Abstract

In recent years, there has been a global trend towards increasing the commercial speed of high-speed rail lines (HSLs) to cope with expected demand problems. Examples include the new HSLs projects for the UK and California, where the design speed reaches 400 km/h. Therefore, one of the biggest challenges currently faced by the railway sector is the determination of the critical speed value of railway sections. This is true both for future HSLs, due to their high design speeds, and for existing rail lines, due to the intended increase in commercial speed. The critical speed is that train speed that produces a dynamic amplification in the medium underlying the track, causing an amplification of the vertical movement of the track components and the supporting ground such that the stability of the infrastructure and the safety of passengers is compromised. This work sets out the methodology necessary to experimentally study the critical speed of a railway section by in situ applying surface wave spectral analysis techniques, SASW and MASW. The critical speed is determined by the lowest phase velocity of the local minima of the modal dispersion curves, or by the minimum of the apparent dispersion curve. Both methods are equivalent. This paper also presents the results obtained on some Spanish HSLs, both on tracks under construction and in operation. In the cases studied, the ballast layer is the one that presents the lowest shear wave velocity and the minimum of the dispersion curve, so it is the layer that determines the critical speed.

Abstract

The Laboratorio de Geotecnia-CEDEX uses some software and tools for studying different phenomena and performance of track sections. For this purpose, it is usually necessary to determine the S-wave (shear elastic wave) velocity of the different layers in the track section which typically are, from bottom to top: natural ground, embankment, form layer, subballast and ballast. The Laboratorio de Geotecnia-CEDEX has experimental S-wave propagation velocity (Vs) values of embankments, form layers and sub-ballast layers, obtained from several campaigns where the Spectral Analysis of Surface Waves (SASW) was applied on the different layers during the construction of several track sections. To complete the studies and to obtain Vs values for the ballast layer, two campaigns were carried out in the CEDEX Track Box (CTB), a railway testing facility where it is possible to test complete railway sections on a 1:1 scale; and a field measurement campaign on an in-service railway track. Due to the discontinuous nature of the ballast layer carrying out tests to obtain Vs presents serious problems. To avoid these problems, a new procedure was developed to obtain these values using the SASW method by installing sensors on top of the sleepers. Through the interpretation of measurements taken with the SASW technique on the ballast, the dispersion curve is obtained, and from it, the values of Vs are calculated. The results are presented in this article and are also compared with values found through a literature review obtained or estimated by other authors.