Fly ash, when mixed with lime, can be usedeffectively to stabilize most coarse- and medium-grained soils. However, the PI should not be greater than25. Soils classified by the USCS as SW, SP, SP-SC,SW-SC, SW-SM, GW, GP, GP-GC, GW-GC, GP-GM,GW-GM, GC-GM, and SC-SM can be stabilized withfly ash.BITUMINOUSMost bituminous soil stabilization has beenperformed with asphalt cement, cutback asphalt, andasphalt emulsions. Soils that can be stabilizedeffictively with bituminous materials usually containless than 30 percent passing the No. 200 sieve and havea PI less then 10. Soils classified by the USCS as SW,SP, SW-SM, SP-SM, SW-SC, SP-SC, SM, SC, SM-SC,GW, GP, SW-GM, SP-GM, SW-GC, GP-GC, GM, GC,and GM-GC can be effectively stabilized withbituminous materials provided the above-mentionedgradation and plasticity requirements are met.Combination stabilization is specifically defined aslime-cement, lime-asphalt, and lime-cement-fly ash(LCF) stabilization. Combinations of lime and cementoften are acceptable expedient stabilizers. Lime can beadded to the soil to increase the workability and mixingcharacteristics of the soil as well as reduce its plasticity.Cement can then be mixed into the soil to provide rapidstrength gain. Combinations of lime and asphalt areoften acceptable stabilizers. The lime addition mayprevent stripping at the asphalt-aggregate interface andincrease the stability of the mixture.SELECTION OF A STABILIZERIn the selection of a stabilizer additive, the factorsthat must be considered are the type of soil to bestabilized, the purpose for which the stabilized layer willbe used, the type of soil quality improvement desired,the required strength and durability of the stabilizedlayer, and the cost and environmental conditions.The soil gradation triangle in figure 18-1 is basedupon the pulverization characteristics of the soil. WhenFigure 18-1.—Soil gradation triangle.18-3
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