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65 DMT test data. Similar correlations were also obtained • Soil-specific correlations are not developed for pre- between DMT moduli and initial tangent moduli. Borden dicting the resilient moduli properties. However, the et al. (1986) suggested the need for additional research to use of DCP correlations should be carefully consid- validate these relationships. No other resilient moduli stud- ered because the configurations of these devices are ies utilizing DMTs in the United States were documented in not standardized in the highway engineering commu- the literature. nity. ASTM recently standardized the DCP method in ASTM D6951-03 for shallow pavement applications, and hence the use of the standardized DCP device will Summary result in a reliable DCP test database. • Quasi-static cone penetrometer test has been used in The previous sections provide comprehensive details of one study, which revealed the potential of this method various in situ methods to determine the resilient moduli to predict the resilient properties of cohesive subgrades. properties of subgrades and bases. Though good correlation was obtained between CPT and MR results, this correlation requires further valida- • The majority of the intrusive equipment is used for tion with the results from clayey soils other than those the interpretation of resilient moduli of subgrades that used in the research. include both coarse-grained, fine-grained, and mixed • PMT, DMT, and PLT have been sparingly used for soils. DCP is the most prominently used in situ intru- the estimation of resilient properties. The results from sive device, and it has been used by several DOTs for these studies are good to promising. No additional compaction QC/QA tests of bases and subgrades and studies are reported, however, probably because of high determination of layer moduli. More than 50% of the costs and lengthy time of operations as well as training state DOTs used this device for the previously men- of operators to perform these tests on a routine basis. tioned applications. With the advances in instrumentation and automation, • Correlations that use DCPs are empirical and site spe- these methods could become attractive and practical cific. Their use for other sites requires considerable for pavement subgrade exploration and in situ evalua- engineering judgment. tion of resilient properties.