Portable FWD (Prima 100) for in-situ subgrade evaluation.
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Portable FWD (Prima 100) for in-situ subgrade evaluation.

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    • Alternative Title:
      Portable falling weight deflectometer (Prima 100) for in-situ subgrade evaluation
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    • Edition:
      Final report.
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    • NTL Classification:
      NTL-HIGHWAY/ROAD TRANSPORTATION-Pavement Management and Performance
    • Abstract:
      Subgrade soil characterization measured in terms of resilient modulus, MR, has been a prerequisite for pavement design. For new pavement design, MR is obtained by conducting repeated load triaxial tests on reconstituted/undisturbed cylindrical specimens, employing AASHTO T-307 test protocol. Because of the complexities encountered with the test, in-situ tests would be desirable, if reliable correlation can be established. Subgrade characterization for rehabilitation selection, however, in-situ tests are the norm than the exception. The focus of this study is to investigate the viability of Prima 100, a Portable Falling Weight Deflectometer (PFWD), for direct testing of subgrade with the objective of estimating resilient modulus, via a correlation between MR and PFWD modulus (EPFWD). Thirteen as-built subgrade sections reflecting typical subgrade soil materials in Mississippi, were selected and tested for elastic modulus employing a Falling Weight Deflectometer (FWD), followed by PFWD. In-situ unit weight and moisture were measured using a nuclear device. Soil samples collected from those sections were subjected to repeated load triaxial test (AASHTO T307) and to other routine laboratory tests for classification purposes. The first step in analyzing the data was to authenticate the Prima elastic modulus, which was accomplished by establishing an acceptable relation between Prima modulus and FWD modulus (EFWD). A statistically significant relation between MR and EPFWD and was derived, though three other explanatory variables emerged in the model equation. Since moisture and density of in-situ material rarely match those prescribed in the repeated load test sample, those two attributes were included in the model. A third variable, which was soil-related (namely, PI/P200), emerged to account for the range of soil types, and intentionally retained in the model equation. A similar, but abbreviated version of the model was also derived, deleting the soil-related variable. An investigation of the significance of unit weight and moisture on the Prima modulus resulted in a correlation equation between EPFWD and those two variables. An exclusive program, PFWDSUBGRADE was developed to analyze Prima modulus and calculate resilient modulus. The program, in addition to calculating station-by-station resilient modulus, relying on what is known as "cumulative difference" technique, delineates 'homogeneous' subsections of the project, outputting mean and standard deviation of the resilient modulus for each homogeneous section. A graphical plot of resilient modulus of each station is another output of the program.
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