GENERATION OF GROUND VIBRATION BOOM BY HIGH-SPEED TRAINS

Railway-generated ground vibrations cause significant disturbance for residents of nearby buildings even when generated by conventional passenger or heavy-freight trains [1,2]. If train speeds increase, the intensity of railway-generated vibrations generally becomes larger. For modern high-speed trains the increase in ground vibration intensity is especially high when train speeds approach certain critical velocities of waves propagating in a track-ground system. The most important are two such critical velocities: the velocity of Rayleigh surface wave in the ground and the minimal phase velocity of bending waves propagating in a track supported by ballast, the latter velocity being referred to as track critical velocity. Both these velocities can be easily exceeded by modern high-speed trains, especially in the case of very soft soil where both critical velocities become very low. As has been theoretically predicted by the present author [3,4], if a train speed v exceeds the Rayleigh wave velocity cR in supporting soil a ground vibration boom occurs. It is associated with a very large increase in generated ground vibrations, as compared to the case of conventional trains. The phenomenon of ground vibration boom is similar to a sonic boom for aircraft crossing the sound barrier, and its existence has been recently confirmed experimentally [5,6] (see also chapter 11). The measurements have been carried out on behalf of the Swedish Railway Authorities when their West-coast Main Line from Gothenburg to Malmo was opened for the X2000 high-speed train. The speeds achievable by the X2000 train (up to 200 km/h) can be larger than lowest Rayleigh wave velocities in this part of Sweden characterised by very soft ground. In particular, at the location near Ledsgard the Rayleigh wave velocity in the ground was around 45 m/s, so the increase in train speed from 140 to 180 km/h lead to about 10 times increase in generated ground vibrations [5] (see chapter 11). The above mentioned first observations of ground vibration boom indicate that now one can speak about “supersonic” (“superseismic”) or, more precisely, “trans-Rayleigh” trains [7-9]. The increased attention of railway companies and local authorities to ground vibrations associated with high-speed trains stimulated a growing number of theoretical and experimental investigations in this area (see, e.g. [10-13]). 2 If train speeds increase further and approach the track critical velocity, then rail deflections due to applied wheel loads may become essentially larger. Possible very large rail deflections around this speed may result even in train derailment, thus representing a serious problem for train and passenger safety [6,14-16]. From the point of view of generating ground vibrations outside the track, these large rail deflections can be responsible for an additional growth of ground vibration amplitudes, as compared to the above mentioned case of ground vibration boom [7,9,17]. In the present paper we review the current status of the theory of ground vibration boom from high-speed trains. Among the problems to be discussed are the quasi-static pressure generation mechanism, effects of Rayleigh wave velocity and track wave resonances on generated ground vibrations, effects of layered geological structure of the ground, and waveguide effects of the embankments. The results of theoretical calculations for TGV and Eurostar high-speed trains travelling along typical tracks are compared with the existing experimental observations.