Håkan Carlsson is the laboratory manager for measurement technology and workshop at VTI. His role gives him responsibility for the activities, and he normally doesn’t have time to take part in the practical work. However, there is one exception, and that is proof load testing for airfield bearing capacity. When VTI is commissioned to measure airfield bearing capacity, it is always Håkan who is called upon to carry out the work.
After more than 30 years, he knows about almost all of the airports and airfields in Sweden, as well as a lot about their bearing capacities.
“Of course, it’s very important that runways, taxiways and aircraft stands can withstand the weight of aircraft without being damaged that would otherwise make it dangerous to operate on them. Potholes, deep ruts and cracks could have devastating consequences.”
The proof load testing is carried out using VTI’s heavy Falling Weight Deflectometer (FWD) and heavy means that the weight that is released down onto the surface of the runway has a load, or force, that exceeds 100 kilonewtons. The falling weight causes a point load that results in a stretch in the structure, and that is what is measured, as a deflection on the surface.
One measurement takes one to three minutes, including moving to the next measuring point, and several hundred measurements are taken on a runway. The distance between measuring points varies but is normally somewhere between 10 and 50 metres. The measurement is taken in the wheel tracks because the aircraft almost always travels in the same tracks; they follow the lines and the runway lights closely.
Isn’t it dangerous to be out on the runways?
“Well, clearly it’s essential to ensure that the measurement is taken under safe conditions, both for me and for air traffic. I always have escorts at Arlanda and at the other major airports but at the smaller airports and military airfields I usually have authorisation to be out on the runway alone. VTI is currently in the process of investing in a new heavy falling weight deflectometer, and the new measuring equipment will speed up the work.”
The recommendation is to conduct a bearing capacity study on an airfield at ten-year intervals, provided that nothing out of the ordinary has happened to the runways in the meantime. Håkan Carlsson visits the airfields on assignment at varying intervals but the trip to Arlanda is usually once a year.
“There are more runways there, and over a year there is always something that requires a new measurement, such as runway repaving or repairs. An assignment at Arlanda can take up to one working week. By comparison, it takes one to two days at the smaller airports.”
According to Håkan Carlsson, a common misconception is that the stress on the runway is greatest while an aircraft is landing, but that is when the aircraft weighs least, still has a lot of lift force remaining, and is rolling at high speed. Instead, stress is highest when an aircraft taxies to the starting point and when it starts. It weighs most in these situations because it is fully fuelled, rolls slowly, and follows the ruts closely.
In addition to proof load testing of bearing capacity, VTI also takes friction measurements at airfields. In which case, it is not a question of routine friction measurements, but VTI is called in as an independent expert to check measurements, such as when an airport orders the inspection of a contractor’s work.
Text: Hillevi Ternström
Translated by: CBG Konsult & Information AB
VTI, Swedish National Road and Transport Research Institute