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Airports

Airport Concrete Slab Stabilisation

Challenge

Taxiway slabs at a major UK airport had settled up to 50 mm due to weak subgrade and water ingress, creating safety risks and requiring rapid remediation.

Solution

Geobear injected Type II geopolymer beneath 392 m² of slabs, restoring level tolerances, achieving >1 MPa compressive strength, and cutting carbon emissions by 46% — all in night shifts.

50 mm

Up to 50 mm of settlement corrected to 0 mm

27 year

design life validated under cyclic aircraft loading

46%

lower CO₂ vs. cementitious grouting

Background

At one of the UK’s busiest airports, a number of airfield pavement slabs had settled excessively due to a combination of weak subgrade soils and drainage-related erosion. In some areas, settlement reached 50 mm, creating steps at slab edges and causing risks to aircraft taxiing between runways and stands.
Timely intervention was required. Traditional remediation methods, slab jacking with cementitious grout or full slab reconstruction, were rejected because:

  • Cementitious grout injection would have required large volumes of material, long curing times, and carried a risk of leakage into critical drainage infrastructure.
  • Full reconstruction would have meant weeks of closure and significant disruption to airfield operations.
  • A faster, more sustainable solution was required.

Challenge

The project presented three major challenges:

  1. Variable settlement conditions: Four slab blocks across the taxiway had different magnitudes and directions of settlement, ranging from 5 mm to 50 mm. The settlement was concentrated at slab edges where sealant failure had allowed water ingress. Combined with dynamic aircraft wheel loading, this had led to sub-base pumping and erosion.
    1. Operational constraints: Works had to be completed without disrupting airfield operations. The method needed to fit within short night possessions, with slabs reopened to aircraft traffic the following morning.

    2. Design requirements: The client specified that the material must:

      • Lift slabs back to level with adjacent pavement.

      • Provide a compressive strength >1 MPa.

      • Deliver a design life of at least 25 years under cyclic aircraft loading.

    These requirements ruled out cementitious grout and demanded a design capable of combining lift efficiency, structural strength, and durability.

Airport slab diagram

  1. Operational constraints: Works had to be completed without disrupting airfield operations. The method needed to fit within short night possessions, with slabs reopened to aircraft traffic the following morning.

  2. Design requirements: The client specified that the material must:

    • Lift slabs back to level with adjacent pavement.

    • Provide a compressive strength >1 MPa.

    • Deliver a design life of at least 25 years under cyclic aircraft loading.

These requirements ruled out cementitious grout and demanded a design capable of combining lift efficiency, structural strength, and durability.

Precision under pressure — Geobear re-levelled airport slabs overnight, restoring safety and strength with zero operational disruption.

Solutions

Geobear’s engineering team carried out a detailed analysis of three different geopolymer formulations to determine the optimum balance between lifting efficiency, compressive strength, and design life.

  • Type I geopolymer offered maximum expansion and high lifting efficiency, but insufficient compressive strength.
  • Type III geopolymer offered the highest compressive strength and a 60+ year design life, but low lifting efficiency, requiring more material.
  • Type II geopolymer (Selected for the treatment) provided the best balance — with a design life of 27 years, sufficient strength for cyclic aircraft loading, and efficient lift performance.

The design life of the geopolymer was calculated using Geobear Design Life Model (GDL), see Equation 1, which was based on extensive laboratory cyclic loading testing and bespoke to Geobear’s geopolymers.  The use of this model requires estimating the vertical stress generated on the geopolymer due to aircraft loading and was done using a Pyramid Load Distribution (PLD) simplified method, as shown in Equation 2, along with inputs from Jacobs and HAL related to aircraft loading and loading frequency. From the calculations it was found that Type III geopolymers had a design life that exceeded 60 years and then followed by 27 years and 12 years for Type II and Type I, respectively.

Geobear design life

Where, 
A: Wheel contact patch.
a,b,c,d: model coefficients.
 : applied vertical stress on the geopolymer. 
s: geopolymer compressive strength.
Na: number of applied loading cycles.
P: aircraft wheel loading. 
IF: Impact factor to account for dynamic nature of the wheel load, typical value of 1.5 for taxiing aircrafts. 
H: slab thickness.

 

Load disruption

Based on the analysis, it was concluded that Type II geopolymer would provide the most cost-effective and optimum design as it would be able to satisfy all the clients requirements along with the high lift efficiency. Type I geopolymer would provide the maximum lift efficiency, but cannot provide sufficient compressive strength and design life. On the other hand, Type III geopolymer would have the maximum compressive strength and design life but with low lifting efficiency. Therefore, Type II seems to provide a well-balanced design that provides sufficient compressive strength and design life while maintaining a high level of lift efficiency, hence it was selected for this application. 


Injection design

  • A 1.5 m grid of injection points was drilled across affected slabs, to a depth 30 mm beneath slab base level.
  • Injection tubes (12 mm) were installed and sealed.
  • Controlled injection commenced, with precision laser monitoring accurate to 0.5 mm ensuring slabs were lifted back to tolerance without over-lift.

 

Execution

  • The works were carried out across 7 night shifts over 10 days, covering 392 m² of pavement.
  • During injection, resin expanded up to 40 times its volume, displacing water, filling voids, and re-establishing slab support.
  • Rapid curing (seconds to minutes) ensured slabs were trafficked immediately after treatment, with no curing delay.
  • Geopolymer was used to ensure lifting the slabs to desired levels
Airport-injections

Results

The project successfully re-levelled and stabilised all affected slabs:

  • Settlement corrected: Slabs with up to 50 mm lips were restored to 0 mm (before/after laser monitoring verified).

  • Structural performance: Resin achieved compressive strength >1 MPa, validated by testing to BS EN 826.

  • Design life: Geobear’s design life model (GDL), based on cyclic aircraft load testing, confirmed 27 years’ life for the selected geopolymer.

  • Carbon savings: Compared to cementitious grouting, geopolymer injection achieved a 46% lower carbon footprint (KLH 2018). Compared to slab replacement, savings were even greater due to avoided concrete and excavation.

  • Minimal disruption: All works delivered in night possessions with no operational impact on airfield traffic.

Pre injection slab

Pre injection

Post injection

Post injectoin

 

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