Reusing Life-Expired Railway Ballast: Laboratory Testing, Shape Analysis, and Petrographic Evaluation

Ballasted railway track is the primary form of existing railway track system throughout the world and is likely to remain so for the foreseeable future. The general track form consists of Vignoles rails running over concrete or timber sleepers on a trackbed. The trackbed is formed of a typically 300-mm thickness of ballast and sometimes a $\sim 100\text{-}\mathrm{mm}\text{-}\mathrm{thick}$ sand blanket placed upon a prepared subgrade. The thickness of the trackbed is specified to reduce stresses to levels acceptable for long-term cyclic loading of the subgrade (Li and Selig 1998a, b). If the subgrade is of sufficient stiffness and strength, the ballast thickness is at least the depth necessary to carry out tamping operations.

Over its life cycle, ballast is subjected to loads from passing trains. Measured in cumulative tonnage, busy routes commonly can receive more than 15 million equivalent gross t/year (e.g., some sections of the United Kingdom’s (UK’s) east coast mainline achieve this) (Le Pen et al. 2018). Usage inevitably leads to track geometry deterioration and periodic maintenance to restore line and level for safe and comfortable train running. Maintenance usually is carried out by mechanized tamping, a process in which vibrating tines are introduced vertically on either side of the railseats of the sleepers to squeeze the ballast together, raising the support beneath the railseats, while the rails and sleepers are lifted and held to a design line and level. Although maintenance by tamping may be a necessary activity, it is intensely damaging to the ballast (Selig and Waters 1994; Aingaran et al. 2018). Tamping consists of two motions: a slower, large-amplitude squeezing motion to raise the ballast level; and a second faster, small-amplitude higher frequency vibration of the tines at $\sim 35–40\mathrm{Hz}$ to allow the times to break the frictional contact between grains and push through the ballast (Aingaran et al. 2018). The squeezing motion rotates the principal stresses from vertical to horizontal, loosening the ballast and causing a reduction in support stiffness and rapid rates of settlement as the newly tamped track beds in. Together, both motions cause abrasion, attrition, and breakage generating fouling material (fines). There may also be environmental sources of fouling such as migration of finer materials from the subgrade. Qian et al. (2022) showed that ballast fouling and the moisture content of the fines affects the strength of the ballast matrix. The damage caused by tamping, the generation of fines, and the addition of environmental fouling leads to an increasing deterioration rate for trackbed performance and requirements for more-frequent maintenance. This partly can be alleviated by ballast cleaning, using specialized machines at mid-life cycle, and includes reintroduction of a proportion of fresh ballast to approximately restore the intended particle-size distribution. Ballast cleaning extends the period before ballast replacement. However, eventually, perhaps after a 20–30-year service life encompassing 10–15 tamps and a midlife ballast clean, the trackbed performance becomes so poor that it requires complete replacement, i.e., renewal. During renewal, the now life-expired and fouled ballast is dug out and replaced with fresh ballast.

Railway ballast as a trackbed-forming material has been in use for centuries. It is relatively inexpensive compared with alternative slab tracks (in terms of up-front cost) and offers the potential for the track geometry to be realigned relatively easily. One potential advantage also has been that ballast may be sourced from the nearest local quarry. This latter advantage no longer necessarily holds, because modern railway ballasts preferably are sourced from crushed igneous or metamorphic rocks because of their greater strength and fracture resistance (Watters et al. 1987; Clifton et al. 1987; Raymond 1985). This means that ballast material may be shipped from the most suitable quarries. In the UK this can mean that ballast sourced in Norway or Scotland ends up in the South of England, as was the case for the construction of High Speed 1 (linking London with Paris), for which only ballast sourced from one quarry in the UK (Glensanda) or ballast from Norway was considered to be of sufficient quality. Generally, the types of ballast materials that are used for railway ballast in many parts of the world are required to meet minimum criteria commonly determined from the Los Angeles abrasion (LAA) (BSI 2020) and Micro-Deval (MD) (BSI 2011) tests. Their application to railway ballast is specified in BS EN 13450 (BSI 2002). These are the UK and European standards; other regions apply similar standards with some variations.

The Micro-Deval and the LAA test are different types of abrasion tests. The Los Angeles abrasion test measures ballast particle toughness or tendency for breakage. The Micro-Deval test measures the durability required to achieve the desired ballast life (Selig and Boucher 1990). The specification of both types of tests is perhaps indicative that they provide complimentary but unequal results, so a material may perform better in one test than the other when ranked against other tested materials (Selig and Boucher 1990).

The criteria specified for candidate ballast materials to pass these tests are likely to eliminate sedimentary rocks from consideration leaving common igneous materials such as granites, granodiorites, basalts, and gabbros (the last two commonly are referred to as types of trap rock) as likely suitable ballast materials.

The relatively new understanding of the most-suitable ballast types (on the scale of the history of the railways) has meant that since the 1980s in the UK, sedimentary ballasts previously present on some railway routes gradually have been replaced during renewals with igneous ballasts. With a rollout covering the previous 40 years, there is a strong likelihood that recovered material from renewals taking place today will be an igneous ballast.

In the long-term, quarrying ever greater quantities of raw ballast materials to replace life-expired material is potentially unsustainable (or, at best, simply inefficient), and there is growing pressure on the railway industry to move toward more environmentally acceptable practice. One means to reduce the demand for raw materials is to reuse life-expired ballast.

In the UK, recovered trackbed material from renewals is sent to processing centers where the material is screened, cleaned, and sieved. Depending on its initial quality and degree of fouling material, much of this processed material is of too fine a gradation for reuse as railway ballast but still may be downcycled, for example, as a fill material for the highways industry. However, a portion of the recovered trackbed material is in a suitable size fraction to be reused as ballast.

Reusing ballast offers a potentially more sustainable future for ballast supplies. However, there is concern about the quality of recovered ballast. There are doubts and/or contradictory opinions about its performance. Many practitioners hold that recycled ballast performs less well than fresh ballast. Research and trials in Australia have shown potential benefits for the use of recycled ballast placed when used alongside geocomposites (Indraratna and Salim 2003). They also found ballast breakage and deterioration of strength to be one of the mechanisms explaining degradation in latite (igneous rock) fresh ballast.

This paper investigated the potential for reusing recovered UK life-expired predominantly igneous railway ballast with a small proportion of basalt by means of comparative tests with a fresh igneous railway ballast. Tests used the Southampton Railway Testing Facility (SRTF), a large triaxial testing apparatus at Nottingham University, surface roughness measurements using optical three-dimensional (3D) surface characterization, and material composition tests by the UK’s National Oceanography Centre (NOC).

The testing methods can be classified depending on whether they investigate the performance of representative volumes of assemblies of grains—referred to as element tests—or are tests at the grain-scale. The test methods were applied to both used and fresh ballast, and the results were compared.

Both fresh and reused ballast were supplied by Network Rail (NR) from stocks held at Eastleigh recycling center, Hampshire, UK. The physical appearance of the two ballast types is shown in Fig. 1. Visually, the fresh ballast was assessed as being entirely from a crushed granite or granodiorite parent rock (Clayton et al. 1995). The used material also was mainly from granite or granodiorite parent rock; however, other rock types also were present, including a proportion of basalt. To quantify the composition of the used ballast, 34 kg of material comprising some 427 individual grains was evaluated visually and the proportions of different rock types were quantified. The reused ballast was found to contain just over 60% granite or granodiorite, 20% basalt, and small proportions of limestone, flint, and miscellaneous particles such as dolomite and volcanic glass (Table 1).

The primary proportion of used ballast material, the granite or granodiorite, is thought to have spent some 20–30 years in the trackbed. The limestone present perhaps was remaining material from a previous trackbed that largely was removed during a previous renewal. The flint and other material present likely were contaminants present in the local soil. The basalt and some portion of the granite material may be additional materials placed during tamping works over the course of the trackbed’s life cycle or from a previous life cycle. The provenance of the used ballast and the fresh material from Eastleigh recycling center was not available. However, the supplied material can be compared with known sources.

Table 2 presents some properties of ballasts commonly available from quarries in the UK. The rock type and its specific gravity were close to those of ballast from a number of UK quarries supplying granite or granodiorite crushed rock ballasts. Sieve tests were carried out to determine the particle-size distributions for comparison with each other and with the current Network Rail ballast specification used in the UK (Fig. 2). Reused ballast 1 was slightly finer than the range normally specified, and Fresh ballast 2 was slightly coarser (Fig. 2). It is not uncommon when receiving relatively small volumes of material that a particular sieved sample is slightly out of the expected range. This can be a result of segregation during transport and also of from the location within a stockpile from which a particular shipment has been collected (finer particles tend to fall to the bottom).

Grain scale tests and shape measurements were carried out to compare particle mineralogy, form, angularity, and surface roughness.

Standard triaxial drained tests were carried out to compare the stress–strain behavior of fresh and life-expired ballast.

The reused ballast in this study contained 63% granite and 21% basalt (Table 1). Studies of strength of intact basalt and granite rocks by Hoek and Brown (1992) and Piasta et al. (2017) showed that the compressive strength and modulus of elasticity of basalt rocks is greater than that of granite. This is mainly because granite is made up of less quartz ($\sim 35\%$ quartz, 40% feldspar), whereas basalt is 90% quartz (Fig. 2 and Table 3). The overall strength and mobilized shearing resistance of the reused ballast might have been improved by the presence of the basalt grains, compared with fresh ballasts formed solely of granite and granodiorite. Compared with other triaxial tests in the literature, these shear strengths fall within the range of expected behavior (e.g., Aursudkij et al. 2009; Indraratna et al. 1998; Suiker et al. 2005; Raymond and Davies 1978). The only other triaxial tests of used ballasts that the authors found were carried out by Ionescu and Indraratna (2003), who compared recovered ballast and a fresh ballast formed of a volcanic basalt (latite). The recovered material was reported as having a 61% angular crushed rock composition and a further 35% crushed river gravel. In their tests the fresh ballast outperformed the recovered material, which was attributed mainly to greater volumetric deformation of the recovered material. This result, which was the opposite of that found in the present tests, highlights that not all recovered material is the same. There also is likely to be a significant relative difference in different fresh ballasts, so that even if a reused ballast appears to perform comparatively well compared with one type of fresh ballast, other fresh ballasts could perform better. In our 50:50 ballast tests we mixed the fresh and life-expired ballast in small batches of 100 kg each to obtain an even mix with hand shovel. Because ballast normally is transported using 1-t bags, we would recommend pouring out both the fresh and spent ballast materials simultaneously from the 1-t bags and mixing the pile manually or using a mixing machine or excavator.

The greater sphericity and the reduced angularity and surface roughness of reused ballast affects its initial packing density and shear behavior. These tests showed that, collectively, the competing influence of these grain characteristics leads to greater initial density, (lower void ratio) suppressing initial dilation and generating a stable support stiffness in the SRTF. Fresh ballast, with its lower sphericity and greater angularity and surface roughness, had a lower initial packing density (higher void ratio) and earlier onset of a dilative response when sheared. The greater volumetric change of the fresh ballast was evident from the particle breakage analysis carried out after each test, in which fresh ballast had greater breakage than reused ballast. Thus, the life-expired ballast can be reused alone or mixed with fresh ballast as long as it is cleaned (without any fouling material) and graded to the trackbed design standards.

The petrographic analysis showed that the majority of the recovered material was granite (60%) with 21% basalt, and therefore it was acceptable to compare its performance with that of a fresh ballast mainly formed of granite. The form analysis showed that there was a small tendency for more-spherical columnar grains in the reused material. The surface roughness analysis showed that there was a major reduction of the roughness of the used ballast compared with the fresh ballast.

In the SRTF tests the reused ballast performed best in terms of having the least permanent settlement compared with the fresh and blended ballasts. Counterintuitively, the fresh material settled the most.

The monotonic triaxial tests results showed that the reused ballast exhibited slightly higher shear strength, had greater initial densities compared with the fresh ballast, and dilated less. The monotonic failure results support the SRTF results.

Taken as a whole, the three strands of this research indicate that a reused ballast appears to perform slightly better than a particular fresh ballast of similar mineral composition. The reduced surface roughness of the used ballast was more than compensated for by other improvements—e.g., the greater initial packing density. The presence of basalt contributed to the increase in strength and mobilized angle of shearing resistance of the reused ballast.

In general, there is no reason not to reuse ballast if it can be cleaned and screened and supplied in an acceptable particle-size distribution, and its mineral composition confirms that it is suitable. The performance of the reused ballast then should be similar to that of fresh ballast of similar mineral composition. The reuse of material potentially is more sustainable and less costly—although this may depend on local circumstances. Ideally, further work should compare the in-field performance of reused and fresh ballast at control sites.

All data, models, and code generated or used during the study appear in the published article.

The work described in this paper was jointly funded by the EU via the project In2Rail (Call identifier H2020-MG-2014, Grant Agreement No. 635900) and the UK’s Engineering and Physical Sciences Research Council (EPSRC) as part of the track to the future (T2F) project (Grant Reference EP/M025276/1). The authors thank Network Rail for support in kind (providing the materials for testing and explaining current practice). The authors also acknowledge staff at the UK’s National Oceanography Centre (NOC) for the mineralogical analyses, in particular Dr. Richard Pearce and Dan Doran.