Railway foundations are critical to maintaining track geometry, safety, and ride comfort. A recurring problem arises at transition zones—locations where the track moves from one structural support type to another, such as embankments to bridges. These areas are prone to differential settlement, voiding, and stiffness mismatches, which result in:
At the UBC4 Sarsfield Road bridge on the Dublin–Cork main line, Iarnród Éireann faced such challenges. The single-span structure, skewed to the track, exhibited perennial twist defects and settlement at abutments. Routine tamping and maintenance only provided temporary relief, with geometry faults returning soon after.
Pictured: UBC4 Sarsfield Road bridge (side view)
Geobear proposed a non-disruptive solution using expansive geopolymer resins. These multi-component structural resins are injected in liquid form into the soil, where they expand, fill voids, and increase density and stiffness.
Key advantages:
Importantly, the design approach used was underpinned by Geobear’s patent GB2614319A, which describes resin injection strategies and modelling techniques for optimised ground strengthening.
Typical transition zone treatment using Geopolymer Injections (after patent GB2614319A)
The design methodology for UBC4 followed a structured geotechnical process:
1. Ground model development: Based on borehole logs and track data, layers of ballast, sub-ballast, clay fills, and stiff clay were defined with associated stiffness (Eu = 12–40 MPa) and shear strengths.
2. Transition zone length: Determined by line speed (60 mph → 13.8 m) using the half-speed rule.
3. Numerical modelling: PLAXIS finite element simulations assessed track deformation under dynamic train loading. Results confirmed that >90% of displacement occurred within 1.5 m of the subgrade surface, focusing treatment depth.
(Figure 1) Detailed design for transition zone treatment (using geopolymer injections) on UB4
(Figure 2) PLAXIS analysis for transition zone treatment (using geopolymer injections) on UB4
The installation of Geobear’s expansive geopolymer system at UBC4 was carried out within restricted nighttime possessions, ensuring that the line remained operational during the day. A total of 528 injection points were drilled across the transition zone, with different injection depths, with resin quantities increased towards the bridge abutments to account for higher stiffness requirements.
The geopolymer was delivered in liquid form and expanded in situ, filling voids, densifying the clay fills, and reinforcing the sub-ballast layer. The expansion process was closely monitored using laser levelling (±0.5 mm accuracy) to prevent over-lift of the track during injection.
By executing the works in 19 consecutive nighttime shifts, Geobear eliminated the need for disruptive excavation or reconstruction. The process was completed with no health or safety incidents, and the treated transition zone was immediately available for train traffic at line speed the following day.
Performance was verified through both geotechnical and track monitoring methods:
Dynamic Cone Penetration (DCP) Testing: DCP testing confirmed that soil stiffness targets of 50–80 MPa were consistently achieved throughout the transition zone. Notably, values increased to approximately 100 MPa near the abutments, indicating effective soil densification.
Track Geometry Monitoring: Track geometry was monitored before and after treatment using laser leveling and geospatial surveying. The results demonstrated a 59–69% reduction in twist faults, with 75% of locations showing a downgrade from red faults to yellow or green. These improvements were achieved without the need for tamping between monitoring events.
These results demonstrate that the expanding resin solution provided a consistent transition stiffness profile, improved track geometry, and reduced the need for ongoing maintenance.
The UBC4 case demonstrates the strategic benefits of Geobear’s system:
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