Selection of design strengths of soil subgrades and subgrades treated with cement or hydrated lime is a problem in pavement design analysis and construction because a variety of different types of soils may exist in a highway corridor and a wide range of different strengths may exist after the soils are compacted to form the pavement subgrade. The selected subgrade strength will largely affect the pavement thickness obtained from the design analysis, the future pavement performances, and the overall bearing capacities of the subgrade during construction and the pavement structure after construction. In developing the proposed selection scheme, a newly developed mathematical model, based on limit equilibrium, is used. Relationships between undrained shear strength (and CBR) and tire contact stresses are developed for factors of safety 1.0 and 1.5. The minimum subgrade strength required to sustain anticipated construction tire contact stresses to avoid bearing capacity failures of the subgrade and partially constructed pavements during construction is determined. Also, a criterion is proposed for determining when subgrade stabilization is needed. Methods of selecting the design subgrade strength are examined. A previously published method, based on a least cost analysis, appears to be an appropriate approach as shown by analysis of a case study involving pavement failures during construction. Two case studies show that soaked laboratory strengths appear to be fairly representative of long-term field subgrade strengths. Hence, using soaked laboratory strengths and least cost analysis appears to be reasonable means for selecting the design strength of subgrades for pavement analysis. When chemical stabilization is used, it is suggested that the net strength gain obtained at the end of a 7-day curing period may be used in the pavement design analysis. To avoid failures of chemically stabilized layers, relationships between thicknesses of chemically treated layers and the CBR values of the untreated subgrade for a factor safety of 1.5 are presented. Layer coefficients (a3), based on 7-day strengths, are also presented for hydrated line-and cement-treated subgrades.

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The Contents of this report reflect the views of the author, who is responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the University of Kentucky, the Kentucky Transportation Cabinet, nor the Federal Highway Administration. This report does not constitute a standard, specification, or regulation.