Geotechnical engineering is the method used to evaluate the physical properties of the ground, stability of slopes and to design earthworks and foundations of structures. Meanwhile, geosynthetics is the name given to products such as geotextiles that separate layers of dissimilar stone or other materials; geogrids that reinforce or stabilise the layers; and geocomposites that can combine the properties of geotextiles with geogrids or drainage layers.
When correctly specified and installed, the use of geosynthetics within permanent-way construction has been proven to enhance trackbed performance and significantly extend overall design life. There are four major functions – separation, filtration, drainage and reinforcement/stabilisation – that geosynthetics fulfil when they are employed within, beneath and around ballast and sub-ballast layers.
The functions of separation and filtration are often considered as singular. Geosynthetics can replace the functionality of traditional construction materials while providing significant construction savings and increased speed of installation. The on-going impact of changes in construction and the requirement to deliver sustainable development is increasingly creating the need for more diverse and innovative geosynthetics solutions across all aspects of rail construction.
No need for a sand blanket
Terram, an organisation that is so closely identified with geosynthetics that its name has become the generic term for the material, has long been an innovator in design of rail materials. The company’s latest is TERRAM Hydrotex, which provides a permanent way solution for trackbed stability that not only acts as a filter and separator for fine soils, but also removes the requirement for a sand blanket. The traditional use of a sand blanket in railway trackbed uses a sub-ballast and/or a graded-sand layer to prevent the upward movement of fine sub-grade particles, while allowing effective drainage and dissipation of pore water pressure.
Unique materials contributing to higher levels of rail safety are PW5 and PW Trace which have a foil strip detectable by ground penetrating radar, so that radar pulses can be used to detect anomalies in the ground. This gives confirmation of the presence of a geosynthetic layer, and accurately measures ballast depth and crossfall. It can also aid in detection of ballast contamination and monitor ballast depth and condition for planned maintenance.
Better preservation of rail line and level
Another recent British made product is the Tensar TriAx® TX190L geogrid which has been developed specifically to improve rail track ballast stability and so maintain track geometry. Triax TX190L can be used for both sub-ballast and ballast stabilisation in new track bed and maintenance upgrades. Its benefits for rail networks include reduced differential settlements, extended maintenance cycles and reduced whole life costs, as well as reduced construction time and traffic disruption with their associated additional costs.
Triax TX190L was awarded a Network Rail Acceptance Certificate in November 2012.
Researchers in the UK and around the world have found that geogrid stabilisation of ballast has led to better preservation of rail line and level. In fact, the improvement is found to be three to five times better than a sleeper settlement without geogrid stabilisation, and therefore allows the time between maintenance cycles to be significantly increased. Tensar geogrids were originally invented by Dr Brian Mercer in the late 1970’s and early 1980’s for the construction industry to provide stabilisation and reinforcement with the underlying concept of simplicity, flexibility and strength, and are now used throughout the world for a wide range of soil reinforcement applications.
Leading-edge technology in concrete
One of the most revolutionary new products currently in use by Network Rail is Concrete Canvas, a ground breaking material technology that allows concrete to be used in a completely new way. Invented at Imperial College London in 2004 by the two company founders, Peter Brewin and William Crawford, Concrete Canvas is a flexible, cement impregnated fabric that hardens on hydration to form a fibre reinforced, waterproof concrete layer.This innovation enables up to 150mm of poured concrete to be replaced with just 8mm for many surfacing applications. There is no need for mixing or measuring, and as the concrete is premixed it cannot be over hydrated. As a result, material saving of 95 per cent can be achieved for a typical construction project.
Furthermore, Concrete Canvas is chemically resistant, has good weathering performance, will not degrade in UV and has an expected life of over 50 years. It will even set underwater as well as in seawater, so is a truly remarkable material. It has dozens of potential uses but is currently primarily used for ditch lining, bund lining and slope protection works, with Network Rail the largest end-user of the product in the UK.
Supplied as either bulk rolls that can be dispensed from track mounted equipment or man-portable batched rolls for remote areas, Concrete Canvas offers site-specific advantages to rail projects.
The speed and ease at which Concrete Canvas can be installed means that fewer and shorter possessions are required when installing as a liner in trackside drainage. In addition, Concrete Canvas allows construction and remediation projects in the rail sector to be completed with more efficiency and with a lesser impact on the environment than traditional concrete solutions.
Although still a relatively new product on the market, Concrete Canvas is fast becoming the engineers’ choice for ditch lining and slope protection, offering key advantages over conventional precast, slabbing or poured concrete solutions. The material requires no mixing on site, will easily negotiate sharp bends and creates a natural looking and long term durable channel lining.
This true British success story is now sold to 40 countries around the world, with exports now accounting for 85 per cent of the company’s turnover.
The increasing use of geosynthetics in track applications and embankments and structures has contributed to a reduction in time on site, increasingly an issue in these days of restricted possession times.
Meanwhile, materials such as the radar detectable geotextile offer a significant contribution to track safety, and allow for better planning of maintenance schedules.
With the advent of higher speed rail lines and rising demand on the network the reliance on engineered solutions to allow for a safer, more reliable railway system will only increase in the future.
Tim Martin is sales and development manager at Keyline – www.keyline.co.uk. Keyline’s National Rail Division is the leading provider of civil engineering and specialist products to the rail industry.