Geotextiles from Marsden to Diggle
Between 1795 and 1894, four parallel tunnels over three miles in length were cut through the millstone grit and strong shale of the Pennines between Marsden in Yorkshire and Diggle in Lancashire, in northern England.
The work on the earliest of the four Standedge Tunnels was particularly arduous – more than the expected amount of water entered the unlined works and had to be removed by buckets on pulleys, as gunpowder was used to blast through the solid rock by candlelight. A further problem was maintaining an adequate air supply for the workers. Work on the tunnel took far longer than anticipated and ran way over budget.
It became the longest and highest canal tunnel in the UK and the other three tunnels built parallel to it – while avoiding many of the problems experienced with the first – were for railway lines, of which only the last to be completed in 1894 is still in use today.
Maldon, Essex-based Terram, a business belonging to the leading nonwovens manufacturer and INDEX™ 17 exhibitor Berry Plastics, is a specialist in the use of nonwovens in projects for the construction and renewal of railway infrastructure.
Track renewal challenge
The full track renewal through the 1894 Standedge Tunnel has been one of the most challenging projects the company has supplied materials for, mainly due to the natural height restrictions and the shallow placement of the catch pits.
The UK’s Network Rail carried out a full report to evaluate how to improve the stability, strength and saturation of the track bed surface after renewal. Due to ground conditions, it was recommended that a geotextile separator should be installed across the site in order to prevent intermixing between the sand blanket and the new ballast layer.
The material selected was Terram’s PW1, a cost-effective needlepunched polypropylene nonwoven specifically designed for the rail industry and also ideal for preventing the loss of rail track alignment caused by sub-grade erosion, negating the need for long term costly maintenance procedures caused by having to plan and temporarily close the track for renewal works.
It greatly improved efficiencies at the Standedge Tunnel site during renewal and without the geotextile layer, Network Rail would have struggled to meet the installation timescales.
Terram geosynthetics provide one or more of four key functions when used in track-bed construction – separation to maintain the integrity of adjacent soil types, filtration to prevent the leaching of soil particles, drainage to allow the free passage of water and reinforcement to provide additional strength.
“The rail industry is now looking for value engineering, meaning great products at competitive prices,” says Neil Stewart, Terram’s Solutions Sales Manager for Rail. “But punctuality is also vitally important when you have short possession times – brief periods when you can access the site. Getting materials where they need to be quickly and on time every time is very important to allow the work to be carried out within the allocated time frame. This influences our approach to delivery. Speed of quote is something we work hard on and we also offer technical support on all our products and are happy to offer free lunchtime technical talks on the site, to develop understanding about the benefits of our solutions.”
Other Terram products regularly employed in railway projects include Hydrotex, which provides a permanent solution for track-bed stability and not only acts as a filter/separator for fine soils, but also removes the requirement for a sand blanket.
Hydrotex nonwoven composites consists of a central filter media thermally bonded to two opposing needlepunched nonwovens, to create a geocomposite with sufficient strength and protection to be durable to the abrasion and point loading of ballast.
The traditional use of a sand blanket in railway trackbed involves a sub-ballast and/or a graded-sand layer being laid to prevent the upward movement of fine sub-grade particles, while allowing effective drainage and dissipation of pore water pressure. Hyrdotex offers this solution in a single geocomposite.
Hydrotex is strong but flexible, allowing the composite to conform to the excavated formation, so that no voids exist below the filter media for the development of pockets of slurry resulting in an unstable track-bed.
The material prevents upwards particle passage smaller than 0.002mm and is a permeable filter, allowing upward and downward water transmission. It functions under ballast without any reduction in performance and reduces the depth of excavation that would be required with a sand blanket. The result is a decrease in the amount of spoil being taken away for landfill and a reduction of vehicles required to deliver materials.
“Hydrotex is a real success story for Terram,” says Neil Stewart. “We spotted the market need for a replacement for sand blankets in preventing clay pumping – something quicker and less expensive to install. Hydrotex achieves those aims and is proving its worth – particularly in possession times – across a variety of rail projects.”
“Our nonwoven-based Geocells are often also used in reinforcing embankments, and preventing erosion on adjoining railway slopes. Geocells are honeycomb-style 3D confinement systems and their shape makes them extremely versatile. They also utilise on-site materials, making them quick and simple to install.
“Terram geotextiles are now needlepunched as opposed to spunbonded which makes them stronger, and delivers a much higher puncture resistance. Also, by manufacturing our own materials in the UK, we can tailor to different widths and lengths depending on the specification from the engineers on site. This ability to customise solutions is something our competitors struggle to match.
“We’ve invested a huge amount of money into machinery and staff training, and this shows in the finished products. Our quality control procedures are also industry-leading. We test all products for mass, thickness, tensile strength, tensile elongation and resistance to static puncture, and carry out additional hourly checks on the manufacturing line. Every product line is also subject to rigorous and regular quality testing to EN ISO standards.”