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Pavement Structural Analysis August 2004 Report No. 15953-2/1 ARA-ERES Consultants C.2 Unbound Layer Modulus Determination The second part of Appendix C includes a summary of the procedure and responses used to determine the resilient modulus for each unbound layer. In other words, select an elastic modulus for the unbound materials and soils to ensure that the theory (EVERSTRS) and laboratory provide consistent values at the same stress state for use in the fatigue analysis. One of the important steps in this process is to include the at-rest stresses with those computed from EVERSTRS. Table 6 provides a summary of the information and data that were used to compute the overburden pressures and at-rest stresses in each unbound layer. Table 6 Data used to calculate the overburden pressure and at-rest stresses in each unbound layer of the design cross section (refer to figure 1). Layer/Material Type Dry Density, pcf 38 Moisture Content, % At-Rest Earth Pressure Coefficient, ko HMA; average of all three layers 150 --- --- 21 AA-MOD Crushed Aggregate Base 130 7.0 0.9 Class II-A Sand Subbase 136 7.0 0.9 Low Plasticity, Firm Silty Clay 116 13.0 0.5 Table 7 summarizes the EVERSTRS computations from two iterations of using trial elastic modulus values for each unbound layer. The two iterations demonstrate the need to check the values used in the elastic layer program to ensure that the theory and laboratory values provide consistent results. As summarized in table 7, the first iteration used the design resilient modulus suggested for use by the Michigan DOT and the maximum layer modulus ratio to estimate the elastic modulus of all other unbound layers (refer to figure 15). The trial or “guessed” resilient modulus values are not consistent with the values that would be measured in the laboratory from repeated load tests (figures 5,7, and8). However, the modulus values used in the second iteration are, for all practical purposes the same values that would be measured in the laboratory at the same stress states. The values from the second iteration were used in the fatigue cracking analysis.
Pavement Structural Analysis August 2004 Report No. 15953-2/1 ARA-ERES Consultants Table 7 Comparison of the trial elastic layer modulus used in EVERSTRS with those measured from repeated load resilient modulus tests in the laboratory at the same stress state. 1 2 Iteration No. & Unbound Layer Trial E- Value, ksi At-Rest Stress @ ¼ Layer Depth, psi XX & ZZ YY Stresses Computed w/EVERSTRS @ ¼ Layer Depth, psi ZZ XX YY 39 Confining Stress Total Stress State, psi Deviator Stress Bulk Stress Lab E- Value, ksi Silty Clay* 3 -4.55 -2.28 -0.30 -0.04 -0.04 2.32 2.53 --- 5.1 Sand Subbase 9 -2.68 -2.41 -0.50 0.37 0.39 2.02 --- 7.24 6.8 Aggregate Base 20 -1.54 -1.39 -1.21 0.42 0.51 0.88 --- 4.60 6.5 Silty Clay* 5.2 -4.55 -2.28 -0.41 -0.07 -0.07 2.69 2.61 --- 5.2 Sand Subbase 7.0 -2.68 -2.41 -0.64 0.02 0.04 2.37 --- 8.08 7.2 Aggregate Base 7.5 -1.54 -1.39 -0.97 -0.11 -0.07 1.46 --- 5.47 7.5 *NOTE: The stress state was determined at the ¼ depth within each unbound layer, with the exception for the foundation or subgrade soils. The stress state was determined 18-inches into the subgrade. These depths are consistent with the values recommended for use by Von Quintus, et al. (10) The at-rest stresses were computed using the information and data included in table 6.
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