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

Appendix C. FWD SN 129, November 3, 1995–April 14, 1996

This previously reported sensor position error was also identified using the automated version of SLIC (see appendix B). This procedure incorporated 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. The first graph shown in this appendix (figure 12) is a plot of all the SN #129 d7 sensor position predictions during 1995 and 1996 for lane 1, drop height 4 FWD tests.

In figure 12, 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 due to the relatively large distance between d6 and d7. However, during the period of time in question (November 3, 1995–April 14, 1996), the average predicted position of d7 is clearly around 121.9 cm (48 inches) (average SLIC prediction for all flagged test dates = 123.2 cm (48.5 inches)). In fact, a sensor holder was positioned at 121.9 cm (48 inches).

The second figure in this appendix (figure 13), clearly shows that the SN #129 predicted positions for sensor 7 in the fall of 1995 are extreme outliers relative to the predicted positions for sensor 7 when other (correctly configured) FWDs are used. For the six test sections shown (from two different FWDs), the average predicted position of d7 was 157 cm (61.8 inches), while the average prediction for SN #129 during the period of time in question was 124.7 cm (49.1 inches).
In the two following figures (see figures 14 and 15), the same results are shown graphically, with the lines and data points labeled 11/9/1995 showing the SLIC plot for d7 in both its actual (121.9 cm (48 inches)) and protocol but incorrect (152.4 cm (60 inches)) offset position. The portions of the 11/9/1995 lines that are parallel to the rest of the protocol position data are the correct plots, with d7 set at 121.9 cm (48 inches).

Because of this information, and the other sensor position information supplied to FHWA, 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 November 3, 1995, and April 14, 1996.

Figure 12. Graph. Predicted position of d7, unit #129, 1995–96.

Figure 13. Graph. Same section data for d7 position, two different FWDs.

Figure 14. Graph. SLIC plots for section 34–0503 including unit #129, November 1995.

Figure 15. Graph. SLIC plots for section 34–0507 including unit #129, November 1995.