Experimental investigation of track substructure bearing pressures with under tie pads (UTPs)

Abstract

The implementation of under tie pads (UTPs) in railway track structures has gained attention due to their potential to mitigate track degradation and extend maintenance intervals. This study evaluates the effects of UTPs on substructure bearing pressure through a combination of laboratory and field experiments. A laboratory experiment was first conducted to establish baseline pressure distributions under different UTP configurations and to provide proof of concept of the sensor arrangements for the subsequent field study. The results indicated that minor adjustments in crosstie position caused significant variability in pressure magnitudes with a maximum difference of 93.4 psi (644.0 kPa). This variability may be attributed to changes in ballast particle engagement that modified the vertical load path (i.e., force-chain), as well as variations in support conditions near the rail seat region, both of which contributed to measurement inconsistencies. To mitigate force-chain effects and ensure consistent pressure measurements during field experiments under heavy axle load (HAL) revenue service conditions, pressure cells were deployed at a depth of 16 in. (41 cm) within the sub-ballast layer. The field experiment included three scenarios: a control track and tracks with UTP Types A and B. Results indicated that UTP Type B exhibited the highest median pressure followed by the control track and UTP Type A. All three cases displayed a reduction in pressure over time, which can be attributed to accumulated tonnage leading to gradual stabilization and consolidation of the substructure. This reduction was more pronounced in the UTP padded tracks, highlighting the long-term benefits of UTPs as their impact became more evident with increased tonnage. Additionally, a Risk-Weighted Pressure Index (RWPI) was introduced to better capture and assess the potential risk associated with high-pressure occurrences. The control track showed the highest RWPI values, indicating a greater likelihood of high-pressure occurrences that could accelerate track degradation and lead to substructure failures. These findings highlight the role of UTPs in enhancing track resilience, optimizing substructure performance, and reducing track maintenance demands under HAL conditions.