Railway Geotechnical Engineering using Geosynthetics

There are a wide range of railway geotechnical engineering issues influencing the construction and maintenance of rail infrastructure. All railways are built upon, cut into, or tunnel through soil and rock. Rail embankments are constructed from natural soils and track foundations constructed from natural aggregates. The condition and properties of these materials will influence the construction process and lifetime performance of a railway.

Where the original geotechnical design is inadequate, major problems arise. This could be a result of incorrect or incomplete knowledge, changing soil conditions due to climate change effects, or increased loading. As a result, railway companies all around the world face expensive and disruptive earthworks repairs, repeated track maintenance, or reconstruction. In many cases line speed restrictions remain in place, caused by poor track geometry and misaligned rails. Disruption of this nature impacts passengers and train operators, as well as the network owners.

To mitigate this, a growing number of railway companies around the globe are incorporating geosynthetic solutions into their infrastructure to address geotechnical problems, both during the initial construction and in renewal and maintenance works. In this guide, we shall explore what these solutions are, the areas in which they are applied, and the numerous benefits they have.

Tensar has developed geogrid solutions that address a number of geotechnical issues in railway engineering. These include railway trackbed stabilisation, foundations for embankments over weak soils, reinforced soil bridge abutments, and steepened embankments.

Looking for something in particular? If so, use the links below, otherwise let’s jump straight in!

• Geotechnical challenges faced in railway design, construction, and maintenance
• Incorporating stabilisation geogrids into trackbed layers to improve performance
• Solutions for railway embankments
• Reinforced soil structures for railways
• Efficiency in design and construction of temporary works that support railway infrastructure projects
• The Tensar difference

Geotechnical challenges faced in railway design, construction, and maintenance

Designing and building safer, better, longer-lasting railways that are more economical, sustainable, and resilient to changing environmental conditions is the top priority for rail engineers. Rail authorities challenged with maintaining existing rail networks are facing a widening gap between the maintenance needs of a deteriorating network and working with tighter budgets, in addition to addressing the critical challenge of addressing climate change and meeting carbon emission targets. As a result, they require solutions that offer greater resilience and require less frequent and disruptive maintenance.

Railway infrastructure, like all infrastructure works, starts at the ground. Railway geotechnical engineers have to address a multitude of issues. Unlike roads, railways need to avoid steep gradients. This requires more extensive earthworks, deeper cuttings through soil and rock, and higher embankments to maintain an acceptable vertical alignment. River and road crossings require major engineering works - bridges, viaducts, and tunnels. Each of these presents geotechnical challenges.

Above the earthworks or ground level, the trackbed foundation presents railway geotechnical engineers with a set of challenges: bearing capacity, settlement, and drainage all have to be addressed. The foundation soil parameters and the trackbed layer material properties at the time of construction must be understood and accounted for, alongside their potential to change during the life of the railway.

We will cover many of these railway geotechnical issues in this article, but let’s start with the trackbed.

Andrew visits Vietnam's famous Hanoi Railway to talk about the different components of a railway trackbed

Incorporating stabilisation geogrids into trackbed layers to improve performance

In order to maximise efficiency, a rail network must maintain speeds and keep service disruptions for maintenance operations to a minimum. Both the quality and durability of the trackbed are critical, not only in determining the safe operating speeds but also in minimising maintenance-related disruption. When designing the trackbed it is essential to accurately determine the required ballast and sub-ballast thickness to eliminate or mitigate aforementioned problems with subgrade bearing failure, track settlements and ballast degradation under repetitive loading, trackbed variability and smooth ride quality over transition zones.

By designing a trackbed with enhanced bearing capacity and stiffness using a geotechnical solution, such as stabilisation geogrids in the trackbed layers, the time between necessary maintenance can be significantly extended. Additionally, ballast degradation and track settlements are reduced, allowing track speeds to remain high and extending maintenance cycles, saving whole life time, cost, and carbon emissions.

Rail trackbed showing the installation of Tensar TX190L-GN geocomposite (large aperture geogrid with laminated non-woven separation geotextile) for ballast stabilisation over weak ground

Tensar has extensive experience in supporting engineers with designing to meet project specific requirements, ranging from simpler specifications in accordance with national standards to complex Finite Analysis of in-service performance. These research-based methods have been well proven and are now widely used. They deliver financial and sustainability gains and offer improved resilience against the effects of climate change.

Powerful Finite Element Analysis capabilities are possible with the Tensar Stabilised Soil Model (TSSM) plugin

Ballast stabilisation

As previously discussed in this article, repetitive dynamic loading of the ballast layer inevitably leads to ballast degradation, settlements/deformations, and variable levels of performance of the trackbed with regard to stiffness and ride quality, amongst other factors.

Mechanical stabilisation of the ballast layer, typically with a large aperture product to account for the ballast material grading, has been proven to significantly reduce the particle movement and rotation within the zone of confinement of the stabilisation geogrid.

Typical installation location of a ballast stabilisation geogrid

‘SmartRock’ research, presented in 2016, demonstrated the scale of this reduction, using a large aperture Tensar stabilisation geogrid within a ballast layer subjected to cyclic loading. This limiting of translational particle movement heavily reduces degradation and maintains the ballast layer’s strength and drainage characteristics. It also controls ballast displacement, maintaining track alignment and significantly reducing the required frequency of ballast tamping maintenance. Research has shown such extension of the ballast’s life through mechanical stabilisation can extend the maintenance cycle by three times, therefore reducing whole life costs in terms of time, cost, and carbon emissions.

‘SmartRock’ research undertaken demonstrated a significant reduction in particle movement and rotation as a result of the introduction of a mechanical stabilisation geogrid

Below-track structures such as culverts and bridge decks or transition between ballasted track and slab track can create weak abrupt changes in trackbed stiffness that leads to a loss of alignment and poor ride quality that limit track speeds.

Engineered trackbed stiffness is possible across transition zone with stabilisation geogrids (right)

The issue can be addressed by constructing a mechanically stabilised transition zone, incorporating geogrids within either the ballast layer, sub-ballast layer as discussed below, or both, and can provide the ability to design for an engineered variable stiffness. This results in smoother and more durable transitions, improving the overall ride quality, something that the Crew Green Link Road project below benefitted from.

Sub-ballast stabilisation

Soft subgrade conditions lead to an increase in deformation which can affect alignment and may even lead to subgrade bearing failure. The primary function of the sub-ballast layer is to distribute loading over the subgrade to increase bearing capacity, therefore mitigating the risk of bearing failure, and limit subgrade deformation. The weaker the subgrade, the thicker the sub-ballast layer required to achieve adequate distribution.

Therefore, increasing the strength of the sub-ballast or foundation layer through geotechnical solutions increases bearing capacity, reduces settlements, and helps maintain track alignment. One such method of achieving this is to strengthen the sub-ballast layer by incorporating a stabilisation geogrid, typically at the base of the granular material, with additional layers introduced where necessary to ensure continuity of mechanical stabilisation throughout the sub-ballast.

Typical installation location of a sub-ballast stabilisation geogrid

This can enable a considerably thinner sub-ballast layer to be used, saving on the construction costs and need for transportation of expensive aggregates. Single or multiple layers of Tensar geogrid can achieve the required bearing capacity with significantly less sub-ballast material, dependent upon project trackbed performance requirements.

Where variable subgrade conditions arise, track stiffness suffers as a result, leading to uneven settlement and track deflections. Trackbed stiffness has a direct impact on track deflection under load, therefore achieving increased stiffness by mechanical stabilisation of the sub-ballast or foundation layers allows trains to run at higher speeds without shortening the lifespan of the track infrastructure. It can also greatly assist with reducing variability, providing a more consistent track support and therefore an improved ride quality for passengers.

Subgrade improvement

A prepared subgrade, or ‘form layer’, is sometimes adopted on projects above embankment fill material to improve the foundation on which the trackbed layers are to be constructed and therefore must provide adequate bearing capacity to support the sub-ballast and ballast components of the track structure. A frost protection/filtering layer is sometimes specified beneath the sub-ballast, combining with it to form the ‘blanket layers’.

Quality control parameters that are typically ensured during railway embankment construction often include a deformation modulus, or E_V2 value, which must be achieved at the top of a prepared subgrade/form layer. The inclusion of a Tensar stabilisation geogrid within this, forming a mechanically stabilised layer, improves the structural performance of the material through increasing particle interlock and reducing movement and rotation, therefore facilitating the use of a thinner layer whilst still achieving the necessary performance characteristics. Alternatively, the same layer thickness can be adopted with the inclusion of a stabilisation geogrid to achieve an increased deformation modulus value. Mechanical stabilisation of subgrade improvement layers can also contribute to maintaining the formation modulus over the lifetime of assets subject to cyclic loading.

In this episode of Ground Coffee Blog, Andrew Lees visits Grand Belfast Central Station. He looks the challenges of building on poor ground conditions and the innovative solution and innovative design method that was used on this project.

Reinforced soil structures for railways

As railway infrastructure projects face increasingly difficult challenges with construction through topographically ambitious and weak-soiled terrains, increasing train loadings, and an industry drive towards efficiency in whole-life cost, the more traditional methodologies, for example reinforced concrete retaining walls, find themselves up against greater scrutiny.

The following section demonstrates some areas in which reinforced soil solutions might provide a viable alternative.

Improving slope stability – a major railway geotechnical engineering problem

The stability of embankment slopes is crucial for railway engineering. Slow movement of railway embankments will affect track alignment, and total slope failure can close a railway line for many weeks or months. Geotechnical engineers are increasingly frequently needing to deal with risks arising from shallow and complete slope failures. Slope stability is influenced by soil properties, the slope geometry, and by the movement of water. The effects of climate change on rainfall can have a major long-term impact. Fortunately, there are geotechnical engineering solutions to improve slope stability and improve the climate change resilience of railways.

Active shallow slope failures can be halted by stabilising the slope using plate piles, for example using the , an efficient and cost-effective solution for the stabilisation of both new slopes and active slides. The closely spaced Plate Pile elements form a series of horizontal barriers where the soil arches between the plates, forming a continuous line of resistance against downslope movement.

Geopier’s Slope Reinforcement Technology (SRT) system can be installed in-situ to stabilise slope failures

Where slopes have failed, the failed soil can be removed and, in some cases, re-installed with the inclusion of geogrid reinforcement and additional drainage to increase stability and prevent future failures. This approach is quick and avoids the need to transport large quantities of material to and from the site.

Minimising land take using steepened embankment slopes

As embankments increase in height with the same side slope gradient, the base of the embankment must widen, therefore taking up more land. In some cases, this can encroach on existing property boundaries, structures, or infrastructure. One geotechnical engineering solution can be to increase the gradient of the embankment side slope, though steepening a slope reduces internal stability and can lead to rapid slope failure.

A commonly used geosynthetic engineering solution is to design the slope as a reinforced soil structure. By incorporating layers of geogrid, the internal stability is increased. Railway embankment slopes may be constructed at angles of up to 70° using this technique. Tensar has two solutions for steep slope construction. – TensarTech Greenslope for slopes up to 70° and TensarTech NaturalGreen for shallow slopes up to 45°.

One such use of these systems was on the Murrayfield Stadium Tram Stop project, in Edinburgh, where a combination of Tensar’s GreenSlope and TW3 block wall reinforced soil structures were used.

Retaining wall systems for railways that are quicker and cheaper to construct

Earth retaining walls are fundamental to rail infrastructure, acting as both track side protection and support for elevated tracks. Retaining walls in the context of railways are especially important where grade separation is required in spatially constrained areas, for example in high-density urban cities.

In the past, retaining walls would typically be constructed from steel piles, masonry, or reinforced concrete gravity walls. The latter two both require substantial foundations, requiring piling or excavation. Today, railway geotechnical engineers are able to adopt geosynthetic solutions, such as geogrid reinforced retaining walls. These systems use reinforced soil techniques to create stable retaining walls that are economical, quicker to construct and lower embodied carbon. In some cases, no foundation works are required, and site-won/marginal fill may be utilised for the structure, greatly reducing local impacts from construction activities.

Much of the existing railway infrastructure was built in the nineteenth century. Masonry walls may need upgrading or replacing to cope with modern railway loading. Railway geotechnical engineers have used reinforced soil systems constructed behind existing walls, designed to relieve thrust onto the ageing structures. In a similar application on the Bermondsey Dive Under project, reinforced soil retaining wall structures were used to support rail loading on the approach structure tie-ins to masonry arch structures.

Railway geotechnical engineering solutions for earthworks and bridgeworks

Almost all rail projects involve major earthworks and the construction of structures such as bridge abutments and retaining walls. Railway geotechnical engineers, faced with difficult terrain and soft or variable ground conditions, are seeking solutions that offer best value at the time of construction as well as durability and resiliency.

Tensar has innovative solutions for all geotechnical challenges arising from railway structures such as bridges and embankments over weak soils, with faster construction, reduced land take, and controlled settlements, for a longer lasting, optimal value, resilient infrastructure.

The Tensar difference

Tensar offers proven, high-performance geosynthetic solutions for railway geotechnical engineering that enable you to deliver your project with confidence. Not only do they provide significant value and sustainability benefits over alternative solutions, but they’re also widely accepted by rail authorities worldwide and have had their performance proven and quantified by extensive research and field trials. 

Our range of geosynthetic solutions for railway embankments and rail trackbed reinforcement includes:

• Tensar® InterAx® (NX) Geogrids
• Tensar® TriAx® (TX) Geogrids
• Tensar® TriAx® (TX-G) Geocomposite
• Tensar® Biaxial BX Geogrids
• TensarTech® Stratum® Cellular Foundation Mattress System
• Tensar® Basetex® Geotextile 
• TensarTech® TW1 Wall System
• TensarTech®TW3 Wall System
• TensarTech®TR2 System
• TensarTech® RockWall™ Rock Cage Retaining Wall System
• TensarTech® NaturalGreen™ Slope System
• TensarTech® GreenSlope System
• Tensar® Basetex® Geotextile