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56 Table 10 Stiffness Results (Nazarian et al. 2004) An implementation project conducted for TxDOT dis- Final Summary on NonDestructive Methods cussed the development of DSPA to determine the moduli of bases and subgrades. A step-by-step measurement procedure The following list summarizes the salient findings from the was also developed (Nazarian et al. 2006). This procedure DOT-funded studies related to resilient moduli property allows a rapid measurement and interpretation of moduli. interpretations using nondestructive test methods: This device was also used for field compaction QC. Typical test data showing the field target moduli are presented in • Among nondestructive field methods, most DOT Figure 65. agencies use FWD tests for moduli determination of pavement layers. Both KUAB and Dynatest devices The techniques can be used to address the stiffness varia- are primarily utilized. Also, these agencies use dif- tions with moisture fluctuations and develop design moduli ferent backcalculation programs to analyze the FWD assessments that account for the variations of moduli with results. Because the predictions of moduli using the moisture changes. Overall, DPSA is still new to the nonde- FWD do not match the laboratory resilient moduli of structive field in pavement geotechnics and more studies are base and subgrade layers, the 1993 AASHTO Guide needed to address the full potential of this handy device, for Pavement Design recommended that a fraction of which can provide stiffness parameters in a quick turn- FWD moduli be used as the design resilient modulus around time. value for the mechanistic pavement design. Research studies supported by Mississippi, Minnesota, Texas, and other DOTs confirmed these variations, and the range of variation appears to depend on several fac- tors, including soil type, backcalculation programs used, and others. • The moduli definitions used by various tests in the mechanistic pavement design explain the variations with respect to strain levels. At small shear strains, the shear moduli and related elastic moduli are high and constant. These moduli will then decrease with an increase in shear strain levels. The strains imposed on bases and subgrades using nondestructive field test methods are close to small strains and hence moduli values are large when compared with laboratory RLT tests, where the strains are of medium range. Nazarian et al. (1996) and Edil and Benson (2005) provided more FIGURE 65 Field seismic modulus for compaction QC insights into the variations in moduli at various shear (Nazarian et al. 2006). strain levels.