A number of airfield pavement slabs at a major London Airport had settled excessively due to soft underlying conditions combined with potential drainage issues, with up to 50mm of maximum settlement on some slabs (see Figure 1). The settlement of these slabs could disrupt aircraft taxiing from/to landing and take-off zones, therefore, timely maintenance intervention was required. Traditionally to treat this problem, two techniques can be used, namely:
- Slab jacking using cementitious grout injections. This technique involves pumping cementitious grout under the affected slabs to lift them to the desired position. The disadvantage in the technique is that it would require a significant amount of pumped grout to generate sufficient lifting pressure. Also, cementitious grout would require a relatively long curing time to harden and therefore this could increase the risk of grouting leaking and blocking nearby drainage assets.
- Slab reconstruction. This involved a complete reconstruction of the affected slabs, which can be disruptive to the airport and expensive.
Alternatively, expansive geopolymer injections can be used to lift the slabs to the desired position and strengthen the sub-base material. The advantages of this technique are:
- Depending on the geopolymer type, they can expand by up to 40 times their original volume, generating sufficient lifting pressure with a much smaller quantity of material compared to cementitious grouting. Figure 2 shows the expanding characteristics of three different geopolymers, supplied by Geobear, at different confinement pressures
- Due to the expansive nature of geopolymers, which would lead to lower injection quantity requirements, the use of geopolymers would generate 46% less carbon footprint as opposed to conventional cementitious grouting (after KLH, 2018).
- Minimal disruption due to rapid hardening time. Geopolymers require a few seconds/minutes to harden. Slabs can be trafficked as soon as works are completed as there is no wait time for concrete curing. Also, rapid hardening time would reduce the likelihood of affecting nearby drainage assets.
- Lower risk and exposure to hazards such as noise, vibration, dust, heat and lifting operations.
- Lower likelihood of foreign object damage (FOD) due to smaller drilling diameter, less arisings, quick setting material easier to clean up.
The airport operator identified four main blocks that were experiencing significant settlement situated in various locations on the taxiways.
To fully understand the extent of the problem and the underlying reason driving the settlement, Jacobs specified a site investigation that included settlement measurements, probing and drainage and buried services investigation.
Each block had a number of pavement slabs that had varying levels of settlement in different directions ranging from 5mm to 50mm and mainly concentrated along the slabs’ edges, as shown in Figure 4. It was believed that the underlying reason for excessive settlement is due to sealant failure between the slabs (see Figure 5), which resulted in high water ingress at these locations, which in combination with the aircraft’s dynamic wheel loading can lead to pumping and sub-base erosion as illustrated in Figure 6.
Design requirements & analysis
The principal designer on this project, Jacobs, provided Geobear with a number of design requirements for the proposed solution which were as follows:
> Lift the affected slabs so they are releveled with adjacent slabs.
> The geopolymer material should have a compressive strength of more than 1MPa.
> The geopolymer material should have a minimum design life of 25 years.
Depending on the geopolymer mix, the structural and functional characteristics of the geopolymer can vary substantially. For example, highly expansive geopolymers can be very efficient in lifting structures, however, they may not have sufficient strength to resist structural and dynamic loading (i.e. wheel loading). On the other hand, low expansive geopolymers may not be very efficient in lifting structures, but they tend to have excellent strength to resist structural and dynamic loading. Therefore, it was necessary to select an appropriate geopolymer type that would provide an optimum balance between lift capability and strength to provide the client with the most cost-effective design solution.
To achieve this, Geobear investigated the use of three different types of geopolymers, with different properties, and carried out a systemic analysis to compare their lifting efficiency (geopolymer mass required to lift a 1m2 area by 1mm), compressive strength and design life (under cyclic loading), as shown in Table 1.
In Table 1, it can be observed that the Geopolymer Type I seems to have the highest lifting efficiency compared to the other geopolymer types as it is the most expansive type of all the other geopolymers. That means it can lift the slabs more efficiently with a lower quantity of geopolymer, 1.8 and 5.2 times lower than Type II and III, respectively. As for the compressive strength for each geopolymer, values were obtained from testing in the laboratory which was carried out in accordance with BS EN 826. As opposed to the lift efficiency, the compressive seems to be the highest for Type III. In fact, the compressive strength for Type III is 19 and 5 times higher than Type I and II, respectively.
Finally for the design life under cyclic loading for each geopolymer, 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 and Figure 7, 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.
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.
Based on the analysis presented in Table 1, 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.
Geobear solution and installation
Based on the analysis provided in the previous section, a full injection design was developed using Type II geopolymer for each slab, as shown in the example in Figure 8. The injection design utilised a 1.5m injection grid where the geopolymer is injected 30mm below the pavement slabs.
For the installation on site, all the injection locations were marked up by the client to avoid services and permits to drill were provided before drilling commenced. Geoebear operatives drilled in the designated injection locations to the required depth and installed the injection tubes with safety mushroom caps, as shown in Figure 9a . Following drilling and tube installation, the injection gun was connected to the tubes and the injection commenced, as shown in Figure 9b. During the injection process, precision laser level monitoring was used to ensure lifting the slabs to desired levels, as shown in Figure 9c.
The work was completed in 7 night shifts within a period of 10 days treating a total area of 392m2 with no safety incidents. The injection treatment was successful and managed to lift the airfield slabs to the desired levels as shown in Figure 10.
The presented case study illustrates the use of geopolymer injections to treat airfield slabs at one of UK’s busiest airports. The treatment solution was developed and optimised using robust engineering analysis to provide the client with the most cost-effective treatment plan. The results of the project confirm that Geobear’s unique expansive geopolymer technology offers a cost-effective alternative solution to re-level and stabilise airfield slabs with no disruption to the airport operations.