LGW Block 45N

Project summary

Gatwick Airport’s Block 45N is located at the intersection of Taxiway Juliet and Taxiway Zulu acting as a main through-way to 08R-26L and subject to regular trafficking by large heavy jets. A recurring issue of sinking AGL seating pots had seen patch repairs and localised fitting replacements taking place regularly over a number of years with the underlying problem not being sufficiently addressed. Consequently, the airport had been spending a significant amount of money on a regular basis which resulted in the unavailability of the intersection during maintenance works.

Scope of assessment

An inspection was undertaken on a night closure where the defects and existing repairs carried out to date were documented. The investigation reviewed the historical AGL installation drawings supplied, the existing pavement construction profile and publicly available borehole logs from the immediate vicinity. An analysis of possible failure modes and scenarios was also carried out.

A range of Code C, D and E aircraft were analysed with the nose gear tyre positioned on a typical seating pot for a characteristic pavement profile. The theoretical pressure at the bottom of each pavement layer and the top of the subgrade was calculated and compared to the likely failure strengths of the various media. This included understanding the failure limit of lean concrete and the bearing capacity of the underlying subgrade.

Aerial overview of Block 45N

Nose gear load path analysis

The results showed that under certain loading conditions there was potential for underlying lean concrete to fail if a stabilisation plug was not installed for the seating pot to be founded upon. It was also generally found that the stiff clays upon which the pavements were constructed were of sufficient bearing capacity for the various loading scenarios. The investigation revealed that the most probable cause for the failures was the insufficient compaction of the repair material during each successive intervention leading to recurring depressions.

Benefit

The final report provided a detailed overview of the likely failure mechanisms of the AGL seating pots within Block 45N. Carried out for a local maintenance contractor at the airport, it offered them with the opportunity to propose more resilient repair methods and materials while providing their client with some clarity over the ongoing maintenance issues with their asset.