Clitheroe
Shap
Horwich, Bolton
Chorley
Horwich, Bolton
Clitheroe
Shap
Horwich, Bolton
Chorley
Horwich, Bolton
Rails and sleepers are the most visible parts of a railway, but much of the track’s performance relies on what sits beneath and around them.
Every passing train transfers significant force into these lower layers, which support the track and help keep it in position.
Railway aggregates are selected to distribute that force, support the sleepers and create a track bed that can be adjusted and maintained. The angular stone seen between and beside the sleepers, known as railway ballast, is the most familiar example. However, aggregates are also used in lower track layers, drainage systems, earthworks and access routes.
In this guide, we'll look at the purpose of each layer and how specification, wear and reliable supply affect railway construction and renewal.
A conventional railway track is a layered structure, and not all the stone beneath it is ballast. Each part has a different job.
From the top down, the structure generally includes:
Drainage systems may also run alongside or beneath the track, using free-draining aggregates to help direct water away from the formation.
The precise build-up and terminology vary between projects, and not every railway uses conventional ballast. Some routes use slab track, where the rails are supported by a rigid concrete structure.
Aggregates may still be required for drainage, earthworks and concrete production, but the upper track construction is different.
The position of railway ballast is closely connected to the jobs it performs. Material beneath the sleepers provides support, while the ballast between and beyond them helps resist movement.
Ballast creates a bed that is firm enough to support the track but can still be adjusted during construction and maintenance.
As a train passes, force travels from the rails into the sleepers and then through the ballast. The ballast distributes it over a wider area before it reaches the lower track layers, reducing the amount concentrated at any one point.
Railway ballast is generally made from crushed, angular stone. Its particles press against and interlock with one another, making them more resistant to movement than smooth, rounded material.
Ballast placed between the sleepers is known as crib ballast, while the material extending beyond their ends forms the ballast shoulders. Together with the ballast beneath the sleepers, these areas help maintain the line, level and alignment of the track.
The open spaces between clean ballast particles allow water to move through the upper track layer towards the drainage system.
Ballast must still work alongside properly designed and maintained drainage. It cannot compensate for blocked drains or a wider drainage failure.
Unlike a completely rigid foundation, ballast can be rearranged to correct the position of the track. Maintenance processes such as tamping and stoneblowing work with the material beneath the sleepers to restore support, level and alignment.
This allows sections of track to be adjusted without rebuilding the complete formation beneath them.
Crushing a hard rock does not automatically make it suitable for use as railway ballast. The finished aggregate must have the right shape, grading, durability and cleanliness for the demands placed on it.
The edges and faces of angular particles allow them to interlock, helping the ballast resist movement under passing traffic. However, angularity is only one part of particle shape.
Excessive flaky or elongated particles may sit unevenly, break more readily or make it harder to achieve consistent support beneath the sleepers.
Railway ballast is produced within a controlled range of coarse particle sizes. This grading affects how the particles fit together and whether sufficient open space remains for water to pass through.
Products described as 32/50 mm track ballast are commonly used in the UK, but this should not be treated as a universal requirement.
The correct grading and permitted proportions of smaller or larger particles must be checked against the applicable ballast specification. Find out more about aggregate grading and gradation reports.
Railway ballast is exposed to repeated loads, handling and track-maintenance activity. It must therefore resist breaking, chipping and wearing down too quickly so that it retains its grading and particle shape over time.
Clay, soil, dust and material from neighbouring stockpiles can affect ballast before it even reaches the railway. Care is therefore needed throughout production, storage, loading and delivery to keep the product clean.
Excessive fines can occupy the spaces between larger particles, restricting drainage and reducing effective interlock. Cleanliness at the point of supply is therefore just as important as the original properties of the rock. Our guide to fines content in aggregates explains this in more detail.
Granite is widely associated with railway ballast because of its strength and resistance to wear. However, describing a material as granite does not by itself prove that it is suitable for a particular railway project.
Basalt, gritstone and other durable rock types may also be suitable where the finished aggregate meets the required performance categories. Suitability depends on the properties of the individual source, the way the material has been processed and the specification it must satisfy.
Shape, grading, durability and cleanliness must be assessed against the applicable railway ballast specification.
BS EN 13450 specifies properties for aggregates intended for use as railway ballast in the upper track layer. The infrastructure owner, client or individual project may then select particular performance categories or impose further requirements covering production controls, testing, conformity information and traceability.
Ballast is the most recognisable railway aggregate, but it is not the only material involved. Rail projects use different aggregates beneath the track, alongside it and throughout the surrounding civil engineering works.
Depending on the track design, granular layers may be installed between the railway ballast and the underlying formation.
These layers help spread loads, provide a stable base for construction and limit the movement of material between different parts of the track structure.
They require different grading and compaction characteristics from the coarse, open-graded ballast immediately beneath the sleepers.
Water needs to move away from the track before it can weaken the formation or carry fine material into the ballast.
Clean drainage aggregates may be used around pipes, in trackside trenches and within other drainage features. The material must be suitable for the drainage design, allowing water to pass through without introducing unwanted fines or becoming blocked.
Much of a railway project takes place beyond the rails themselves. Aggregates may be needed to improve weak ground, create capping layers, build or repair embankments and backfill structures.
The required material will depend on factors such as ground conditions, loading, compaction and drainage.
Rail construction and maintenance also require safe access for heavy plant, deliveries and site teams. Aggregates can be used to create haul roads, compounds, working platforms and other temporary or permanent surfaces.
These materials must provide a stable working area, but they do not require the same grading or performance characteristics as railway track ballast.
Aggregates are also used within concrete and asphalt for platforms, bridges, retaining structures, drainage works and service roads.
This means aggregates for rail infrastructure are not interchangeable. Track ballast, drainage stone, structural fill and concrete aggregate each perform a different job and must meet the specification for that particular application.
Railway ballast does not remain unchanged once the track enters service. Every passing train applies repeated loads that can cause particles to rearrange, settle, wear or fragment.
Over time, smaller particles can be produced as the stone wears. Soil, debris and material from the surrounding track environment may also enter the spaces between the ballast particles. This gradual build-up is commonly known as ballast fouling.
As those spaces become filled, the ballast may:
Maintenance may involve repositioning the ballast through tamping, removing fines through ballast cleaning or replacing material that can no longer perform as required.
Recovered ballast does not necessarily have to become waste. Network Rail reports that substantial quantities of used ballast have been recovered and returned to the track. However, material can only be reused in this way where it continues to meet the necessary product-quality requirements. Find out more about how Network Rail recovers and reuses track ballast.
Railway construction and renewal work often takes place within tightly controlled access periods. Materials therefore need to arrive in the correct quantities, sequence and condition for the planned work.
Product consistency is particularly important. Repeated deliveries must maintain the required grading and performance characteristics, while different aggregate products need to remain separated during production, storage, loading and delivery.
Supply planning must also account for the overall volume of material, available storage space, site access and the rate at which ballast or other aggregates will be placed.
A supplier may need to coordinate deliveries across several phases rather than provide the full quantity at once.
Testing, product information and delivery records provide a link between the aggregate produced at the quarry and the material received on site.
This traceability helps project teams confirm what has been supplied and investigate any questions that arise during the work.
Our guide to why consistent aggregate supply matters for infrastructure projects explores these project-delivery considerations in more detail.
Railway infrastructure relies on a complete system of aggregates rather than one universal product. Ballast supports and restrains the track, while lower granular layers, drainage stone, structural fill and construction aggregates perform their own roles elsewhere in the project.
Choosing the right material means considering where it will be used, how it must perform and which specification applies. Size or rock type alone is not enough to establish suitability.
Explore Armstrongs’ aggregate range or contact our team to discuss the material requirements for your next infrastructure or civil engineering project.
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