A chronic track defect on the Dublin–Cork mainline has been successfully remediated using geopolymer ground improvement technology, eliminating the need for excavation and avoiding prolonged line closures.
The challenge
UBC 4 is a single-span bridge carrying the railway over Sarsfield Road in Dublin. The bridge deck sits at a skew to the track direction, creating a long-standing twist and warp fault at the transition zone.
Differential settlement at the abutments had progressively distorted the rail and fastening systems, requiring frequent maintenance interventions. While tamping provided short-term correction, it could only act as a temporary measure, with defects repeatedly reoccurring.
A long-term ground engineering solution was required that could stabilise the transition zone without major track reconstruction or extended disruption to rail operations.
The engineering approach
Geobear designed a targeted ground improvement solution to progressively increase trackbed stiffness across the transition zone.
The design aimed to achieve a starting stiffness of 50-80 MPa, increasing towards the bridge abutments across a 13.8m transition zone, based on established transition design principles described by Burrow et al. (2009).
The engineering process included:
Ground model validation using image correlation analysis of passing train footage, identifying 5.4 mm dynamic deflection, corresponding to a subgrade modulus of 11.7 MPa
- PLAXIS modelling, confirming that more than 90% of strain occurred within 1.5 m below the subgrade;
- Geopolymer dosage design based on the methodology of Dominijanni & Manassero (2014);
- The final treatment design specified 528 injection points across four depth levels, with geopolymer volume increasing progressively toward the abutments. In total, approximately 10 tonnes of geopolymer material were injected to strengthen the trackbed soils.
The results
All works were completed over 19 limited night-time possessions during November and December 2024, with no safety incidents.
Post-treatment Dynamic Cone Penetrometer (DCP) testing confirmed that stiffness targets were achieved at 11 of 12 locations at the transition start, rising to 100% compliance at the transition end.
Track monitoring – carried out without intermediate tamping – demonstrated significant improvements:
- 59-69% reduction in twist and warp at monitored rail positions;
- 75% of previously defective locations downgraded from red fault to yellow or green condition.
The results confirmed that targeted ground improvement could effectively stabilise the transition zone and significantly reduce the ongoing maintenance burden on this section of track.
Engineering verification: Monitoring data, ground testing and track geometry measurements confirmed the predicted stiffness improvements and validated the design assumptions used in the numerical modelling.
Wider applications across rail infrastructure
While this project focused on a bridge transition zone, geopolymer ground improvement has broad applicability across railway assets, including:
- Embankment stabilisation;
- Bridge abutment settlement remediation;
- Void filling beneath trackbeds, culverts, and structures;
- Subgrade strengthening to increase trackbed bearing capacity;
- Tunnel and TBM ground support;
- Level crossing stabilisation;
- Platform and station infrastructure re-levelling.
In each case, the key advantage is the ability to deliver engineered ground improvement without excavation, often within short possession windows while keeping rail operations running.
As rail infrastructure across Ireland continues to age, solutions that extend asset life while minimising operational disruption are becoming increasingly important. For infrastructure owners managing ageing networks under tight constraints, geopolymer ground improvement provides a rapid, lower-carbon and verifiable alternative to traditional ground engineering approaches.
🌐 www.geobear.com
📧 richard.dee@geobear.com
Figure 1: PLAXIS finite element modelling showing strain distribution beneath the track transition zone, confirming that more than 90% of deformation occurs within approximately 1.5m of the subgrade – informing the geopolymer injection depth design.