Standards Development

Scope

This part of ISO 14837 gives guidelines for the quantification of the effectiveness of mitigation measures for controlling vibration and ground-borne noise from rail transport systems in the form of vibration-attenuating track support systems. The main quantity to evaluate is the insertion gain. In the railway context, the insertion gain of a track support system is defined as the difference, in dB, between the transmitted vibration with the track components relative to those without the component. It can be expressed as a spectrum (e.g. in 1/3 octaves). While this can refer to an actual track replacement, it can also be used to express the predicted vibration that occurs with one track system in comparison with a hypothetical base case.

The design of track for vibration isolation may differ significantly in terms of the number of stages, i.e. single stage involving merely resilient rail baseplates, or multistage involving rails resiliently supported from blocks, sleepers or slabs in turn resiliently supported from the tunnel invert or ground formation. Designers of proprietary systems or components use several ways of expressing their performance, either in terms of dynamic stiffness and loss factor of components, or insertion loss or insertion gain (insertion loss with opposite sign) obtained by subtracting a spectrum with the system in place from a spectrum for a reference system. The reference system can be described as “ballasted track” or may be a hypothetical extremely stiff track support system. The spectra concerned may be calculated using simple transmissibility equations involving lumped parameters, or may be more thoroughly computed spectra of lower spectral density for a length of track.

Insertion Gain plots shown relative to another track support system such as ballasted track may be unhelpful since they show a large gain where the loaded track natural frequency was in the reference case, and a reduced gain at higher frequencies where the reference track was providing some vibration isolation.

There are two principal cases where the evaluation of insertion gain is required

- Replacement of, or substitution of an alternative to, an existing track support system

- Design of a new railway and selection of an appropriate track support system.

In the former case where measurement data exist for the vibration and/or groundborne noise occurring in one or more receptor locations due to the passage of trains on the existing system the end requirement is a spectrum indicating the predicted difference between received vibration as a result of replacing/substituting the alternative track support system.

In the latter case, it is necessary to include information about the performance of the proposed track support system in the design model being employed to make overall predictions of the vibration and groundborne noise that will occur at receptors.

The requirements of both cases can be satisfied by the use of standardized insertion gain spectra. In the former case, however, two spectra are required, one calculated for the existing track support system and the other calculated for the proposed alternative/substitute, and provided that the Insertion Gains are both defined in the same way, and correctly related to the generation and transmission of vibration into the surroundings of the railway, the difference between the two spectra may be applied in the process of determining the outcome of implementing the proposed change. In the case of a new railway, one spectrum is required, capable of being incorporated into the prediction model employed for predicting the vibration and groundborne noise occurring at receptors.

Purpose

It is frequently necessary for the designers of new railways to comply with requirements imposed for the purpose of limiting levels of groundborne noise and vibration in sensitive receptors. Operators of existing railways may also be required to install track components or new track which includes provision for controlling or reducing levels of groundborne noise and vibration. It is essential that a method is available for specifying the properties of track support components, or of complete track support systems, in an unambiguous way.

Current practice in specifying the performance of track components for this purpose varies widely, and may often be incompletely described. Furthermore, in the design of a complete track support system it may be necessary to break down the performance of the complete system into a set of specific dynamic requirements for individual elements, and as part of this process it may be necessary to determine the contribution of individual elements to the overall performance of the track support system with regard to groundborne noise and vibration.

The purpose of this Part is to reduce the uncertainty and error which may be associated with data produced by track designers by setting out a method of determining insertion gain in a controlled way, including information on the limitations of applicability of the data and on the expected accuracy resulting from the use of insertion gain data in predicting compliance with imposed requirements.