What is an Underdrain?

An underdrain is a perforated pipe in a trench of porous backfill that collects water from under a pavement and carries it to an outlet. FAA guidance also calls it a subdrain or a subsurface drain.

Underdrain, edge drain and drainage layer

Three terms appear together in FAA documents.

• Underdrain or subsurface drain. FAA AC 150/5380-6C describes subsurface drains as perforated pipes in a permeable sand or gravel trench. Geotextile encases the trench, and outlet pipes carry the water away.
• Edge drain. It is an underdrain along the edge of the pavement. FAA AC 150/5320-5D places the collector drains normally along the shoulder. Drains under the pavement can cause settlement or heave, so the AC says to avoid them when possible. Large pavements may need them. Then the designer places them away from high traffic areas.
• Drainage layer. It is a layer in the pavement structure that lets water drain fast to the side. It is also a structural layer and can serve as part of the base or subbase. AC 150/5320-5D says that the drainage layer feeds a collection system of trenches with a drain pipe, backfill and filter. A pavement without a collection system needs a base and subbase that daylight at the shoulders. The layers then reach the shoulder surface, and the water leaves there.

AC 150/5320-6G says that edge drain systems work best together with a subsurface drainage layer. AC 150/5320-5D lists a manufactured geocomposite edge drain as an option in place of a trench with a pipe. It needs a modification to FAA standards. The AC says to consider it only for pavements without a drainage layer.

What an underdrain contains

The FAA construction specification for airports, Item D-705 Pipe Underdrains for Airports, sets out the parts:

• Pipe. The item lists several pipe types. They include corrugated aluminum, corrugated polyethylene, PVC, steel reinforced polyethylene, corrugated steel, perforated or porous concrete and fiberglass. The engineer selects one. AC 150/5320-5D notes that most state highway agencies use corrugated polyethylene or PVC.
• Porous backfill. It must be free of clay, humus and other objectionable matter. It must meet one of the two gradations in the table. When the plans call for two courses, the finer material follows No. 1 and the coarser material follows No. 2.

• Filter fabric. It must meet AASHTO M288 Class 2 or equivalent. AC 150/5320-5D describes a nonwoven needle-punched geotextile. It lines the trench and keeps fines out of the backfill without blocking the flow to the pipe.
• Trench, bedding and cover. The trench is at least as wide as the pipe plus 6 inches (150 mm) on each side. Perforated pipe sits on a 4 inch (100 mm) bed of granular backfill. At least 12 inches (300 mm) of granular backfill stays over the underdrain. For concrete pipe, the holes in perforated pipe face down.
• Outlets. The pipe discharges at outlets built where the plans show them. Porous backfill is not needed around or over the outlet pipes.

For a collector drain, AC 150/5320-5D recommends a minimum pipe diameter of 150 mm (6 in) and a minimum slope of 0.15 percent. The pipe size follows the peak flow from under the pavement. The AC asks for at least 80 mm (3 in) of aggregate under the pipe. Item D-705 specifies a 4 inch (100 mm) bed unless the plans show otherwise.

A guidance note in Item D-705 covers pipes in runway and taxiway safety areas. They must carry the largest of three loads: the single wheel load of the largest aircraft, the largest maintenance equipment load or the earth load.

Why airport pavements have them

Water weakens the base and subgrade of a flexible pavement. FAA AC 150/5380-6C names this weakening as one of the main causes of failure in a flexible pavement. In a rigid pavement, traffic forces free water under the slab out through joints and cracks. This is pumping. It erodes the material below the slab and leaves voids. In frost areas, subsurface water also causes heave when it freezes and loss of support when it thaws. Poor subsurface drainage can also contribute to durability cracking (D-cracking) and to swelling of subsurface materials.

AC 150/5320-5D names two sources of water. Infiltration is surface water that enters through cracks, joints, the joint between the pavement and the shoulder, and pores in the pavement. The AC calls it the most important source. Subterranean water comes from a high water table, capillary forces, artesian pressure and freeze-thaw action. It matters most in frost areas.

The criteria under runways and taxiways are stricter than for aprons. For a rigid pavement, pumping grows with the volume and speed of traffic. For a flexible pavement, high-volume and high-speed traffic builds up pore pressure and weakens the structure.

When a pavement needs a subsurface drainage system

AC 150/5320-5D, Appendix G, gives the rule for a pavement that water can leave by gravity flow. A system is not required in these two cases:

• The pavement is in a non-frost area and the subgrade has a permeability above 6 m/day (20 ft/day).
• The pavement is flexible, in a non-frost area, and has a total thickness above the subgrade of 200 mm (8 in) or less.

All other pavements need a subsurface drainage system under the appendix. Even when a pavement qualifies for the exemption, the designer should analyse the possible benefit. A rigid pavement needs special care so that water leaves the bottom of the slab fast.

AC 150/5320-6G has a related recommendation. It recommends a drainage layer for pavements that serve aircraft over 60,000 pounds. The area must have excessive subsurface moisture, and the soils a permeability below 20 ft/day (6 m/day). The geotechnical report must support the need.

Design criteria

The design basis in AC 150/5320-5D, Appendix G, is as follows.

• Design storm. The system handles infiltration from a storm of 1 hour duration and a return period of 2 years. The inflow is the storm index times an infiltration coefficient. The AC allows a coefficient of 0.5 where a precise value is hard to find.
• Drainage time. A saturated drainage layer must reach 85 percent drainage within 24 hours under runways and taxiways. For aprons and other low-speed, low-volume areas the time is 10 days. AC 150/5320-6G gives the same two times.
• Drainage layer materials. The AC defines an open-graded material as a granular material with a permeability above 1,500 m/day (5,000 ft/day). A permeable base has a permeability of 305 m/day (1,000 ft/day) or more. A rapid draining material has 300 to 1,500 m/day (1,000 to 5,000 ft/day) and is stable enough to serve as a base or subbase. AC 150/5320-6G allows a drainage layer with a permeability of 500 to 1,500 ft/day without calculations.
• Separation layer. A layer under the drainage layer keeps fines from entering it and gives a working platform. A filter fabric can replace it where the foundation is already firm.
• Placement. Under a rigid pavement, the drainage layer goes directly under the slab. Under a flexible pavement, AC 150/5320-6G puts it directly above the subgrade, unless the geotechnical engineer recommends otherwise. If the whole structure is thinner than 300 mm (12 in), it can go directly under the surface layer.

More open material drains faster, but it is less stable and more prone to rutting. The AC therefore says to keep the drainage material as dense as possible.

Outlets and access

Outlets let the water leave and let the crew clean and inspect the system. AC 150/5320-5D recommends these details:

• Lateral outlet pipes every 90 to 150 m (300 to 500 ft) along the edge drain. Add one at the low point of each vertical curve.
• Outlet pipes of metal or rigid solid-wall pipe. They sit at about 45 degrees to the flow in the collector and slope 3 percent to the outlet structure.
• A discharge end at least 150 mm (6 in) above the 2-year design flow in the ditch.
• Dual outlets, so that crews can flush a section of the collector from both ends.
• A headwall flush with the slope, so that mowing is not impaired, and a removable rodent screen at the pipe outlet.
• Reference markers for each outlet. The AC calls them recommended but not required.
• Manholes, observation basins or risers, at intervals of 300 m (1,000 ft) or less, with a flushing riser between manholes and at dead ends.

In frost areas, the AC says to place the collector below the depth of frost penetration where possible. It also says to place outlet pipes so that ice and snow do not clog them.

Inspection and maintenance

A drain that is not maintained can harm the pavement more than no drain. FAA AC 150/5380-6C lists the faults: clogged or silted outlets and edge-drain pipes, missing rodent screens, vegetation that blocks outlets, and vegetation in side ditches. They let water back up in the pavement.

AC 150/5380-6C asks for periodic inspection of runway and taxiway edge drains, for example each season, and after unusually heavy rain. The signs to look for include:

• ponding of water
• soil built up at the pavement edge
• eroded ditches
• blocked subsurface drainage outlets
• broken or deformed pipes
• backfill that settled over pipes
• discoloration of the pavement at joints or cracks

AC 150/5320-5D splits the inspection into two parts.

• Visual inspection. The inspector measures ditch depths. The inspector looks for crushed outlets, too much vegetation, debris in daylighted openings, headwall condition, erosion and missing rodent screens. The AC asks for this at least once a year. It also asks for a pavement condition evaluation for moisture-related distress. Examples are pumping, faulting and D-cracking in concrete and fatigue cracking in asphalt. It recommends this every 2 years, as a full PCI survey or a brief overview.
• Video inspection. A camera head on a flexible probe goes into the pipe. It finds silting, soil intrusion, ruptured pipes and broken connections. The AC asks for it when there is evidence of a drainage problem. It also finds construction faults before the owner accepts the project.

For maintenance, AC 150/5320-5D says to flush collector drains and outlets with high-pressure water jets to remove sediment. Dual outlets make this possible. Keep the area around outlets mowed and clear the ditches and daylighted openings. Replace missing rodent screens and markers and damaged pipes and headwalls.

EASA guidance asks the aerodrome operator to inspect drainage and storm water collection systems at least weekly (AMC1 ADR.OPS.B.015). FAA AC 150/5380-6C also tells airports to design and maintain drainage features so that they do not attract hazardous wildlife. For design detail, both FAA pavement circulars point to AC 150/5320-5, Airport Drainage Design.

What a failed underdrain does

AC 150/5320-6G warns that an overlay on a pavement with poor subsurface drainage usually performs poorly. Fixing the drainage may need reconstruction of the whole pavement structure.

Pumping is the visible sign in a rigid pavement. AC 150/5380-6C and the PAVER manual describe it as ejection of water and material through joints or cracks. It leaves surface staining and base or subgrade material near the joints or cracks. Pumping alone does not prove that an underdrain failed. AC 150/5380-6C says pumping near joints indicates poor load transfer, a poor joint seal or the presence of ground water. PAVER rates pumping only where the joint seal is defective.

Drone surveys

Several signs in the inspection list show on the surface: ponding, soil built up at the pavement edge, and discoloration at joints and cracks. A drone survey can record these. It cannot see the pipe, the backfill or the fabric under the pavement. Those need a camera in the pipe or an excavation.