Evaluation of the LWD for in-situ determination of pavement layer moduli

1. Evaluation of the LWD for in-situ determination of pavement layer moduli M. Shabbir Hossain, Ph.D., P.E. Alex K. Apeagyei, Ph.D, P.E. GRAC Meeting APRIL 5, 2010

2. Introduction • Soil modulus is a key input parameter for design. • Current QC/QA specifications is density-based. • Two main reasons account for this: • Long history of and experience with density-based specs and, • Lack of a reliable field soil modulus measurement device. • The LWD is being promoted as a QC tool because: • It is portable, and • It is able to measure soil modulus directly

3. Purpose and Scope • The purpose was to evaluate the LWD for in-situ determination of soil modulus. • The objectives were to: • Determine soil modulus using LWD, GeoGauge, and DCP. • Compare measured modulus between devices • Investigate effects of density and moisture on modulus.

4. Geo Gauge DCP

5. LWD

6. Test Sections Seven sections in five VA counties

7. Results and discussions • Field testing: • Effects of density. • Effects of moisture. • Spatial variability. • Laboratory testing: • Effects of density. • Effects of moisture.

8. Field testing – Density effects Route 644 -subgrade

9. Field testing – Density effects Route 743 - subgrade

10. Field testing – Density effects Route 782 – gravel road

11. Field testing – Density effects Route 785 – gravel road

12. Field testing – Moisture effects Route 644 -subgrade

13. Field testing – Moisture Effect Route 743 - subgrade

14. Field testing – Correlation among devices 85th percentile

15. Field testing – Correlation among devices Field testing – Correlation 85th percentile

16. Field testing – Correlation among devices Field testing – Correlation 85th percentile

17. Lab testing: Effect of Moisture and Density 85th percentile

18. Conclusions • All pavement sections exhibited high spatial variability in the measured stiffness modulus. • Modulus values measured by the three devices do not appear to be correlated to each other. • Good correlation observed when the 85th percentile modulus of the three devices are compared. • None of the devices showed any consistent influence of dry density on the measured stiffness. It is important to note that the dry densities also did not vary significantly.

19. Conclusions (2) • Moisture content had a significant effect on the measured stiffness values for all three devices especially the LWD. • From the limited laboratory testing, the high LWD moduli could be attributable to the effect of pore water pressure buildup during testing.

20. Recommendations • VDOT’s Materials Division should not use LWD for QC/QA of pavement subgrade and bases until further research has determined the causes of the high spatial variability in measured soil modulus observed in this study. • VTRC should consider more investigation of LWD to find out the effect of moisture content on measured modulus values. Once this investigation is complete, VDOT Materials Division and VTRC should come up with a specification to use LWD for construction QC/QA for both base and subgrade based on VTRC findings from further research.

21. Suggestion for further research • The high variability in LWD moduli was found to be related to the development of pore water pressure during testing and the presence of soil capillary suction, even within allowable M.C. (+/- 20% or 2%). • Additional studies are needed to evaluate the effect of saturation/moisture on LWD moduli before the LWD could be adopted for routine testing by VDOT. • Therefore, it is recommended that the Virginia Transportation Research Council (VTRC) and VDOT’s Materials Division perform additional well-controlled laboratory testing to evaluate further the effect of soil saturation on LWD moduli along with field verification.

22. The End Thank you!