Speed (km/h) v ≤ 80
80 < v ≤ 120 120 < v ≤ 160 160 < v ≤ 230 230 < v ≤ 300
Standard deviation (mm) D1
2.3 to 3
1.8 to 2.7 1.4 to 2.4 1.8 to 2.7 1.0 to 1.5
Table 2: alert limits based on standard deviation of longitudinal level (wavelength interval D1: 3m < λ< 25m).
running at 300km/h, higher wavelengths are of interest, namely the range 0.3-21m (see Table 1). In addition impacts at rail joints or worn welds produce a high wheel-rail impact load with broad-band frequency content. The longitudinal level corresponds to the standard deviation of vertical irregularities from the two rails identified by a track geometry recording car (Table 2). Initial vertical track irregularities may be induced by tolerances in the rail manufacturing process or by surface irregularities in the ballast bed after track construction or tamping takes place.
The longitudinal level may also deteriorate because of irregularities in track support stiffness such as hanging sleepers and transition zones. Differential settlement of the track embankment such as non-elastic deformations of the ballast and subgrade caused by repeated static and dynamic loads from passing trains may have an effect. EU standards define a minimum track quality for safe operation of trains on high-speed and main lines. The wavelength range D1 corresponds to track irregularity wavelengths λ in the interval 3m < λ < 25m, and such wavelengths lead to low-frequency excitation of the train/track-system that
10−6
may be important for ground vibration. Welding of rails in the field may
result in a cusp-like discontinuity. As is the case with rail joints, it is suggested that the discontinuities on each side are approximated by a quadratic function. Such discontinuities in the rail induce high-frequency vertical wheel-rail contact forces. Simulations carried out within Rivas show that for dipped rail, large contributions to the dynamic contact force in the frequency interval 50-100Hz are observed, and that the vibration level increases with an increasing dip depth.
The gap between rail joints is typically 4-20mm and the height difference (misalignment) between adjoining rails may be 0-2mm. In addition, on each side of the joint, there may be a dip in the rail. Such defects induce low-frequency vibrations, with wider gaps creating higher levels. The vertical irregularity for a loaded rail joint can be observed by measurement vehicles with a sampling distance of 5cm (Graph 1).
Track design optimisation Many track defects have been
presented above to show their impact on ground vibration generation. Once these defects are run by a wheel, the track design can also have an impact on track receptance at the contact point (the ratio between the excitation force produced at the top of the rail and the track displacement at this contact point) which partially drives the wheel-rail interaction force generation, and on track mobility as a whole so that less vibration is transmitted to the sub- layers.
The mitigation measures implemented on the track can therefore
8m 10−7 10−6
act either on interaction force generation, track mobility, or both, to reduce vibration transmitted to the sub- layers.
Rivas is going to test two mitigation measures for straight ballasted track. Graphs 2 and 3 show the influence of the mitigation measures on different dynamic indicators of the track which have been derived from a numerical parametric study carried out within Rivas to optimise these mitigation solutions.
The first mitigation measure consists of installing soft or very soft pads under heavy sleepers. This system decouples the rail and sleepers in the upper part of the track from the ballast and sub- layers which now “capture” the energy from these upper layers while reducing mobility between the track and the free- field, as shown in Graph 2. In this context, the heavy sleepers, with a larger contact area between the sleeper and the ballast, allow a reasonable rail deflexion at the contact point, even with very soft under-sleeper pads. Different combinations of soft or very soft under-sleeper pads and heavy sleepers are being tested on the Eiffage Rail Test Rig in Bochum, Germany, this spring.
The second mitigation measure consists of using a new rail fastening system that uses very soft railpads. In fact, a reduction of railpad stiffness implies an increase of the track receptance at the contact point, as shown in Graph 3-left. With a higher rail receptance, the resonance of the wheel-rail interaction force is shifted to the lower frequency ranges, reducing the interaction force for the frequencies above this resonance (Graph 3-right). Rivas plans to test different rail fastening systems which use soft to
32m 10−7
10−8
10−8
10−9
10−9
10−10
0
50
100 Frequency [Hz]
150
10−10
0
50
100 Frequency [Hz]
Graph 2: mobility between the track and the free field at 8m (left) and 32m (right) with an under sleeper pad stiffness of 50MN/m (solid line), 100MN/m (dashed line), 500MN/m (dashdotted line) and no under sleeper pad (dotted line) (specific soil conditions corresponding to a previous test site).
IRJ June 2013 39
150
Mobility [m/s/N]
Mobility [m/s/N]
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52