabilized soil increases exponentially for 4 weeks; thereafter the rate of increase in CBR is reduced but keeps on increasing perpetually. In the countries that are using bio-enzyme stabilizers, the usage costs are much lower than conventional chemical stabilizers. This is because their cost is lower, the application is simpler and less expensive, and transport is cheaper. Even when modest increments in the road stability occur, there is an increased cost-effectiveness and improvement in performance and durability of the road. Brazetti and Murphy have conducted a lot of field studies in Brazil. Three stretches of the road are selected. The soils are tested for plasticity, gradation and CBR. These stretches of the road are treated with organic soil stabilizer (Bio enzyme). After more than 7 months of usage, without any required maintenance, road sections treated with soil stabilizer showed the following improvements. Increased CBR (or) capacity to support traffic loading to more than 15 times that of the soil not treated with soil stabilizer.
19 Preserved the structural integrity of the surface (no permanent deformation or plastic effects) with increased cohesion of the base layer material and consequent increase in CBR. No Accumulation of material at the pavement edge. No peeling off material due to erosion (or) abrasion from traffic. Absence of corrugation on the surface of the pavement after repeated loading condition. Also absence of tire marks. Minimized creation of dust Non destructive test was done using Dynamic cone penetrometer (DCP) and found that CBR increases considerably. Hitam and Yousof (1998) of palm oil research institute of Malaysia conducted field studies on improvement of plantation roads. The road was unpaved road and it affects badly due to adverse weather conditions. Terrazyme was treated to 27.2 km of the road and the sections were then monitored on the surface erosion for two monsoon seasons. No damage was noticed and the researches have concluded that Terrazyme stabilization can convert the road to all weather roads that has minimum destruction in hot and wet seasons. Sureka Naagesh and Gangadhara (2011) conducted a number of experiments on an expansive soil treated with a bio enzyme stabilizer the expansive soil obtained from Davangere in Karnataka state, India. One dimensional odometer test was used to conduct swell consolidation test. The specimens are directly prepared in a consolidation ring using static compaction. The treated specimens were kept for curing in desiccators and tested after the curing period. Soil Specimens are treated with different dosages of bio enzyme (0.25%, 0.50%, 1% and 2%) The swelling potential and swelling pressure are measured in the one dimensional consolidation load
20 cell using swell and load procedure. Specimens with 29% initial water content exhibited 45% reduction in swell potential with 30 days of curing upon further increasing the curing period to 60 days, the reduction was to be about 50%. The scanning electron microscopy (SEM) studies on untreated and bio-enzyme treated specimens were conducted in order to ascertain the reasons for reduction in swelling characteristics. The following conclusions are drawn: The soil specimens treated with bio enzyme exhibit lesser percent swell and swell pressure compared to untreated soil specimens. Scanning electron microscopic studies indicate occurrence of changes in the fabric of bioenzyme treated soil specimens the untreated specimens displayed flocculated structure and bio enzyme treated specimens exhibited dispersed structure. X-ray diffraction studies indicate that noticeable mineralogical changes did not occur in bio-enzyme treated soil on treatment with bio-enzyme the intensity of the minerals showed marked reduction up to 80%. Mihai et al 2005 have done experiments on two types of soils with two enzyme products. The main objective is to investigate the stabilization mechanism of some of the commercially available enzyme based products to understand their potential value for road construction. The soil tested for sieve analysis and compaction. Then the soils are treated with two types of Enzyme with variable dosage 0.5 cc, 1cc, 1.5cc/5 l. The result of the test indicated that soils treated with enzymes shows marginal improvements in resilient modulus. The resilient modulus represents the stiffness of the material tested. Testing results shows increase in the values of resilient modulus for the soils treated with Enzyme. After the addition of enzyme the shear strength of soil also increases considerably.
21 Kyle M.Rollins (2010) has done experimental investigation on 3 types of soils (coarse medium, fine) using Terrazyme. Tests were performed on samples which were allowed to cure for 1, 4 and 14 weeks after compaction. The results from these tests were compared with untreated samples the CBR values for all 3 Terrazyme treated gradation, (coarse, medium, and fine) increased significantly at 4 weeks and 14 weeks over the untreated soil samples. Isaac et al (2003) have done experimental studies in the area of Bioenzymatic soil stabilization in the laboratory for different types of soils from Kerala. The soil samples were laterite, clay and sand. These samples were tested for grain size analysis and Atterbergs limit. The optimum moisture content of the soil was determined from modified compaction test. The soil was treated with different dosage of enzyme. CBR test were conducted on each soil samples at different curing period under soaked condition. From the study they found that Terrazyme treatment is very effective, economical and environment friendly technique for the stabilization of fine grain soil. Lacuoture and Gonzalez (1995) conducted a comprehensive study of the TerraZyme soil stabilizer product and its effectiveness on sub-base and sub-grade soils. The reactions of the soils treated with the enzyme was observed and recorded and compared to the untreated control samples, the variation in properties was observed over a short period only and it was found that in cohesive soils there was no major variation in properties during the early days but the soil showed improved performance progressively. Sharma (2001) has conducted laboratory studies on use of Bio-Enzyme stabilization of three types of soils namely, clay of high plasticity, clay of low plasticity and silt of low plasticity. It was found that soil shows a marginal improvement in CBR value and substantiates reduction in
22 saturation moisture after four weeks of stabilization. The soil shows a marginal improvement in unconfined compressive strength, direct tensile strength and fatigue strength. Roger Bergmann (2006) has studied on soil stabilizers on trail surface and concluded that Bioenzymes requires some clay content in the aggregate material in order to create the reaction that will strengthen the material. Also reports showed that successful stabilization with as little as 2% clay in the aggregate material but best result seem to be achieved with 10 to 15% clay, upon completion of construction the trail looked very good but like the other trail sections it did not hold up over the first winter. Manoj Shukla et al (2003) have been made as extensive study on five types of soils with low clay content to very clay content. These soil samples were tested for engineering properties, Atterbergs limit, specific gravity, OMC and maximum dry density using modified proctor test. The strength test such as CBR, UCC with and without the use of bio-enzyme at different curing period. The pavement design thickness is evaluated for enzyme made soil samples for maximum CBR at optimum dosage and cost comparison chart made with conventional design and bioenzyme design. He concluded that bio-enzyme stabilization has shown little to very high improvement in physical properties of soil. Saad Aiban (2006) studied the compressibility and swelling characteristics of eastern Saudi Arabian expansive soil. Odometer free swell test were conducted on undisturbed and re-molded playgorsite expansive soil. The mineralogical composition and fabric of soil was analyzed. The swelling to re-molded samples were higher than undisturbed samples. It was found that Pre-wetting of undisturbed samples this swell potential.
23 Andrew R. Tolleson et al (2003) in their research on An Evaluation of Strength Change on Sub grade Soil Stabilized with an Enzyme Catalyst Solution Using CBR and SSG Comparisons, a laboratory bench scale testing program was conducted to evaluate the effectiveness of enzyme treatment on sub grade soil. Their objective was to study the potential applicability of tested enzyme for unpaved road in-situ stabilization. The effectiveness of enzyme treatment was evaluated on the basis of statistical measurement of change in CBR strength, soil stiffness and soil modulus. It was concluded that the CBR test appear to be a relatively poor indicator of direct soil strength for testing conditions. Not with standing, the test result showed CBR strength gain and to a lesser degree strength gain measured by the means of the SSG equipment resulting from the application of the enzyme solution on most soils tested, indicating a promising potential for sub grade stabilization using the enzyme solution. Effects of terrazyme on increase of CBR was studied Soil mechanics laboratory in National Road Department of Thailand (1996) and they found that after one week, two week, three week, and 14 week periods CBR was found as 37, 62, 66 and 100+ respectively as compared to 28% of untreated soil, investigators also reported reduction in gravel loss, road roughness, dust levels on the Terrazyme treated road sections. Patel and Desai (2010) proposed a method for correlating CBR values with the liquid limit, plastic limit, and plasticity index, optimum moisture content and maximum dry density of cohesive soil of various zone of Surat City of Gujarat state. The results were analyzed statistically. A correlation is made between CBR and the soil index properties using linear regression model through excel and SPSS software.
24 Sridharan and Nagaraj (2005) have made a compaction study with 5 different types of soils; the correlation is made between OMC and Plastic limit, maximum dry density and plastic limit. A mineralogical analysis was performed using an X-ray Diffracto Meter. Taskiran (2010) used artificial intelligence methods for prediction of CBR. Artificial neural network and Gene expression programming were applied for the prediction of CBR of fine grain soils from Southeast Anatolia region Turkey. He concluded that both ANN and GEP are found to be able to learn the relation between the CBR and basic soil properties and can be used for the predicting CBR values of soils. Vipulanandan (2010) has studied laboratory and field compacted soil samples falls in the group CL, CH and SE. He concluded that the undrained shear strength had better correlation with the CBR. Addo has made Non-Destructive study using spectral pavement analyzer (SPA) at the two pavement sites are located in the city of Surrey in British Colombia (Canada). He concluded that if the Asphalt is in good condition, the Asphalt thickness estimated from SPA tests implies to an accuracy of less than 6%. Mohamed Ali (2001) as per laboratory test to establish the deformation modulus of sub-graded soil as well as inter relationship between CBR and modulus of elasticity for the structural design of highways in Sudan. He proposed an equation between modulus of elasticity and California Bearing Ratio. The study conducted in China says that lime is the common material used for stabilization. But it destructs and pollutes environment. Further when lime is used to produce the dust and affect the environment heavily in the construction site. The mineral constituent is one of the main characteristics, which illuminates the soils formation. The mineral constituent has the critical
25 effect to the soil s characteristic. The clay which belongs to secondary mineral is the most activist mineral constituent. So, the clay is mainly solidified when the soil is solidified. The soil which was collected from Thong chuan in Shanxi was treated with Terrazyme. The measurement of CBR, studies on x-diffraction, and scanning election microscope shows the increase of strength after adding Terrazyme. Tewodros Alene (2010) has done Extensive study in Ethiopia on expansive soils. The construction of test sections is carried out on a road with Expansive sub grade along the Chancho Ginchi road. Different types of soil stabilization techniques have been used (Mechanical, lime and chemical stabilization). The expansive sub grade is mechanically stabilized by mixing it with non plastic gravel with a proportion of 50% by volume and compacted to the required density. The expansive soil is treated with semi processed lime with an application rate of 10% by weight further the expansive sub grade was treated with a combination of hydrated lime and a Terrazyme. Nine trial sections were formed by using different materials. A comparative study on different trial sections reveals that Terrazyme stabilization gives fruitful results. Shankar et al (2009) have done experimental work on lateritic soil collected from Dakshina Kannada and Udupi districts. The lateritic soil and blended lateritic soil are tested for Engineering properties, CBR and UCC. Then these soils are treated with different dosages of Enzyme, Again these soils are tested for Engineering properties, CBR and UCC over different curing periods, By comparing CBR, UCC values the soil treated with Enzyme shows a marginal improvement.
26 2.7 General findings based on literature review Most researchers found that the application of Terrazyme on the soil improves the CBR to a large extent. Improves the soil structure sufficiently which results in cost savings up to 25%. Improves structural integrity and load capacity, and reduces the occurrence of serious defects such as pot holes ad rut formation, resulting in fewer maintenance needs. Minimized creation of dust. Suitable for road to all weather seasons. Application of Terrazyme offer good result on an expansive soil. Improves CBR, UCC, and Shear strength of soil Decrease of plasticity index 2.8 Objectives of the Present research work The objectives of the present research work are: 1. To study the effect of Bioenzyme on the quality of sub grade/base course layers and its influence on pavement system. 2. To optimize the quantity of Bioenzyme to be used as stabilizing agent and the extent of stabilization of strength gain with time. 3. To develop suitable specifications and recommendations for evolving proportions for its use in subsurface layers. 4. To validate mathematical model with experimental results