laboratory simulation of field loading conditions and ...

LABORATORY SIMULATION OF FIELD LOADING CONDITIONS AND MAINTENANCE OPERATIONS Giacomo D’Angelo Nicholas Thom Davide Lo Presti University of Nottingham Nottingham, NG7 2RD UK KEYWORDS: Railway, Ballast, Ballast box, tamping, stoneblowing ABSTRACT Ballasted track is the most common solution worldwide over other alternatives such as concrete slabs since it provides less stiff support, produces less noise and is more economical. Despite its benefits, a large portion of the track maintenance budget is spent on ballast related problems. However, it is not yet clear which is the best management strategy for maintenance operations. For this reason a test that reflects performance in track, allowing consistent comparisons between materials and practices is needed. In this study a ballast box has been used to simulate loading conditions and maintenance by tamping and stoneblowing in a simplified and controlled manner. The aim of this paper is to demonstrate how the box test can be used to replicate ballast field conditions to compare different materials and maintenance processes. INTRODUCTION The railway plays a fundamental role in most transportation systems. It provides a fast means of transportation by a durable and economical system. Ballasted track, which consists of track superstructure supported on a layer of granular material (ballast), is the most common solution worldwide over other alternatives such as concrete slabs. This is because ballast provides less stiff support (which is an important factor in the case of differential settlement or subgrade failure), is more economical, and produces less noise [1]. Despite its benefits, a large portion of the track maintenance budget is spent on ballast related problems [2]. Ballast deterioration, in fact, has been identified as the main cause of average and differential settlement of railway track under cyclic rail loadings. When geometric parameters exceed certain tolerances, maintenance operations such as tamping or stoneblowing are needed to restore the track geometry for safe and efficient running of trains. Tamping machines squeeze the ballast up below the sleepers after they have been raised to the desired level, whereas stoneblowing machines add stone to the existing ballast surface after lifting the sleeper up to the required level (Figures 1 and 2). However tamping, which is the most commonly used method, loosens and damages the ballast, serving further breakdown and leading to loss of strength and stiffness of the layer. It is estimated that a tamper generates around 4 kg of fines per sleeper for one tamping insertion, whereas only 0.5 kg of fines per sleeper are produced by an equivalent stone-blowing cycle [3]. Figure 1: Tamping maintenance on ballast [3] (Esveld, 2001)

Figure 2: Principle of stoneblowing [3] (Esveld, 2001)

Stoneblowing maintenance process involves adding crushed rock to the surface of ballast under the lifted sleepers: preliminary, the existing geometry of the track is measured; then, the vertical adjustment at each sleeper required to achieve the desired geometry is calculated. finally, the volume of stone to be blown beneath each sleeper to achieve this adjustment is determined and the stones are then blown under the sleeper [4]. When the ballast breakage reaches a critical level, the track needs to be maintained either by ballast cleaning or ballast renewal, which requires more fresh ballast. Thus, it is important to study the ballast behaviour, the effect of maintenance operation and investigate the possibility of ballast reuse. This can increase ballast life on the track, reduce waste ballast generated from ballast cleaning or ballast renewal, minimise the frequency and cost of ballast replacement, and lead to further developments in the railway industry. For this purpose a test that reflect performance in track, allowing consistent comparisons between materials and practices is needed. For this reason a ballast box has been used to simulate loading conditions and maintenance by tamping and stoneblowing in a simplified and controlled manner. The aim of this paper is to demonstrate how the box test can be used to replicate ballast field conditions to compare different materials and maintenance processes. MATERIALS TESTED AND BALLAST BOX The first material tested (Ballast 1) was granite aggregate sourced from Cliffe Hill Quarry in Leicestershire, locally known as Markfieldite, containing principally microdiorite, an intrusive igneous material of superior uniformity, strength and durability [5]. The second material tested (Ballast 2) was granite aggregate sourced from Bardon Hill Quarry, Leicestershire. Their physical properties are shown in Tables 1 and 2. Table 1: Physical properties of Cliffe Hill Quarry granite ballast [6]

Table 2: Physical properties of Bardon Hill Quarry granite ballast [7]

Gradation of specimens is reported in Figure 3. It can be noted that the materials chosen are of different gradation (specimens from Bardon Hill are more uniform) in order to compare results for different ballast type. Figure 3: Specimens grading for Ballast 1 and Ballast 2

The box used is a case-hardened steel box [8]. The dimensions of the box are 460 mm, 200 mm and 300 mm as regards length, width and height, respectively. This box can be considered a scaled (by a factor 2/3) version with respect to the box used by McDowell et al. [9]. A piece of synthetic rubber was used as a standard elastic mat beneath the ballast in order to generate realistic mechanical behaviour in the ballast. The mat is approximately 20 mm thick. Its static bearing modulus and its dynamic modulus at 5 Hz were obtained according to the Standard DBS 918 071-01 [8]; their values are 350 kPa/mm and 850 kPa/mm respectively. Four pieces of synthetic rubber, 3 mm thick, were placed on the sides of the box to simulate the continuity of field conditions. An aluminium section 190 mm long, 148 mm wide, and 100 mm deep, simulating a sleeper, was used to transmit the load to the ballast layer. TEST PLANNING AND PROCEDURE In total 29 box tests were performed: 7 tests (Series 1) for Specimen A (Ballast 1), 7 tests (Series 2) for Specimen B (Ballast 1), 8 tests (Series 3) for Specimen C (Ballast 2) and 7 tests (Series 4)for Specimen D (Ballast 2). After each box test a maintenance operation was performed in order to restore the initial position of the sleeper. Table 3 gives the details of the test planning.

Each specimen, after sieve analysis, was poured into the box reaching a thickness of 50 mm, and then it was slightly compacted by a weight of 5 kg. This operation was repeated until the level of the sleeper was reached (depth of 200 mm) as shown in Figure 4. Table 3: Ballast box test plan

Figure 4: Specimen preparation at 50 mm (left) and 100 mm (right)

The set-up of the ballast box test prior to loading is shown in Figure 5. For establishing the maximum sleeper/ballast contact stress to apply in the laboratory cyclic load tests, a nominal axle load of 250kN was assumed, which corresponds to a static wheel load of 125 kN. Figure 5: Specimen set-up before the test

The corresponding average contact stress at the sleeper/ballast interface is approximately 200 kPa. Based on these estimates, in order to apply the same contact stress, the maximum cyclic vertical load was selected to be 6.5 kN, whereas the minimum load was 0.5 kN. The ballast was loaded cyclically with sinusoidal pulses [10] at a frequency of 3Hz by means of a hydraulic apparatus (MAND) as shown in Figure 6. Figure 6: MAND apparatus used for the box test

This frequency was chosen as compatible with the actuator characteristics; moreover there would appear to be evidence that the frequency of testing does has little influence on the resilient behaviour of granular materials [11]. Tests were carried out applying 50,000 cycles of load. Although this number is not sufficient to fully replicate ballast performance in real track (where several millions of cyclic loads take place) it was deemed to be adequate to obtain stable behaviour in the ballast layer [12], and to fit each test into a reasonable period. Load and displacement were measured by means of a transducer integrated with the actuator. The ballast tamping operation was simulated by inserting a 2.5 cm wide chisel using a Kango hammer into the ballast specimen (at approximately 1 5 o to the vertical). The insertion was applied 50 mm from the edge of the sleeper for around 2 seconds per side [9]. Before this operation, the sleeper was lifted and held at the level occupied at the beginning of the test, to an accuracy of 1.5 mm. Stoneblowing was also simulated in a similar manner. The sleeper was removed, taking care not to disturb the compacted ballast in the process [13]. Extra material was then added and its amount was calculated based on the displacement that had occurred in the previous test. The stones used came from the same source as the ballast with a size between 14 and 20 mm [14] (Figure 6). The sleeper was then re-set at its original position, with an accuracy of 1.5 mm. Figure 6: Stones used for stoneblowing in Specimen C

RESULTS AND DISCUSSION The main parameters measured are the settlement (change in level of the sleeper) and the vertical stiffness of the ballast layer. In Figures 7 and 8 results of settlement against number of cycles for all test series are reported. The plots show the sleeper level starting from zero. This choice was taken to allow a clearer comparison between tests. However, it has to be remarked that simulated maintenance operation (represented in plots as ‘spikes’) could only be set to an accuracy of ± 1.5mm. The sleeper level has been defined as the sleeper deflection at the minimum load (0.5 kN) applied by the actuator. It represents the plastic displacement after each cycle that cannot be recovered, as shown in Figure 9. For clarity, only a limited number of the data points are shown to define the test curves. From Figures 7 and 8 (Specimen A and Specimen C) it can be noted that the tamping simulation tends to become gradually less effective, i.e. the settlement at the end of each tamping interval increases with increasing number of tamps. Otherwise, stoneblowing simulation, performed before the last test in Series 1, the last two tests in Series 3, and before all tests in Series 2 and 4, seems to gradually improve ballast performance. Moreover, apart for the initial compaction, the average settlement after this second operation is around half that after tamping. This latter result agrees with McMicheal and McNaughton [14]. A plot of stiffness, C , against number of cycles for all specimens is shown in Figure 10.

It is defined for each cycle as: 𝐶=

𝜎𝑚𝑎𝑥 − 𝜎𝑚𝑖𝑛 𝛿𝑟

where 𝜎𝑚𝑎𝑥 and 𝜎𝑚𝑖𝑛 are the maximum and minimum applied stresses respectively, and 𝛿𝑟 is the sleeper resilient displacement. In the plot is also reported the range of stiffness measured by the Falling Weight Deflectometer (FWD) on railway tracks in the United Kingdom. The FWD stiffness range in terms of force applied has been found to be 30 to 100 kN/mm/sleeper end [15]. Since it applies a 125 kN load on each sleeper through a beam, which is shaped to distribute the load to both ends of the sleeper, the displacement range is: 𝛿=

125 1 ( ) = 0.625 → 2.083 𝑚𝑚 2 30 → 100

Figure 7: Settlement of the sleeper against number of cycles for box test on Specimen A and Specimen B

Figure 8: Settlement of the sleeper against number of cycles for box test on Specimen C and Specimen D

Figure 9: Box test 1.5 for Specimen A, detailed curve

Thus the range of FWD stiffness in the UK in terms of the box test is: 𝐶=

200 = 96 → 320 𝑘𝑃𝑎/𝑚𝑚 0.625 → 2.083

Table 4 shows stiffness mean values for all specimens tested. It can be noted that stiffness results for all tests are within the range as above. Thus, it can be deduced that, in this aspect at least, this test is reasonably repeatable and test can replicates field conditions.

Figure 10: Stiffness values against number of cycles for Ballast 1 and Ballast 2

CONCLUSIONS A box test was used to simulate ballast loading and maintenance conditions in the trackbed in a simplified and controlled manner. With the aim of comparing maintenance operation effectiveness on ballast settlement two different igneous ballasts were tested. Specimens were cyclically loaded with a sinusoidal pulse with a minimum load of 0.5 kN and maximum load of 6.5 kN, at a frequency of 3 Hz. The tamping process was simulated by inserting a 2.5 cm wide chisel using a Kango hammer into the ballast, whereas stoneblowing was simulated by placing a certain amount of 14 mm to 20 mm size stones below the sleeper. Both maintenance processes allowed the sleeper to be re-set in its original position. It can be noted from Figures 7 and 8 that for both materials tested, tamping operations worsened performance, whereas stoneblowing gradually improved it in terms of final settlement and deterioration rate. Both findings agree with the field behaviour reported by McMichael and McNaughton [14]. Furthermore, all specimens, independent of past maintenance history, after the last test, and following a stoneblowing simulation, exhibited almost the same settlement. Overall, it may be argued that these tests did not replicate exactly the situation in real track because the sleeper used was smaller than a real sleeper section and the effects on the ballast from neighbouring sleepers was not simulated. However, test materials and stress levels have not been scaled down. Furthermore, materials were tested under the same conditions in terms of preparation, loading, and maintenance simulations. The tests are therefore considered to give a valid insight into the behaviour of a ballast layer under repeated loading and maintenance processes. They indicated that stoneblowing can result in significantly improved post-maintenance track performance.

The final finding of this investigation is that the box test can be used to replicate field conditions reasonably closely.

ACKNOWLEDGEMENTS This project has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement n. 607524 – SUP&R ITN (www.superitn.eu) REFERENCES [1] Profillidis, V. A. (2000). Railway Engineering. Ashgate [2] Indraratna, B., Salim, W. and Christie, D. (2002). Improvement of recycled ballast using geosynthetics. In 7th International Conference on Geosynthetics, pages 1177–1182, Nice, France. [3] Claisse, P. A., Keedwell, M. and Calla, C. (2003). Tests on a two layered ballast system. Procedings of the ICE - Transport, vol. 156 (2): 93-101 [4] Selig, E. T. and Waters, J. M. (1994). Track Geotechnology and Substructure Management, Thomas Telford [5] Whateley, M. K. and Barrett, W. L. (2009). Cliffe Hill Quarry, Leicestershire-development of aggregate reserves. Introduction to Mineral Exploration [6] Laryea, S., Baghsorkhi, M. S., Ferellec, J., Mcdowell, G. R. and Chen, C. (2014). Experimental and numerical discrete element analyses have been performed to investigate. Transportation Geotechnics, 1(4):225–240. [7] Aggregates Industries UK Ltd (2013). Aggregates for railway ballast, Bardon Hill Quarry [8] Sol-Sánchez, M., Thom, N., Moreno-Navarro, F., Rubio-Gámez, M., and Airey, G. (2015). A study into the use of crumb rubber in railway ballast. Construction and Building Materials, 75:19–24. [9] McDowell, G. R., Lim, W. L., Collop, A., Armitage, R. J., and Thom, N. H. (2005). Laboraratory simulation of the effects of train loading and tamping on ballast. Proceedings of the Institution of Civil Engineers, (May):89–95. [10] Brown, S. F. (1996). Soil mechanics in pavement engineering. Géotechnique, 46(3):383– 426. [11] Lekarp, F., Isacsson, U., and Dawson, A. (2000). State of the Art I: Resilient Response of Unbound Aggregates. Journal of Transportation Engineering, 126(1):66–75. [12] Indraratna, B., Khabbaz, H., Salim, W., and Christie, D. (2006). Geotechnical properties of ballast and the role of geosynthetics. Ground Improvement, 10(3):91–101. [13] Anderson, W. F. and Key, A. J. (2000). Model Testing of Two-Layer Railway Track Ballast. Journal of Geotechnical and Geoenvironmental Engineering, 126(April):317–323.

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