Use of Matrix-Based Tactile Surface Sensors to Assess Fine-Scale Ballast–Tie Interface Pressure Distribution in Railroad Track

The pressure distribution at the ballast–tie interface of conventional railroad track plays a key role in overall track support. Loads exceeding the strength of the ballast or tie can contribute to degradation of track quality. In this study, matrix-based tactile surface sensors (MBTSS) were used to study the load distribution at the ballast–tie interface. MBTSS allows for fine-scale pressure distributions to be measured unobtrusively and in a dynamic load environment. In this application, the loads imparted by individual ballast particles can be measured. Laboratory ballast box testing and in-track testing were conducted at the Transportation Technology Center. Ballast gradation at the interface was varied for both laboratory and in-track testing. Laboratory results indicated that under nominal heavy axle loads, average peak ballast–tie pressures ranged from 284 psi (1,960 kPa) on sand to 1,450 psi (10,000 kPa) on new ballast. In-track testing found that six of the 10 ties tested showed higher pressures adjacent to the rail and not directly underneath it. In both cases, the contact area was shown to increase under an increasing applied load, in part because of additional ballast particles being engaged as the tie deflects. The high peak pressures observed in the laboratory and the variability of pressure distribution along the tie observed in-track significantly varied from the ballast–tie pressure distribution recommended by the American Railway Engineering and Maintenance-of-Way Association's Manual for Railway Engineering. Ballast–tie interface characterization has implications for tie structural design, ballast degradation, and under-tie pad design.