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Qualitative Rating System for Drainability of Roadway Base Materials

Poor drainage of roadway base/subbase materials can lead to increased pore water pressure, reduction of strength and stiffness, and freeze-thaw damage. Base course drainability is dependent on physical properties of the material that affect its water flow and retention behavior including particle size distribution, fines content, density or porosity, the geometric and boundary conditions of the pavement system, and site-specific environmental conditions. Objectives of this project are to quantitatively assess permeability and water retention characteristics of representative roadway base materials, to derive predictive equations for indirect estimation of material properties that control drainability, and to develop and recommend rating systems for assessing more general base materials. Laboratory tests were conducted on 16 samples of materials used in or considered for use in roadway applications to determine grain size distribution, hydraulic conductivity, and soil-water characteristic curves. Results are correlated to grain size characteristics including percent gravel, percent fines, grain size indices (e.g., D10, D30), and unit weight. Procedures are provided to qualitatively assess drainability as “excellent,”“marginal,” or “poor,” from grain size, thereby offering a rationale to reduce pavement life cycle costs, improve safety, realize material cost savings, and reduce environmental impacts.