Assessment of Aseismic Performance of Ballasted Track with Large-scale Shaking Table Tests

Transcription

1 PAPER Assessment of Aseismic Performance of Ballasted Track with Large-scale Shaking Table Tests Takahisa NAKAMURA Researcher, Track Structures and Geotechnology Laboratory, Track Technology Division Etsuo SEKINE, Dr.. Eng. Senior Researcher, Track Technology Division Yusuke SHIRAE Former Railway Technical Research Institute, Presently Shikoku Railway Company The authors performed shaking table tests to evaluate the aseismic performance of ballasted track by using full-scale models. The test results showed that in the case of ballasted track with no countermeasures, the amplitude of lateral displacement and the residual displacement increased because the lateral resistance of ballasted track significantly decreased when the earthquake acceleration acting on the ballasted track exceeded around 800 gals. Furthermore, the authors concluded that the decrease in lateral ballast resistance force could be prevented by applying adequate countermeasures against track buckling. Keywords: ballasted track, aseismic performance, large-scale shaking table test, lateral ballast resistance force, countermeasures against track buckling 1. Introduction The aseismic performance of railway structures in Japan is a key issue since the country is often subject to earthquakes. In the 1980 s, a survey of earthquake damage to ballasted track was carried out [1]. Out of the cases of damage found, which had reduced train-running safety, a number were reported to be due to a degradation of railway infrastructure and track buckling caused by a decrease in lateral ballast resistance force beneath non-degraded railway infrastructure. Based on the results of this survey, research was carried out into the fall in lateral ballast resistance force caused by earthquakes using a large-scale shaking table test, to confirm the relationship between the lateral ballast resistance force reduction ratio and vibration acceleration [2][3]. However, this research did not go as far as to determine track ballast deformation characteristics under the shaking and the post-shake track ballast resistance force. Meanwhile, in the wake of the 1995 Hyogo earthquake, design standards for railway infrastructures (seismic design) were updated [4]. Seismic design standards for track structure however, were not reviewed and so the need was felt to have a type of aseismic performance evaluation that took into account post-seismic restoration. The authors therefore performed the shaking table test using full-scale model to evaluate the aseismic performance of ballasted track. Furthermore, the authors performed the test for lateral ballast resistance force with ballasted track model after the shaking. 2. Large-scale shaking table test The large-scale shaking table test equipment was Fig. 1 Outline of shaking table test equipment therefore employed to exert sine wave and earthquake wave at right angles to the rail on the horizontal plane. Figure 1 shows an outline of the shaking table test equipment. An iron body frame measuring: X-axis 7m, Y-axis 5m, height 0.6m was bolted rigidly to the to the shaking table. 2.1 Outline of full-scale track model The full-scale track model has a ballast thickness of 300mm, and a ballast shoulder width of 500mm. To reproduce ballasted track without any countermeasures, the track model was set up using type-4t and type-3h PCsleepers as shown in Fig.2 (a). In case of ballasted track with countermeasures against track buckling, the model was built with plate for buckling prevention and a ballast retaining wall as shown in Fig.2 (b). Type-4T PC- 156 QR of RTRI, Vol. 52, No. 3, Aug. 2011

2 500mm 3H:2400mm, 4T:2350mm 500mm 1200mm Fig. 2 Outline of full-scale track model 600mm D 50 =33.9(mm) U c =1.55 Fig. 3 Grain size distribution of ballast sleepers were used for both setups with countermeasures against track buckling. The ballast was compacted into place with a shaking vibrator, to achieve a density of 1.6t/ m 3. The ballast material is Andesite, the grain size distribution is shown in Fig Outline of countermeasures applied against track buckling The authors confirmed existing studies which demonstrated that the lateral ballast resistance force was expressed by the sum of the resistance force of each sleeper surface: the ends, sides and bottom [5][6]. The plate for buckling prevention, which was fixed with bolts to both sleeper ends, were designed to improve resistance forces of sleeper end. Iron plate for buckling prevention have a surface area that is 3.5 times that of type-4t PCsleepers. Ballast retaining walls were installed to prevent the fall in lateral ballast resistance force caused by ballast flow due to shaking, and were fitted to the shaking table at a distance of 500mm from each sleeper end. Both countermeasures aimed mainly to improve resistance force of sleeper end. Type-3H PC sleeper, which is 1.26 times heavier than type-4t PC sleepers, and which end area has 1.16 times that of type-4t PC sleepers, was expected to improve the resistance forces of the sleeper end and bottom. 1: mm 2.3 Test conditions Table 1 shows the configurations tested on the shaking table. Sine and earthquake waves were applied during the tests. In the case of sine wave shaking, tests were performed with a track model composed of one sleeper as shown in Fig.4, to clarify the basic properties of ballasted track during shaking. A loading frequency of 3Hz was used with a shaking of ten sine waves. Shaking was increased in 200gal steps from 200gal to 800gal. In the case of earthquake wave shaking, tests were performed with a track model comprising five sleepers as shown in Fig.5, to clarify the basic properties of ballasted track under real earthquake conditions. The wave used reproduced those of the Kushiro-Oki earthquake, which is assumed to have been a trench-type earthquake (Specter Ⅰ ) of Level Ⅱ earthquake motions. The shaking lasted for a considerable time, with a high number of cycles as shown in Fig.6. The test equipment for these trials used the acceleration method control to control shaking. Sleeper displacement and the sleeper acceleration were measured by displacement meters and accelerometers set at both sleeper ends as shown in Fig.2. Ballast acceleration was measured with accelerometers set un- Fig. 4 Outline of track model (sine wave shaking) Fig. 5 Outline of track model (earthquake wave shaking) PC sleeper Type 4T Table 1 Test est cases of large-scale shaking table test Track model Countermeasure against track buckling Shaking wave Sine wave(gal) Earthquake wave Kushiro Oki No countermeasure Plate for buckling prevention Ballast retaining wall Type 3H No countermeasure QR of RTRI, Vol. 52, No. 3, Aug

3 der the center of the sleepers, and shaking table acceleration was measured with an accelerometer set on the shaking table. Furthermore, the cross section shape of ballast was measured before and after shaking. 2.4 Test results Fig. 6 Kushiro-Oki earthquake wave Relationship between vibration acceleration and sleeper displacement Figure 7 shows that relationship between the sleeper displacement amplitude and the vibration acceleration for each sine wave step and the earthquake wave shaking, and Fig.8 shows the relationship between the residual sleeper displacement and the vibration acceleration after each shaking step. Here, the sleeper displacement amplitude shows relative displacement between displacement amplitude of the shaking table and displacement Fig. 7 Fig. 8 Relationship between vibration acceleration and sleeper displacement amplitude Relationship between residual sleeper displacement and vibration acceleration amplitude of the sleeper, using the maximum displacement amplitude of the tenth wave for sine wave shaking, and maximum displacement amplitude for earthquake wave shaking. In the case of sine wave shaking, the sleeper displacement amplitude tended to increase with vibration acceleration in all cases, and increased even more rapidly when vibration acceleration exceeded 600gal. In addition, the sleeper residual displacement increased rapidly, when the vibration acceleration was beyond 600gal. In the case of earthquake wave shaking, the sleeper displacement amplitude was around double that under sine wave shaking at 800gal on track with no countermeasure and equipped with type-4t PC sleepers, and was around half of that under sine wave shaking at 800gal with plate for buckling prevention. Meanwhile, residual sleeper displacement was at the same level at 800gal as sine wave shaking in all cases Relationship between acceleration reply magnification and area ratio In previous studies, the authors had found that the ballast conditions around sleeper ends affected lateral ballast resistance force [7]. The authors therefore calculated the area ratio of the ballast shoulder before and after shaking, by measuring the cross section shape of ballast shoulder after each shaking step. Figure 9 and 10 show the relationship between the area ratio and the acceleration reply magnification for sleeper and ballast. Here, the acceleration reply magnification is the ratio between shaking table acceleration and acceleration of each sleeper and the ballast. In addition, the vibration acceleration shows the peak acceleration during the tenth wave during sine wave shaking, and the maximum acceleration concerning the earthquake wave shaking. The acceleration reply magnification of the sleeper tended to increase as the area ratio of the ballast shoulder fell with shaking as shown in Fig.9. The area ratio marked a sharp decline after shaking at 800gal and acceleration reply magnificent of the sleeper markedly increased. This is consistent with what could be expected, because ballast rigidity fell sharply at the ballast shoulder and under the sleeper due to a collapse of the ballast shoulder during shaking at 800gal. Fig. 9 Relationship between area ratio and acceleration reply magnification for sleeper 158 QR of RTRI, Vol. 52, No. 3, Aug. 2011

4 The acceleration reply magnification of the ballast under the center of the sleeper was constant until shaking at 600 gal as shown in Fig.10, while the area ratio of the ballast shoulder declined, then, it marked a sudden increase around 800gal. This again is consistent with what could be expected because the ballast rigidity declined significantly at the ballast shoulder and under the sleeper due to a collapse of the ballast shoulder at 800gal sine wave shaking. Ballast rigidity under the center of the sleeper also declined. Both acceleration reply magnification for sleeper and ballast under earthquake shaking showed the same tendencies as sine wave shaking at 800gal. 3. Test for lateral ballast resistance force This test exerted a load in a longitudinal direction on the sleepers of the ballasted track model on the largescale shaking table. 3.1 Test conditions Fig. 10 Relationship between area ratio and acceleration reply magnification of ballast 16 test configurations were tried in total. Tests were performed on both types of track model without countermeasures (type-4t and type-3h PC sleeper) and both types of track model with countermeasures against track buckling (plate for buckling prevention and ballast retaining wall). Figs 11 and 12 show a cross section and a photograph of tests for the lateral ballast resistance force set up, respectively. In the test, a load at sleeper longitudinal direction was exerted onto each sleeper of the ballasted track model with loading equipment, at a loading speed of 2mm/min. The lateral displacement of the sleeper was Fig Outline of test for lateral ballast resistance force Fig. 12 Lateral ballast resistance force test measured by displacement meters placed at each sleeper end, and the load was measured by a load cell as shown in Fig Test results Lateral ballast resistance force Figure shows the test results for lateral ballast resistance force after sine wave shaking. Figs 13 and 15 show the relationship between the load and lateral sleeper displacement. type-3h PC sleepers, which are heavier and have a larger cross than type-4t PC sleepers, also had higher lateral ballast resistance force as a whole. At a vibration acceleration of up to 600gal, the load was the same level as in the absence of no shaking. When vibration acceleration reached around 800gal, the load fell from its initial level. Previous studies have shown that lateral ballast resistance force is affected mainly by the condition of ballast around sleeper ends and sides when load is first applied, and was then affected by the state of ballast at the sleeper ends with the increase in lateral sleeper displacement. The lateral ballast resistance force therefore decline gradually from 600gal shaking upwards, due to a decline in ballast rigidity around the sleeper ends, then it falls sharply due to a decline in ballast rigidity around not only the sleeper ends but also the sides and the bottom of the sleeper at 800gal shaking. Figs 14 and 16 show the relationship between the spring constant (calculated between the load and the lateral sleeper displacement), and lateral sleeper displacement. In both cases, PC sleeper Type 4T Table 2 Test est cases of lateral ballast resistance force test Track model Countermeasure against track buckling no shaking Shaking wave Sine wave(gal) Earthquake wave Kushiro Oki No countermeasure Range of area calculation Ballast retaining wall Type 3H No countermeasure QR of RTRI, Vol. 52, No. 3, Aug

5 Fig. 13 Fig. 15 displacement (No countermeasure 4T) displacement (No countermeasure 3H) Fig. 14 Relationship between spring constant and lateral sleeper displacement (No countermeasure 4T) Fig. 16 Relationship between spring constant and lateral sleeper displacement (No countermeasure 3H) Fig. 17 Relationship between area ratio and ratio of lateral ballast resistance force ino countermeasure 4T,3H) as vibration acceleration increased, the spring constant tended to decline especially at 800gal shaking. Figure 17 shows the relationship between 'the area ratio of the ballast shoulder before and after shaking' and 'the ratio of decline in the lateral ballast resistance force (load for 2-mm lateral sleeper displacement) no shaking and after each step of shaking'. It shows that this is a relative relationship and the lateral ballast resistance force declined sharply with the collapse of the ballast shoulder in both cases at 800gal shaking. Next, Fig.18 shows the relationship between lateral sleeper displacement and load in the case of track with countermeasures against buckling. In case of the plate for buckling prevention, the load declined under shaking at 800gal however, it displayed higher lateral ballast resistance force as a whole than the ballast retaining wall. Furthermore, in case of the plate for buckling prevention, when the vibration acceleration reached 800gal, the load was the same as the case tested without any countermeasures, up to 0.2mm lateral sleeper displacement. The load then declined as lateral sleeper displacement increased. It is assumed that this is due to ballast rigidity around the sleeper end declining and ballast rigidity around the side and bottom of sleeper remaining stable. In case of the ballast retaining wall, cross section shape of ballast remained completely unchanged, and lateral ballast resistance force showed almost no decline at 800gal shaking. This is thought to be because of the ballast retaining wall confining the ballast shoulder, and the effect of countermeasures against track buckling. Figure 19 shows the relationship between lateral sleeper displacement and the spring constant where countermeasures were used against track buckling. The spring constant of the plate for buckling prevention was much larger than for the ballast retaining wall. When vibration acceleration reached 800gal, the spring constant of the plate for buckling prevention declined slightly as did the load, however, 160 QR of RTRI, Vol. 52, No. 3, Aug. 2011

6 Fig. 18 displacement (countermeasure) Fig. 19 Relationship between spring constant and lateral sleeper displacement (countermeasure) the spring constant of ballast retaining wall hardly changed Lateral ballast resistance force Figs 20 and 21 show the test results of lateral ballast resistance force after the earthquake wave shaking on a track model composed of five sleepers. In case of both type- 4T PC-sleepers without countermeasures and with plate for buckling prevention, the load and the spring constant Fig. 20 displacement (Earthquake wave shaking) Fig. 21 Relationship between spring constant and lateral sleeper displacement (Earthquake wave shaking) declined from the initial load, in comparison with absence of shaking. In addition, this tendency towards decline in the lateral ballast resistance force was smaller in case of the type-4t PC-sleeper and was larger where plate for buckling prevention were used, after 800gal sine wave shaking Influence of vibration acceleration for lateral ballast resistance force Figure 22 shows the relationship between vibration acceleration and the lateral ballast resistance force (load for 2mm lateral displacement). In the case of type-4t and type-3h PC-sleepers without any countermeasures, the lateral ballast resistance force declined as vibration acceleration increased, with a particularly steep progression under 800gal shaking. Next, in the case of plate for buckling prevention, the lateral ballast resistance force tended to decline after 800gal shaking, though it remained around twice that of the type-4t PC-sleeper model with no countermeasure after 800gal sine wave shaking and around 1.5 times of the type-4t PC-sleeper model after earthquake wave shaking. Where the ballast retaining wall was employed, the lateral ballast resistance force hardly declined at all after 800gal shaking. Fig. 22 Relationship between vibration acceleration and lateral ballast resistance force QR of RTRI, Vol. 52, No. 3, Aug

7 4. Conclusion To assess the aseismic performance of the ballasted track, the authors performed large-scale shaking table tests and lateral ballast resistance force tests. Conclusions based on results are as follows: (1) Cross section shape of ballast hardly changes under vibration acceleration up to 600gal, however, the ballast shoulder collapses and ballast rigidity declines after shaking at 800gal in the case of no countermeasures. (2) Lateral ballast resistance force hardly declines at all under vibration acceleration up to 600 gal, however, it declines sharply after shaking at 800gal shaking in the case of no countermeasures. (3) Under L2 earthquake wave shaking, the ballast shoulder collapses at the same rate as under 800gal sine wave shaking and the lateral ballast resistance force declines at the same rate as under 800gal sine wave shaking. (4) The authors confirmed that both countermeasures against track buckling had a constant effect. In particular the lateral ballast resistance force of the plate for buckling prevention was twice that of the model without countermeasures under 800gal shaking. The plate for buckling prevention proved to be the most effective countermeasure against track buckling. (5) There is a correlation in the relationship between the degree of ballast shoulder collapse and the rate of lateral ballast resistance force decline. In this study, the authors confirmed the deformation characteristics of ballasted track subject to shaking, the lateral ballast resistance force after shaking and the effect of countermeasures against track buckling. A further study to develop a method to evaluate the lateral ballast resistance force after shaking should be conducted. References [1] Sato, Y., Miura, S., Deformation of Railway Track and Running Safety of Train Earthquake, RTRI Pre- Report, No.82-45, (in Japanese). [2] Sato, Y., Kobayashi, S., Nagata, M., Stability of Track with Concrete Ties and Crushed Stone Ballast in Earthquake, RTRI Report, No.675, (in Japanese). [3] Sato, Y., Takatani, H., Suzuki, S., Study on Buckling Stabikity of SHINKANSEN Track in Earthquake, RTRI Report, No.1334, (in Japanese). [4] Railway Technical Research Institute, Design Standard for Railway Stractures -Seismic Design, August 1999 (in Japanese). [5] Sato, Y., Miyai, T., Lateral ballast Resistance Force and the Performance Various Types of Ballasted Track, RTRI Report, No , (in Japanese). [6] Sekine, E., Nakamura, T., Hirao, H., Model Test for Lateral ballast Resistance Force of Ballasted Track, 65th JSCE Annual Meeting 4, pp , (in Japanese). [7] Hirao, H., Sekine, E., Shaking Table Test with Ballasted Track Model -Lateral ballast Resistance Force, 63th JSCE Annual Meeting 4, pp , (in Japanese). 162 QR of RTRI, Vol. 52, No. 3, Aug. 2011