Innovation Showcase | In partnership with Geobar
Stable solution
Geobear undertook a pioneering railway embankment stabilisation project in 2022.
In July 2022 Geobear designed and implemented a world-first embankment stabilisation scheme on the Severn Valley Railway (NCE, September 2022). Eight months after the work began, the latest results from the project have been released.
The scheme featured a Geobear design to stabilise a railway embankment at Alveley Wood in Shropshire where the track was showing a 300mm deviation from design geometry. The embankment was constructed on a slip plane where a formation of stiff clay is slipping over a softer clay beneath. The solution “stapled” together the slip planes using ground injection technology comprising 300 individual injections of expansive geopolymer to lock the two planes into place.
MODELLING AND DESIGN
Geobear developed a Plaxis model of the existing conditions which confirmed shear zone movement hypothesis as the underlying mechanism of failure. The treatment design focused on strengthening this layer.
Back analysis of the slope failure using Plaxis showed that the embankment had a factor of safety of between 0.8 and 0.9. To achieve a factor of safety of more than 1, the effective cohesion of the shear zone had to be increased from 0 to 5 kN/m2. Consequently, the mass of the geopolymer required to be injected was calculated to achieve that.
The Geobear team used a purpose built mobile injection unit mounted on a train wagon
TREATMENT INSTALLATION
Geopolymer injections were made into different sections of the embankment, with the configuration combining the most cost-effective design with the maximum benefit.
The work took place across two 10-hour shifts in July 2022. Geobear’s team used a purpose-built, mobile injection unit mounted on a train wagon. Holes of 12mm diameter were drilled into the embankment, with 136 tubes installed and 2.5t of expanding geopolymer introduced through 300 separate injections.
VALIDATION RESULTS
The process was validated using pre- and post-dynamic probing which recorded the improvement in soil strength. Tilt measurements were assessed for a period of six months.
SHORT TERM PERFORMANCE
The probing results show a significant increase in undrained shear strength in the depth range of 3.7m to 3.9m below ground level, from around 17kN/m2 to peak value of 120kN/m2, with a mean increase to 70kN/m2.
The results indicate across the shear zone the effective cohesion would have increased from around 1.7kN/m² to an average value of 7kN/m².
LONG TERM PERFORMANCE
Existing tilt sensors were utilised to monitor track performance at the treated area and understand the long-term impact of the treatment. After discounting the e›ect of the start tilt value, the rate of deterioration before treatment is approximately 1.75mm/ month. The rate of deterioration after treatment with 2.5t of geopolymer is approximately 0.39mm/month.
Modelling predicts that with 5t of geopolymer, deterioration could be reduced to 0.19mm/month. The lower volume was used due to time constraints although long term monitoring has shown signs of the deterioration slowing down by up to 77%.
CONCLUSION
Conventional rail embankment stabilisation methods can be time consuming, expensive and disruptive to passengers. This project has illustrated that the Geobear solution enables engineers to surgically treat problem areas without major disruption and, in many cases, no major possession will be required.