Two different remedies to reduce track stiffness and increase track damping for the track on the bridges were implemented for two separate ballast deck bridges with standard concrete ties located on a high tonnage heavy haul revenue service route. One remedy used concrete ties fitted with rubber pads on the bottom surface and the other used ballast mats between the ballast layer and bridge deck. The ballast sections were increased to a minimum depth of 12 inches below the bottom of the ties, and drainage improvement was made to ensure that water would not accumulate on the bridges or in the approaches.
The two bridge locations were selected in September 2007 and June 2009, for remediation and long-term monitoring of performance as part of the heavy axle load revenue service mega site testing program conducted by Transportation Technology Center, Inc. and Union Pacific Railroad.
Before remediation, these two locations experienced excessive track geometry degradation, mud pumping, and track component failure that required localized maintenance work on a quarterly basis (approximately 63 MGT).
After remediation, no localized maintenance (except yearly surfacing operations for the entire line) has been required for more than 1,000 MGT.
The main root causes of these problems were determined to be high track stiffness and low track damping for the track on the bridges, which adversely affected dynamic vehicle-track interaction when differential track settlement started to occur at the bridge approaches. Some of these ballast deck bridges with concrete ties had track modulus measured at 12,000 lb/in/in, which is considered too high to accommodate dynamic vehicle-track interaction.
Long-term performance of these remedies has been excellent, resulting in significant benefits from reduction of slow orders, train delays, and major track maintenance activities.