Keywords: microtremor, wave propagation, train-induced ground vibration process, railway vibration transmition, traffic effect on structures and environment, railways structures dynamics.

The operation of modern railways has been attended by an increase in train speed, train weight and wagon axle load. There seems nevertheless to be a few scattered complaints by wayside residents of vibration due to the passage of trains. The experimental evidence points out that the impact from the wheels passing over the rail joints have significant influence on the ground vibration transmitted from railway to nearby regions and the spectral characteristics of the ground-borne vibration can be significantly dependent on: (i) a unit train of identical vehicles produces ground vibration at frequencies which are related to wagon length, (ii) vibration which are produced by passing a steady wheel load over the discrete support provided by sleepers this effect is independent of inertia effects, (iii) vehicle vibrations (inertia effects) and (iv) track irregularities. The transient stress variations are produced in the ground below the track, they will propagate away from the track as ground-borne vibration. In the ideal case when the ground is homogeneous the compression and shear waves propagate in all directions away from the source, and thus they have substantial geometric attenuation, as well as losses due to the damping properties of the ground. The Rayleigh's waves, being surface waves, do not have the same geometric attenuation, but are still subject to loss by damping. In practice the ground is far from homogeneous: it can be stratified and possess discontinuities. In such a case additional modes of vibration can propagate along the interfaces of strata, and mode conversion from one type of wave to another may be encouraged.

The experimental tests have been performed in the several test fields near the railways of the ZSR (Slovak Railway Company). The object of the experimental measurements was to find: (i) spectral characteristics of the vibration components of the railway structure near the nearest rail joints and in the long distance from rail joints (rails, heavy concrete ties, ballast, roadbed and ground) by the power spectral densities , , (ii) the soil frequency characteristics expressed by frequency response function or by the gain factor of the response function , respectively, (iii) the Rayleigh´s wave velocities , by the cross correlation function and then calculation the initial tangent shear modulus , and (iv) the attenuation coefficients from the equation , where , are standard deviations of the amplitude of vibration at the distance , from the source of the vibration obtained from power spectral densities , , and represents the effect of geometrical attenuation [1,2].

According to performed of the theoretical and the experimental analysis of the railway track and ground vibrations, the following summary and conclusions can be made:

• The results of the experimental measurements indicate that the analysis of random ground vibration due to railway traffic provides a useful and required information on the frequency spectral characteristics of ground vibration , , the vibration characteristic of soils , the surface wave velocities propagation , the soil attenuation coefficients ; and damping parameters as well as the viscoelastic properties of soils, E, , , , .
• From the spectrum analyses of the accelerations of the vibrations induced in the rail and rail pad, we can observe the strong peaks area found at frequencies 70.0 Hz, to 180.0 Hz, (near the welded rail joint), but in the tie, the roadbed and the ground spectra the strong peaks area cannot be found at the same range frequencies. In comparison of these spectra we see that the accelerations of the vibrations with lower frequencies induced in the rail are transmitted to the sleepers and the roadbed with little damping, but the vibrations with the frequencies over 100 Hz are transmitted downwards with large damping.
• Power spectral densities (PSD) of the ground accelerations of the vibrations due to transmitted Rayleigh's waves indicate higher damping in the higher frequencies e.g. over 70.0 Hz. It proves the viscoelastic properties of the ground and roadbed respectively. The strong peak area in the ground accelerations spectra are in the frequency range from 30.0 Hz to 70.0 Hz, which indicates the significant influence of the vibrations induced after the impulse into the track and ground. Experimental analysis has proved when the spring constant of the tie pads increases and bed materials become softer, neither exist the first nor the second kind of vibration and only this third kind of vibration is induced after the impulse.
• As a result a construction of the railway track, which, has the resistance to the vibration from 35 to 70 Hz is the most important problem to extend the durability of the track materials and to diminish the maintenance-of-way expenses.

1

Bencat, J. "Research in Railway Structure Dynamics". Research Report III-4-6/03.03. Volume II. University of Transport and Communications, Zilina, (1989), (in Slovak).

2

Bencat, J. "Microtremour due to Traffic", Research Rep. A-4-92/b, Zilina, U.T.C, 1992 (In Slovak).

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