Modeling Railway Track Mechanical Behavior with Under Tie Pads and Under Ballast Mats

Abstract

Under tie pads (UTPs) and under ballast mats (UBMs) are used in rail track construction to reduce track stiffness and increase the size of the ballast contact area where it meets track support elements either above the ballast (ties) or below the ballast (e.g., subgrade, bridge deck, or tunnel invert). Limited information on the in-track performance of UTPs or UBMs is available. The University of Florida recently completed a research program to monitor track performance and track support changes using a UBM and UTPs in the Virginia Avenue Tunnel in Washington, DC. Researchers measured parameters to estimate the effect of a UBM and UTPs on tunnel invert pressure, tie bottom pressure, and track deflection. A control condition without a UBM or UTPs was not available for testing; however, Volpe developed a finite element track model for estimating track behavior without a UBM or UTPs. The modeling results highlighted the individual contributions of both a UBM and UTPs to stress reduction on the ties and tunnel floor. The model included features that simulate the load chains that can develop in clean granular materials leading to high peak stresses and ballast and structure degradation. The modeling results revealed that a UBM and UTPs might reduce the average stress on the ties and tunnel floor by 5% to 20% when the ballast was well compacted. The peak stresses from ballast load chains, or edges, may be reduced by 50% or more.