Keywords: under sleeper pad, ballast bed, railway superstructure, load transfer, contact surface area, static loading, impact loading, bedding modulus.

The paper is focused on the issue of the magnitude of the contact area between the sleeper loading area and the ballast bed when under sleeper pads are applied. The application of under sleeper pads results in a significant change in the transfer of loads. Under loading, the ballast bed aggregates are pushed into the elastic material of under sleeper pads thus increasing the area through which the load is transferred onto individual grains of the ballast bed.

The measurement of the contact area between the sleeper loading area and the ballast bed aggregates is a relatively demanding job; therefore, it is not commonly performed. The easiest way of determining the contact surface area is directly from the permanent imprint left by the ballast bed aggregates in the under sleeper pad.

The method of determining the contact area used by Monaco [1] was enhanced and applied in measurements performed both in laboratory and in-situ conditions. The principle of the new method consists of applying a suitable coat of paint on the aggregates that get into contact with the under sleeper pad, and making an imprint of the contact surface onto a thin industrial foil fixed onto the bottom surface of the under sleeper pad. The colour imprint is further subjected to automated electronic processing using a digital camera and software for the computation of two-colour images.

Five types of under sleeper pads differing by their thickness and rigidity were used for the experiments. The measurement in laboratory conditions was performed on non-consolidated sharp edged aggregates and on round edged aggregates. The measurement on consolidated aggregates was performed as part of maintenance works of the railway superstructure in the Prague Metro depot premises.

The test specimens were loaded with a falling weight deflectometer, or a device for the static plate load test. In both cases, the load was transferred onto the test specimen by means of a rigid circular plate with a diameter of 300 mm.

The advantage of this method as compared to the method used by Monaco [1] is its higher sensitivity, more distinct output and automated computation of the contact area. The use of a standardized impact loading device for loading the test specimens brings additional benefits of ensuring constant loading and a potential for use in both laboratory, as well as in-situ conditions. The determined values of the contact area are in a good correlation with the data in the literature.

1

V. Monaco, "Untersuchungen zu Schwellenbesonhlunger im Oberbau", dissertation thesis, TU Graz, 2003.

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