Long-term performance of flexible pavements located on cement-treated soils and the longevity of soil-cement subgrades are examined at sections of four highway routes. Ages of the soil-cement subgrades range from 6 to 30 years. Field and laboratory studies were conducted at each section. Generally. the soil-cement subgrades were non-plastic and were classified as SM, or sandy silt, according to the Unified Soil Classification System, although the classification of the untreated soil subgrades located below the treated layers ranged from CL (clay) to GC (clayey gravel). Plasticity index of the untreated soils ranged from non-plastic to 44 percent. Generally, the untreated soils were moderately plastic. In situ bearing ratios of the soil-cement subgrades were generally very large. Based on a percentile test curve, the in situ bearing ratios of the cement-treated subgrades at the 90th and 50th percent test values were about 24 and 90, respectively. Based on overlay histories of the routes, flexible pavements located on the soil-cement subgrades have performed well. In the older sections, overlays had been constructed every 11 to 14 years. Use of cement to construct stabilized subgrades represents a good design alternative when compared with other stabilizing methods and other design alternatives.

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