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PAPER Performance of ballast mixtures

By Glenn R McDowell, Jason Buchanan and Wee L Lim, Nottingham Centre for Geomechanics, School of Civil Engineering, University of Nottingham.

Introduction

It has been shown (McDowell et al, 2004) that the behaviour of different ballasts varies enormously. McDowell et al (2004) examined the performance of each of four different ballasts in a box test which simulates train loading and rearrangement caused by tamping, and were able to correlate the results in terms of settlement and degradation with simple index tests such as the Los Angeles Abrasion value and the Micro-Deval Attrition value (as defined in BS EN 13450: 2002 (British Standards Institution, 2002)).

In that paper, one ballast was clearly inferior to the others in terms of performance. The question arises as to whether a weak ballast could be mixed with a strong ballast to give a ballast mixture which meets current specifications, and how much weak ballast could be included without adversely affecting the performance of the mixture to a significant extent. This would allow the selection of an appropriate amount of weaker, cheaper aggregate for inclusion in rail trackbeds, reducing ballast cost.

This paper examines the performance of ballast mixtures containing different quantities of weak ballast. Performance is measured in terms of permanent settlement and degradation in box tests, degradation in oedometer tests, and other simple index tests.

Ballast A (strong) has been mixed with various proportions (10%, 20%, 30%, 40%) by mass of ballast B (weak) and tested using the box test to ascertain how much of ballast B can safely be added without adversely affecting performance too much.

In addition, the paper examines whether the strategic placing of weak ballast at the bottom of the ballast layer in the box test, with the good quality ballast at the top of the ballast layer underneath the sleeper, may give acceptable performance. This may provide an acceptable way of reducing costs without compromising performance in rail trackbeds.

Ballast properties

In the UK, until recently, the Wet Attrition Value (WAV) and Aggregate Crushing Value (ACV) have been used to assess the potential performance of ballast (Railtrack, 2000). The Los Angeles Abrasion (LAA) test and Micro-Deval Attrition (MDA) tests have now been adopted in the new European specification BS EN 13450 (2002) (British Standards Institution, 2002).

McDowell et al (2003, 2004) showed that because the ACV test is performed on 10mm to14mm particles, this test is of little use because of the size effect on particle strength, and track ballast consists of much larger particles.

The WAV, LAA and MDA tests each involve measuring the degradation of ballast in a revolving drum. Table 1 shows the WAV, LAA and MDA values for ballasts A and B. In addition, the flakiness index and particle length index are given according to the BS EN 13450 (2002) specification. A full description of the tests is given in McDowell et al (2004).

It can be noted that Appendix E of the Railtrack Line Specification (Railtrack 2000) states that from 1 April 2005, the LAA value must not exceed 20, and the MDA value must not exceed 7.

However, this specification has been adopted early and is already now in use.

Ballast B therefore clearly does not meet the new specification, but the question arises as to whether some of this ballast may be introduced into a mixture with ballast A without adversely affecting the performance too much.

Ballast A is a coarse-grained granodiorite containing 30% plagioclase, 25% quartz, 20% alkali feldspar; ballast B is also a granodiorite from a different quarry containing 50% plagioclase which has been mainly altered to clay and mica, 30% quartz and 10% hornblende (Large, 2003).

Box tests

McDowell et al (2004) describe a box test for simulating train loading on ballast and rearrangement by tamping. The box (Figure 1), is 700mm long, 300mm wide and 450mm high, and can be envisaged as representing a section of ballast underneath the rail seat (Figure 2). It is made mainly of casehardened steel with one side (a longer side) made of reinforced Perspex, so that degradation can be observed during the test. The base of the box is made of wood and a 10mm thick rubber sheet was placed between the ballast and the wood to replicate a typical stiffness of sub-ballast and subgrade.

To compare performance consistently in the box tests, all ballast samples had the same grading that conformed to the new BS EN 13450 (2002) specification for track ballast grading. Figure 3 shows the grading used and the permissible range of particle size distributions.

The box tests were conducted with wet ballast because track ballast is often in the wet condition, and ballast in the wet condition is considered to be more critical (Selig and Waters, 1994).Thus, each ballast sample was soaked in water for 48 hours to ensure that all ballast particles were fully saturated before pouring into the box. A controlled amount of water was also added at various stages during the testing, as described later.

Each prepared ballast sample was poured into the box until the ballast thickness reached approximately 300mm. The top of the ballast was then levelled by hand. A rectangular hollow steel section with dimensions 250mm by 300mm by 150mm, representing a section of a sleeper, was placed on the ballast and additional ballast was poured around both sides of the sleeper to the top of the box to represent crib ballast.

To restrain the sleeper from moving horizontally or tilting, a steel piston was attached to it and a guide plate for the loading piston was attached to the box frame to guide it during cyclic loading (Figure 4).

Ballast settlement was measured by measuring the displacement of the top side of the bottom flange of the sleeper using an LVDT.The ballast was loaded cyclically with a sinusoidal load pulse with minimum load of approximately 3kN and maximum load of approximately 40kN (equivalent to an axle load of 20-25t) for 100,000 cycles, at a frequency of 3Hz.

The ballast tamping process was simulated in the box test by inserting a one inch (25.4mm) wide chisel into the ballast using a Kango hammer (see McDowell et al, 2004). Before the chisel was inserted into the ballast, the sleeper was lifted until its top was level with the top of the box.

At this level, the bottom of the sleeper was 300mm from the bottom of the ballast layer.Thus, the ballast could be tamped to regain (approximately) its original thickness.The chisel was then inserted towards the ballast underneath the sleeper through a guide hole, 160mm from the sleeper edge, at an angle of approximately 10infinity to the vertical. Figure 4 shows the guide holes: one on each side of the sleeper.

Three 'tamps' were applied at each side of the sleeper in a number of different locations (95mm, 150mm and 205mm from the Perspex wall). Each insertion took approximately two seconds and tamping was conducted at 100; 500; 1,000; 5,000; 10,000 and 50,000 cycles.

Prior to tamping, 2 litres of water were poured evenly on each side of the sleeper to ensure the ballast remained wet during the test.Water and fines which drained out of the box were retained on an aluminium tray underneath. To ensure the same amount of ballast was available to be 'pushed' underneath the sleeper, and to maintain the correct amount of crib ballast, additional ballast was added to the top of the box after tamping.

Ballast mixtures

Mixtures were created using ballast A containing 10%, 20%, 30% and 40% of ballast B to ascertain whether there is some obvious critical proportion above which the performance in the box tests deteriorates considerably.

Figure 5 shows the settlement of each ballast as a function of number of cycles. The data points at one cycle and 100 cycles represent the settlement since the start of the test. Each subsequent data point represents the settlement which occurred after each simulated tamping operation (eg the settlement at 10,000 cycles is the settlement which occurred after the simulated tamping at 5,000 cycles).

The mixture with 40% of ballast B clearly shows larger settlements than any of the other mixtures.Figure 6 shows the change in the particle size distribution for each mixture: again, the mixture with 40% of ballast B degrades significantly more than the other mixtures (note that the sieve size has been plotted on a linear scale).

The amount of degradation was also measured using Hardin's total breakage B t(Hardin, 1985) which is the area swept out by the particle size distribution (with sieve size plotted on a log scale in the usual way). The values are given in Table 2.

It should be noted that Lim (2004) has found that for ballast A alone, the total breakage B t(after 10 6cycles) was 0.022 in one test and 0.021 in another test (demonstrating good repeatability), although this was for ballast graded to the old (Railtrack, 2000) specification.

It can be seen that as the proportion of ballast increases beyond 30%, significant degradation occurs. In particular, it was found that for the ballast directly beneath the sleeper, the fines percolated downwards so that the percentage by mass passing each sieve size was found to increase more significantly at the bottom of the ballast layer.

The ballast column directly beneath (ie in vertical alignment with) the sleeper was divided into two layers of equal thickness, and the aggregates sieved separately. Figure 7 shows the changes in the particle sizes for each layer.

Large oedometer tests

McDowell et al (2004) found there was some correlation between the values of total breakage in the box tests and those in large oedometer tests for single ballasts (ie not mixtures) graded to the old (Railtrack, 2000) specification.

The oedometer is 300mm in diameter, and samples are compressed to a stress of 21MPa (as in the standard ACV test on 10mm to14mm particles) in an Instron testing machine following compaction on a vibrating table. The oedometer is described by McDowell et al (2003).

It was considered that the oedometer test might be suitable for distinguishing between the different ballast mixtures used in the box tests in terms of total breakage. As for the samples described by McDowell et al (2003) and McDowell et al (2004), sample heights were limited to approximately one half of the diameter so as to minimise the effect of wall friction. As for the box tests in this paper, samples were graded according to Figure 3 and soaked for 48 hours prior to loading.

Values of total breakage B tare given in Table 3 for ballast A alone, and mixtures containing 20%, 30% and 40% ballast B.The mixture containing 40% ballast B degrades significantly more than the others, which show similar amounts of degradation. It would appear that under the much higher stress levels encountered in the oedometer tests which induce bulk fracture of particles (compared with the box tests which cause mainly abrasion), the mixture containing 40% ballast B exhibits a much higher degree of bulk fracture than the other mixtures.

It should be noted that Lim (2004) found that for an oedometer test on 100% ballast B, the total breakage was 0.71, though this was for a sample graded to the old (Railtrack 2000) specification.

Index tests

Los Angeles Abrasion tests and Micro-Deval Attrition tests were performed on the mixtures containing 20%, 30% and 40% ballast B according to the Standards BS EN 1097-1 (British Standards Institution, 1998) and BS 1097-2 (British Standards Institution, 1996) respectively.

The LAA and MDA values are given in Table 4. It can be seen that the mixtures containing 20% and 30% ballast B just meet the requirements of the new BS EN 13450 (2002) specification (which states that the LAA value must not exceed 20, and the MDA value must not exceed 7). However, the mixture containing 40% ballast B does not meet the requirements.

Although the MDA values and LAA values each differ only by one for the mixtures containing 30% and 40% ballast B, the tests are considered repeatable since only one test is required by each of the standards to return the LAA or MDA value. In addition, the LAA and MDA results are consistent with the oedometer tests and box tests, so that all tests are in agreement that 30% of ballast B can be included without adversely affecting the performance of the mixture to a significant extent.

Layered ballast

An additional box test was performed on an aggregate with approximately 40% ballast B positioned at the bottom of the box, and approximately 60% ballast A in the upper layer. Thus the ballast disturbed by the simulated tamping was mainly ballast A, which was noticeably less susceptible to breakage during the simulated tamping operation.

At the beginning of the test, the percentage by mass of ballast A was less than 60%; more ballast A was added to the crib ballast after each tamp, so that at the end of the test the percentage by mass of ballast A present was approximately 60%. Thus, in the early stages of the test, the percentage by mass of ballast B was actually greater than 40%.

Figure 8 shows the resulting settlements as a function of number of cycles. The layered ballast performs much better than the uniformly mixed sample, and the performance compares well with the other uniform mixtures in Figure 5.

It was also found that for the layered ballast, there was much less breakage than for the uniform mix. The total breakage B tfor the whole sample of the uniform mixture containing 40% ballast B was 0.036 (Table 2) but only 0.019 for the layered aggregate, which compares well with the other values in Table 2 for the uniform mixtures containing smaller proportions of ballast B.

It therefore seems the strategic placing of weak ballast could present significant savings in cost without compromising performance. Further work on a larger scale is required to verify this. It seems that alternative weak aggregates may be suitable as ballast materials as long as they are placed at the bottom of the ballast layer.

Conclusions

Box tests have been performed on uniform binary mixtures of ballast aggregates (one strong ballast and one weak ballast) to simulate the effects of train loading and tamping, to determine the proportion of weak ballast which can be included without affecting the performance of the mixture to a significant extent in terms of settlement characteristics and degradation.

Large oedometer tests and Los Angeles Abrasion and Micro-Deval Attrition tests have also been performed. All the tests are in agreement that for the ballasts tested here, 30% of the weak ballast can reasonably be included; the mixture containing 40% of weak ballast was shown to exhibit significantly poorer performance.

However, a box test was also performed on a layered aggregate with approximately 40% weak material at the bottom of the ballast layer. This ballast was found to perform well, in terms of settlement and degradation, comparing favourably with the uniform mixtures containing smaller proportions of weak ballast. This suggests that weak aggregates may be suitable for use as ballast so long as they are placed at the bottom of the ballast layer in the trackbed.

References British Standards Institution (1996). BS EN 1097-1: Tests for mechanical and physical properties of aggregates - determination of the resistance to wear (micro-Deval).

British Standards Institution (1998). BS 1097-2: Tests for mechanical and physical properties of aggregates - methods for the determination of resistance to fragmentation.

British Standards Institution (2002). BS EN 13450 (2002): Aggregates for railway ballast.

Hardin BO (1985). Crushing of soil particles. Journal of Geotechnical Engineering, ASCE 111, No. 10, pp. 1177-1192.

Large D (2003). Petrographic description of railway ballast samples. University of Nottingham (confidential report).

Lim WL (2004). Mechanics of railway ballast behaviour. PhD dissertation, University of Nottingham.

McDowell GR, Lim WL, Collop AC, Armitage R and Thom NH (2004). Comparison of ballast index tests with performance under simulated train loading and tamping. Proceedings of the Institution of Civil Engineers - Geotechnical Engineering 157 GE(3), pp151-161.

McDowell GR, Lim WL and Collop AC (2003). Measuring the strength of railway ballast. Ground Engineering 36, No.1, pp. 25-28.

Railtrack (2000). Railtrack Line Specification RT/CE/S/006 Issue 3: Track Ballast.

Selig ET and Waters JM (1994). Track geotechnology and substructure management. Thomas Telford, London.

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