Case Study: Geopolymer Injection for Track Transition Stabilisation
Location: UBC4, Sarsfield Road, Dublin
Client: Irish Rail
Sector: Rail Infrastructure
Service: Ground Improvement
Background
UBC4 is a single-span railway bridge on the busy Dublin–Cork line, carrying trains over Sarsfield Road in Dublin. The bridge deck sits on four longitudinal steel box girders and reinforced concrete shelves, set at a skew to the track alignment.
This skewed geometry had caused persistent issues at the track transition zone for many years, with differential settlement at the bridge abutments leading to rail distortion and twist faults. Regular maintenance was required, but a permanent fix was not possible without addressing the underlying subgrade stiffness.
The Challenge
Irish Rail needed a long-term solution to eliminate twist and warp faults in the track geometry. However, any intervention had to avoid major disruption to services on this key national route. Traditional reconstruction would have required weeks of line closure — not a viable option.
The challenge was to stabilise the transition zone by improving the subgrade stiffness, using a fast, non-intrusive method that could be completed during night-time possessions.
Objectives
- Create a consistent, gradual stiffness transition approaching the bridge
- Achieve subgrade stiffness of 50–80 MPa, increasing towards the abutments
- Deliver the works within limited night-time possessions
- Avoid excavation or track removal
Geobear Solution
Geobear used high-density geopolymer injection to strengthen and stabilise the subgrade beneath the track. The geopolymer is a multi-component resin that expands and cures rapidly after injection, filling voids and increasing ground stiffness.
Design Process:
- Ground Model defined using borehole data and validated via PLAXIS analysis
- Transition Zone Length calculated as 13.8 metres, based on line speed
- Treatment Depth focused on the top 1.5 metres, where >90% of strain was observed
- Injection Layout included 528 injection points, spaced across four depths (0.1m to 1.6m)
Implementation
The project was delivered over 19 night-time possessions in November and December 2024. There was no need for excavation or major disruption to the line. Real-time lift monitoring ensured accurate and controlled ground improvement, with laser levels used to maintain precision to within ±0.5mm.
Geobear installed approximately 10 tonnes of geopolymer safely and with zero incidents.
Results
Soil Stiffness
- Verified using Dynamic Cone Penetrometer (DCP) testing
- 11 of 12 test locations at the start of the transition met the 50–80 MPa target
- 100% of locations at the end of the transition achieved the required stiffness
- A consistent increase in Young’s Modulus was recorded across all tracks
Track Geometry
- Post-treatment monitoring showed:
- 59–69% reduction in short twist and long twist faults
- 75% of red fault locations were downgraded to yellow or green
- No tamping was required between monitoring intervals
Outcome
Geobear successfully stabilised the transition zone beneath UBC4 without excavation or prolonged closure. The geopolymer solution provided a long-term, low-disruption fix for a critical section of Ireland’s rail network.
Key Benefits:
- Elimination of persistent twist and warp faults
- Minimal disruption to rail operations
- Enhanced safety and reliability of the line
- Environmentally sustainable ground improvement
📩 Want to learn more about Geobear’s solutions for rail infrastructure?
Contact us today or book a CPD session with our rail engineering team.
