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 5 used to analyze the resilient moduli data of subgrades and correlations for resilient modulus property of subgrades and bases. Irwin (1995) also noted the need for semi-log mod- unbound bases. els that capture nonlinear subgrade material behavior under traffic loading and can provide moduli when tensile stresses Both laboratory and field methods as well as correlations are encountered in the subgrades or at the pavement layer are then summarized with respect to their application for interfaces. Other methods of determining subgrade and base pavement design. Reliability of the test procedure, repeat- layer moduli or stiffness properties deal with backcalcula- ability, and limitations as well as new research directions tion of the moduli in the field using nondestructive tests such in these methods are also presented. Hence, this synthesis as Falling Weight Deflectometer (FWD). This approach has is aimed at assisting pavement design engineers in evaluat- been used by several agencies and used in both pavement ing direct resilient property measurements against empirical design and rehabilitation design tasks. correlations-based resilient moduli predictions. The recom- mended practices that could provide high-quality predictions In MEPDG, for pavement design, material and geotech- using the available empirical correlations are also included nical engineers can provide three types of resilient moduli in the synthesis. or stiffness input for both unbound bases and subgrades. The first one is referred to as Level 1 input and it requires Information collected in this synthesis was based on a the determination of moduli of the materials from labora- comprehensive literature review, surveys of pavement design tory or field tests. The second one is known as Level 2 in and materials/geotechnical engineers from state DOTs, and which users determine resilient moduli values from vari- selected interviews. Information collected here included ous empirical relationships developed locally or elsewhere various research reports prepared on resilient moduli stud- to estimate the resilient properties of soils. The last type is ies conducted by several DOTs. referred to as Level 3 in which users assume resilient modu- lus values based on threshold default values for different soil types. Outline of Chapters For more than 30 years, researchers have been develop- This synthesis contains six chapters. Chapter two presents a ing and revising procedures to measure resilient modulus of survey description, survey respondents’ information, survey subgrades and unbound bases either by performing tests on results, and summary statistics. This information is included specimens in the laboratory or using a nondestructive test in in chapter two before other chapters primarily to give a pic- the field or using empirical or semi-empirical relationships ture about how the DOT engineers perceive the current test between moduli and material parameters. Although a large procedures, correlations, and field methods for the determina- number of correlations currently exist, their accuracy and tion of resilient properties of subgrades and unbound bases. robustness are still unknown to the pavement designers who routinely use them. Chapters three and four describe laboratory and field test methodologies and chapter five presents correlations currently provided in the literature for the determination of Synthesis Objectives and Overview the resilient properties. This is followed by a comprehen- sive summary discussion that enlists various DOTs and the This synthesis was initiated to summarize various resilient test practices they followed to determine resilient properties. modulus test procedures using direct and indirect methods Similar discussion is made with respect to field test methods for either measuring or interpreting the resilient properties of both nondestructive and destructive or intrusive methods. of unbound pavement materials and subgrades. For correlations, both direct and indirect correlations cur- rently used or recommended by various DOTs are listed in In direct test measurements, testing procedures currently this chapter. Both advantages and limitations of these prac- reported in the literature are covered; in indirect test meth- tices are included in this chapter. ods, laboratory-based stiffness measurements using geo- physical methods, in situ intrusive tests such as the dynamic Chapter six presents findings from the literature infor- cone penetration (DCP) and cone penetration tests (CPTs), mation compiled in chapters three, four, and five, and sum- and backcalculation approaches utilizing nondestructive marizes the “useful practices” for better determination of tests were summarized and discussed. In each method, both resilient properties of unbound bases and subgrades. This advantages and disadvantages are mentioned. This section is chapter provides a summary of the key findings and oppor- followed by a comprehensive summary of various predictive tunities for additional research needs.