What is a Pavement Edge Drain?
A pavement edge drain is a perforated pipe in a trench along the pavement edge that carries water out of the pavement layers.
How it works
FAA AC 150/5320-5D, Appendix G, places the collector drains normally along the shoulder of the pavement. The drain system has a collector pipe, backfill and filter fabric. Lateral outlet pipes then take the water out of the system. The pipe can be corrugated polyethylene, smooth PVC or ABS.
The same appendix says an edge drain must handle the maximum flow from the drainage layer of the pavement. A drainage layer is a free-draining layer in the pavement structure. AC 150/5320-6G says an effective drainage layer reaches 85 percent drainage in 24 hours on runways and taxiways. The drainage layer needs outlets so that water is not trapped in it.
Design values in FAA guidance
AC 150/5320-5D recommends these values:
- A collector pipe of at least 150 mm (6 in) diameter.
- A slope of at least 0.15 percent.
- Lateral outlet pipes every 90 to 150 m (300 to 500 ft), and at the low point of each vertical curve.
- An outlet pipe invert at least 150 mm (6 in) above the 2-year design flow in the ditch.
- Dual outlets, so that sections of the collector drain can be flushed.
- Easily removable rodent screens at the pipe outlet.
The AC also says not to put geotextile between the drainage layer and the edge drain. AC 150/5370-10H, Item D-705, is the construction specification for pipe underdrains. It requires filter fabric of AASHTO M288 Class 2 or equivalent.
What happens when water stays in the pavement
PAVER defines pumping as the ejection of material by water through joints or cracks, caused by slab deflection under passing loads. Surface staining and base material near the joints are evidence of it. PAVER defines faulting as a difference of elevation at a joint or crack. AC 150/5320-5D lists pumping, faulting and D-cracking as the moisture-related distresses in concrete pavement. For asphalt pavement it lists fatigue cracking and stripping.
Inspection and maintenance
AC 150/5320-5D gives three levels of inspection:
- Visual inspection of the outlets and ditches, at least once a year. It looks for crushed outlets, vegetation, clogged openings, headwall condition, erosion and missing rodent screens.
- A pavement condition check for moisture-related distress, recommended every 2 years.
- Video inspection of the pipe, whenever there is evidence of a drainage problem. It finds clogging, soil intrusion, ruptured pipes and broken connections.
AC 150/5380-6C adds that crews should inspect edge drains in each season and after unusually heavy rainfall. Its list of warning signs includes ponding of water, blocked subsurface drainage outlets, broken or deformed pipes and backfill settlement over pipes.
Maintenance includes periodic flushing with high-pressure water jets. It also includes repair of damaged pipes and headwalls and replacement of missing rodent screens and outlet markers.
Frequently Asked Questions
- Does every airport pavement need an edge drain?
- No. FAA AC 150/5320-5D exempts a pavement from a subsurface drainage system in one case. The pavement must be in a non-frost area, and the subgrade permeability must be more than 6 m/day (20 ft/day). A flexible pavement in a non-frost area is also exempt if its total thickness above the subgrade is 200 mm (8 in) or less. All other pavements need a system. The AC also says to analyse the benefit of drainage for an exempt pavement.
- How often must an edge drain be inspected?
- AC 150/5320-5D says to inspect the external drainage features at least once a year. It recommends a check of the pavement for moisture-related distress every 2 years. It says to use video inspection of the pipe whenever there is evidence of a drainage problem. AC 150/5380-6C adds that edge drains should be inspected in each season and after unusually heavy rainfall.
- What is the difference between an edge drain and a drainage layer?
- A drainage layer is a free-draining layer inside the pavement structure. An edge drain collects the water that leaves that layer. AC 150/5320-6G says edge drain systems remove free water best when combined with a subsurface drainage layer. AC 150/5320-5D says the edge drains must handle the maximum flow from the drainage layer.
- What happens when an edge drain blocks?
- Water can back up inside the pavement system. AC 150/5320-5D says an improperly maintained drainage system can cause more damage to the pavement structure than no drainage at all. Moisture-related distress in concrete pavement includes pumping and faulting.
References
- faa-ac-150-5320-5d
- AC 150/5320-5D, Airport Drainage Design (15 August 2013), Appendix G: Design of Subsurface Pavement Drainage Systems FAA
- faa-ac-150-5320-6g
- AC 150/5320-6G, Airport Pavement Design and Evaluation FAA
- faa-ac-150-5370-10h
- AC 150/5370-10H, Standard Specifications for Construction of Airports, Item D-705 Pipe Underdrains for Airports FAA
- faa-ac-150-5380-6c
- AC 150/5380-6C, Guidelines and Procedures for Maintenance of Airport Pavements FAA
- faa-paver-concrete
- PAVER Concrete Surfaced Airfields Distress Identification Manual FAA
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