Heathrow Airport operates a parallel runway system alternating the departure runway between the northern and southern runways halfway through the day. This only occurs however when westerly operations are in effect. When aircraft depart to the east, an historical agreement, known as the Cranford Agreement, restricts departures to the southern runway and arrivals to the northern runway. Heathrow Airport requested a high-level investigation into the little-used concrete extension which forms part of the northern runway and determine its suitability for regular departure traffic in a post-Cranford scenario.
The report examined three distinct aspects: a review of previous literature on the subject and their conclusions; an analysis of the pavement’s structural suitability; and the most likely rehabilitation scenarios required to deal with more onerous design requirements. The report’s conclusion aimed to further inform future deliberations on the matter and provide scope for budgeting on the various options put forward.
Initially a thorough examination of historic records and reports by the British Airports Service, TPS Consult and URS was carried out with their key findings summarised and reported. These findings were contrasted with more recent surveys undertaken in 2012 as part of the preliminary work associated with the 2013/14 rehabilitations project. Each determined that the structural suitability of the 1969 extension was not in question.
A number of potential issues were identified however, including the likelihood of poor friction arising early in the revised operational scenario. Although little-utilised from an operational point of view, over forty-five years of weathering and the exposure of smooth textured gravel aggregate could yield friction values consistently near the minimum friction level or maintenance planning level. The PQ concrete bay sizes were also noted as being larger than current specifications for gravel aggregate. Curling stresses were therefore seen as a risk and due consideration given to this behaviour.
While the ‘do-nothing’ scenario was deemed operationally acceptable in the short term, rehabilitation options in the medium term needed to be considered. The solutions discussed encompassed a traditional asphalt overlay greater than 100mm or full-depth concrete reconstruction, both of which would involve significant disruption to CAT III operations. With the concrete located in the pre-threshold region, a number of inset approach light fixtures would require decommissioning. In this respect the report was able to generate further discussion on preferences for short term commissioning with intervening maintenance or a more long-term approach with the accompanying operational disruption.
Summary of key points
- Thorough review of historical pavement reports and conclusions
- Identified key properties to examine including friction and PQ bay sizes
- Rehabilitation options proposed for consideration