Study of Long-Term Pavement Performance (LTPP): Pavement Deflections

Appendix D. FWD SN 129, April 15, 1997–May 21, 1997

This sensor position error was recently identified using an automated screening version of SLIC, and reported in Feedback Report RNS–2M (see appendix A). This relatively short-lived misreporting of the actual position of d7 was not detected during the previous work done that resulted in Feedback Report RNS–2.

The automated version of SLIC used a model that was specifically chosen to predict the position of sensor 7 with a close to zero overall bias and the best possible precision (see appendix B). The first graph shown in this appendix (figure 16) is a plot of all the SN #129 d7 sensor position predictions during 1997 and 1998 for lane 1, drop height 4 FWD tests.
In figure 16, it can be seen that the average prediction for this 2-year period was around 152.4 cm (60 inches), as expected. The predictions are somewhat scattered, mainly due to the relatively large distance between d6 and d7. However, during the period of time in question (April 15, 1997–May 21, 1997), the position of d7 is clearly around 121.9 cm (48 inches) (average SLIC prediction for all flagged test dates = 121.9 cm (48 inches)), where in fact an empty sensor holder was usually positioned.

In figure 17, it can be seen clearly that the SN #129 predicted positions for sensor 7 in the spring of 1997 are outliers relative to the predicted positions for sensor 7 when other (correctly configured) FWDs are used. For the five test sections shown (from two different FWDs), the average predicted position of d7 was 154.2 cm (60.7 inches), while the average prediction for SN #129 during the period of time in question was 123.2 cm (48.5 inches).

In the two following graphs in this appendix, figures 18 and 19, respectively, the same results are shown graphically, with lines and data points labeled 5/13/1997 in figure 18 and 4/15/1997 in figure 19 showing the SLIC plot for d7 in both its actual position (121.9 cm (48 inches)) and the protocol but incorrect offset position (152.4 cm (60 inches)). The portions of the 5/13/1997 and 4/15/1997 lines that are parallel to the rest of the data are the correct plots, with d7 set at 121.9 cm (48 inches). Because of this information, it can be concluded with certainty that d7 was not positioned correctly at 152.4 cm (60 inches); rather, it was positioned at 121.9 cm (48 inches) (or very close to 121.9 cm (48 inches)) on FWD SN #129 between April 15, 1997 and May 21, 1997, inclusively. These dates correspond to the dates when lane 1 tests were conducted at drop height 4. This period of time may need to be extended somewhat if other tests were conducted along different lanes or at different drop heights. In any case, FWD tests conducted on or before March 25, 1997, and on or after June 3, 1997, clearly show d7 positioned at 152.4 cm (60 inches) (as per protocol).

Figure 16. Graph. Predicted position of d7, unit #129, 1997–98.

Figure 17. Graph. Same section data for d7 position, two different FWDs.

Figure 18. Graph. SLIC plots for section 24–0509 including unit #129, May 1997.

Figure 19. Graph. SLIC plots for section 36–4017 including unit #129, April 1997.