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84 following methods have been found useful, both as a result of measuring or interpreting the resilient properties of unbound high coefficient determination values or the potential to cap- pavement base materials and subgrades. In laboratory test ture the nonlinear resilient behavior of subgrades and bases: measurements, laboratory direct test methods and labora- tory indirect test methods using geophysical measurements • Direct Models for Subgrades and Unbound Bases: are covered. MRDS 3, MRDS 4, and MRDS 6 (Laboratory-Based Correlations) (MR stands for Resilient Modulus and DS In field methods, nondestructive tests, such as FWD, stands for Direct Correlations and Soil Properties–Based GeoGauge, seismic pavement analyzer (SPA), and LWD Relationship); and MRDI-1 (Field Method Correlation) methods, as well as in situ intrusive tests, such as DCP and (MR stands for Resilient Modulus and DI stands for cone penetration tests, are summarized and discussed. In each Direct and In Situ Test–Based Relationship). method, both advantages and disadvantages are mentioned. • Indirect Models for Subgrades and Unbound Bases: This section is followed by a comprehensive summary of MRI2-5 (two-parameter model); and MRI3-3, MRI3-4, direct and indirect correlations for the determination of resil- and MRI3-5 (three-parameter models). ient modulus properties of subgrades and unbound bases. Following are some of the major findings from the present Useful Approaches to Pavement Design synthesis: For new pavement design projects and overlay design, the • The use of MR properties of bases and subgrades in moduli of unbound bases and subgrade are needed. In such pavement design has been increasing among trans- designs, Level 1 input parameters are needed, which can be portation agencies, with some preferring to use direct determined by performing laboratory-recommended RLT testing utilizing triaxial equipment, others using field tests on the quality core specimens retrieved from field devices, and the rest using correlations to predict mod- sites. The AASHTO T-307 method is useful for determining uli. However, there remains a certain amount of skep- moduli values at various combinations of stresses for both ticism among the engineers and practitioners, which unbound granular and subgrades. The design moduli values could be attributed to the constant modifications to the can be established from the measured moduli based on the resilient moduli test procedures, development of new confining and deviatoric stresses that are representative of equipment and devices for laboratory and field mea- the base and subgrade locations in the pavement systems. If surements of moduli, and confusion caused by various several soils exist at a project site, the preferred practice is definitions of moduli measured at different strain lev- to determine the moduli for each soil type. If such practice els. Other factors include poor reproducibility of test is not practical, engineers can select a weak soil type along results and backcalculations and lack of standardized the pavement project site and then test that soil for design procedures. Through the use of surveys and literature moduli. This may result in uneconomical yet safe pavement reviews, it is clear that certain state agencies prefer design sections. using modified standard test procedures, new equip- ment, or both to measure moduli parameters for the Level 1 moduli parameters of bases and subgrades can pavement design practice. States such as Minnesota, also be established from field studies using nondestructive Louisiana, and Texas belong to this category. Such FWD studies or intrusive DCP methods. Field test meth- practices should be encouraged because these states ods discussed in chapter four are useful here. Level 2 design address the appropriate use of measured modulus as a parameters can be determined based on correlations between design modulus for flexible pavement design. stiffness properties and soil conditions. A few correlation • Certain survey respondents expressed concerns with types are given in chapter five. Validation of the correla- respect to long correlations and complicated triaxial tions with local soil database including stiffness properties test methods. Oversimplification will ruin the whole is highly recommended before using them to determine the design practice; however, the use of empiricism to a stiffness values. Level 3 design parameters use assumed soil certain level to simplify the current correlations will properties for pavement design and such practice may not help several DOT agencies better implement the mod- be useful except in cases in which such soil parameters are uli in design practices. approved by the local geotechnical/material engineers in the • The synthesis identified different laboratory methods department. and nondestructive methods as well as field intrusive and in situ LWD methods for determining soil moduli. Among them, the RLT test is the most preferred form of Conclusion laboratory test for repeatable and reliable moduli prop- erty determination. In the field, nondestructive tools This synthesis covers various resilient moduli tests and field such as FWD and an intrusive method such as DCP are procedures through direct and indirect methods for either preferred field test methods. The latter was also used