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Chapter 6. Conclusions and Recommendations

Summary

In this research project, comprehensive field and laboratory evaluations were performed on subgrade soils at nine different sites representing typical primary roads in the State of Wyoming. Resilient modulus (MR) values were obtained from: laboratory testing based on 41.4-kPa (6-psi) and actual field deviator stresses, back calculation based on three different computer programs, and the AASHTO equation based on deflection measurements. In addition, several laboratory tests were conducted to examine the fundamental soil properties of the subgrade soils included in this study. These soil properties included: water content (actual and optimum), AASHTO soil classification, group index, and plasticity index. Three different MR values (AASHTO, LAB, and FIELD) obtained at each site were then used to determine the required overlay thicknesses. Finally, all of the resulting data were used in conducting comprehensive data analyses. The following conclusions can be drawn from this research:

  1. Subgrade soil samples should be extracted from Shelby tubes shortly after obtaining them from the field.
  2. MR measurements made with the LVDT's on the ring located inside the testing chamber consistently gave higher values compared to the actuator LVDT's located on the loading piston.
  3. The EVERCALC back calculation program appears to give somewhat better MR values than do the MODULUS and BOUSDEF programs.
  4. Some fundamental soil properties do influence the measured MR value. Resilient modulus values for type A-4 and A-6 subgrade soils in this study decreased as water content increased.
  5. Layers within Shelby tubes do not differ significantly from one another. Therefore, averaging the resilient modulus values from all layers will give more reliable results compared to the value from one layer.
  6. The recommended correction factor (C) of 0.33 or less appears to be adequate for cohesive subgrade soils in the State of Wyoming.
  7. MR values based on actual deviator stresses did not statistically differ from values based on the assumed deviator stress of 41.4-kPa (6-psi). However, by computing actual deviator stresses, the resulting MR values within each testing site were more consistent.
  8. The three MR values calculated based on AASHTO equation, laboratory with 41.4-kPa (6-psi) deviator stress, and laboratory with actual deviator stress did not result in significantly different overlay thicknesses. Among the three, however, the AASHTO MR value gave the lowest overlay thicknesses.

Recommendations for Needed Future Research

  1. Because this study was limited to cohesive subgrade soils, it would be of interest to conduct a similar research project on granular subgrade soils.
  2. The effect of resilient modulus selection on the design of new pavement structures should also be evaluated.

References

  1. American Association of State Highway and Transportation Officials (AASHTO). (1993). AASHTO Guide for Design of Pavement Structures. Washington, DC: Author.
  2. American Association of State Highway and Transportation Officials (AASHTO). (1992). Interim Specifications for Transportation Materials and Methods of Sampling and Testing, Part II, Interim Test Methods. Washington, DC: Author.
  3. American Association of State Highway and Transportation Officials (AASHTO). (1986). Standard Specifications for Transportation Materials and Methods of Sampling and Testing. Washington, DC: Author.
  4. Ali, N.A. & Khosla, N. P. (1987). Determination of Layer Moduli Using a Falling Weight Deflectometer. In Transportation Research Record (No. 1117, pp. 1-9). Washington, DC: Transportation Research Board, National Research Council.
  5. The Asphalt Institute. (1982). Research and Development of the Asphalt Institute's Thickness Design Manual (Manual Series No. 1, 9th ed.). College Park, MD: Author.
  6. The Asphalt Institute. (1978, March). Soils Manual for the Design of Asphalt Pavement Structures (Manual Series No. 10). College Park, MD: Author.
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  8. Darter, M.I., Elliott, R.P., & Hall, K.T. (1992). Revision of AASHTO Pavement Overlay Design Procedures, Appendix: Documentation of Design Procedures, Final Report. Washington, DC: Prepared for National Cooperative Highway Research Program, Transportation Research Board, National Research Council.
  9. De Jong, D.L., Peatz, M.G.F., & Korswagen, A.R. (1973). Computer Program BISAR Layered Systems Under Normal and Tangential Loads (External Report AMSR.0006.73). Amsterdam: Konin Klijke Shell-Laboratorium.
  10. Elfino, M.K. & Davidson, J.L. (1989). Modeling Field Moisture in Resilient Moduli Testing. Resilient Moduli of Soils: Laboratory Conditions (ASCE STP No. 24, pp. 31-51) New York, NY: American Society of Civil Engineers.
  11. Elliott, R.P. (1992). Selection of Subgrade Modulus for AASHTO Flexible Pavement Design. In Transportation Research Record (No. 1354, pp. 39-44). Washington, DC: Transportation Research Board, National Research Council.
  12. Heukelom, W. & Klomp, A.J.G. (1962). Dynamic Testing as a Means of Controlling Pavements During and After Construction. Proceedings of the 1st International Conference on the Structural Design of Asphalt Pavements (pp. 667-685).
  13. Highway Research Board (HRB). (1962). Special Report 61E - The AASHO Road Test, Report 5: Pavement Research. Washington, DC: Author, National Research Council.
  14. Huang, Y.H. (1993). Pavement Analysis and Design. Englewood Cliffs, NJ: Prentice Hall, Inc.
  15. Hveem, F.N. (1949, April 14). A New Approach to Pavement Design. Engineering News-Record, p. 77.
  16. Lee, S.W., Mahoney, J.P., & Jackson, N.C. (1988). Verification of Backcalculation of Pavement Moduli. In Transportation Research Record (No. 1196, pp. 85-95). Washington, DC: Transporation Research Board, National Research Council.
  17. Mahoney, J.P., Newcomb, D.E., Jackson, N.C., & Pierce, L.M. (1991, September 17-19). Pavement Moduli Back Calculation Shortcourse. Presented in Reno, NV.
  18. Neter, J., Wasserman, W., & Kutner, M.H. (1990). Applied Linear Statistical Models: Regression, Analysis of Variance, and Experimental Designs (3rd ed.). Homewood, IL: Richard D. Irwin, Inc.
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  20. Seed, H.B., Chan, C.K., & Lee, C.E. (1963). Resilience Characteristics of Subgrade Soils and Their Relation to Fatigue Failures in Asphalt Pavements. In Proceedings, International Conference on Structural Design of Asphalt Pavements (pp. 611-636). Ann Arbor, MI: University of Michigan.
  21. Snedecor, G.W. & Cochran, W.G. (1989). Statistical Methods (8th ed.). Ames, Iowa: Iowa State University Press.
  22. Stoffels, S.M. & Lytton, R.L. (1987). Development of a Utility Evaluation for Nondestructive Testing Equipment Used on Asphalt Concrete Pavements. In Transportation Research Record (No. 1117, pp. 134-142). Washington, DC: Transportation Research Board, National Research Council.
  23. TAFFY [computer program]. (1988, February). Denver, CO: Colorado Department of Transportation.
  24. Thompson, M.R. & Robnett, Q.L. (1976). Resilient Properties of Subgrade Soils, Final Report-Data Summary (Transportation Engineering Series No. 14, Illinois Cooperative Highway Research and Transportation Program Series No. 160). Urbana-Champaign, IL:  University of Illinois at Urbana-Champaign.
  25. Uddin, W. (1984). A Structural Evaluation Methodology for Pavement Based on Dynamic Deflections. Unpublished doctoral dissertation, The University of Texas at Austin, TX.
  26. Uzan, J., Scullion, T., Michalek, C.H., Paredes, M., & Lytton, R.L. (1988, September). A Microcomputer Based Procedure for Backcalculating Layer Moduli from FWD Data (Research Report 1123-1). College Station, TX: Texas Transportation Institute.
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  30. Zhou, H., Hicks, R.G., & Bell, C.A. (1990). BOUSDEF: A Backcalculation Program for Determining Moduli of a Pavement Structure. In Transportation Research Record (No. 1260, pp. 166-179). Washington, DC: Transportation Research Board, National Research Council.

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Acknowledgment | Disclaimer | Preface

MPC Report No. 94-34
Selection of Subgrade Modulus for Pavement Overlay Design Procedures

Khaled Ksaibati
Michael L. Whelan
James M. Burczyk
Michael J. Farrar

August 1994


Mountain-Plains Consortium
www.mountain-plains.org