Suitability of earth and granular materials pressure cells for measuring railway trackbed tie/ballast interfacial pressures

Abstract

The magnitudes and relative distributions of pressures at the tie/ballast interface are important trackbed engineering design and analysis aspects. The pressures produced by millions of load applications ultimately affect the long-term performance of the track and the service lives of the component materials and layers. Ideally the interstitial pressure intensities can be reduced by distributing pressures uniformly over large areas thereby reducing abrasion of the bottom of the tie and crushing of the surface layer of the ballast. Multiple earth pressure cells and granular materials pressure cells were used in a series of laboratory tests to ascertain the applicability of using these types of sensors for accurate measurement of the vertical pressures at the tie/ballast interface. Test loads were applied through a section of a tie that was positioned on the ballast. In addition, the effects of several variables were evaluated;-primary variables included the type of granular support-ranging from new and worn ballast size to fine sand size aggregate and the type of cushioning-including resilient pads and thin rubber membranes. These tests were conducted to validate or negate the relative effects of variables on test results. Pressures calculated from the controlled testing, using a 50,000-lbf (222 kN) MTS hydraulic servo system test machine, were compared to simultaneously measured pressures indicated by the outputs from the pressure cells for machine test loads typical of in-track train loadings. Near perfect correlations were obtained. Furthermore, test repeatability was consistent with little variability of replicated tests. The results indicated that the cells were capable of accurately recording known pressure inputs and therefore applicable for in-track measurement of tie/ballast interfacial pressures. In-track tests were conducted under typical locomotive and freight car loadings with cells positioned at the tie/ballast interfaces directly under the rail/tie intersection-the point of maximum pressure intensities on the ballast layer. These values were very consistent over periods of elapsed time as the track experienced typical loadings from normal train operations. These test results are described in detail as related to measured pressure intensities and distributions from typical locomotive and freight car wheel loadings of 33,000 and 36,000 lbs. (15,000 and 16,300 kg).