Rockdelta

Fouled ballast (often quantified using the Fouling Index) describes the presence of fine-grained clay and silt particles in the ballast voids. Caused by ballast breakdown, infiltration, and sleeper wear, these particles reduce permeability and can lead to the ballast layer becoming saturated. Saturation reduces the ballast’s ability to spread the forces applied to the sleepers under train loadings, resulting in track geometry problems, mud pumping, and high stresses transmitted to the subgrade (contributing to subgrade failures). The impact of fouling can be particularly destructive when combined with freeze-thaw cycles in cold climates. 

Fouled ballast problems can be treated by removing the degraded ballast and replacing it with a more durable material, such as a resilient sub-ballast mat. 

Shear failure occurs in fine-grained clay and silt subgrades, often as a result of inadequate ballast thickness and/or drainage. The failure develops at the subgrade surface as the soil shears and remoulds due to repeated overstressing. The soil moves outwards and upwards, becoming apparent as soil heave along the track shoulders and causing a cross-level in the track. In addition, as the subgrade moves, a depression, known as a ballast pocket, can form below the track. Water can become trapped in the ballast pocket, causing a further strength reduction in the subgrade soil, worsening the situation.

Several methods can be used for treating progressive shear failure and ballast pockets, including increasing ballast thickness, placing an asphalt layer between the ballast and subgrade, and installing geosynthetics or resilient confinement layers over the subgrade. Another method involves pressure injection of various mixes (e.g. lime or cement based) to fill ballast pocket voids and/or chemically stabilise the subgrade soils.

Each of the above methods may be appropriate under certain circumstances; however, each has its limitations. Increasing the ballast thickness may be limited by clearance constraints, particularly in tunnels, underpasses, and on bridges. Installing asphalt layers or geosynthetic products requires removing the entire track structure. Slurry injection methods can be used without removing the track; however, their effectiveness depends on mix design, subgrade soil properties, and injection patterns and methods. Proper implementation of these methods requires a thorough understanding of the subsurface conditions, with subsurface explorations and laboratory testing undertaken and proper design procedures used when available.

A possible simpler remedy is the use of resilient sub-ballast mats. The ballast pockets should also always be drained, or the resilient mats may not solve the problem.

Embankment slope failure threatens track stability. It most commonly occurs when the shear strength of the embankment material decreases due to a rise in pore-water pressure during heavy rainfall. If the shear strength falls below the shear stresses acting on the slope, sliding occurs.

As the slope fails, tension cracks typically develop at the track level or near the crest of the embankment. These cracks fill with water, exerting additional hydrostatic pressure on the sliding mass and accelerating the movement. The resulting embankment deformation causes severe track geometry problems, including differential settlement, lateral displacement, and loss of support beneath the sleepers.

Slope failures can also be triggered when shear stresses in the slope increase, such as through excavation at the toe, or from additional loading at the crest from track maintenance activities or construction equipment.

Common slope stabilisation methods include the removal and replacement of the slide mass, toe buttresses, various retaining wall schemes, pile driving, excavation to unload the head of the slide, and subsurface drainage techniques. The appropriate stabilisation method will depend on site constraints and subsurface conditions. In many cases, several methods may be appropriate for a particular site and the method selected is based on cost and technical suitability.

Attrition occurs when the subgrade is a soft rock or a hard clay layer in direct contact with the ballast and water is present above the contact between the two materials. During loading, the ballast is pushed into the subgrade, causing localised subgrade failure. This process produces fine subgrade particles, which can form a mud slurry that migrates upward during subsequent loading cycles, fouling the ballast and eventually pumping at the ground surface. If left unaddressed, attrition can also lead to significant track settlement. 

Placing a protective blanket of sub-ballast or resilient mat between the hard subgrade and the ballast can often control subgrade attrition.

When most people think of stone wool, they think of the insulation used to protect buildings against fire and extreme temperatures. That’s understandable. Stone wool insulation is used widely in virtually every type of building today. But its unique properties and versatility make it ideal for a range of lesser-known applications, including reinforcing car brakes, stormwater management systems, horticultural substrates, and noise and vibration control solutions. Although stone wool insulation and stone wool resilient mats are both produced from the same starting material, they are different products. Stone wool mats differ substantially from their insulation counterparts and are engineered to meet the high-performance requirements of railway systems.

Quite the opposite! Stone wool resilient mats have been used to control ground-borne noise and vibrations on railways in some of the harshest conditions since the 1970s, starting in Scandinavia, before spreading around the world. We now have over 40 years of expertise in railway noise and vibration control solutions. Today, stone wool mats are the preferred solution for rail track vibration control and structure protection in Scandinavia and many other countries.

In most cases, this is actually true. Generic stone wool is not suitable for railways. However, unlike the stone wool insulation that can be found in DIY shops, Rockdelta stone wool resilient mats are highly engineered products designed to meet the extreme demands of the railway industry, including all types of rolling stocks, track constructions, and project conditions. All Rockdelta products have been tested internally and by recognised laboratories according to the most relevant material and application standards. In fact, our stone wool mats are the only mineral fibre material proven through experience and testing to be suitable for controlling ground borne noise and vibrations in railway applications.

The cornerstone of this performance is their stiffness. The mats are produced with up to 10 times the density of regular stone wool insulation products. This is critical as, at any given time, a typical rail structure must support different kinds of rolling stock—from tramways and light rails, all the way to freight and heavy tonnage trains. When used for ballasted tracks, our products can be manufactured with a dual density: an upper layer provides higher puncture resistance for the side in contact with the ballast, while the lower layer provides the desired elastic properties. This makes them highly effective when controlling vibrations and protecting the track infrastructure.

To ensure that the mats deliver the desired performance from installation through to the end of their lifetime, each undergoes a unique ageing process to ensure consistent performance from day one. Finally, since it is a stone wool-based material, the resilient mats have outstanding fire resistance, are safe for humans and the environment, and have a high degree of material stability.