A section of track at Chorley Tunnel in the northwest of England was subject to line speed restrictions due to subsidence. The section in question was at the end of the slab track running through the Chorley Tunnel, a line notable for its electrification scheme in the previous decade. The subsidence was caused by a blocked drainage channel, which resulted in excess water saturating and washing out elements of the sub-base towards the end of the slab near the transition zone, affecting both the upmain and downmain lines.
The route asset team at Network Rail sought a solution to improve the ground and lift the slab track, enabling line speeds to return. Consequently, they approached Geobear to design an advanced geopolymer injection solution to lift the slabs by 40-50mm over 5m on the Upline and 10m on the Downline.
In a live railway environment, the options for remediating sinking slab track are limited due to the impact on passengers and potential penalties. In this instance, a complete dig-out and replacement would require a full line closure for a substantial time. Similarly, piling would require extensive works, meaning a faster alternative solution was needed.
Geobear Solution
The Geobear-engineered design solution utilised their fast geopolymer injection process. Originally pioneered by the company forty years ago, the geopolymer injection method is now widely applied across rail networks, highways, and airports.
Addressing the challenges faced by Route Asset Directors is one of the primary merits of the Geobear system. A purpose-built rail container houses all of the material, plant, and equipment necessary to deliver the designed geopolymer solution. The operational delivery team mobilises with the container and conducts the works within possession times, ensuring there is no downtime in railway operations.
The speed at which the Geobear treatment can be carried out ensures routes are not subject to lengthy delays. Geobear proposed a detailed methodology for the geopolymer injection process that could be delivered within several possessions, ensuring minimal disruption to ongoing rail operations. Geobear used trackbed investigation data to inform a design that involved injecting a specialist geopolymer resin below the slab to lift it back to the original level.
Precise laser-level monitoring was used to ensure the slabs were lifted to within 0.5-1mm of the required levels. The achieved lift is the result of a two-component material injection that polymerises (or expands) beneath the slab. The expansion filled the microvoids left by the washout of material and created vertical lift. Geobear’s geopolymer material expands within 30 seconds and reaches 90% strength within 15 minutes, meaning sites can be handed back for use almost immediately if required.
The project was executed within the program over two phases: firstly, midweek possessions over four nights of 3.5-hour working windows for drilling, followed by a weekend possession for the actual injection process.
Low Carbon Solution
Along with addressing Network Rail’s primary engineering challenge, the Geobear solution is also the optimal method when considering sustainability. Carbon Footprint Ltd produced a report in 2023 that illustrated the difference in the level of carbon emissions of Geobear solutions compared to traditional methods. By comparing the carbon output on a level crossing replacement with a Geobear life extension solution, it showed the Geobear solution released 75% less carbon emissions.
In the context of the Chorley project, and many similar rail schemes, the carbon emission reduction is comparable to the 75% indicated in the carbon footprint report.
Outcomes
The project was delivered in accordance with the program and was completed on schedule, with the line handed back for normal operations.
Critically, the outputs of subsequent track level monitoring exceeded the expectations of Network Rail, allowing them to restore full line speed ahead of target. This reduced the need for rail vehicles to slow down, thereby avoiding route delays.