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32 Standard Recommended Practice for Stabilization of Subgrade Soils and Base Materials AASHTO Designation: R Draft (2008) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001
33 Standard Recommended Practice for Stabilization of Subgrade Soils and Base Materials AASHTO Designation: R Draft (2008) INTRODUCTION Engineering design of pavement structures relies on the assumption that each layer in the pavement possesses the minimum specified structural quality to support and distribute the super imposed loads. But the available earth materials do not always qualify to be used directly as a construction material, but instead may require modification to improve their engineering properties in order to achieve the target strength requirements set for pavement materials. An economical way of addressing these strength deficiencies can be through chemical modification or stabilization. Successful stabilization of soils depends on the physio-chemical properties of soil as the soil-stabilizer interactions can vary with soil composition. This is particularly important if the treatment is performed with the intent of achieving long term benefits. Soil-stabilizer interactions are complex and may vary among soils due to heterogeneity in soil composition, differences in micro and macro structure of soils, heterogeneity of geologic deposits and differences in physical and chemical interactions between soil particles and additives. Stabilization projects are site specific and require integration of standard test methods, analysis procedures and design steps to develop acceptable solutions. This recommended practice provides a simplified protocol to be followed in selecting the appropriate calcium-based stabilizer and achieving a viable and economic design for the use of the selected stabilizer for a specific subgrade soil or base material. SCOPE This standard of practice discusses the techniques to successfully achieve the required engineering properties for individual soil groups. This recommended practice also addresses the basic mechanism(s) of stabilization when using lime, Portland cement and fly ash; soil exploration and sampling techniques; guidelines and techniques for identifying an effective additive for individual soil types; and techniques for validating the selection of the stabilizer through mixture design and testing. Construction practices are not addressed in this document. This standard may involve hazardous materials, operations and equipments. This standard does not address all the safety problems associated with their use. It is the duty and responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. REFERENCED DOCUMENTS AASHTO Standards: M 295, Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete R 13, Conducting Geotechnical Subsurface Investigations
34 T 27, Sieve Analysis of Fine and Coarse Aggregate T 87, Dry Preparation of Disturbed Soil and Soil Aggregate Samples for Test T 89, Determining the Liquid Limit of Soils T 90, Determining the Plastic Limit and Plasticity Index of Soils T 99, Moisture Density Relationship of Soils Using a 2.5-kg (5.5-lb) Rammer and a 305-mm (12-in.) Drop T 134, Moisture Density Relations of Soil-Cement Mixtures T 136, Freezing and Thawing Tests of Compacted Soil-Cement mixtures M 145, Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes T 180, Moisture Density Relationship of Soils Using a 4.54-kg (10-lb) Rammer and a 457- mm (18-in.) Drop T 248, Reducing Samples of Aggregates to Testing Size T 290 (modified), Determining Water-Soluble Sulfate Ion Content in Soil R(Draft), New AASHTO Standard Recommended Practice for Stabilizing Sulfate Rich Soils ASTM Standards: E 105, Standard practice for Probability Sampling of Materials E 122, Standard Practice for Calculating Sample Size to Estimate, With Specific Precision, the Average for a Characteristic of a Lot or a Process E 141, Standard Practice for Acceptance of Evidence Based on the Results of Probability Sampling D 559, Standard Test Method for Wetting and Drying Compacted Soil-Cement Mixtures D 560, Standard Test Method for Freezing and Thawing Compacted Soil-Cement Mixtures C 593, Standard Specification for Fly Ash and Other Pozzolans for Use With Lime for Soil Stabilization D 1632, Standard Practice for Making and Curing Soil-Cement Compression and Flexure Test Specimens in the Laboratory D 1633, Standard Test Method for Compressive Strength of Molded Soil-Cement Cylinders D 2974, Standard Test Method for Moisture, Ash, and Organic Matter of Peat and Other Organic Soils D 3551, Standard Practice for Laboratory Preparation of Soil-Lime Mixtures Using Mechanical Mixer D 3665, Standard Practice for Random Sampling of Construction Materials D 5102, Standard Test Method for Unconfined Compressive Strength of Compacted Soil- Lime Mixtures D 5239, Standard Practice for Characterizing Fly Ash for Use in Soil Stabilization D 6276, Standard Test Method for Using pH to Estimate the Soil-Lime Proportion Requirement for Soil Stabilization TERMINOLOGY Soil - All deposits of loose material on the earthâs crust that are created by weathering and erosion of underlying rocks. Coarse-grained soils - Soils with less than 50 percent of the mass passing 75 μm sieve. Fine-grained soils - Soils with more than 50 percent of the mass passing 75 μm sieve. Kinetics - Rate of progress of a chemical reaction.
35 ppm - Parts per million or milligram per liter; concentration of solute, sulfates, in milligrams per liter of solvent, water. Isomorphous substitution - Substitution of one atom by another of similar size within a crystal lattice and without disrupting the crystal structure of the mineral. Short-term strength - Strength derived immediately, within about 7-days of compaction. Modification - Improvement that occurs in the short term, during or shortly after mixing (within hours). This modification reduces the plasticity of the soil (improves the consistency) to the desired level and improves short-term strength to the desired level. Even if no significant pozzolanic or cementitious reaction occurs, the textural changes that accompany consistency improvements normally result in measurable strength improvement. Stabilization - A longer term reaction that is derived from the 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, when treated with a stabilizer, of at least 50 psi greater than that of the untreated soil fabricated and cured under the same conditions is used in this document to consider soils to be a stabilized material. Soil cement - Stabilized soil in which the coarse aggregate, sand size and larger, is surrounded and bonded by a matrix of cement paste and fine soil particles. SIGNIFICANCE AND USE Although stabilization is an effective alternative for improving soil properties, the engineering properties of individual soils vary widely with changes in soil composition. Soil-stabilizer interactions also change with soil composition as does the extent of improvement of soil properties. Therefore, the ability to identify the most appropriate stabilizer for use with an individual soil and a thorough understanding of the techniques needed to validate the selection of a stabilizer are critically important to the success of a stabilization project. This recommended practice provides the information needed for stabilizer selection and validation of that selection through mixture design and field testing. BACKGROUND Soil texture is largely related to its appearance and is dependent on the size, shape and distribution of particles in the soil matrix. Soil particle sizes may vary from boulders or cobbles, roughly a meter in diameter, to very fine clay particles, roughly a few microns in diameter. Strength and stiffness development in coarse-grained soil fractions is primarily dependent on physical interlocking of particles and varies in degree with the size, shape and relative amount of coarse-grained particles present. The behavior of finer fractions, silts and clays, is influenced more by electro-chemical and physio-chemical properties and varies with the quantity and type of individual minerals present in the soil. This is largely due to the high specific surface area of the fine grained fractions. Weathering processes impact soil composition creating variability in surface soils. These processes primarily influence the composition of fine fractions in soil as these fine fractions are easily transported away from parent rock formations. These transport processes often result in mixing of soil minerals and may
36 introduce salts or organic material of a variety of species and concentrations. These processes create differences in soil compositions within soil groups which can influence the behavior of individual soils. Due to the higher specific surface area of fine soil fractions, they are more reactive in the presence of chemical modifiers when compared to coarse fractions in the soil. Among these fine fractions, the specific surface area of clay particles is orders of magnitude larger than that of silt fines in soil. This difference is part of the reason that clay particles are more reactive than silt particles. Clays also exhibit varying levels of consistency and engineering behavior and demonstrate various levels of plasticity and cohesiveness in the presence of water. Clay minerals have a unique sheet particle structure and a crystalline layer structure that is amenable to significant isomorphous substitution. As a result of this isomorphous substitution, clay mineral surfaces carry a significant negative surface charge that can attract positively charged ions and dipolar water molecules. The cumulative effect of high surface area and surface charge makes clay particles particularly reactive with water, and is the root cause of the propensity of clay particles to shrink and swell depending on the availability of water. Due to a higher reactivity of fine fractions in soils, altering their physio-chemical properties by using chemical stabilizers/modifiers is often a more effective form of durable stabilization than mechanical stabilization in subgrade soils where the concentration of finer fractions is dominant. The fines content can become significant when as little as 10 percent of the soil is comprised of fines, smaller than about 75 μm. Soil characteristics including mineralogy, gradation and physio-chemical properties of fine-grained particles can all influence soil-additive interactions. Hence stabilizer selection should be based on the effectiveness of a given stabilizer in improving the physio-chemical properties of the selected soil. MECHANISMS INVOLVED IN CHEMICAL STABILIZATION Chemical stabilization using traditional, calcium-based stabilizers involves mixing or injecting the soil with chemical compounds such as Portland cement, lime or fly ash. Traditional stabilizers generally rely on pozzolanic reactions and cation exchange to modify and/or stabilize soil properties. Lime-soil reactions are complex and involve multiple, synergistic processes. These reactions can broadly be grouped into two parts: initial and longer-term. The initial reactions involve cation exchange and flocculation/agglomeration of soil particles that result in textural and plasticity changes in the soil. These
37 processes also make the soil more friable and workable. Longer-term reactions involve interactions between free lime (Ca(OH)2) and soil particles. These interactions are referred to as pozzolanic as they involve pozzolans, the alumina and silica made available from the soil by the high pH lime-water solution. When these pozzolans react with free lime and water, a cementing effect among particles as well as an alteration of surface mineralogy occurs. These pozzolanic reactions contribute to an increase in strength which can be considerable depending on the degree of pozzolanic reaction in lime-soil mixtures. These pozzolanic reaction products, calcium-silicate-hydrates and calcium-aluminate-hydrates, are similar to the cementitious products formed when Portland cement hydrates. These pozzolanic reaction processes are slow when compared to flocculation/agglomeration reactions in soil. Therefore, mellowing periods, normally about one-day, but up to about 4-days, are sometimes prescribed to maximize the effect of short term reactions in reducing plasticity and increasing workability. The mellowing period also affords time for re-mixing after initial reactions have taken place. This can result in more intimate mixing and a more thorough degree of modification prior to compaction. However, the pozzolanic reaction process may progress relatively quickly in some soils depending on the reactivity of the soil minerals with lime or if a cementitious hydration reaction, such as with Portland cement or some class C fly ashes, accompanies the pozzolanic reaction. In this case, the pozzolanic reaction as well as cation exchange contributes to plasticity reduction. In fact, long term plasticity reduction is primarily due to pozzolanic effects. The extent of formation of pozzolanic reaction products depend primarily on the rate and degree of dissolution of the soil minerals from the soil matrix. The pozzolanic reaction process can therefore be modest or quite substantial depending on the mineralogy of the soil. Maintaining a high enough pH condition, generally agreed to be above 10.5, is required in order to solubilize soil pozzolans that participate in these reactions. Portland cement is comprised of calcium-silicates and calcium-aluminates that hydrate to form cementitious products. This cementitious reaction is the primary mode of strength gain in soil cement. Free lime, Ca(OH)2, produced during the hydration process can comprise up to about 25 percent of the cement paste (cement and water mix) on a weight basis. This free lime can produce a concomitant pozzolanic reaction between the lime and soil, which can continue as long as the pH is high enough to solubilize the soil minerals. Cement hydration is rapid and causes immediate strength gain in stabilized layers. Therefore, a mellowing period is not typically allowed between mixing and compaction. The general practice is to compact soil cement before or shortly after initial set, preferably within 2 hours of mixing. An extended mellowing period, beyond 2 to 4 hours, may be acceptable if an improved uniformity of the mix is required. The soil-cement mixture, in this case, should be
38 remixed after the mellowing periods to achieve a homogeneous mixture before compaction. The ultimate strength of a soil-cement mixture with an extended mellowing period may be lower when compared to mixtures where compaction is achieved before initial set. It is important to realize that intimate mixing after extended mellowing may require equipment with more power than is normally used in mixing due to the cementitious effect. Traditional stabilizers such as fly ash and by-product stabilizers like lime kiln dust and cement kiln dust also rely on pozzolanic reactions and cation exchange to modify and/or stabilize soil properties. Each of these by products may be highly variable. Based on AASHTO M 295, fly ashes can be classified as either class C (self-cementing) or class F (non-self cementing) fly ash. Class C fly ash contains a substantial amount of lime, CaO, most of which is combined with glassy silicates and aluminates in the ash. Upon mixing with water, hydration reactions produce free lime that either can combine with other unreacted pozzolans, silicates and aluminates, available within the fly ash, or may react pozzolanically with silicates and aluminates available in soil to form cementitious reaction products. Formation of these cementitious products contributes to strength gain in fly ash stabilized soils. Pozzolanic reactions between soil particles and free lime released from the ash during the hydration process can alter soil properties and increase strength just as they do in soil cement. Class F fly ash contains very little lime, CaO, when compared to a class C fly ash and most of the glassy silica and/or alumina exists as pozzolans in the ash. Activation of these pozzolans requires additives such as Portland cement or lime, which provide a sufficient source of free lime. The pozzolanic reactions that occur when this fly ash-activator blend is mixed with water form the products that bond soil grains or agglomerates the soil particles together to develop strength within the soil matrix. Kinetics of the cementitious reactions and pozzolanic reactions that occur in fly ash stabilized soils vary with the type of ash and with the composition of ash used in stabilization. Therefore, the allowable compaction time for fly ash soil blends vary with the type of ash and depends on whether or not an activator is used. However, the standard practice is to compact the mixture within 6 hours of initial mixing. Lime kiln dust and cement kiln dust are by-products formed during the production of lime and Portland cement. As such they may be highly variable. Lime kiln dust (LKD) normally contains between about 30 to 40 percent lime which may either be free lime or combined with pozzolans in the kiln. LKDs may be somewhat cementitiously and/or pozzolanically reactive because of the presence of pozzolans or they may be altogether non reactive due to the absence of pozzolans or due to the low quality of the pozzolans contained in the LKD.
39 Cement kiln dust (CKD) generally contains between about 30 and 40 percent lime and about 20 to 25 percent pozzolanic material. CKD is more likely to contain reactive calcium-silicates and/or calcium-aluminates and/or pozzolans when compared to LKD and therefore may be able to support some level of cementitious and/or pozzolanic reactivity. SOIL EXPLORATION, SAMPLING AND CLASSIFICATION Soil exploration involves assimilating information regarding conditions of the underlying strata that can affect the performance of pavement structures. This also involves recovery of representative soil samples for classification and testing purposes. Successful soil exploration requires careful consideration of certain selected features as listed and discussed in the following sections and subsections. Preliminary data collection involves acquiring all pertinent information that can influence the outcome of a stabilization project. Geological and pedological information from the location provides the basis for differentiating earth materials and to identify problem areas. A pedological approach to soil classification can be used as a basic approach in assessing the impact of the soil profile on the stabilization project. The reactivity of soils with calcium-based stabilizers is known to vary with depth within the pedological profile or with the soil horizons based on the changes in mineralogy and/or soil chemistry within these profile horizons. Furthermore, pedological profile data provides pertinent information for assessing the presence and form of minerals, such as sulfates or certain sulfide forms that might deleteriously affect the modification or stabilization process. Estimates of the soil compositional characteristics based on pedological profiles can be acquired from the National Resources Soil Conservation Service (NRSCS) county soil survey reports. Data from these reports can be used as a guideline for sampling efforts and to identify the required depth and frequency of sampling as well as to establish the expected results of the sampling process. Geological data, which is available from the NRSCS, the U.S. Geological Survey (USGS) and the State Geologic Survey reports, is valuable as it provides a basis from which to interpret the impact of land forms and to identify materials below the soil layer that might impact the stabilization process via migration or diffusion of ions with moisture fluctuations. Data from geological documents along with the soil surveys are particularly useful in identifying the presence of sulfate seams below the surface that can provide a source for sulfate diffusion into the stabilized layer. If potential sources of sulfates are identified during preliminary data collection, a risk assessment should be made prior to undertaking the stabilization process and
40 should be performed in accordance with the AASHTO standard recommended practice for stabilizing sulfate rich soils. Subsurface investigations should be guided by the purpose, requirements, and geographical settings of the project location. This involves reviewing all available data, described in Section 7.2, regarding the project location prior to beginning the field investigation and collecting and reviewing all pertinent information from investigation reports from adjoining projects, if available. A comprehensive exploration plan should be developed as a part of subsurface investigation to communicate the intent and level of testing required for the project. Subsurface investigation should be conducted in accordance with AASHTO R 13. The success of a subsurface investigation depends primarily on the effectiveness of the geotechnical engineers and technicians involved in field operations and therefore should only be performed by responsible, well-trained and experienced people. Direct observation of subsurface conditions and retrieval of field samples can be achieved by examination of soil formations from accessible excavations, such as shafts, tunnels, test pits, or trenches, or by drilling and sampling to obtain cores or cuttings. A properly designed sampling plan should be developed to minimize sampling error and optimize sampling efficiency. Samples should be taken in a manner that minimizes the bias of the person taking the samples. The sampling plan is required to randomize sampling locations. Sample units of roadway materials should be selected randomly in accordance with ASTM D 3665. The number of field samples to be collected depends on the level of confidence required by project specifications. Guidance in determining the number of samples required to obtain the desired confidence levels are detailed in ASTM test methods E 105, E 122 and E 141. A sufficiently large quantity of soil should be sampled to allow adequate testing to determine engineering properties of the soil that are pertinent to the proposed design (e.g. mix design). Undisturbed soil samples are not usually required to evaluate the efficacy of soil stabilization as stabilization operations involve mixing and compaction operations that destroy the original soil fabric. Therefore, any mechanical disturbance of samples during extraction does not normally compromise the quality of the sample or its acceptability for testing. Frequency of sampling should be based on the continuity of soil and rock formations observed during subsurface investigations. Sample should be collected every time there is a change in observed physical characteristics of the soil.
41 Subsurface conditions identified at the individual test pits, boring holes or by examining open cut sections during subsurface investigations should be used as a guideline to decide the frequency of sampling. The uniformity of the soil and the potential for the soil profile to contain minerals that may cause a deleterious reaction with the stabilizer form the basis for determining the frequency of sample collection. A general recommendation regarding frequency of sampling based on varying soil conditions is given in Table below. Soil condition Frequency of sampling Uniform 0.5 to 1.0 mile Non-Uniform 0.25 to 0.5 mile Highly variable 1,000 ft to 0.25 mile Sulfate bearing 500 ft Sampling should be deep enough to identify all strata that can significantly influence the outcome of the stabilization. AASHTO R-13 recommends that the depth of exploratory borings or test pits for road beds be at least 5 feet below the proposed subgrade elevation. For cuts exceeding these depths, sampling should be done to the road bed depth plus an additional 2 feet. The spacing requirements and boring depths mentioned above should not be considered as either a minimum or a maximum, but instead should be used as a guide. The final decision regarding sampling should be based on field conditions and expertise of the geotechnical engineer such that the sampling operation and collected soil samples provide a basis for capturing all pertinent information regarding the engineering and hydro geologic properties relevant to the project design. GUIDELINES FOR SOIL STABILIZATION Soil-stabilizer interactions vary with soil type and so does the extent of improvement in soil properties as a result of stabilization. Hence the efficacy of using a stabilizer must be evaluated prior to the treatment. A generalized flowchart detailing the steps to be followed in evaluating the effectiveness of a stabilizer is presented below.
42 Soil Exploration/Sampling Soil Classification/Sieve Analysis/Atterberg Limits Sulfate Test Refer Sulfate Guidelines Additive selection Mix Design Evaluation of Properties Proceed to Construction Acceptable Base Material No treatment unless required for project Additive selection Mix Design Evaluation of Properties < 25 % passing No. 200 ⥠25 % passing No. 200 > 3000 ppm < 3000 ppm No Change Additive(s) if Yes No No Yes The key decision making factor prior to deciding stabilization techniques is classification of selected soil type.
43 Classification of soils should be done in accordance with AASHTO M 145. As a candidate for stabilization purposes, the soil should first be classified as either a subgrade category or base category material. This is based on the fraction passing No. 200 sieve. If 25 percent or more of the soil mass passes the No. 200 sieve, then the soil is classified as a âsoilâ. Otherwise, it is classified as a base for stabilization purposes. Base materials must also satisfy plasticity and gradation requirements and restrictions which vary among agencies. Base materials that qualify can be used directly for field applications without stabilization (Note 1). Note 1. In this standard, it is assumed that the candidate aggregate base material is of at least moderate quality, otherwise, the material should be treated as a soil. Moderate quality is defined as: (1) not more than 40 percent finer than the no. 40 sieve (0.425 mm or 0.0165 in.), (2) a maximum plasticity index of 12 percent, and (3) a maximum liquid limit of 40 percent. The second factor to be considered when using traditional soil stabilizers is the presence of and the concentration of sulfate salts and organic materials in the soil. Water soluble sulfate levels in soils should be identified in accordance with the modified version of AASHTO T 290 or equivalent test methods. Soils with sulfate levels above 3,000 ppm have the potential for formation of significant levels of expansive minerals, like ettringite and/or thaumasite, which may result in disruptive volume changes within the stabilized layer. The standard practice for stabilization of high sulfate soils should be consulted if the soluble sulfate level exceeds 3,000 ppm. Screening for organic contents in soil should be done in accordance with ASTM D 2974. Soils with an organic content of greater than 1 percent as determined by ASTM D 2974 may be difficult to stabilize or uneconomical quantities of additives may be required in order to stabilize them. However, the impact of the organic content varies considerably with the type of organic present, and a full testing regime is usually required to assess the impact. TECHNIQUES AND GUIDELINES FOR STABILIZER SELECTION Characteristics of fine-grained soils including mineralogy, gradation and physio-chemical properties influence the soil-additive interaction. The preliminary selection of the appropriate additive(s) for soil stabilization should therefore be based on soil index properties (i.e., particle size (sieve) analysis and Atterberg limit data) and should be identified following the test procedures detailed below. Soil samples should be prepared following AASHTO T 87. The initial processing of most soils requires thorough air drying or assisted drying at a temperature not exceeding
44 60oC (140oF). Aggregations of soil particles should be broken down into individual grains to the extent possible. A representative soil fraction should then be selected for testing following AASTHO T 248 which should be used to determine index properties of the soils. Sieve analysis should be performed following AASHTO T 27. Liquid limit testing should be performed following AASHTO T 89 and plastic limit and plasticity index testing should be measured following AASHTO T 90. Plasticity index (PI) and the percent passing the no. 200 sieve (percent smaller than 75 μm) are index properties that have been successfully used to identify the appropriate stabilizer for a given soil. This process should be performed following the decision tree for stabilizer selection presented in Sections 9.3 and 9.4. Individual agencies are encouraged to use these decision trees as guidelines and to incorporate local experience into revise and improve these decision trees. The decision tree for selecting stabilizers for use in subgrade soils is given in figure below. Sieve Analysis ⥠25% Passing No. 200 sieve Subgrade Atterberg Limits PI < 15 PI ⥠3515 ⤠PI ⤠35 Cement Asphalt (PI< 6) Lime-Flyash (Class F) Flyash (Class C) Lime Lime - Cement Lime â Flyash (Class F) Flyash (Class C) Cement Lime Lime - Cement Lime-Flyash (Class F) Lime - Flyash (Class C) The decision tree for selecting stabilizers for use in Base materials is given in figure below.
45 Sieve Analysis < 25% Passing No. 200 sieve Base Material Atterberg Limits PI ⤠12 PI ⥠12 Lime Cement Asphalt (PI< 6) Flyash (Class C) Lime Cement Lime-Cement Lime â Flyash (Class F) Flyash (Class C) Figures in sections 9.3 and 9.4 identify more than one potential stabilizer for a given soil. Additional guidelines detailed in the following sub sections may also be used to refine the process of stabilizer selection prior to validation testing. These guidelines specify the stabilizer (lime, Portland cement, or fly ash) best suited for the soil in question. Lime has been found to react successfully with medium, moderately fine and fine grained soils resulting in a decrease in plasticity and swell potential of these soils. Lime is an appropriate stabilizer for most cohesive soils but the level of reactivity depends primarily on the type and amount of clay minerals in the soil. Soils with a plasticity index of 10 or greater and with a minimum of 25 percent passing the No. 200 sieve are typically considered to be candidates for lime stabilization. Lime is generally considered to be a suitable, if not the most suitable, stabilizer for soil types that belong to AASHTO classifications A-4, A-5, A-6, A-7 and some of A-2-6 and A-2-7 soils. Cement can be successfully used to stabilize a wide range of soils. However, it is particularly well suited to stabilize well graded soils that contain sufficient amount of fines to effectively fill the available voids space and float the coarse aggregate particles. Silty soils (A-2-4 to A-4) have been documented to derive the highest degree of improvement (when stabilized with Portland cement) among the soils amenable for cement stabilization. Most soil types, except those with high organic content, highly plastic clays and poorly reacting sandy soils, are amenable to stabilization with Portland cement.
46 Portland cement is generally considered to be a good candidate stabilizer for soils with less than 35 percent passing the no. 200 sieve. General gradation specifications limit the nominal maximum aggregate size at 2-inches with at least 55 percent passing the no. 4 sieve. For uniformly graded materials, the addition of non plastic fines like fly-ash, aggregate screenings, cement and lime kiln dust may help fill the voids in the soil structure and help reduce the required cement content. For fine-grained soils, the general consistency guidelines are that the plasticity index (PI) should be less than 20 and the liquid limit (LL) should be less than 40 in order to ensure proper mixing of cement and soil. A more specific guideline defining the upper limit of PI for soils is given in equation below: 4 )075.0(%5020. mmthansmallerIP â+⤠However, depending on the efficiency of mixing equipment and expectations of the stabilization process, soils with PIs above 20 percent may also be stabilized with Portland cement. The ability to stabilize soils with plasticity indices above about 20 using cement is based on the ability to intimately mix cement with the soil to a degree that will produce a reasonably homogeneous and continuous stabilized matrix of the agglomerates. This requires a certain efficacy of mixing, which is in turn associated with the energy imparted to the soil by the mixing equipment and by the time span over which mixing occurs. Fly ash can be used effectively to stabilize coarse grained particles with little or no fines. In these soils, fly ash generally acts as a pozzolan and/or filler to reduce the void spaces among larger size aggregate particles to float the coarse aggregate particles. Fly ash may be effective in silty soils or soils that have low clay content or when the clay is not pozzolanically reactive. Fly ash can help enhance the pozzolanic reactivity of these fine-grained soils. The presence of organic matter and sulfates in the soil to be treated or stabilized must be evaluated as part of the stabilizer selection process. Techniques for measuring sulfate contents in soils and recommendations for stabilizing soils with high sulfate contents are detailed in Sections 8.3.1 and 7.2.4. Soil organic content should be measured following procedures detailed in Section 8.3.2. Additional steps and considerations for soils with higher organic content are detailed in the mix design procedure for each specific stabilizer. It is important to understand that the decision trees and the guidelines presented in previous sections of the standard are only the first step towards stabilizer selection. Once the stabilizer is selected, specific laboratory tests are required
47 in order to determine whether the strength and performance characteristics of the stabilized soil are satisfactory. Specific test methods required to validate the use of a selected stabilizer are discussed in later sections of this standard. VALIDATION TECHNIQUES FOR STABILIZER SELECTION: LABORATORY TESTING METHODS Stabilization projects are site specific and the soil-stabilizer interactions vary with soil types. Therefore the extent of improvement in soil properties is dependent on the interactions of the selected stabilizer with the soil. The following sections and sub-sections detail the techniques for validating the use of individual stabilizers and outline the requirements for selecting individual stabilizer type for field applications. The preliminary stabilizer selection process is outlined in Section 9. In the lime stabilization process, the extent of lime-soil interactions depends on the type and amount of clay minerals present in the soil. The mix design protocol given below is designed to optimize the potential for long-term strength gain and durability of lime stabilized soils. The candidate soil for lime treatment should be identified following the steps detailed in Section 9.2. Organic content in soil should be determined by following steps detailed in Section 8.3.2 (Note 2). Note 2. If the organic content of the soil is above one percent, additional compression strength samples should be prepared and tested with higher lime contents, i.e., at least one to two percent above optimum lime content. The purpose of this testing is to determine whether the strength and durabilty of the lime-soil mixture can be enhanced with additional lime and that the additional lime compensates for the loss of free calcium due to adsorption of calcium by organic functionalities, which interupt cation exchange and pozzolainc reactons between calcium and the soils being treated. Water soluble sulfate levels in the soil to be treated with lime must be evaluated following steps detailed in Section 8.3.1. Soils with sulfate contents above 3,000 ppm may be considered problematic and should be addressed separately as detailed in the AASHTO Standard Recommended Practice for Stabilizing Sulfate Bearing Soils. The trial stabilization lime content should be determined following ASTM D 6276. For reliable test results, the lime used in the pH test should be the same as that to be used in construction and this lime should be carefully stored to avoid carbonation.
48 Lime-soil mixtures should be fabricated following ASTM D 3551 for moisture density and compressive strength testing. For compressive strength testing, samples are not required to mellow before fabrication unless it is difficult to achieve satisfactory homogeneity during laboratory mixing. If so, mellowing periods of up to 24 hours can be included to simulate field mellowing. The moisture-density relationship of the lime-soil mixtures should be evaluated for the amount of lime identified by the Eades and Grimm pH test, ASTM D 6276. The moisture-density relationship of lime-soil mixtures should be determined in accordance with AASHTO T 99. Triplicate samples should be prepared for compressive strength testing following ASTM D 5102, procedure B with the lime content determined following ASTM D 6276 (Note 3). Samples should be fabricated at optimum moisture content (OMC) ± 1 percent. Note 3. It is recommended to prepare and test additional mixtures with lime contents one to two percent higher than the optimal lime content as determined following ASTM D 6276. Preparation and testing of these samples should also follow the procedures detailed in Sections 10.3.5 and 10.3.6. After compaction the test specimens should be wrapped in a plastic wrap and stored in an air tight moisture proof bag with about 10 ml of free water to ensure proper moisture for pozzolanic reactions. The specimens should be cured at 40oC (1040F) for 7 days before compression testing (Note 4). Note 4. Since the accelerated cure is not always a good approximation of strength gain at long term, normal cure, it is appropriate to subject one set of lime soil samples to normal cure for 28 days before compression testing. After the specified curing period, the specimens should be removed from the storage bags and plastic wraps and then wrapped with a wet absorptive fabric or geotextile and placed on a porous stone for capillary soak (Note 5). A soaking period of at least 24 hours is recommended. But capillary soaking should continue for as long as it takes for the moisture front to move to the top of the sample or until the moisture front ceases to move. Note 5. During capillary soak, the water used in soaking should never come in direct contact with the specimen. The water level should be maintained at the top of the porous stone and kept in contact with the fabric wrap. Samples prepared for compression testing may also be used to evaluate volume changes in lime stabilized expansive soils (Note 6). Vertical and circumferential measurements of samples before and after soaking should be made to calculate volume changes between the dry and soaked conditions. A three dimensional volumetric expansion of up to 2 percent is typically regarded as acceptable.
49 Note 6. This step is applicable only for expansive clay soils and sulfate bearing soils. If the expansion in the treated soil is higher than the recommended value, then 1 to 2 percent additional lime should be added and procedures detailed in Sections 10.3.5 to 10.3.11 should be repeated. Swell test can also be used to determine the extent of swelling in sulfate bearing soils. But the period of exposure to moisture required for sulfate bearing soils is considerable longer and should be continued until swelling ceases. Additional samples should therefore be prepared for swell testing in case of sulfate bearing soils. These samples should be prepared in accordance with steps discussed in in Section 10.3.5 and 10.3.6. Following capillary soaking, the specimens should be subjected to unconfined compression strength testing in accordance with ASTM D 5102 procedure B. The suggested minimum requirements for lime stabilized soils are given in table below. Anticipated Use of Stabilized layer Compressive strength recommendations for different anticipated conditions (psi) Extended Soaking for 8 Days Cyclic Freeze-Thaw 3 Cycles 7 Cycles 10 Cycles Sub-base Material Rigid Pavements/Floor Slabs/Foundations 50 50 90 120 Flexible Pavement (>10 in.) 60 60 100 130 Flexible Pavement (8 in. - 10 in.) 70 70 100 140 Flexible Pavement (5 in. - 8 in.) 90 90 130 160 Base Material 130 130 170 200 For cyclic moisture conditioning the samples should be made to reach 80 percent saturation upon âwettingâ followed by 50 percent saturation upon âdryingâ. The strength guidelines in section 10.3.12 are minimum vales. If more than one lime content is considered in compression testing, then the lime content that produces the highest strength and durability, and of course that meets minimum strength requirements, should be selected as optimum. The compressive strength values in Table 10.3.12 may vary depending on the purpose of stabilization, exposure conditions, expected number of freeze-thaw cycles and the insulation effect of pavement layers over the stabilized layer.
50 The purpose of adding lime as a stabilizer for base materials is for lime to interact with the fine material to form a matrix and provide improved strength, stiffness, and durability for the aggregate base. Since the fine material (smaller than 75 µm material) comprises no more than about 10 percent of the of the entire mixture by weight, the amount of lime used by weight of the total aggregate base will be considerably less than that used in soils. The following steps should be followed for base course stabilization with lime. Add incremental concentration of lime to the base materials, generally starting with 1 percent by weight of the entire mixture and increasing in 1 percent increments to a maximum of 4 percent. Moisture density relationships for each aggregate-lime blend should be determined following either AASHTO T-99 or AASHTO T-180 based on agency requirements. Determine unconfined compressive strengths of the lime-aggregate blends following curing for 7-days at 400C (1040F) followed by capillary soak. Procedures described in the Section 10.3.10 should be used to moisture condition the samples prior to compressive strength testing. The compressive strength testing procedure and target compressive strength requirements should be based on specifications defined by the user agency. In the absence of such guidelines, the residual strengths in the table in section 10.3.12 can be applied. The lowest lime content that meets the compression strength requirement is considered as the required lime content for stabilization purposes. Steps involved in developing mix designs for Portland cement stabilized base materials are given in the following sub sections. Soils must be screened for organic content following Section 8.3.2. Soils with more than two percent organic content are usually considered unacceptable for cement stabilization. If the presence of organic mater is detected in soils, a pH test should be conducted to verify if organic matter will interfere with the hydration processes in soil. In the pH test, a soil-cement paste should be prepared by mixing 10 parts soil to one part cement (by weight). Determine the pH of the paste after 15 minutes of mixing. If the pH is 12.0 or higher after 15 minutes of mixing, then it is probable that organics will not interfere with the normal cement hydration process. If not, then a higher cement content than those recommended based on AASHTO soil groups given in Section 10.4.3.1 may be required for stabilization. Soils must also be screened for sulfate content prior to using Portland cement for stabilization. Sulfate contents in soil should be determined in accordance with AASHTO T 290 (modified) test method or equivalent test methods.
51 If the soluble sulfate concentration of the soil to be treated is greater than 3,000 ppm, then the standard practice that deals with stabilization of high sulfate soils should be consulted before proceeding. The standard practice for stabilization of high sulfate soils identifies the risk associated with stabilization of these soils and describes steps to reduce the risk of treatment of these soils. As a first step towards identification of cement content for stabilizing a given soil, classify the soil based on AASHTO M 145. A preliminary estimate of the cement content should then be made based on the following table. AASHTO Soil Groups Usual Range in Cement Requirement Estimated Cement Content, Percent by Weight Percent by Volume Percent by Weight A-1-a 5-7 3-5 5 A-1-b 7-9 5-8 6 A-2 7-10 5-9 7 A-3 8-12 7-11 9 A-4 8-12 7-12 10 A-5 8-12 8-13 10 A-6 10-14 9-15 12 A-7 10-14 10-16 13 These cement contents are only preliminary estimates and are proportioned on a weight basis in terms of the percent of oven dry soil. These estimates should further be verified or modified based on additional testing detailed in the following sections. It is important to understand that the estimated cement contents given in the table in section 10.4.3.1 are based on durability tests, ASTM D 559 and D 560, and that many soils can be successfully stabilized with considerably lower cement contents. After the required amount of cement is added to the soil, the blend should be mixed thoroughly until the color of the mixture is uniform. Soil samples used to establish the moisture density relationship should be fabricated following AASHTO T 134. The median cement content as estimated in Section 10.4.3.1 should be used to determine moisture density relationships of soil cement mixtures. However, if it is expected that acceptable treatment can be achieved with considerably lower cement contents, then that cement content should be used to determine the moisture-density relationship. Three cement contents should be used to determine the compressive strength of soil cement mixtures. Compression test samples should be prepared with the median cement content and with cement contents two percent above and below the median content. Sample should be prepared following AASHTO T 134.
52 Preparation and curing of samples for compressive strength testing should follow ASTM D 1632. The samples should be moist cured through out the curing periods and immersed in water for four hours prior to compression testing. Compressive strength testing should follow ASTM D 1633. Typical acceptable ranges of unconfined compressive strength criteria of moisture conditioned soil cement specimens for various soil classifications are given below. Soil Type AASHTO Classification Soaked Compressive Strength (psi) 7 Days 28 Days Sand and A-1, A-2, A-3 300-600 400-1,000 Silty A-4, A-5 250-500 300-900 Clayey A-6, A-7 200-400 250-600 Strength requirement for stabilized layers may vary considerably from agency to agency. A range of compressive strengths that are typically considered to be acceptable lower limits are presented in the table below. Purpose of Stabilized Layer Minimum 7 day Unconfined Compressive Strength (psi) Flexible Pavement Rigid Pavement Base Course 300 - 750 300 - 500 Sub base, select material or subgrade 250 200 The lowest cement content in the mixture design that meets the requirements in Section 10.4.11 or that satisfies the agency requirements should be used as the design cement content. If the tested samples does not confirm to these minimum requirements, then higher cement contents may be added to soil and Sections 10.4.4 to 10.4.9 should be repeated until the strength values meet minimum strength and durability requirements. In environments where significant freeze-thaw activity is expected, durability testing following AASHTO T 136 can be used. In this case the freeze-thaw activity is mimicked in the laboratory by freeze-thaw cycling as described in AASHTO T 136 but a residual strength criterion following the freeze-thaw activity is used in lieu of loss after brushing. The residual strength requirements should be established by the user agency, but in the absence of such criteria, the values in section 10.4.10 may be used. In cement stabilization of base materials, the fine material (materials smaller than 75 µm) comprises no more than about 10 percent of the entire mixture by weight. Therefore, the amount of cement used by weight of the total aggregate base will be considerably less than that used in soils. Steps to be followed in successful stabilization of base materials are detailed below.
53 Add incremental concentrations of cement to the base materials, generally starting with 1 percent by weight of the entire mixture and increasing in 1 percent increments to a maximum of 3 percent. Moisture density relationships for each aggregate-lime blend should be determined following either AASHTO T-99 or AASHTO T-180 based on agency requirements. Determine the unconfined compressive strengths of the cement-aggregate blends following moist cure for 7-days followed by a 4 hours soak as recommended by ASTM D 1633. The compressive strength testing procedures and the target compressive strength requirements should be based on specifications defined by the user agency. The lowest cement content that meets the compression strength requirement is considered as the required cement content for stabilization purposes. Fly ash can be effectively used to stabilize coarse grained soils or bases. In coarser aggregates where a substantial improvement is strength is the target, fly ash generally acts as a pozzolan and/or filler to reduce the void spaces among larger size aggregate particles and to float the coarse aggregate particles. In fine-grained soil, fly ash is typically used as a standalone stabilizer or in conjunction with lime or cement to enhance the reactivity of the fine-grained soil with lime or cement. Steps involved in developing mix designs for use of fly ash in soils and base materials are given in the following sub sections. The fly ash to be used in stabilization should be classified as either class C fly ash or a class F fly ash following AASHTO M 295. The cementitious properties of the fly ash to be used in stabilization should be characterized following ASTM D 5239 (Note 7). Note 7. ASTM D 5239 does not evaluate the interaction between fly ash and soil or aggregate. This must be verified separately based on mix design procedures outlined in the following paragraphs. Selection of the optimal fly ash content for soil modification or stabilization is a function of the purpose of stabilization; i.e., whether the purpose is to achieve maximum strength or to achieve a target level of strength. To achieve maximum strength and durability, fill the voids in the matrix with fly ash to achieve maximum density and then determine the moisture density relationship for this optimal blend. To achieve a target level of strength, experience or a trial and error process should be used to identify trial fly ash percentages and activator contents. These estimates are then used to establish moisture density relationships and to determine the compressive strength of the mixture.
54 Steps for developing effective mix designs in soils using class C fly ash are detailed in sub sections 10.5.4.1 through 10.5.4.4. Development of mix designs to meet the strength requirement is dependent on moisture density and moisture strength relationships. Compactive energies should be selected based on the intended application of these materials. Mix and mold the samples to be used for moisture-density determination following ASTM C 593. Determine the moisture-strength relationship for all the selected mixes. To determine the moisture-strength relationships, compact the samples at different moisture contents to determine the moisture content that will produce the maximum compressive strength (Note 8). Prepare compressive strength specimens by blending soil, fly ash, and water and mold the specimens after the specified compaction delay. The compaction delay should ideally be kept below 2 hours. Sample preparation and testing should be done in accordance with ASTM C 593. Cure the specimens for 7 days at 38oC (100oF) in accordance with ASTM C 593 before compression testing. Note 8. Optimum moisture content for highest strength gain may typically be 1â8 percent below optimum moisture content needed to attain maximum dry density. This value may vary with soil type and the mineralogy of ash particles. The strength developed by adding the class C fly ash may be sufficient. However, if substantially higher strength development is required, the addition of an activator may optimize the pozzolanic activity and substantially increase the strength and durability of the mixtures. Add activator (lime or Portland cement) using a trial and error approach to the mixture of soil, fly ash and water. Add the activator in 2 percent increments beginning with an activator content of 2 percent. The goal is to determine the amount of activator required to maximize the pozzolanic reaction based on strength gain. The activator content required to develop maximum pozzolanic activity may be as much as 20 to 40 percent of the ash, and hence it may be necessary to replace some of the fly ash with the activator. The following recommendations are applicable to cases where class C or class F fly ash is added to fine-grained soils that are not pozzolanically reactive, meaning soils where lime is effective in reducing plasticity and improving workability but not in providing the target strength. Determine the lime content required for soil stabilization based on the Eades and Grimm pH test, ASTM D 6276.
55 Add increasing concentrations of fly ash to the soil with the designated amount of lime, as determined in the preceding section, starting with 4 percent by weight of the mixture and at increments of 2 percent. Follow the steps outlined in sections 10.3.7 through 10.3.10 and 10.3.12 for compressive strength and durability testing (Note 9). Note 9. Fly ash, activator, and soil samples may also be conditioned following agency directives prior to compression strength and durability testing. The conditioning processes vary from the severity of vacuum saturation as prescribed in ASTM C 593 to a capillary soak as described in section 10.3.10. A compressive strength test following moisture conditioning prescribed by the agency to mimic the climatic conditions of the region may also be considered acceptable. When fly ash, normally class F fly ash, is used to stabilize base course materials (pozzolanic stabilized mixture or PSM), the goal may be either to achieve maximum density and optimal strength or to achieve a target strength. Determine the optimum fly ash content or target fly ash content for the mixture following a trial and error process to achieve a maximum dry density for the mixture. Determine the optimum moisture content of the selected mixture following ASTM C 593. Determine the optimum activator content or target activator content for the selected blend by trial and error method (Note 10). Prepare at least three replicates for compression testing for each blend of fly ash and activator. Note 10. Required lime or Portland cement contents to activate Class F fly ash are typically between one part lime/cement to three parts fly ash to 1 part lime/cement to four parts fly ash. If lime kiln dust (LKD) or cement kiln dust (CKD) is used as activator, then higher activator ratios may be required. Cure the fly ash-lime-soil mixes for 7-days in sealed containers. Samples prepared with lime and kiln dust activators are cured at 37.8oC (100o F) for 7-days. Moisture conditioning prior to testing should be performed following the protocol presented in sub sections of 10.3.10 (Note 9). Cure the fly ash-cement mixes at a 100 percent relative humidity environment at 22.8oC (73o F) for 7-days. The samples should be moisture conditioned by soaking in water for 4 hours prior to testing. Determine the compressive strength of the blends following test methods ASTM C 593 and ASTM D 1633. The conditioning processes prescribed by the agency to mimic the climatic conditions of the region or capillary soak as described in section 10.3.10 may also be considered acceptable. Compare the compressive strength values with target values to determine if the blend produces acceptable strengths to function properly under the design loading and environmental conditions.
56 Typically, a 7-day compressive strength of 400 psi is considered acceptable for field applications. But this requirement may vary with field conditions and among agencies. A mix that attains the required properties with the lowest percentage activator is selected as the design mix for use in field. Durability testing should be performed at the end of curing periods based on specifications defined by the user agency (Note 9). In areas where there is no freeze-thaw effect, durability testing may be waived in accordance with local practice. REPORT The report for stabilization of soils and base materials should include: Identification of sampling locations, details of locations of test pits and bore holes, and details of all other sampling sources used to obtain soil for test purposes. Details of subsurface conditions identified at the individual test pits, boring holes or by examining open cut sections during subsurface investigations. Approach used to select the stabilizer and method used to validate the selection of the stabilizer and perform mixture design. Tabulation of test data supporting the stabilization decisions. PRECISION AND BIAS This standard provides qualitative data only; hence, precision and bias are not applicable. KEYWORDS Soil stabilization; mineralogy; traditional stabilizers; soil sampling REFERENCES: American Coal Ash Association, âFly Ash Facts for Highway Engineers.â Federal Highway Administration, FHWA Report No.IF-03-019, Washington DC (2003). American Coal Ash Association, âSoil Stabilization and Pavement Recycling with Self- Cementing Coal Fly Ash.â American Coal Ash Association Education Foundation, Colorado (2008). American Concrete Institute, âState-of-the-Art Report on Soil Cement.â ACI 230.1R-90, ACI Materials Journal, Vol. 87, No. 4, (1990), pp. 23. ASCE, âTechnical Engineering and Design Guides as Adapted from US Army Core of Engineers, No. 30 - Soil Sampling.â American Society of Civil Engineers, Virginia (2000).
57 Little, D. N. âEvaluation of Structural properties of Lime Stabilized Soils and Aggregates.â http://www.lime.org/soil.pdf (March 2008). Little, D. and S. Nair, âBackground for Development of Standard Practice for Modification and Stabilization of Subgrade Soils and base Courses.â NCHRP 20-07, (2008). National Lime Association, âTechnical Brief: Mixture Design and Testing Procedure for Lime Stabilized Soils.â http://www.lime.org/Oct2006VerMix Design.pdf (March 2008). Portland Cement Association, âSoil-Cement Laboratory Handbook.â Portland Cement Association, Illinois (1992). Terrel, R. L., J. A. Epps, E. J. Barenberg, J.K. Mitchell, and M. R. Thompson, â Soil Stabilization in pavement Structures - A User Manual.â FHWA Research Report No. FHWA-IP-80-2, WA (1979). Texas Department of Transportation, âGuidelines for Modification and Stabilization of Soils and Base for Use in Pavement Structures.â ftp://ftp.dot.state.tx.us/pub/txdot-info/cmd/tech/stabilizati on.pdf (March 2008) (March 2008). United Facilities Criteria (3-250-11). âSoil Stabilization for Pavements, TM 5-822- 14/AFJMAN 32/1019.â (2004). http://www.wbdg.org/ccb/DOD/UFC/ufc_3_250_11.pdf (Jul. 16, 2006).
