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1 INTRODUCTION The purpose of this document is to support and serve as background for the draft Standard Recommended Practice for Modification and Stabilization of Subgrade and Base Soils in Pavement Structures. The Recommended Practice and this background report address soil exploration, modifier selection, verification of stabilizer selection, and mix design. This document addresses use of the traditional calcium-based stabilizer: lime (CaO or Ca(OH)2), Portland cement, and fly ash. Long term performance of pavement structures often depends on the stability of the underlying soils. Engineering design of these constructed facilities relies on the assumption that each layer in the pavement has the minimum specified structural quality to support and distribute the super imposed loads. These layers must resist excessive permanent deformation, resist shear and avoid excessive deflection that may result in fatigue cracking in overlying layers. Available earth materials do not always meet these requirements and may require improvements to their engineering properties in order to transform these inexpensive earth materials into effective construction materials. This is often accomplished by physical or chemical stabilization or modification of these problematic soils. Although the solution appears simple and straight forward, engineering properties of individual soils may vary widely due to heterogeneity in soil composition, difference in micro and macro structure among soils, variability and heterogeneity of geologic deposits and due to differences in physical and chemical interactions of air/water with soil particles. These differences necessitate the use of site-specific treatment options for stabilization. Over the years engineers have tried different methods to stabilize soils that are subject to fluctuations in strength and stiffness properties as a function of fluctuation in moisture content. Stabilization can be derived from thermal, electrical, mechanical or chemical means. The first two options are rarely used. Mechanical stabilization, or compaction, is the densification of soil by application of mechanical energy. Densification occurs as air is expelled from soil voids without much change in water content. This method is particularly effective for cohesion less soils where compaction energy can cause particle rearrangement and particle interlocking. But, the technique may not be effective if these soils are subjected to significant moisture fluctuations. The efficacy of compaction may also diminish with an increase of the fine content, fraction smaller than about 75 µm, of the soil. This is because cohesion and inter particle bonding interferes with particle rearrangement during compaction. Altering the physio-chemical properties of fine-grained soils by means of chemical stabilizers/modifiers is a more effective form of durable stabilization than densification in these fine-grained soils. Chemical stabilization of non-cohesive, coarse grained soils, soils with greater than 50 percent by weight coarser than 75 µm is also beneficial if a substantial stabilization reaction can be achieved in these soils. In this case the strength improvement can be much higher, greater than ten fold, when compared to the strength of the untreated material. This report discusses key factors associated with stabilizing soils using chemical modifiers including: Mechanisms of Stabilization Soil Classification Soil Exploration Guidelines for Stabilization and
