Interface - The Journal of Wheel/Rail Interaction

PERFORMANCE-BASED ASSESSMENT

Using Performance-Based Assessment of Reserve Gauge Strength to Enhance Tie-Replacement Planning (continued)

The strip chart shown in Figure 3a shows the measurements of unloaded and loaded gauge, curvature and cross level, and GWP and “Projected Loaded Gage” (PLG), which are both GRMS measurements. The bottom line on the chart represents crosstie counts selected on the basis of the measured reserve gauge-strength performance.

This is essentially a visual tie plan for one mile of track. The thresholds selected for this mile are: GWP> 0.85”, cluster lengths of 4 or more ties in a row exceeding the GWP threshold, and an estimated replacement rate of 33% within the clusters that are found. This plan was developed on a Class 4 track for the purpose of improving short-term track strength condition. Under this plan, minimal tie maintenance was required.

Figure 3b illustrates the same track with a major tie replacement plan. This visual tie plan was developed around the following parameters: GWP > 0.525” (which is well below the minimum safety standard), clusters of 3 or more ties (to focus in at short lengths of track that are weak), and a higher estimated replacement rate of 45% with the clusters. The Holland/Industrial Metrics Rangecam software program that was utilized provides full flexibility when it comes to all of these parameters. Obviously, much more track comes into requirements for replacement ties on the basis of the thresholds featured in figure 3b.

Both of the preceding models were purely theoretical and did not include any visual tie inspection component. The models were based on the common experience that significant detrimental track conditions in wood tie track are almost always associated with the need to replace crossties. To verify the findings, these concepts were taken into the field for evaluation against visual inspection. The inspection was conducted on class 3 and 4 track with curves of up to 6 degrees and annual tonnage of approximately 15 mgt.

After the completion of a track-strength evaluation of 57 track miles, the performance-based assessment was compared to a normal walking visual inspection. The railroad’s cluster length (4 ties in a row or more) and replacement percentage logic (45%) was input into the software, without knowing where to set the GWP threshold. During this trial, the approach was to back into the GWP threshold for this evaluation by matching up to the total tie counts generated from the tie inspecor’s visual inspection.

The railroad’s tie-replacement count, which was based on a tie inspector’s visual inspection, was 7,981 ties over 57 miles of Class 4 track. With this information, the GWP threshold was modified in the software until we got close to that total tie count for the 57 track miles. Figure 4 shows the 57 track miles in question laid out in a comparison on a mile-by-mile basis. As expected, the software’s ties counts were very close to those of the railway’s tie counts over many miles. But the counts varied significantly within several miles.

In order to sort out the differences, the data from the software was taken to the field. Fifteen track segments were inspected. Figure 5a shows mile 173 of the track that is quite strong, based on the relatively low GWP readings throughout most of the mile. (“TS” represents performance-based TrackSTAR® data generated by the GRMS system. “TI” represents the visually assessed condition, and tie counts generated by Zeta-Tech Associates, Inc.’s TieInspect® program.) The chief tie inspector concluded that although this section of track exhibited excellent reserve gauge strength, the tendency of inspectors who are grading ties is to sometimes alter their grading standards when they have walked long lengths of track without classifying ties as “failed” or “weak”. This might have accounted for part of the 211 ties selected by the visually based method.

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