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vii EXECUTIVE SUMMARY Long-term performance of pavement structures is significantly impacted by the stability of the underlying soils. In situ subgrades often do not provide the support required to achieve acceptable performance under traffic loading and environmental demands. Although stabilization is an effective alternative for improving soil properties, the engineering properties derived from stabilization vary widely due to heterogeneity in soil composition, difference in micro and macro structure of soils, heterogeneity of geologic deposits, and due to differences in physical and chemical interactions between the soil and candidate stabilizers. These variations necessitate the consideration of site-specific treatment options validated through testing of soil-stabilizer mixtures under simulated field conditions. This report addresses soil treatment with the traditional calcium-based stabilizers: Portland cement, lime, and fly ash. The report describes and compares the basic reactions that occur between these stabilizers and soil and the mechanisms that result in stabilization. The report presents a straightforward methodology to determine which stabilizers should be considered as candidates for stabilization for a specific soil, pavement, and environment. The report then presents a protocol for each stabilizer through which the selection of the stabilizer is validated through mixture testing and mixture design. The mixture design process defines an acceptable amount of stabilizer for the soil in question based on consistency testing, strength testing, and in some cases (resilient) modulus testing. Within each additive validation and mixture design protocol, an assessment of the potential for deleterious soil-additive reactions is made. For successful soil stabilizer applications it is imperative to understand the mechanism of stabilization of each additive. A basic understanding of stabilization mechanisms assists the user agency in selecting the stabilizer or additive best suited for a specific soil not only from the standpoint of developing the engineering properties desired for the pavement sublayers but also to minimize the risk of long-term deleterious reactions that might compromise pavement structural capacity or even induce disruptive volumetric changes such as sulfate-induced heave. In order to determine an appropriate soil-additive combination and to reduce the risk of deleterious reactions for a specific soil-stabilizer combination field exploration is required. For soil stabilization operations, the exploration process is less complex than for structural foundations as the depth of the influence zone is less. Therefore, although geological data are valuable, the most important data come from pedological profiles that are available, for example, in the National Resources Conservation Service (NRCS) County Soil Surveys. This report describes how the NCRS surveys and geological data sources should be used to plan an effective exploration plan to more clearly define the extent and boundaries of soil series and the depth of soil horizons that may affect chemical stabilization. The report provides a protocol for mixture design for each additive type. This protocol begins with stabilizer selection and then proceeds to the verification step in which the selected stabilizer is evaluated based on consistency and strength testing. An indispensable part of the verification protocol is mixture design in which the amount stabilizer required to provide long-term, durable performance is determined. A separate protocol is presented for the most widely used traditional, calcium-based stabilizers: Portland cement, lime, and fly ash.