Soil improvement and ground modification methods chapter 11 admixture soil improvement Soil improvement and ground modification methods chapter 11 admixture soil improvement Soil improvement and ground modification methods chapter 11 admixture soil improvement Soil improvement and ground modification methods chapter 11 admixture soil improvement Soil improvement and ground modification methods chapter 11 admixture soil improvement Soil improvement and ground modification methods chapter 11 admixture soil improvement Soil improvement and ground modification methods chapter 11 admixture soil improvement
CHAPTER 11 Admixture Soil Improvement Admixture soil improvement refers to any improvement application where some material is added and mixed with existing or placed soil to enhance the engineering properties or engineering behavior of the soil This chapter provides an overview of the improvement objectives, mixing methods, and some common applications for admixture treatments There is also a discussion of the various materials used, including natural soils, chemical additives, and waste products, along with a discussion of applicability to different soil types Included in the chapter are some case studies exemplifying some of the successful possibilities of utilizing admixture stabilization 11.1 INTRODUCTION TO ADMIXTURE SOIL IMPROVEMENT The engineering properties of soils can be dramatically enhanced through the addition (or subtraction) of materials to (or from) the soil The mechanics of the improvements may be physical or chemical in nature In most cases, changes in the soil properties are permanent Improvement of soils through the use of admixtures (material added to the soil) is often called soil stabilization, as it may in many instances result in the soil being rendered more “stable” by being less susceptible to engineering property fluctuations (e.g., strength fluctuations, volume stability, moisture content change, etc.) Soil admixtures may include a wide array of materials such as natural soils, chemical reagents, binders, polymers, industrial by-products (waste or recycled materials such as fly ash, slag, shredded rubber, crushed glass, etc.), salts, poly-fibers, and bitumen/tar Soil stabilization with admixtures has been used for economical road building, conservation of materials, investment protection, and roadway upgrading In many instances, soils that are unsatisfactory in their natural state can be made suitable for subsequent construction by treatment with admixtures, and/or by the addition of natural aggregate or other soil materials Admixture improvement has also been used for repair of geotechnical failures by providing a rebuilt soil structure that is much stronger and more robust than Soil Improvement and Ground Modification Methods © 2015 Elsevier Inc All rights reserved 231 232 Soil improvement and ground modification methods the original construction Admixture soil improvement is now routinely used for site and roadway rehabilitation as well as new construction projects Use of admixtures can improve engineered soil and in situ ground conditions so that significant cost savings may be possible This can be achieved by requiring less costly foundation schemes, using a smaller volume of select fill material, utilizing lower-quality soils, and realizing economic savings over conventional excavation/replacement methodologies This is especially important and useful for fine-grained soils, but also has numerous applications for coarser granular materials Another driving force behind using admixture treatments is the shortage of available, conventional aggregates in many locales Environmental concerns, regulations, and land use patterns have also severely impacted the availability of useable aggregate Physical improvements can be made by altering the soil gradation (or soil grain-size distribution) by adding or subtracting certain soil grain sizes, or by adding materials that physically “bind” soil particles together without causing any chemical reactions or changes to the mineralogical structure of the soil Conversely, chemical improvements can be made by adding materials that intentionally cause reactions to occur, resulting in physiochemical changes in the mineralogical structure of the soil These changes can have pronounced improvements in the characteristics of the soil, even leading to a change in the fundamental classification of the soil 11.1.1 Benefits of Admixture Soil Improvement In so many parts of the world, poor soil conditions inhibit sound construction and development of quality infrastructure For many people, transportation lifelines are severely impacted by poor subgrade soils and lack of quality fill or roadway materials These conditions often exist in underdeveloped or developing areas where soil improvement engineering practice is sorely lacking or nonexistent In many of these cases, relatively simple and inexpensive soil improvement techniques, using readily available admixture materials and equipment, can dramatically enhance conditions and reduce the degradation that otherwise would require continual repairs Such improvements can lead to an improved quality of life and more efficient movement of needed supplies as well as the mobilization of emergency transportation Mixing admixtures into soil has been shown to be greatly beneficial in: • Drying up wet soil • Improving strength (including “solidification” of wastes for disposal assistance) • Providing volume stability (reducing swell, controlling shrinkage) Admixture soil improvement 233 • Reducing soil deformations (reduced compressibility/settlement concerns, minimizing differential settlement effects; increasing stiffness) • Reducing erodibility (through increased surface strength and water repulsion) • Improving durability to dynamic/repeated loads, including freeze-thaw (through increased intergranular strength and decreased degradation of aggregate) • Permeability (or moisture) control (either reduced permeability for water conveyance or retention structures, moisture consistency, or water repulsion) • Dust control As a consequence, soil improvements have been responsible for: • Improved working platforms and workability of soils • Reduced thickness of roadway layers • Slope stabilization • Foundation/structural support • Excavation support • Liquefaction mitigation • Reduced leakage/seepage from hydraulic retention/conveyance structures • Stabilization of marine sediments • Environmental (contamination) remediation Increased soil strength allows steeper slopes to be constructed Increased slope angles result in less volume of engineered fill required to attain a desired embankment height, less area (footprint) needed for the same embankment height requirements, economic savings from faster construction, and so on Stabilization projects are almost always site-specific, requiring the application of standard test methods, along with fundamental analysis and design procedures, to develop workable solutions A number of standards for materials and testing related to soil stabilization with admixtures have been developed and are available from ASTM and others A listing of some of the relevant ASTM test standards is provided at the end of this chapter 11.2 ADMIXTURE MATERIALS The materials that may be added to a soil for stabilization are wide-ranging and have a variety of properties, forms, and attributes These generally range from naturally occurring soils (different in grain size distribution to those being treated) to chemical additives and even reused waste products Class C fly ash, a by-product of coal combustion, has been widely used as a soil admixture either by itself or in addition to lime and/or cement The type 234 Soil improvement and ground modification methods of admixture material to be used will depend on a number of variables including: • Soil type to be treated • Purpose of use • Engineering properties desired • Minimum requirement (or specification) of engineering properties • Availability of materials • Cost • Environmental concerns The selection of an appropriate additive may begin by following some general guidelines based on soil gradation and plasticity, which have been well-documented in the literature (e.g., Federal Highway Administration, 1992a,b; Joint Departments of the Army and Air Force, 1994) For example, while cement can be used with a variety of soil types, it is critical that it be thoroughly mixed with any fines fraction (grain sizes 90%, Admixture soil improvement 239 meeting ASTM C977 Quicklime is a white, caustic, alkaline crystalline solid that is most commonly made by thermal decomposition of limestone or other calcium carbonate materials containing the mineral calcite (CaCO3 or MgCO3) Because of this, it is also sometimes called burnt lime Quicklime is made by heating the source material to above 825 C (1517 F) in a process called calcination, which drives off CO2 leaving calcium oxide: CaCO3 + MgCO3 ! CaO + MgO + CO2 (11.2) One of the advantages of using quicklime is the intense heat generated during hydration, which can reach temperatures above 150 C This basic hydration reaction is CaOðsÞ + H2 OðlÞCaðOHÞ2 ðaqÞ ðDH r ¼ 63:7kJ=mol of CaO; 490BTU=lb ¼ 273cal=gÞ (11.3) (11.4) The drying effect of adding quicklime to a wet soil can be attributed to two cumulative processes First, the water content reduces due to hydration of the lime Knowing that kg CaO absorbs 0.32 kg of water through hydration, the decrease in moisture from hydration (Dw) is given by the following equation (Kitsugi and Azakami, 1982): Dw ¼ wo À ðw o À 0:32as Þ=1 + 1:32as Þ (11.5) where wo is the original soil water content and as is the mass ratio of lime to soil Second, moisture is also lost due to evaporation from the heat generated by hydration of quicklime Hausmann (1990) showed that the additional moisture loss is equal to Dw ¼ 0:45as (11.6) Quicklime is volatile and must be kept sealed until use It is perishable and must be “fresh” (typically