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83 CHAPTER SIX USEFUL PRACTICES, CONCLUSIONS, AND FUTURE RESEARCH NEEDS Useful Practices for Determining Resilient Properties Level 2 and Level 3 types of MR design inputs for Mecha- nistic-Empirical Pavement Design Guide (MEPDG). Seis- mic laboratory methods using bender element or resonant One of the objectives of this synthesis is to list various prac- column tests are also successfully used by certain agencies tices for determining the resilient properties of materials. Of to determine soil moduli for pavement design and other these, certain practices are identified as particularly useful for applications. Such practices are best left to those agencies laboratory methods, field nondestructive and intrusive testing, for their consideration. and modeling correlations to determine resilient properties. Identification of these methods is based on the comprehensive Field Methods—Nondestructive (Level 1 Parameters) literature review as well as the survey responses from vari- ous departments of transportation (DOTs). These methods Falling Weight Deflectometer (FWD), a field nondestructive should not be construed as endorsements by this synthesis test, is useful as a field method for determining moduli of study. Instead, the state DOTs are strongly urged to develop both subgrades and bases. This approach has been used by their own practices by considering those recommended here several DOTs with reasonable success, as observed from the and then evaluating them to determine the realistic resilient survey results presented in chapter two. Different backcal- moduli or stiffness results of their local soils. culation software is successfully used by DOTs to analyze FWD data to determine the backcalculated moduli. Hence, Laboratory Methods (Level 1 Parameters) no single software is singled out for FWD backcalculation analysis. Among the laboratory tests, a useful method is to perform a standardized test method utilizing the repeated load triaxial Also, several portable Light Falling Weight Deflecto- (RLT) equipment. AASHTO test procedure T-307 can be meters (LWDs) are used for both estimation of moduli employed to perform Resilient Modulus (MR) tests on both and determination of compaction quality of subgrades and subgrades and unbound bases under laboratory conditions. bases. More research on these LWD methods will provide For unbound bases, procedures developed for granular mate- better evaluation of their potential to interpret field moduli, rials in the T-307 method can be used. which may lead to future implementation of this method for pavement design. At least three tests might be conducted on identical soil specimens for each type of soil encountered at the project Field Methods—Intrusive (Level 1 Parameters) site, and the average moduli results from these three tests can be used for the pavement design. For subgrade specimen The dynamic cone penetration (DCP) method is a useful tool preparation, an impact compaction method may be used. for in situ evaluation of stiffness and also to address com- For base specimens, a similar method or vibratory compac- paction quality of subgrades. Several local correlations are tion is recommended. To reduce system compliance errors, developed and used by various DOTs for determining the in particular when testing stiffer and base materials, end stiffness properties of local soils, and hence there is no single grouting may be used for the soil specimens. Both internal best practice or correlation type for determining moduli of and external displacement monitoring systems may be used. local soils. Though other in situ test methods have been used Although the former method provides slightly higher mod- for MR studies, their usage and application potentials are yet uli, the practical problems of installation and slipping during to be addressed in a comprehensive manner. testing necessitate the use of external linear variable differ- ential transformers (LVDTs) for displacement monitoring. Correlations—Direct and Indirect (Level 2 Parameters) Other laboratory testing methods, including California Although a large number of correlations currently exist, their Bearing Ratio value, R value, and Soil Support Value, and accuracy is still unknown to pavement designers. Indepen- other soil properties, are used to determine MR by means of dent assessments or validation studies with the local soil indirect correlations. Such practice will lead to the use of database may be conducted before using them. Overall, the