Abstract Summary
The limit values commonly used in Sweden and Norway for structure borne noise from railroad are strict when seen in an international context. At the same time, tunnels with low coverage are regularly built through densely populated areas, which can cause complaints about high structural borne noise levels in nearby dwellings. Traditionally, ballasted tracks are used in most railway tunnels in Norway and Sweden and under ballast mats (UBM) are the usual structure borne noise mitigation measure. Since UBMs that meet technical regulations for rail in many cases do not provide sufficient effect, dispensations have been granted for the use of softer ballast mats in some projects. However, there is limited information about the effect of softer UBMs on maintenance and about whether soft UBMs lead to excessive rail deflections, which could affect the track adversely. There is therefore a need for more knowledge about how UBMs affect the track and especially at higher speeds. When assessing the effect of UBMs on the track, both information about the track displacement amplitude and the shape of the deflection curve is important. As a first step, a pilot study has been carried out to find a suitable method to measure rail deflection, which can be used for long-term monitoring in tunnels with UBM to gather more information. Three measurement methods were compared in a laboratory test rig: Linear Variable Differential (LVDT) displacement sensors, Fiber Bragg Grating (FBG) optical strain sensors, and high sensitivity low frequency accelerometers. The results indicate that by using a high-sensitivity, low-frequency (DC) accelerometer in combination FBG strain sensors, both an accurate determination of the displacement amplitude and information about the shape of the deflection curve can be obtained.
