What is Differential Settlement?
Differential settlement is the sinking of parts of a pavement foundation by different amounts, which leaves the surface uneven.
How it arises
The foundation of a pavement is the subgrade and the layers below the surface. When these layers consolidate by different amounts, the surface above them is no longer level.
The FAA lists causes for pavement distortion. For flexible pavement, they are foundation settlement, poor compaction of the pavement courses, swelling soils and frost action in the subgrade. For rigid pavement, they are foundation settlement, expansive soils, frost-susceptible soils and loss of fines through badly designed subdrains.
Some ground gives this problem even when the builder does everything right. AC 150/5320-6G says that a 1.5 m granular fill over muskeg still settles differentially and needs considerable maintenance. In permafrost regions, new construction can thaw the ground and cause severe differential settlements.
How it shows on the pavement
On concrete, it shows as a step at a joint or crack. The PAVER manual rates this as settlement or faulting. Severity depends on the height of the step:
| Severity | Runways and taxiways | Aprons |
|---|---|---|
| Low | under 6 mm | 3 to 13 mm |
| Medium | 6 to 13 mm | 13 to 25 mm |
| High | over 13 mm | over 25 mm |
The manual says to use a straightedge or level to measure the step. It does not rate a step that the builder made during construction.
On asphalt, it shows as a depression. This is a low area that is often noticeable only after rain, when water stands in it. Depressions cause roughness, and water in them can cause hydroplaning.
What the standards say
EASA and ICAO both give the same guidance. Aircraft operation and differential settlement of surface foundations will, over time, increase surface irregularities. The guidance sets acceptable, tolerable and excessive limits for the height of an irregularity, and the limits depend on its length. Above the acceptable limit, plan maintenance. Above the tolerable limit, the guidance calls corrective action mandatory. Above the excessive limit, the area warrants closure.
A new runway wearing course must meet a different test. The EASA guidance gives a limit of 3 mm under a 3 m straight-edge.
Frequently Asked Questions
- Is differential settlement the same as faulting?
- No. Faulting is the visible step in the surface, and differential settlement is one possible cause. The PAVER manual rates the step as one distress called settlement or faulting. The FAA says non-uniform consolidation of the layers below can cause it. Upheaval from frost heave or swelling soils can cause it too.
- Does water that stands on a pavement prove differential settlement?
- No. Standing water shows a low area, and the PAVER manual says a depression can come from settlement of the foundation soil. It can also be built into the surface during construction. Find the cause before you choose a repair.
- What fault height gives High severity?
- On a concrete runway or taxiway, a fault above 13 mm is High severity. The limit is 25 mm on an apron. These are PAVER severity levels for the Pavement Condition Index (PCI), not limits for operations.
References
- faa-ac-150-5380-6c
- AC 150/5380-6C, Guidelines and Procedures for Maintenance of Airport Pavements FAA
- faa-paver-concrete
- PAVER Concrete Airfield Pavement Distress Manual (distress 71, Settlement or Faulting) FAA
- faa-paver-asphalt
- PAVER Asphalt Airfield Pavement Distress Manual (distress 45, Depression) FAA
- faa-ac-150-5320-6g
- AC 150/5320-6G, Airport Pavement Design and Evaluation FAA
- easa-ear-aerodromes
- Easy Access Rules for Aerodromes (Regulation (EU) No 139/2014), March 2026 EASA
- icao-annex-14-vol-i
- Annex 14 to the Convention on International Civil Aviation, Volume I, 9th edition (July 2022) ICAO
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