Foundation measurement, design for special trackwork: One often-overlooked key to extended service life for special trackwork lies beneath the surface, out of sight, but not out of mind - NRCNews - Transportation Technology Center Inc - Brief Article

Railway Track and Structures, May, 2001 by David D. Davis, Satya P. Singh, Don Guillen

While this system is not a real-time production-measurement device, it is a useful research tool that will assist the industry in developing special-trackwork foundation-design policies.

There are differences in the damping characteristics of the four FAST test sections. The GEOWEB section had the highest damping, but this section still had less damping than the theoretical optimal value determined from modeling.

While the GEOWEB layer increased damping by 24 percent, it only moved the track section 14 percent closer to the predicted optimal value.

Significant findings include:

* Typical ballasted track has a damping value of 111 lbs/in/sec per tie. This is below the predicted optimal value of 300 lbs/in/sec/tie needed to minimize frog flangeway gap dynamic loading.

* Of the typical subgrade improvement methods tested, the GEOWEB was able to increase track damping (as compared to the control section) the most, by about 27 lbs/in/sec or 24 percent. The eight-inch HMA pavement did not significantly affect track damping (110 lbs/in/sec) and the 12-inch concrete slab decreased damping by 30 percent to 78 lbs/in/sec.

* The impact hammer/accelerometer field measurement method combined with NUCARS simulation is effective in determining the damping characteristics of ballasted track. A preload is needed for wood-tie, cut-spike-fastener track to eliminate any track structure gaps (e.g., between rail and tie).

The fact that subgrade strengthening methods did not bring the track to optimum damping conditions is not surprising. It was likely not a consideration in their designs. However, now that methods to measure damping and to determine what damping value is optimal are available, progress can be made toward an optimally damped foundation.

Damping may also be added in the superstructure of the switch or frog with railseat and tie pads.

Track damping measurements

Measurements of the track response were made using an instrumented impact hammer and an accelerometer system (Figure 4). A track preload is essential. This preload takes the "slop" or vertical gaps out of the track, making the actual track more like the track in the dynamics model used.

Each test section was modeled in NUCARS to replicate the track response measured in the field. By varying the damping in the model, the actual track response was obtained. In this manner the damping of each section was estimated.

Damping test results

Figure 5 shows the calculated damping values for each sections. (3) The results suggest that the GEOWEB subgrade-reinforcement system added damping to the track; whereas, the concrete slab reduced damping. The HMA section had damping characteristics similar to the sub-ballast control section.

References

(1.) Davis, David, Singh, Satya and Guillen, Don, "Evaluation of Foundations for Special Track Work," TTCI/AAR Technology Digest 00-023, November 2000.

(2.) Singh, Satyn and Davis, David, "Reducing Impact Forces on High-Angle Crossing Diamonds," TTCI/AAR Technology Digest 98-021, August 1998.