SOIL STABILIZATION AND DRYING USING FLY ASH by H. A. Acosta, T. B. Edil, and C. H. Benson Geo Engineering Report No. 03-03 Geo Engineering Program University of Wisconsin-Madison Madison, Wisconsin 53706 USA January 5, 2003
i EXECUTIVE SUMMARY The objective of this study was to evaluate the effectiveness of stabilizing soft subgrade soils with self-cementing fly ashes. A laboratory testing program was conducted using seven soils and four fly ashes. The soils varied in terms of source, composition, and texture. The fly ashes differed in terms of geographical availability and composition. Both Class C and off-specification fly ashes (i.e., ashes that do not meet the C or F classification) were used for stabilization. All of the soils were fine-grained and represented poor subgrade materials. Test specimens were prepared with each soil using a range of fly ash contents (0, 10, 18 and 30%) at different soil water contents ranging from optimum water content to 18% wet of optimum water content. Specimens prepared at optimum water content were used as controls, whereas those compacted at higher water contents were intended to simulate the soft and wet subgrade conditions often observed in the field. Three tests were performed on each soil-fly ash mixture: California Bearing Ratio (CBR), resilient modulus (M r ), and unconfined compression. Tests were also conducted on the fly ash alone for comparison. All of the soils had very low CBR (0-5) in their natural condition. A substantial increase in the CBR was achieved when the soils were mixed with fly ash. Specimens prepared with 18% fly ash at optimum water content showed the best improvement, with CBRs ranging from 20 to 56. Specimens prepared with 18% fly ash and compacted at 7% wet of optimum water content showed significant improvement compared to the untreated soils, with CBR ranging from 15 to 31. Specimens prepared with 18% fly ash at very high water content (e.g., 18% wet of optimum) had lower CBRs (8 to 15), but were appreciably stronger than untreated soils at the same water content. Soil-fly ash mixtures prepared with 18% fly ash and compacted 7% wet of optimum water content had similar or higher resilient modulus than untreated specimens
ii compacted at optimum water content. The resilient modulus of specimens prepared at very high water contents generally had lower resilient moduli compared those prepared at optimum water content, but much higher resilient modulus than untreated soils having high water contents. The resilient modulus also increased with increasing curing time, with increases as large as 40% between 14 and 28 d of curing. Unconfined compressive strength of the soil-fly ash mixtures increased with increasing fly ash content. Soil-fly ash specimens prepared with 18% fly ash and compacted 7% wet of optimum water content had unconfined compressive strengths that were 4 times higher than those of untreated specimens compacted at the same water content. The effectiveness of fly ash stabilization generally depended on soil type. Greater increases in CBR and resilient modulus were obtained for inorganic soils having more fines and higher plasticity (i.e., poorer subgrade materials). However, the improvement in compressive strength was independent of soil type. Organic matter also affected stabilization. All but one of the fly ashes was ineffective in stabilizing the soft organic soil used in the testing program. The one fly ash that was effective had high organic content, which suggests that high carbon fly ashes may prove useful for stabilizing problematic organic soils and sludges The CBR and resilient modulus data were used to illustrate how stabilizing soft subgrades with fly ash can make flexible pavement designs more economical. Design calculations showed that the base course thickness could be reduced by as much as 40% when a soft subgrade is stabilized with fly ash.
iii ACKNOWLEDGEMENT Financial support for this study was provided by the US Department of Energy through the Midwestern Combustion Byproducts Recycling Consortium in Carbondale, Illinois, the University of Wisconsin-Madison Consortium for Fly Ash Use in Geotechnical Applications (funded by Mineral Solutions, Inc., Alliant Energy Corporation, and Excel Energy Services, Inc.), and the Wisconsin Department of Transportation. The opinions and conclusions described in the paper are solely those of the authors, and do not necessarily reflect the opinions or policies of the sponsors.
TABLE OF CONTENTS iv ABSTRACT... i ACKNOWLEDGEMENT...iii TABLE OF CONTENTS... iv LIST OF FIGURES...vii LIST OF TABLES... xi SECTION 1-INTRODUCTION...1 SECTION 2-BACKGROUND...3 2.1 NEED FOR SUBBGRADE STABILIZATION...3 2.2 WHAT IS FLY ASH?...3 2.3 TYPES OF COAL...6 2.4 TYPES OF FLY ASH...7 2.5 FACTORS AFFECTING PHYSICAL AND CHEMICAL PROPERTIES OF FLY ASHES...7 2.6 STABILIZATION EFFECTS ON CLAYS...11 2.7 REVIEW OF RECENT LIME AND FLY ASH STABILIZATION RESEARCH...12 2.7.1 Lime Treatment...12 2.7.2 Lime-Fly Ash Treatment...13 2.7.3 Fly Ash Treatment...15 SECTION 3-MATERAILS AND METHODS...21 3.1 FLY AHSES...21 3.1.1 Sources and Compositions...21 3.1.2 Index Properties and Compaction Characteristics...27 3.2 WISCONSIN SUBGRADE SOILS...29 3.2.1 Sources...29 3.2.2 Soil Index Properties...29 3.2.3 Compaction Characteristics and California Bearing Ratio...30 3.3 TEST PROCEDURES...35 3.3.1 CBR Test...35 3.3.2 Resilient Modulus Test...35 3.3.3 Unconfined Compressive Strength...42 SECTION 4-CALIFORNIA BEARING RATIO...46
4.1 CBR OF FLY ASHES...46 4.2 CBR OF UNTREATED SOILS...48 4.3 CBR OF SOIL-FLY ASH MIXTURES...51 4.3.1 General Effects of Fly Ash Stabilization...52 4.3.2 Effect of Soil Water Content...54 4.3.3 Effect of Soil Type...58 4.3.4 Effect of Organic Content...61 4.4 SYNTHESIS...66 v SECTION 5-RESILIENT MODULUS...67 5.1 RESILIENT MODULUS OF FLY ASHES...67 5.2 RESILIENT MODULUS OF UNTREATED SOILS...69 5.3 RESILIENT MODULUS OF SOIL-FLY ASH MIXTURES...75 5.3.1 General Effects of Fly Ash Stabilization...75 5.3.2 Effect of Soil Water Content...80 5.3.3 Effect of Curing Time...80 5.3.4 Effect of Soil Type...83 5.3.5 Effect of Organic Content...87 5.4 SYNTHESIS...99 SECTION 6-UNCONFINED COMPRESSIVE STREGTH...91 6.1 UNCONFINED COMPRESSIVE STRENGTH OF FLY ASHES...91 6.2 UNCONFINED COMPRESSIVE STRENGTH OF UNTREATED SOILS...93 6.3 UNCONFINED COMPRESSIVE STRENGTH OF SOIL-FLY ASH MIXTURES...95 6.3.1 General Effects of Fly Ash Stabilization...96 6.3.2 Effect of Soil Water Content...101 6.3.3 Effect of Curing Time...101 6.3.4 Effect of Soil Type...103 6.3.5 Effect of Organic Content...103 6.4 SYNTHESIS...109 SECTION 7-PRACTICAL APPLICATION...111 7.1 FLEXIBLE PAVEMENT DESIGN...111 7.2 BASE THICKNESS DESIGN COMPARISON...112 SECTION 8-CONCLUSIONS...117 8.1 CBR...117 8.2 RESILIENT MODULUS...118 8.3 UNCONFINED COMPRESSIVE STRENGTH...119
8.4 PRACTICAL APPLICATIONS FOR FLEXIBLE PAVMENT DESIGN...119...... vi SECTION 9-REFERENCES...120 APPENDIX A...126 APPENDIX B...128 APPENDIX C...130 APPENDIX D...132
LIST OF FIGURES vii Fig. 2.1. Poor subgrade soils in Wisconsin (WisDOT 1997)...4 Fig. 2.2. Fig. 3.1 Fig. 3.2. Fig. 3.3. Comparison of a fly ash grain size distribution with those of several soils...5 Particle size distribution curves for Dewey, Columbia, King, and Edgewater fly ashes...26 Compaction curves for Dewey, Columbia, King, and Edgewater fly ashes...28 Location where soils were sampled...31 Fig. 3.4. Particle size distributions for the Wisconsin soils...33 Fig. 3.5. Compaction curves for Wisconsin soils...34 Fig. 3.6. Fig. 3.7. California Bearing Ratio test being conducted on soil-fly ash mixture...36 Resilient modulus soil-fly ash specimen being compacted inside a mold...37 Fig. 3.8. Specimen undergoing resilient modulus test...39 Fig. 3.9. Resilient modulus of synthetic specimens A and B...41 Fig. 3.10. Fig. 3.11. Fig. 4.1. Fig. 4.2. Unconfined compression test being conducted on soil-fly ash mixture...43 Comparison between Proctor and Harvard compaction methods...44 Stress-penetration data from CBR tests for different fly ashes prepared at 35% water-fly ash ratio for a curing period of 7 days...47 The CBR of soil-fly ash specimens normalized to CBR of untreated soil for soil-fly ash mixtures prepared at 7% wet of optimum water content. (RSCT (CBR = 5), LRC (CBR = 2), and BS (CBR = 3))...53
viii Fig 4.3. Fig. 4.4. Fig. 4.5. Fig. 4.6. Fig. 4.7. Fig. 4.8. Fig. 4.9. Fig. 4.10. Fig. 5.1. Fig. 5.2. Fig. 5.3. CBR gain as a function of fly ash content of red silty clay till, lacustrine red clay, and brown silt prepared with Columbia, Dewey, and King fly ashes compacted at 7% wet of optimum water content...55 CBR of specimens prepared with (a) Dewey (b) King fly ashes normalized to the CBR specimens prepared with Columbia fly ash as a function of fly ash content (Specimen compacted with 7% wet of optimum water content)...56 Effect of water content on CBR of soils prepared with 10% and 18% (a) Columbia and (b) Dewey fly ash...57 Effect of water content on CBR for red silty clay till (CL) and brown silt (CH) at 10% Columbia and Dewey fly ashes...59 CBR as a function of soil water content (w SOIL - w OPT ) and fly ash content...60 Effect of (a) liquid limit (LL) and (b) plasticity index (PI) on the CBR ratio of soil-fly ash specimens prepared 7% wet of optimum water content and 9-12% wet of optimum water content, normalized to the CBR of the untreated soil specimen...62 Effect of (a) group index (GI) and (b) liquidity index (LI) on the CBR ratio of soil-fly ash specimens prepared 7% wet of optimum water content and 9-12% wet of optimum water content, normalized to the CBR of the untreated soil specimen...63 CBR of organic Theresa Silt Loam with three fly ashes...65 Resilient modulus versus (a) deviator stress and (b) bulk stress for fly ashes prepared at 35% water fly ash ratio for a curing period of 14 days...68 Resilient modulus of untreated soil specimens compacted at optimum water content as a function of (a) deviator stress and (b) bulk stress...70 Comparison between resilient modulus estimated from the CBR (7 days curing) and measured resilient moduli (14 days) at a deviator stress of 21 kpa and confining pressure of 21 kpa.
The conventional Modulus (E DYN ) CBR relationship reported by Heukelom and Foster (1960) is shown as the solid line....74 ix Fig. 5.4. Fig. 5.5. Fig. 5.6. Fig. 5.7. Fig. 5.8. Fig. 5.9. Fig. 5.10. Fig. 5.11. Fig. 5.12. Fig. 6.1. Resilient modulus of three soils prepared with Columbia, Dewey, and King fly ashes compacted at 7% wet of optimum water content and cured for 14 days...76 Moduli of the untreated soil compacted at W OPT to soil-fly ash mixtures prepared with Columbia, Dewey, and King fly ashes compacted at 7% wet of W OPT. Moduli of untreated soil at 7% wet of W OPT were estimated based on CBR...78 Moduli of soil-fly ash specimens prepared with (a) Dewey and (b) King fly ashes normalized to the moduli of the soil-fly ash specimen prepared with Columbia fly ash. All specimens cured for 14 days. (All resilient moduli are at deviator stress of 21 kpa)...79 Effect of water content on resilient modulus of several soils at 18% (a) Columbia and (b) Dewey fly ashes...81 Effect of water content on resilient modulus of several Plano Silt Loam (CL) prepared with 12% Columbia fly ash and cured for 7 days...82 Resilient modulus tested at different curing time (i.e.7, 14, 28, and 56 days) for red silty clay till prepared with Columbia and Dewey fly ashes at 18% fly ash content, normalized to the resilient tested at 14 days curing time...84 Effect of (a) liquid limit (LL) and (b) plasticity index (PI) in the resilient modulus ratio of soil-fly ash mixtures prepared at water contents at 7% wet of optimum and the very wet condition...85 Effect of (a) group index (GI) and (b) liquidity index (LI) in the resilient modulus of soil-fly ash mixtures prepared at 7% wet of optimum water content and the very wet condition...86 Resilient modulus of organic Theresa Silt Loam and in combination with 30% Dewey fly ash at different water content and cured for 7 days...88 Stress-strain data of unconfined compressive strength tests performed on fly ash specimens prepared at 35% water-fly ash
ratio for a curing period of 14 days...92 x Fig. 6.2. Fig. 6.3. Fig. 6.4. Fig. 6.5. Fig. 6.6. Fig. 6.7. Fig. 6.8. Fig. 6.9. Fig. 7.1. The unconfined compressive strength of soil-fly ash specimens normalized to the unconfined compressive strength of the untreated soil compacted at 7% wet of optimum water content...97 Strength gain as a function pf fly ash content for red silt clay till, lacustrine red clay, and brown silt prepared with Columbia, Dewey, and King fly ashes compacted at 7% wet of optimum water content...99 Unconfined compressive strength prepared with (a) Dewey and (b) King fly ashes normalized to the unconfined compressive strength prepared with Columbia fly ash as a function of fly ash content. (Specimens prepared at 7% wet of optimum water content)...100 Effect of soil water content on the unconfined compressive strength of soil-fly ash specimens prepared with 10 and 18% fly ash content and cured for 14 days....102 Effect of fly ash content and curing time on the unconfined compressive strength of soil-fly ash specimens prepared with Columbia, Dewey, and King fly ashes at 18% fly ash content...104 Effect of (a) liquid limit (LL) and (b) plasticity index (PI) on the unconfined compressive strength ratio of soil-fly ash specimens...105 Effect of (a) group index (GI) and (b) liquidity index (LI) on the unconfined compressive strength ratio of soil-fly ash specimens...106 Effect of fly ash content on unconfined compressive strength of organic Theresa Silt Loam prepared with Columbia and Dewey fly ashes in very wet conditions and cured for 7 days...107 Diagram of a typical flexible pavement structure with its four components...113
LIST OF TABLES xi Table 2.1 Table 2.2 Table 3.1 Table 3.2 Chemical requirements for fly ash classification...8 Typical chemical composition of fly ash...8 Physical properties and chemical properties of fly ash...22 Factors affecting physical and chemical properties of fly ash...23 Table 3.3 Chemical composition of fly ashes...25 Table 3.4 Index Properties of the soils...32 Table 3.5 AASHTO T 292-91 (1996) test sequence for cohesive soils...40 Table 4.1 Table 4.2 CBR of soil-fly ash mixtures compacted 2-hours after mixing and cured for 7 days prior to CBR test...49 CBR of soil-fly ash mixtures compacted 2-hours after mixing and cured for 7 days prior to CBR test...50 Table 5.1 Resilient moduli (MPa) of soil-fly ash mixtures compacted 2 hours after mixing at a deviator stress of 21 kpa...71 Table 5.2 Table 6.1 Table 7.1 Table 7.2 Resilient modulus coefficients K 1 and K 2 for soil and soil-fly ash mixtures...72 Unconfined compressive strength (kpa) of soil and soil-fly ash mixtures...94 Recommended subbase layer coefficients (a 3 ) of soil and soil-fly ash mixtures from CBR and resilient modulus tests at different water content...114 Base course thicknesses calculated using CBR and resilient modulus subbase layer coefficients (a 3 )...116
1 SECTION 1 INTRODUCTION Every year burning coal for production of energy in power plants in the United States produces over 72 Mg of coal ash, including bottom ashes and fly ashes (Collins and Ciesielski, 1992). Traditionally the fly ash has been disposed in landfills at considerable cost. However environmental regulations in many states now promote the reuse of fly ash and many other industrial by products, in a variety of applications. One application of considerable interest is stabilization of soft soils for roadway construction. Soft subgrade soils are a common problem in Wisconsin. The typical approach for remediating soft subgrade has consisted of removal of poor soil, and replacement with large quantities of crushed rock, also known as breaker run. The high cost for removal of poor soils and transportation of select aggregates, along with increasing interest in re-used industrial by products, has prompted investigations to find solutions that complement the needs of highway construction with those of the environment. Use of fly ash for stabilization of soft subgrade is one these solutions being evaluated. The objective of this study was to determine how the use of different types of fly ashes could improve the engineering properties of several soft Wisconsin soils. To achieve this objective, a laboratory-testing program was conducted where compacted soil-fly ash mixtures were prepared at several fly ash contents, and then tested to determine their engineering properties relevant to highway construction. The laboratory program included California Bearing Ratio (CBR),
2 resilient modulus, and unconfined compressive strength tests. Results of the tests were used to formulate guidelines for engineers designing pavements with fly ash stabilized soils in Wisconsin.
3 SECTION 2 BACKGROUND 2.1 NEED FOR SUBGRADE STABILIZATION The distribution of poor subgrade soils in Wisconsin is shown in Fig. 2.1. Nearly 60% of the surficial soils is classified as poor subgrade, with two-thirds of which are soft silts in the southern and central regions, and one-third of which is soft clay in the northern and eastern regions (Edil et al. 2002). Poor subgrade soils tend to have low shear strength and are highly compressible. As a result, they must be removed and replaced or stabilized before a highway can be constructed. One stabilization method is to mix poor subgrade soil with fly ash to improve the engineering properties. Reactions that occur in the soil-fly ash mixture result in lower water contents, higher shear strength, and lower compressibility. 2.2 WHAT IS FLY ASH? Fly ash is a by-product produced from burning coal in electric power plants. In 1993, approximately 43 million Mg of fly ash was produced in the United States (Palmer et al. 1995). A comparison of fly ash particles sizes to those of several types of soils is presented in Fig. 2.2. Fly ash is a fine residue composed of unburned particles that solidify while suspended in exhaust gases. Fly ash is carried off in stack gases from a boiler unit, and is collected by mechanical methods or electrostatic precipitators. Because it is collected from
4 Poor Subgrade Soil Good Subgrade Soil Fig. 2.1. Poor subgrade soils in Wisconsin (WisDOT 1997).
5 Fig. 2.2. Comparison of fly ash particles to those of several soils (from Meyers et al. 1976).
6 exhaust gases, fly ash is composed of fine spherical silt size particles in the range of 0.074 to 0.005 mm (Ferguson 1993). Fly ash collected using mechanical precipitators usually has coarser particles than fly ash collected using electrostatic precipitators. 2.3 TYPES OF COAL Electrical power generation produces fly ash from four major types of coal: bituminous, anthracite, sub-bituminous, and lignite. Bituminous and anthracite coals have low amounts of calcium oxide (CaO) (usually less than 10%). Subbituminous and lignite coals have higher amounts of calcium oxide (CaO), in the range of 20% or more (Meyers et al. 1976, Ferguson 1993, ACAA 1995). Bituminous and anthracite coals are usually found in the eastern United States. Sub-bituminous and lignite coals are usually found in the western United States. Fly ashes produced from bituminous and sub-bituminous coals can be used in several civil engineering applications. One common application is an admixture to Portland cement to increase workability, strength, and reduce heat of hydration of concrete. Fly ash can be used in combination with lime, or by itself for soil stabilization of road base and subbases to increase the bearing capacity of soil. Fly ash is also combined with water, Portland cement, and sand to produce flowable fills that flow like liquid and set up like a solid. Other fly ash applications that have been reported include use in grouts, fast-track concrete pavements, and as structural fills and backfills (ACAA 1995).
7 2.4 TYPES OF FLY ASH ASTM C 618-99 (AASTO M 295) provides the classification requirements for fly ash. The chemical requirements for classification and typical chemical compositions are summarized in Tables 2.1 and 2.2. There are two types of ashes: C and F. Class F fly ash is produced from bituminous coals and does not have self-cementing properties due to its low calcium oxide (CaO) content (Table 2.2). Class F fly ashes are usually mixed with an activator such as lime or Portland cement to generate self-cementing properties. Class C fly ash is produced from sub-bituminous coals, and exhibits self-cementing properties. When exposed to water, Class C ashes form cementitiuos products similar to those produced during hydration of Portland cement. As a result, Class C fly ash is convenient for soil stabilization (Ferguson 1993). 2.5 FACTORS AFFECTING PHYSICAL AND CHEMICAL PROPERTIES OF FLY ASH Variability of the chemical and physical properties of fly ash depends on several factors such as coal type and source, type of boiler, conditions during combustion, type of emission control devices, and storage and handling methods (Toth et al. 1988). Changes in any of these factors affect the characteristics of the fly ash, and its engineering properties. Coal type is one of the factors having the greatest effect on the fly ash characteristics. Coal source affects the calcium oxide (CaO) content, also eastern ashes typically have considerably higher sulfur (S) contents compared to
8 Table 2.1. Chemical requirements for fly ash classification. Chemical Requirements Silicon Dioxide (SiO 2 ) plus Aluminum Oxide (Al 2 O 3 ) plus Iron Oxide (Fe 2 O 3 ), min (%) Class of Fly Ash F* C* 70 50 Sulfur Trioxide (SO 3 ), max (%) 5 5 Moisture Content, max (%) 3 3 Loss on Ignition, max (%) 6 6 *After ASTM Standard C 618-99 Table 2.2. Typical chemical composition of fly ash. Compounds Class of Fly Ash F* C* SiO 2 54.9 39.9 Al 2 O 3 25.8 16.7 Fe 2 O 3 6.9 5.8 CaO (Lime) 8.7 24.3 MgO 1.8 4.6 SO 3 0.6 3.3 *After Ferguson et al. (1999)
9 western fly ashes, (Adriano and Weber 2001). In general as the amounts SiO 2, Al 2 O 3 and free lime (CaO) increases, the pozzolanic activity of the fly ash increases (Meyers et al. 1976). The chemical composition of fly ash influences its color. The color of fly ash ranges from light brown or cream to dark brown or gray. Fly ashes with lighter color have higher calcium oxide content, whereas fly ashes with darker colors have higher carbon content (Meyers et al. 1976). The specific gravity of fly ashes varies depending on the coal source. Specific gravities typically range from of 2.11 to 2.71 (Chu and Kao 1993). The compaction characteristics of fly ash are similar to those of cohesive soils. A typical compaction curve is obtained, but the curve is flatter relative to the typical bell shape curve of most cohesive soils. Fly ashes with higher carbon contents and less calcium oxide generally have a flatter compaction curve, lower dry unit weight, and higher optimum water content. The maximum dry unit weight for standard Proctor generally ranges from 8 to 17 kn/m 3, and optimum water contents range of 15 to 35%. Both of these properties vary with composition of the fly ash (Meyers et al. 1976). There are three main categories of boilers in which coal is combusted: pulverized coal-fired units, stoker-fired units, and cyclone furnaces. Pulverized coal-fired units are the most common (ACAA 1995). The combustion temperature inside the boiler affects the degree to which many minerals elements in the coal may volatilize (Adriano and Weber 2001). The mineralogy and crystallinity of the fly ash is controlled by the boiler design and operation, since the boiler controls
10 the rate at which the fused matter is cooled. Boiler type accounts for the differences in the ash chemistry; this difference can be observed in fly ashes from different sources (Ferguson et al. 1999). Collection method affects fineness of the ash. Fly ashes collected mechanically are typically coarser, and have fineness in the order of 1700 cm 2 /gm. Fly ashes collected using electrostatic precipitators are finer, having fines of approximately 6400 cm 2 /gm. The fineness and gradation of the fly ash are influenced primarily by the degree of pulverization of the coal. Fineness is important because it affect the pozzolanic activity, and the workability when the fly ash is used in Portland cement. As the fineness of the fly ash increases, the pozzolanic activity also increases (Meyers et al. 1976). A fineness specification for concrete typically requires at least 66% of the ash to pass 325 sieve (ACAA 1995). Carbon content in the fly ash is also controlled by the fineness of the coal, and the efficiency of the boiler unit. In general, old boilers are less efficient and produce fly ashes with higher carbon contents (Meyers et al. 1976). There are two primary techniques for handling fly ash, dry and wet methods. Dry methods usually involve short-term storage of the fly ash. The fly ash is stored in hoppers or silos, and is discharged through gates or doors into trucks or rail cars for distribution. The wet method involves addition of water to the fly ash to form slurry that is pumped to settling ponds or lagoons. Slurry disposal affects the compaction curve significantly. The compaction curve becomes flat; there is no variation in dry unit weight over a broad water content (Meyers et al. 1976).
11 2.6 STABILIZATION EFFECTS ON CLAYS Fly ash has been used extensively for stabilization of high plasticity clays (Torrey 1978, Lamb 1985, Ferguson 1993, Turner 1997). Even though the calcium oxide content of fly ash typically is not as high as in lime, fly ash often stabilizes high plasticity clays as well as lime. One of the major benefits of using self-cementing coal fly ash for soil stabilization in lieu of lime is the bond formed between the soil grains and cementitiuos products in the fly ash when it is mixed with water (Ferguson et al. 1999). There are three primary mechanisms that contribute to stabilization when soil is mixed with fly ash. The strength of the soil increases due to the cementation resulting from hydration of tricalcium aluminate present in the fly ash. Free lime (CaO) in the fly ash also reacts with the clay minerals, causing compression of the absorbed layer and a corresponding reduction in plasticity. Free lime not reacting with the clay minerals is available for additional cementation through pozzolanic reaction with silica and alumina compounds. The pozzolanic reaction is the primary factor responsible for the long-term stability of soil fly ash mixtures (Turner 1997).
12 2.7 REVIEW OF RECENT LIME AND FLY ASH STABILIZATION RESEARCH 2.7.1 Lime Treatment Prakash et al. (1989) evaluated the behavior of a montmorillonitic soil treated with different percentages of lime (2 to 12%) and cured for different periods (0 to 60 days). Three tests were conducted on the mixture: liquid limit test, shrinkage limit test, and standard Proctor compaction test. The initial effect of lime treatment was an appreciable decrease in the liquid limit with increasing lime content. However, over time the liquid limit was increased with an increase in lime content due water entrapment between large void spaces caused by flocculation of the soil fabric. Lime treatment produced an immediate increase in the shrinkage limit, and the effect on the shrinkage limit was a more pronounced as time increased. For compaction, the maximum dry unit weight decreased with lime content, and curing time. Optimum water content increased with lime content, and decreased with curing time. Robnett and Marshall (1976) studied the effects of lime treatment and freeze-thaw on the resilient characteristics of several fine-grained subgrade soils. Specimens were treated with 5% lime and compacted at dry and wet of optimum water contents. The immediate response was a 27-34% increase in modulus compared to untreated specimens. The resilient modulus of untreated specimens ranged from 80 to 90 MPa at low deviator stress, whereas the resilient modulus for uncured treated specimens ranged from 120 to 140 MPa. Freeze-thaw was found to have a detrimental effect on the subgrade moduli. The resilient modulus of untreated specimens ranged between 20 to 41
13 MPa after one freeze-thaw cycle. In contrast, uncured compacted soil-lime specimens had moduli ranging from 96 to 140 MPa, even after 10 freeze-thaw cycles. Specimens treated with 5% lime and cured for a period of 20 days had even higher moduli (124 to 165 MPa) after 10 freeze-thaw cycles. 2.7.2 Lime-Fly Ash Treatment Nalbantoglu and Tuncer (2001) and Nalbantoglu and Gucbilmez (2002) evaluated swell potential, compressibility, hydraulic conductivity, and cation exchange capacity (CEC) of an expansive clay from Cyprus that was chemically treated with fly ash and lime. A Class C fly ash with a calcium oxide (CaO) content of 15% was used. Swell testing showed that the same treatment level could be achieved with either lime or fly ash, at different treatment percentages. Swelling of the expansive clay decreased with increasing amounts of lime or fly ash, and also with increasing curing time. A decrease in compressibility of the compacted treated soil was also observed, but the initial void ratio increased with increasing lime and fly ash content. The pre-consolidation pressure was found to increase with increasing lime and fly ash content. A combination of 3% lime and 15% fly ash yielded approximately the same pre-consolidation pressure as was obtained using only 7% lime. The hydraulic conductivity of compacted treated soil increased with increasing lime and fly ash content, and with increasing curing period. The CEC
14 was reduced as the lime and fly ash content increased. The largest reduction in CEC was obtained using a combination of 15% fly ash and 3% lime. Nicholson and Kashyap (1993) and Nicholson et al. (1994) evaluated how fly ash and lime affect the engineering properties of tropical soils from Hawaii. The fly ash did not meet the requirements for Class C or F fly ash, and contained 19% calcium oxide (CaO). Tests that were conducted included Atterberg limits, compaction, California Bearing Ratio (CBR), free swell, and unconfined compression. Soils with a high liquid limit that were treated with 15% fly ash showed a sharp decrease in liquid limit when fly ash was added. An additional small decrease in liquid limit was observed when the fly ash content was increased to 25% fly ash. The reduction in plasticity index was as large as 50% for smectitic clays. For the other clays the reduction in liquid limit and plasticity index was more gradual with the addition of fly ash. For all soils, compaction testing showed that the maximum dry unit weight decreases and optimum water content increases with increasing fly ash content. Swell potential was reduced for all soils when fly ash was added. The reduction in swell potential was as large as 25% for the smectitic clays. Little improvement in CBR was found when the soils were prepared with up to 25% fly ash. However, for several soils prepared with 15% fly ash and 3% lime, a CBR of 23 was obtained, which is appreciably larger than the CBR of 4 obtained for untreated soil.
15 Unconfined compression testing of the soil-fly ash mixtures showed that the compressive strength depends on the fly ash content and the length of the curing period. The stabilization effect also varied considerably with soil type. The unconfined compressive strength ranged between 300 to 1993 kpa for soil fly ash mixture with 25% fly ash at 28 days. Combinations of 15% fly ash and 3% lime yield unconfined compressive strengths ranging between 470 to 1951 kpa at 28 days. Unconfined compression with 7% lime and no fly ash at 28 days yielded compressive strengths between 1110 and 2306 kpa. 2.7.3 Fly Ash Treatment Ferguson (1993) evaluated the effectiveness of self-cementing coal ashes from Kansas City for several geotechnical applications such as water content reduction by addition of solids, shrink-swell reduction, and stabilization for improvement of engineering properties such as bearing capacity, and unconfined compressive strength. Using fly ash as a drying agent showed that water contents could be reduced by 10 to 20% during construction and that compaction of soil fly ash mixtures could be performed within an hour or less. Laboratory tests were conducted on soils of three different textures, a shale clay, a glacial clay, and a fat clay. Stabilization was performed with different percentages of Class C fly ash that had a calcium oxide (CaO) content between 28.0 to 33.0%. Evaluation of the shrink-swell potential showed that the amount of fly ash required for stabilization depends on the properties of the fly
16 ash and the type of soil. Swell potentials between 0.8 and 2.4% were obtained using 16% fly ash, as compare to untreated specimens, which had swell potentials between 8.9-14.7%. CBR tests showed that using 16% fly ash could increase the CBR from 2-5 to 19-34, respectively. Compaction and unconfined compression strength tests showed that the dry unit weight decreases, optimum water content increases, and the unconfined compressive strength decreases when stabilized soils are compacted two hours after mixing rather than immediately. The unconfined compressive strength for a 16% soil fly ash mixture after 7 days showed that the strength reached 780, 790, and 820 kpa, as compared to the untreated specimens with unconfined compression strengths of 260, 190, and 240 kpa, respectively. Turner (1997) conducted a laboratory study to evaluate the effectiveness of using low sulfur western coal fly ashes from Wyoming for stabilization of subgrade soils. Tests were performed to evaluate improvements in unconfined compressive strength, resilient modulus, and wet-dry and freeze-thaw durability. Five low plasticity clayey soils were used, along with one off-specification fly ash, four Class C, and two Class F fly ashes. Unconfined compressive strengths of the compacted soil-fly ash mixtures ranged between 380 to 780 MPa when mixtures were prepared with Class C fly ash and cured for a period of 7 to 28 days. Resilient modulus of the fly ash treated soils ranged between 834 to 6237 MPa, whereas the untreated soils failed during the pre-conditioning stage of the test. Wet-dry and freeze-thaw durability tests showed that compacted soil fly ash mixtures exhibited a cement loss of more than 14%.
17 Toth et al. (1988) evaluated Canadian Class F fly ash and bottom ash as an alternative to natural materials for the construction of structural fills. Several case studies of embankments projects constructed with fly ash and bottom ash were monitored for several years to see how fly ash performs as a structural fill, and also to evaluate how leachate from the ash affects groundwater quality. Monitoring data has shown that the ash embankments behave similar to embankments constructed with soils. The fly ash and bottom ash were considered to be similar based on ease of compaction during construction, and the amount of settlement of the fill and underlying soil. One of the case studies involved a fly ash embankment 1.2 m high was constructed using a Class F fly ash brought to the construction site with a moisture content in the range of 16 to 18%. During compaction operations, the dry unit weight and water content were monitored to evaluate the effect of number of passes on the dry unit weight of the fly ash. Average dry unit weights of 10 to 12 kn/m 3 were achieved with 2 passes of the equipment over the fly ash. A second case study described the use of Class F fly ash and bottom ash of two bridge approaches that were 8 m high. Construction of the embankments was performed using a lower layer of bottom ash, a fly ash core, and an upper outside layer of bottom ash. Up to 2.45 mm of settlement was measured within the 8 m high embankment. Settlements of an underlying clay layer were 295 mm after 10 months monitoring. A third case study described an open area (20 ha) where an embankment of fly ash and bottom ash fill 12 m high was placed for agricultural purposes. The
18 landowner wanted to construct an industrial building partially located over the fly ash fill and partially over the native soil. A geotechnical investigation showed that N values ranging from 10 to 55 for the fly ash, and up to 75 for the bottom ash. Consolidated undrained triaxial tests yielded effective friction angles of 35 to 36 o. To control settlements, the fly ash and bottom ash were excavated and recompacted to 100% of standard Proctor maximum density. After construction, settlements were monitored for two foundations constructed over the ashes, settlements of 1.14 and 1.62 mm were recorded over a 10-month period. An environmental case study is described were a Class F fly ash fill (17 m high) constructed over an abandoned area. An extensive hydrogeology study was performed at the site to evaluate the impact of the fly ash on the groundwater quality. A silt layer with a thickness of 5 m underlay the fly ash. A 3 to 4 m thick layer of clayey silt was beneath the upper silt layer. Concentrations of calcium, sulphate, potassium, and boron were reported. Leachate from the ash had sulphate and calcium concentrations of 1230 and 485 mg/l, and potassium and boron concentrations of 24 and 5.4 mg/l. The sulphate concentrations exceeded the maximum concentration stipulated in the Guidelines for Canadian Drinking Water Quality (GCDWQ). Concentrations of trace elements and metals in the leachate complied with requirements in the GCDWQ. Lee and Fishman (1992) evaluated two fine-grained industrial by products for potential use in pavement construction. One by-product was a Class F fly ash, and the other a fine-grained residue from processing aggregates. The Class F fly ash was non-plastic and had a fines content of 76%. The residue was clayey,
19 and had a liquid limit of 31 and plasticity index of 14. Resilient modulus, unconfined compression, and CBR tests were conducted on the materials alone, and mixtures of the two materials. Resilient modulus of the residue was found to range between 17 to 43 MPa for a deviator stress of 100 kpa, which corresponds to a soft to medium stiff subgrade (Asphalt Institute 1982). Resilient modulus of the fly ash was comparable to that of a granular material, with dependency on bulk stress. Resilient modulus of the fly ash ranged from 11 to 21 MPa for bulk stresses ranging from 21 to 138 kpa, which corresponds to very poor subgrade (Asphalt Institute 1982). The resilient modulus of the compacted mixture of residue and fly ash (10:3 by weight) ranged between 31 to 48 MPa for bulk stresses ranging from 21 to 138 kpa, which corresponds to good subgrade. Unconfined compression strength of the mixtures was approximately 200 kpa for specimens cured for 7 days and compacted wet of optimum water content. CBRs of the residue, fly ash, and the mixture (10:3) were reported as 2, 7, and 13 for compaction at optimum water content. Edil et al. (2002) conducted a field evaluation of several alternatives for construction over soft subgrade soils. The field evaluation was performed along a 1.4 km segment of Wisconsin State Highway 60 and consisted of several test sections. By products such as fly ash, bottom ash, foundry slag, and foundry sand were used. A Class C fly ash was used for one test section. Unconfined compression testing showed that 10% fly ash (on the basis of dry weight) was sufficient to provide the strength necessary for construction on the subgrade.
20 Data were obtained before and after fly ash placement by testing undisturbed samples in the laboratory and by using a soil stiffness gauge (SSG) and a dynamic cone penetrometer (DCP) in the field. Unconfined compressive strength, soil stiffness, and dynamic cone penetration of the native soil before fly ash placement ranged between 100 to 150 kpa, 4 to 8 MN/m, and 30 to 90 mm/blow, respectively. After fly ash addition, the unconfined compressive strength reached as high as 540 kpa, the stiffness ranged from 10 to 18 MN/m, and the DPI was less variable and ranged between 10 and 20 mm/blow. A CBR of 32 was reported for the stabilized subgrade, which is rated as good for subbase highway construction. CBR of the untreated subgrade was 3, which is rated as very poor (Bowles 1992).
21 SECTION 3 MATERIALS AND METHODS 3.1 FLY ASHES 3.1.1 Sources and Composition Four different fly ashes were used in this study: Columbia, Edgewater, Dewey, and King. Columbia fly ash is from Unit-2 of the Columbia Power Plant in Portage, Wisconsin. Edgewater fly ash is from Unit-5 of the Sheboygan Power Plant in Sheboygan, Wisconsin. Dewey fly ash is from the Nelson Dewey Power Plant in Cassville, Wisconsin. King fly ash is from the Allen S. King Plant in Bayport, Minnesota. The Columbia, Edgewater, and Nelson Dewey Plants are operated by Alliant Energy. The King plant is operated by Xcel Energy. These fly ashes collectively provide a wide geographical area coverage as a source and also represent a wide range characteristics of fly ashes available in Wisconsin. Physical properties of the fly ashes are summarized in Table 3.1 along with typical physical properties of Class C and F fly ash. Columbia, Edgewater, and King fly ashes have a powdery texture. Dewey fly ash has a more granular texture. Both Columbia and Edgewater are light brown in color, which indicates these fly ashes have higher calcium oxide content (Meyers et al. 1976). The Dewey and King fly ashes are dark gray and dark brown in color, which indicates higher amounts of carbon (Meyers et al. 1976). Factors affecting physical and chemical properties of fly ashes are shown in Table 3.2. Some of these factors are fineness, coal type and source, collection method, storage method, and type
22 Fly Ash Table 3.1. Physical properties and chemical composition of fly ashes. Classification (ASTM C 618) G s C u Percent Fines Moisture Content (%) LOI (%) Lime (CaO) (%) Other Oxides (SiO 2 + Al 2 O 3 + FeO 3 ) (%) Class C** - - - - 3 6 24.3* 50 5 Class F** - - - - 3 6 8.7* 70 5 Sulfur Trioxide (SO 3 ) (%) Columbia Dewey King C 2.70 9 95.3 0.09 0.7 23 55.5 3.7 Off-Spec 2.53 103 39.6 0.23 16.2 9.8 38.7 11.8 Off-Spec 2.68 11 91.9 0.44 5.4 23.7 49.5 6.4 Edgewater C 2.71 14 92.8 0.03 0.1 20.8 62.3 1.0 Notes: G s = Specific gravity, LOI = Loss on ignition, C u = Coefficient of uniformity. Minimum and maximum percentages for fly ash classification refer to Table. 2.1. *After Ferguson et al. (1999), **After ASTM Standard C 618-99.
23 Table 3.2. Factors affecting physical and chemical properties of fly ash. Properties Columbia Dewey King Edgewater Fineness (%) 14.4 12.7 10.4 24.8 Pozzolanic Activity at 7 days (%) 95.8 82.7 77.7 71.7 Type of Coal and Source Collection Method Storage Type Type of Boiler Notes: Sub- Bituminous Wyoming PRB with Colorado or Petroleum Coke Sub- Bituminous 80% Montana Coal with Colorado or Petroleum Coke Sub- Bituminous 30% Montana PRB 60% Wyoming PRB 10% Petroleum Coke Sub- Bituminous Wyoming PRB with Colorado or Petroleum Coke Electrostatic Electrostatic Electrostatic Electrostatic Dry Silo Dry Silo Dry Silo Dry Silo Pulverized Cyclone Cyclone Pulverized PRB = Powder River Basin coal, Percentage remaining varies throughout the year, information source: Randy Polleck (Alliant Energy) and Michael Thomes (Xcel Energy).
24 of boilers used. Chemical composition of the fly ashes is summarized in Table 3.3, along with typical compositions of Class C and F fly ashes. The Columbia and Edgewater fly ashes are classified as Class C fly ashes following ASTM C 618. These fly ashes have high calcium oxide (CaO) content (23.0 and 20.8%, respectively) and exhibit self-cementing characteristics. Dewey fly ash classifies as off-specification fly ash, has a calcium oxide content of 9.8%, and an organic content of 16.2%. Dewey fly ash is off-specification because the SiO 2 + Al 2 O 3 + Fe 2 O 3 content is below 50%, the sulfur trioxide (SO 3 ) content is above 5%, and the loss on ignition exceeds 6%. King fly ash also classifies as off-specification fly ash because the SiO 2 + Al 2 O 3 + Fe 2 O 3 content is less than 50%, and the sulfur trioxide (SO 3 ) content exceeds 5%. King fly ash has 24% calcium oxide, and is close to a Class C fly ash. Dewey fly ash is closer to a Class F fly ash. The silicon dioxide (SiO 2 ) contents for the Columbia, Dewey, and King fly ashes are below the typical amounts for Class C fly ash. Only Edgewater fly ash has a SiO 2 content (39%) close to typical Class C fly ash. All of the fly ashes have Al 2 O 3 and Fe 2 O 3 contents characteristic of Class C fly ashes, and the magnesium oxide (MgO) contents for Columbia, Dewey, and Edgewater are close to typical Class C fly ash. The MgO content of King fly ash is closer to that of a typical Class F fly ash. The sulfur trioxide (SO 3 ) content is higher for both Dewey and King fly ashes (11.8% and 6.4%), compared to typical SO 3 contents for Class C and Class F ashes (3.3% and 0.6%).
25 Table 3.3 Chemical compositions of fly ashes. Chemical Compounds Percent of Composition Columbia Dewey King Edgewater Typical* Class C Typical* Class F CaO (Lime) 23.1 9.8 23.7 20.8 24.3 8.7 SiO 2 31.1 19.8 27.3 38.7 39.9 54.9 Al 2 O 3 18.3 13.0 16.3 15.8 16.7 25.8 Fe 2 O 3 6.1 6.0 5.9 7.8 5.8 6.9 MgO 3.7 3.1 1.8 3.4 4.6 1.8 SO 3 3.7 11.8 6.4 1.0 3.3 0.6 *After Ferguson et al. (1999)
26 100 80 Percent Passing (%) 60 40 20 Columbia Dewey King Edgewater 0 10 1 0.1 0.01 Particle Size (mm) 0.001 0.0001 Fig. 3.1. Particle size distribution curves for Dewey, Columbia, King, and Edgewater fly ashes.
27 3.1.2 Index Properties and Compaction Characteristics Specific gravities and the percent fines for the fly ashes are summarized in Table 3.1. The specific gravity of the Dewey fly ash is low relative to the other three fly ashes, because it has higher carbon content. For the other fly ashes, the specific gravity ranged from 2.68 to 2.71. The percent fines for Dewey fly ash is also lower (39.4%); the other fly ashes have at least 90% fines. Particle size distributions for the fly ashes are shown in Fig. 3.1. Columbia fly ash is slightly finer than the other ashes. The King and Edgewater fly ashes have similar particle size distributions. The Dewey fly ash is appreciably coarser than the other fly ashes, and is also gap graded. The Columbia, Edgewater, and King fly ashes have particles ranging from fine sand to silt and clay size, whereas Dewey fly ash has particles as large as medium sand and as small as clay. The unusual characteristics of the Dewey ash are also reflected in the coefficients of uniformity (C u ), which are summarized in Table 3.1. The C u for Columbia, Edgewater, and King are similar, (9, 11, and 14), where as the C u for the Dewey fly ash is 103. Compaction characteristics of the fly ashes using the standard Proctor compaction procedure (ASTM D 698) are shown in Fig. 3.2. The compaction curves are flat relative to the typical bell-shaped curves of fine-grained soils. The variation in dry unit weight between the fly ashes is partly due to variations in organic content. The Columbia and Edgewater fly ashes have the lowest organic content, and the highest dry unit weight. Dewey fly ash has the highest organic content, and the lowest dry unit weight. Furthermore, it has a coarser texture.
28 Dry Unit Weight (kn/m 3 ) 18 16 14 12 10 Columbia Dewey King Edgewater 8 6 0 10 20 30 40 50 60 70 Water Content (%) Fig. 3.2. Compaction curves for Dewey, Columbia, King, and Edgewater fly ashes (Compacted immediately after adding water).
29 King fly ash falls in the middle in terms of optimum water content, dry unit weight, and organic content. 3.2 WISCONSIN SUBGRADE SOILS 3.2.1 Sources Seven subgrade soils were considered for the testing program. The locations where the soils were collected are shown in Fig. 3.3. These locations were identified through discussions the Wisconsin Department of Transportation, chief Geotechnical Engineer and are intended to represent the range of soft subgrades typically encountered in Wisconsin. Samples of each soil were collected along the highway shoulder, at a depth of 0.6 m to 0.9 m. 3.2.2 Soil Index Properties Index and compaction properties and classifications of the soils are summarized in Table 3.4. All of the soils are fine-grained and classify as clays according to the Unified Soil Classification System. One of the soils (organic Theresa silt loam) is a highly plastic organic clay (LOI = 10%). The Theresa silt loam and the red silty clay till are low plasticity clays. The Brown Silt and Lacustrine Red Clay are high plasticity clays. The inorganic clays have LOI less than 4%. Particle size distributions for the soils are presented in Fig. 3.4. Lacustrine Red Clay is finer than the other soils. The Brown Silt, Red Silty Clay Till, and
30 Theresa Silt Loam have similar particle size distributions. All of the soils contain at least 90% fines, all but the red silty clay till which has 70% fines. Natural water contents of the soils are presented in Table 3.4. All natural water contents are above optimum water content. On average the natural water content is 7% wet of optimum water content, except one soil which has a water content near its optimum water content. Thus, when the specimens were prepared to simulate the natural wet condition observed in the field, they were prepared approximately 7% wet of optimum water content. 3.2.3 Compaction Characteristics and California Bearing Ratio Compaction curves corresponding to standard Proctor effort were determined for each soil following the procedure in ASTM D 698. Typical bellshaped compaction curves were obtained (Fig. 3.5). The maximum dry unit weights and the optimum water contents are summarized in Table 3.4. Organic Theresa silt loam has the highest optimum water content (29%) and the lowest dry unit weight (13.5 kn/m 3 ), because of its high organic content. Red silty clay till has the highest dry unit weight (18.4 kn/m 3 ), and the lowest optimum water content (13%), which reflects the larger fraction of coarse particles in the till. California Bearing Ratio (CBR) tests were conducted on each soil following the methods described in ASTM D 1883-87. The CBRs are summarized in Table 3.4. The CBR of each soil was measured on a specimen prepared 7% wet of optimum water content using standard Proctor effort. The CBRs range from 0 to 5, which implies the soils classify as very poor to fair subgrades (Bowles 1992).
31 Lacustrine Red Clay Red Silty Clay Till Brown Silt Organic Theresa Silt Loam and Theresa Silt Loam Plano Silt Loam Joy Silt Loam Fig. 3.3. Locations where soils were sampled.
32 Table 3.4. Index properties of the soils. Soil Name Sampling Location LL PI Percent Fines G s LOI (%) Classification USCS AASHTO CBR w N (%) d (kn/m 3 ) w OPT (%) Organic Theresa Silt Loam Theresa Silt Loam Brown Silt Lacustrine Red Clay Red Silty Clay Till Joy Silt Loam STH 28 Mayville, WI USH 151 Platteville, WI STH 13 Cloverland, WI STH 54 Luxemburg, WI STH 60 Lodi, WI 61 45 60 69 47 39 19 97 2.24 10 OH A-7-5 0.3 35 13.5 29 19 99 2.58 2 CL A-7-6 3 19 15.9 18 35 97 2.58 4 CH A-7-6 0.4 32 16.4 19 38 97 2.71 2 CH A-7-6 2 35 15.7 24 22 71 2.69 2 CL A-6 5 19 18.4 13 15 96 2.70 1 ML A-6 3 25 16.5 19 Plano Silt Loam Scenic Edge in Cross Plains, WI 44 20 96 2.71 2 CL A-7-6 1 27 16.2 20 Notes: LL = Liquid limit, PI = Plasticity index, Percent Fines = percentage passing No. 200 sieve, G s = Specific gravity, LOI = Loss on ignition, CBR = California Bearing Ratio (performed approximately 7% wet of optimum water content), w N = Natural water content, d = Maximum dry unit weight, w OPT = Optimum water content.
33 100 Percent Passing (%) 80 60 40 Red Silty Clay Till Lacustrine Red Clay Organic Theresa Silt Loam 20 Theresa Silt Loam Brown Silt Joy Silt Loam Plano Silt Loam 0 10 1 0.1 0.01 0.001 0.0001 Particle Size (mm) Fig. 3.4. Particle size distributions for the Wisconsin soils.
34 Dry Unit Weight (kn/m 3 ) 19 18 17 16 15 14 Red Silty Clay Till Lacustrine Red Clay Organic Theresa Silt loam Theresa Silt Loam Brown Silt Joy Silt Loam Plano Silt Loam 13 12 0 5 10 15 20 25 30 35 40 Water Content (%) Fig. 3.5. Compaction curves for the Wisconsin soils.
35 3.3 TEST PROCEDURES 3.3.1 CBR Test Specimens for CBR testing were prepared in accordance with ASTM D 3668. A typical soil fly-ash specimen undergoing a CBR test is shown in Fig. 3.6. Specimens were prepared 7% wet of optimum water content using standard Proctor effort to simulate the wet and soft condition typically observed when the soils were collected in the field. Additional specimens were prepared at optimum water content, as well as 9 to 18% wet of optimum water content to simulate a very wet and soft subgrade. For the soil fly-ash mixtures, specimens were left in the mold, sealed using a plastic wrap, and left to cure for 7 days at 25 o C and 100% relative humidity prior to testing. 3.3.2 Resilient Modulus Test Fig. 3.7 shows a soil-fly ash specimen being compacted inside a mold. Specimens for resilient modulus testing were prepared using the same compactive effort as specimens prepared using the standard Proctor procedure. The effort was matched by adjusting the numbers of blows per layer so that the same energy per volume was delivered (600 kn/m 3 ). The mold used to prepare the resilient modulus specimens had a diameter of 102 mm, height of 203 mm, and volume of 1.65 L. Specimens were compacted in the mold in 6 layers with 22 blows per layer using a standard Proctor hammer. After compaction, the specimens were extruded. Soil specimens were tested shortly after compaction. Specimens prepared with soil-fly ash mixtures
Fig. 3.6. California Bearing Ratio test being conducted on soil-fly ash mixture. 36
37 Fig. 3.7. Resilient modulus soil-fly ash specimen being compacted inside a mold.
38 were sealed with plastic wrap and cured at 25 o C and 100% humidity. Most specimens were cured for 14 days. However some specimens were cured for as long as 56 days to understand how the resilient modulus changes over time as the fly ash cures. The procedure described in AASHTO T 292-91 (1996) was followed for the resilient modulus test. A photograph of the resilient modulus cell in the loading frame is shown in Fig. 3.8. The loading sequence for cohesive soils was used, as is summarized in Table 3.5. Before testing began, several tests were conducted using synthetic specimens to assess the repeatability of the procedure. The synthetic specimens (A and B) had different aspect ratios (1:1 and 2:1), and were made out of different materials, but had similar density (1.18 Mg/m 3 and 1.21 Mg/m 3 ), as shown in Fig. 3.9. The resilient moduli for Specimen B are in the range of 10 MPa. For Specimen A, the resilient moduli range from 140 MPa to 160 MPa, with higher moduli obtained with increasing deviator stress. In general, similar moduli were measured in each replicate test. Except for the one measurement of Specimen B during the second replication, the resilient moduli differ by no more than 2% at a given deviator stress.
39 Linear Variable Displacement Transducers Load Cell for applied deviator stress ( d) Compacted soil-fly ash specimen encased in latex membrane Line for confining pressure ( 3 ) Fig. 3.8. Specimen undergoing resilient modulus test.
40 Table 3.5. AASHTO T 292-91 (1996) test sequence for cohesive soils. Phase Specimen Conditioning Sequence Number Deviator Stress (kpa) Number of Repetitions 0 41 1000 1 21 50 2 34 50 Testing 3 48 50 4 69 50 5 103 50 Note: A confining pressure of 21 kpa and a seating load of 13.8 kpa were used.
41 200 Resilient Modulus (MPa) 150 100 50 A A-First Test A-Second Test B-First Test B-Second Test B-Third Test B 0 0 20 40 60 80 100 Deviator Stress (kpa) Fig. 3.9. Resilient modulus of synthetic Specimens A and B.
42 3.3.3 Unconfined Compressive Test Unconfined compression tests were conducted following the procedure in ASTM D 5102. A photograph of a soil-fly ash specimen being subjected to unconfined compression is shown in Fig. 3.10. ASTM D 5102 recommends that the strain rate should be between 0.5% and 2%/min. However, a strain rate of 0.21%/min was used because the soil-fly ash mixtures were expected to be stiffer than typical cohesive soils. This reduction in strain rate is consistent with Note 7 in ASTM D 5102, which suggests that stiffer specimens be tested at lower strain rates. Unconfined compression tests were performed on the same specimen used for the resilient modulus test. The specimen was removed from the resilient modulus cell, and then subjected to unconfined compression in a load frame. Additional unconfined compressive tests were conducted on small specimens prepared using Harvard compaction equipment following the procedure in ASTM 4609. The Harvard mold is 33 mm in diameter and 72 mm in height. To achieve approximately the same dry unit weight as in a standard Proctor test, a trial-and-error procedure was followed to find the appropriate number of layers and tamps per layer. Testing showed that 3 layers with 25 tamps/layer were adequate to achieve approximately the same dry unit weight as obtained with the standard Proctor effort. The similarity of the compaction curves obtained with the standard Proctor and Harvard compaction methods is shown in Fig. 3.11 for organic Theresa silt loam, Theresa silt loam, and silt brown clay soils. The differences in maximum dry unit weight and optimum water content are
43 Fig. 3.10. Unconfined compression test being conducted on soil-fly ash mixture.