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AC 150/5300-4B 6/24/75 d. No special design criteria are required for rigid pavements because the FAA standard 6-inch-minimum thickness of concrete pavement will satisfactorily serve airplanes with gross weights up to 30,000 pounds. 66. SOIL STABILIZATION. Soil stabilization is the procedure whereby the properties of a soil are improved to the extent that it will meet the requirements for pavement bases or subbases. Stabilized soils are not intended to serve as a surface course and must be provided with a sur- face in order to resist the abrasive action of operating vehicles or airplanes. To be effective, stabilization should provide a foundation which will eliminate or reduce to an appreciable extent the detrimental effects of volume changes occuring in the soil due to climate influ- ences, or moisture variations. Mechanical and chemical stabilization are the two general types currently employed. a. Mechanical stabilization on airports follows standard practices developed over the years, and requirements regarding materials as well as construction methods are quite definitely established. Per- formance studies have disclosed that the success of a granular stabilized base course depends on the gradation of the mixture and the physical properties of the material. b. Bituminous stabilization is the combining of bituminous material with soil, soil aggregate, or sand to produce the desired soil characteristics. Bituminous stabilizing agents include cutback asphalts, slow -curing asphalts or road oils, emulsified asphalts, and tars. Methods of construction vary with the type of equipment available but, regardless of the equipment, the different steps consist essentially of soil preparation by scarifying and pulver- izing, thorough and uniform mixing of the bituminous material with the soil, curing of the mixture to get rid of excess moisture and volatile constituents, and compaction to a predetermined density. c. By the addition of portland cement in the correct quantity, many types of soils and materials such as shale, gravel, sand, screenings, slag, and mine tailings can be stabilized. Construction of soil -cement bases has been standardized to a large degree. d. Lime, in small percentages, has been added to base course materials such as gravel, disintegrated granite, crusher -run stone containing appreciable amounts of soil -type overburden, and caliche in order to reduce the plasticity index to meet specification requirements. Performance records of highway pavements indicate this reduction in plasticity index accomplishes a marked improvement in the stability of the base course. Lime could also be used to stabilize a clay subgrade. Page 76 Chap 12 Par 65 •