Summary

It has been made clear that acoustic-absorptive materials (recycled) are effective for reducing the wheel/rail noise on slab tracks. They are products of crushed glass and ceramics. They are produced from household trash, but are devoid of injurious elements such as metals and salt. Some devices are needed to hold them in place without being blown away by train passeage. To confirm their noise reducing effect, they were applied on paved track of existing line, and the noise level was measured. The noise level decreased approximately 3dB(A)at 2m away from track center. Further, a test was done to confirm their noise-reducing effect in comparison with a sound protection wall.

Key word: Wheel/Rail noise, slab track, recycled material, sound protection wall

1. Introduction

It has become more and more important to cope with environmental problem such as noise since the introduction of new higher-speed trains. The problems facing JR companies in the construction of the SHINKANSEN network are not only environment preservation but also cost-cutting in construction and maintenance. Slab track can reduce maintenance work, but compared with ballast track, slab track has higher noise level. It is main cause of higher noise level on slab track that the ballast track can absorb the wheel/rail noise, but concrete surface on slab track reflects the wheel/rail noise and doesn't absorb it. Generally speaking, it would be too expensive to set many absorptive blocks in advance along the operating track. So it was preferred to develop a low-cost countermeasure.

2. Investigation of low-cost materials

Fig.1 illustrates the sound absorbing rate of several materials. It shows that standard ballast(400mm thickness) of ballasted track has good acoustic-property, but sound absorbing rate of recycled materials(100mm thickness) is higher than that of standard ballast. Recycled materials are produced by crushing incineration residues of municipal waste which are mainly inoganic materials such as pottery, glass and fine aggreate. Fig.2 illustrates their grain size accumulation curve. They originate from household trash, and are devoid of injurious elements such as metals and salt. It is believed that the application of these materials can reduce the cost and contribute to environmental conservation.

3. Application of recycled materials to the track

3.1 Requirements for application to the track

When we set recycled materials as the acoustic-absorptive materials candidate on the slab track, they must fill the following requirements:

1) To have enough ability to absorb sound

2) To have enough ability to stand winds from high speed vehicles

3) To be strength enough to stand the shock load of an obstacle being dropped from the vehicle

3.2 Methods for preventing recycled materials from being blown away

When a high speed train runs on the track many particles of recycled materials will fly away. As one of the methods for preventing recycled materials from being blown away, recycled materials are packed into bags for trial. In order to check the influence of bags we measured sound absorbing rate of recycled materials packed into 2 kinds(TypeA,TypeB) of polyester bags which are different in mesh size. Fig.3 shows comparison of sound absorbing rate. It made clear of that even if recycled materials are packed into bags, their sound absorbing rates hardly change.

3.3 The test of strength

Some bags packed recycled materials are installed on the slab track, to test their ability to stand the shock from an obstacle that SHINKANSEN vehicle drops. When a block of ice (mass:1.4kg, speed:270km/h) hit a bag, polyester bags were damaged, but those covered with ballast net were little damaged. (Fig.4 Damage of bag)

4. Noise reducing effect of the recycled materials around operating line

To confirm the noise reducing effect of the recycled materials, they were set on paved track (operating line, cutting section, Fig5), and the noise under train running was measured. Fig.6 shows the result. (:before setting of recycled materials :after setting of recycled materials). It was clear that noise reducing effect of the recycled materials was approximately 3dB at 2m away from track center.

5. Investigation of noise reducing effect of recycled materials

5.1 Influence of the method of setting

To confirm the relationship between noise reducing effect and the method of setting on track(area, form), we carried out a moter car running test. Noise was measured for four absorbor setting conditions shown in Fig.7. Fig.8 shows position of noise level measurement. The result of the experiment shows that noise levesl of TypesB,C,D are approximately 3-4dB(A) are lower than that of TypeA at 2m away from track center.(Fig.9-a). It suggests that influence of the absorbor setting conditions is very small, and TypeB(minimum area) is recommended as a reasonable setting method of absorbing materials.

Fig.9-b shows that noise reducing effect at 6.25m away from track center is 2-3dB(A). Fig.10 shows frequency analysis of noise level of this case. The noise reducing effect was recognized at frequencies 400Hz-4000Hz, which pretty well coincide with frequency of high sound absorbing rate of recycled materials (Fig.1)

5.2 Comparison with sound protection wall

Fig.11(a)-(b) shows noise reducing effect of sound protection wall and recycled materials. Fig.11(a) shows the following:

Noise reducing effect of sound protection wall is 8dB(A).

Noise reducing effect of recycled materials is 3dB(A).

When there is a sound protection wall, noise reducing effect of recycled materials is +2dB(A). Total effect(protection wall used together) is 10dB(A).

Table 1 shows noise reducing effect at respective positions of noise level measurement.

Fig.11(b) shows that noise reducing effect of recycled materials equals that of sound protection wall at the higher position where sound protection wall can't reduce the noise. It suggests that absorbing materials set on the track would be effective for reducing the wheel/rail noise at a high position of buildings existed near the railway.

6. Noise analysis using FEM

Now, we are trying noise analysis using FEM(Fig.12). If the results of simulation agree with above mentioned result of the experiment sufficiently, it would be possible based on the present results to seek for the best countermeasure.

7. Conclusions

In this investigation it was made clear following was made clear:

1) Recycled material is effective for reducing the wheel/rail noise on slab tracks.

2) As one of the methods for preventing recycled materials from being blown away, to pack them into bags is effective.

3) Noise reducing effect of the recycled materials around operating line is approximately 3dB(A) at 2m away from track center.

4) At the higher position where sound protection wall can't reduce the noise (such as a building exists near the rail way), absorbing materials set on the track would be effective for reducing the wheel/rail noise.

BIBLIGRAPHY

1. K.Ando, T.Nagafuji, S.Miura: "Present Status on Slab Track and Environmental Conutermeasure", The 3rd Transportation and Logistic Conference, Railway Technology Symposium '94 summary (in Japanese), 6-12, pp.335-340,1994