Pavement Structural Analysis of the Design Recommendations for ...

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Pavement Structural Analysis August 2004 Report No. 15953-2/1 ARA-ERES Consultants Michigan’s experience and should be conservative for the existing foundation soil that will not be removed during the reconstruction of this segment along I-96. Repeated load resilient modulus tests of similar soils, however, are available in the LTPP database for various sites in Michigan. Figure 5 shows the average test results for this type of soil, which are similar to those estimated from correlations developed by Von Quintus, et al. (4) Thus, the repeated load resilient modulus test results shown in figure 5 were used to estimate the design resilient modulus for the foundation soil for both M-E design procedures. Resilient Modulus, ksi 10 9 8 7 6 5 4 3 2 1 0 0 5 10 Cyclic Deviator Stress, psi 6 Confinement = 2 psi Confinement = 4psi Confinement = 6 psi Figure 5 Average repeated load resilient modulus test results recovered from the LTPP database for silty clay soils, similar to those encountered along I-96. The correlations noted above only represent a best-guessed value for design. The design values used in the M-E design procedures are greater than the value suggested for use by the Michigan DOT (3,000 psi or 20,684 kPa) in the AASHTO DARWin program. The difference in these design resilient modulus values will be discussed in greater detail in a latter section of this report. To confirm the design resilient modulus values for the in place soils, however, it is suggested that repeated load resilient modulus tests be performed in the laboratory on undisturbed or re-compacted test specimens. If repeated load resilient modulus tests are not possible, deflection basins can be measured along the existing roadway and the elastic modulus back-calculated for the subgrade soils using the procedure used by Von Quintus et al, for LTPP. (5,6) If modulus values are back-calculated from deflection basins along I-
Pavement Structural Analysis August 2004 Report No. 15953-2/1 ARA-ERES Consultants 96, they should be adjusted using the correction factors recommended by Von Quintus, et al. (6) Based on the boring logs provided, the subgrade soils encountered along the section of I- 96 are believed to have a frost susceptibility classification of moderate or medium using the Corps of Engineers classification system (refer to figure 6). (7) It is suggested that a non-frost susceptible material be placed above the subgrade to minimize the potential for frost heave over time. Thus, a minimum of 36 inches (914 mm) of non-frost susceptible materials were included in the pavement cross-sections analyzed in this study. Results from the subsurface investigations did not indicate ground water at the time of drilling. Seasonal variation in ground water is expected for this area. Thus, subsurface drains were assumed in the design computations for determining the required layer thickness. It is understood that subsurface drains and a geotextile fabric-separator are included in the planned reconstruction. 3.3 Non-Frost Susceptible Material For this climatic area, the Michigan DOT requires that 36 inches (914 mm) of non-frost susceptible material be placed above any frost-susceptible soil based on historical data and experience. The thickness of non-frost susceptible material requirement was assumed for this design study and not re-evaluated, as noted above. Two unbound aggregate materials are available for use in the reconstruction of this segment along I-96: a class IIA sand subbase and a 21AA-MOD crushed stone aggregate base. Resilient modulus tests were completed and are available for similar materials from the FHWA-LTPP database for test sections in Michigan. This laboratory data was used to estimate the resilient modulus for each of these materials, similar to the method used to develop Pavement Structural Design Study – A Simplified Catalog of Solutions. (2) A geotextile fabric should be used as a separator layer between the crushed aggregate base and sand subbase. Sand Subbase Material It is understood that the existing sand material encountered in the borings along I-96 will be replaced with Class IIA material for the flexible pavement design option. Resilient modulus tests on the sand proposed for use were unavailable. However, repeated load resilient modulus tests performed on materials classified as sand subbases were previously extracted from the LTPP database. Figures 17 to 20 in Appendix B graphically present the distribution of the resilient modulus measured at specific stress states. It is important to note that the distributions appear to be normal for those groups with a sufficient number of tests. This normal distribution of resilient modulus values at specific stress states is also applicable to other unbound materials that have a sufficient number of resilient modulus tests. 7
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