Airports are critical infrastructure to all countries as they connect us to the rest of the world for travel, commerce as well as connecting to family with ever-increasing globalisation, Dr. Theuns Henning stated in his case study on the Funafuti airport runway.
The Funafuti airport is in Tuvalu, an atoll island in the middle of the Pacific Ocean. Atolls are highly vulnerable to climate hazard especially that these natural events are exacerbated by climate change.
The airport is critically significant for the community because it not only provides travel needs, but it is a link to work opportunities, medical care, and at times a link to relief services to its residents, the report says.
Damages to Tuvalu’s airport infrastructure can have significant effects on the economic and social aspects of the community thus it is important that the airport remains functional and operational.
Dr. Henning, in his case study, “Implications of Sea Level Rise on Coastal Pavement Infrastructure for the Funafuti Airport Runway,” investigates these impacts on Funafuti’s runway and airport apron. Some of his observation includes heaving on the surface of the runway, as well as surface depression, cracks and holes in the pavement.
Case study objectives
The case study aims to:
- For damaged runways and apron to carry out detailed measurements on atmospheric pressures, vapour pressures, groundwater levels, and pavement properties to know the impacts on tidal fluctuations and atmospheric pressures on the airport pavement.
- To provide a basis for appropriate investments based on identified risks
- Come up with solutions specific to Funafuti airport, that can also be applied to other airports also located in low-lying areas.
Tests done and findings at the Funafuti airport
Atolls, are originally volcanic islands that get eroded away over time until a ring-shaped coral reef island remains. The coral reef being porous absorbs fresh water and stores it beneath the ground like an aquifer.
Freshwater being less dense than saltwater, floats above saltwater during high tides and heavy rainfall, pushing the ground above it.
Tests were administered to monitor and measure water levels, vapour pressure, and surface temperatures in Funafuti airport. Data collected from these tests were analysed and used to determine the relationships of these elements in causing pavement rise.
The data collected shows that water table levels and sea tidal fluctuations are factors that can cause sub-surface pressures and can be enough to lift the pavement surface. Blistering on the surface is observed during spring tides especially when it coincides with heavy rainfall.
Also, another factor that contributes to the blistering is the creation of vents when vapour escapes from the pavement and when the surface temperature increases.
Findings from other airports
The study also looked at other airports with similar situations and having the same problems with rising waters most notably the Hong Kong Airport, Albany city airport in the US, and the Barrow island airport in Australia.
Hong Kong airport is built on reclaimed soil, layered with meters of rock fill, marine sand, and covered with a flexible asphalt and crushed aggregate pavement. Heaving in the pavement was observed almost after the airport’s opening and following prolonged rain spell. Cracks and potholes followed the heaving.
After a rigid investigation, it was found out that the “heaving was caused by the build-up of air pressure underneath the pavement due to the rising tide and blockage of free airflow in the saturated capping sand layer.”
In the case of Albany airport and Barrow Island airport, both pavements experienced blistering on the surface. Similar pavement materials are used for the two airports– double chip seal, to provide a water-tight surface.
They found out that the built-up of vapour pressure lifted the seal and caused the blistering. This was the same problem that was observed in Funafuti airport.
The only difference is that these two airports are located inland with no tidal or sea-level factors and that the blistering was caused by vapour pressure from heavy and prolonged rainfall.
Analysis of data collected in Funafuti airport showed that vapour pressure alone is not likely to cause the blistering, but when it is occurring together with either of the following events: rising water levels, tidal peaks, and rainstorms can cause enough pressure or ‘peaks’ under the pavement to cause heaving and blistering.
What the case study recommends
Given the unique location and problem of Funafuti airport, that being subjected to high tidal levels these are possible solutions being offered:
The pavement materials and pavement itself of the Funafuti airport are porous because it is of corral origin so grouting for existing drainage paths is not advisable.
A water-tight surface like asphalt paving will not work because rising tides, water levels, and vapour pressure below it will be sufficient to lift the pavement, which is similar to what happened in Hongkong airport. The heaving of pavement can further cause secondary failures like vent holes, surface cracks and depression.
A mechanism to relieve pressure from beneath the surface must be provided one that will have subsurface side drains to remove water from the pavement quickly.
It is further recommended to monitor the water table levels and vapour pressure. Also to install a fish-bone type of drainage system using directional drilling technique without replacement of the current base course provided that the integrity of the base course is intact.
Lastly, more research is needed for developing low-cost permeable paving in situations where water-table fluctuations exist.
To read the full report, CLICK on the link below:
World Bank Blogs. Dr. Christ Bennet. “Considering social impacts of investments in Tuvalu: is it a runway or a recreational area?.” Accessed 18 September 2019.