A series of specially designed granular material pressure cells were precisely positioned directly below the rail at the tie/ballast interface to measure typical interfacial pressures exerted by revenue freight trains. These vertical pressures were compared to the recorded wheel/rail nominal and peak forces for the same trains traversing nearby mainline wheel impact load detectors (WILDs). The cells were imbedded within the bottom of new wood ties so that the surfaces of the pressure cells were even with the bottoms of the ties and the underlying ballast. The cells were inserted below consecutive rail seats of one rail to record pressures for a complete wheel rotation.

The stability and tightness of the ballast support influenced the magnitudes and consistencies of the recorded ballast pressures. Considerable effort was required to provide consistent ballast conditions for the instrumented ties and adjacent undisturbed transition ties. Norfolk Southern (NS) crews surfaced and tamped through the test section and adjacent approach ties. This effort along with normal accruing train traffic subsequently resulted in reasonably consistent pressure measurements throughout the test section.

The impact ratio (impact factor) and peak force values recorded by the WILDs compared favorably with the resulting magnitudes of the transferred pressures at the tie/ballast interface. High peak force and high impact ratio WILD readings indicate the presence of wheel imperfections that increase nominal forces at the rail/wheel interface. The resulting increased dynamic impact forces can contribute to higher degradation rates for the track component materials and more rapid degradation rates of the track geometry.

The paper contains comparative WILD force measurements and tie/ballast interfacial pressure measurements for loaded and empty trains. Typical tie/ballast pressures for locomotives and loaded freight cars ranges from 20 to 30 psi (140 to 210 kPa) for smooth wheels producing negligible impacts. The effect of increased wheel/rail impacts and peak force values on the correspondingly transmitted pressures at the tie/ballast interface is significant, with increased pressures of several orders of magnitude compared to nominal impact forces from wheels.

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