Airport designs can vary significantly due to the size and nature of the project. Trench drains are a natural fit for airport projects because there is often little or no longitudinal slope to the sites. Trench drains can provide slope inside the channels to evacuate water from this type of site without complex grading. The most typical applications for airport trench drains are at hangar doors, inside the aircraft hangar, in the apron or taxiway, around de-icing areas, in terminal areas, vehicle parking areas, and in fueling and fuel storage areas.The size and strength of the trench drain can vary significantly depending on the location and depending on if it is a regional, military, or international airport.
Trench Drains and Slotted Drains for Airports & Aircraft Hangars
Sizing the drain
- Trenches inside the hangar are typically of two types, one being utility trench for fire suppression lines, compressed air, electrical, or fuel. The other being a trench for drainage of the floor due to the fire suppression system, spills, and wash down water. For further design information on utility trenches use this link.
- For trench drains inside hangars you will typically be sizing the trench drain and outlet piping for the fire suppression system discharge. Often times the trench drains are not sized to remove 100% of the fire suppression liquid instantaneously. Sometimes the designs require that the liquid be removed within 5-10 minutes. Choose the length of time allowable and then use the flow rate charts to determine the proper size trench that you need for the application.
- For exterior applications, where you will be collecting storm water, you should use the minimum size that you calculate for the site. Over-sizing the drains will cause them to have a decreased flow velocity which will not allow them to reach cleaning velocities.
- Be sure that the outlet capacity does not limit the flow of the drain.
Choosing the channel
- If you are in a deicing or fueling area and the pavement will be coated with a chemical resistant coating then you should choose channels that have similar chemical resistance to that of the pavement coating. Often in these areas the chemicals are so diluted that the concrete is not coated and if it is not required to be coated then the drains also can be made of concrete or a light chemically resistant material (such as polyester fiberglass).
- In exterior applications you need to ensure that the drain will not degrade with exposure to ultraviolet light (UV light). Many larger airports use a fiberglass precast trench drain system or a concrete forming system so that they get a radius bottom.
- In municipal airports where loadings are lighter, precast systems are often chosen, however attention should still be paid to the consolidation of the concrete around the channels.
- The channels will receive various loadings depending on location and airport type. In military and international airports the channel can take severe loads. Be aware of the dynamic loads that can be placed on these systems. Often turning or breaking on the trench grates can transfer severe loads through the grates into the frame. A heavy duty or extreme duty frame is necessary on all but the smallest airports. The frame should have sufficient bearing area under it to support the entire wheel load. Several studies have been done that show the bearing area under the grate to be one of the leading causes of airport trench drain failures. Learn more about loading and material properties of grates and channels.
- Sealing channel joints is not necessary in this application unless you are in an area where the pavement is also being coated with a chemical resistant material. If the pavement is being coated make sure that the sealant you use can handle at least the same chemicals as the sealant for the pavement surface. If you want the joints properly sealed you must specify such and stress to the contractor that this be done properly. Stainless channels should be welded together with a continuous bead and then ground smooth. Polymer concrete and Fiberglass channels should be properly prepared by roughing up the joint and then using an appropriate two part sealant. HDPE channels should be welded together with a continuous heat welding process.
- Exterior applications or indoor applications that will not be temperature controlled will see severe changes in temperature and should have similar thermal coefficients of expansion to that of the surrounding concrete encasement.
Selecting the grate
- Grates for aircraft traffic need to be rated to handle the severe loadings that can be present under this type of traffic. For international airports the load rating is usually guided by the FAA to be 100, 0000 lbs for grating. Smaller airports should be guided by the type of aircraft that will be traversing the trench drain. Many of the smaller airports can use HS-25 rated grates.
- Make sure that the openings on the grate are as large as possible to ensure that clogging is not a factor. In aircraft traffic there should be little if any pedestrian traffic. The smaller the openings the more frequent cleaning of the grates will be necessary. The larger the openings the more likely the debris is to flush through the system.
- Always use ductile iron instead of cast iron in airport applications for the grates. Cast iron can have a brittle failure allowing the aircraft tire to fall through the grate.
- Grates should be locked down for jet aircraft applications. The most widely accepted lock down method are bolts at all four corners of the grates. Without proper locking of the grates the incredible force of the air behind the jets could lift and remove the grates causing a serious safety hazard.
Designing the layout
- Apron and taxiway trench drains at airports can be lengthy runs due to the flat longitudinal slope that is often present. Due to the long runs, be aware of the depths of the trench drain systems. Use systems that can handle the long runs or choose to have multiple outlets.
- The depth of the system that you are going to use can conflict with piping and other buried utilities.
- Outlets should be placed to minimize piping.
- The FAA requires fire breaks at 125′ to keep flammable liquids from spreading across apron areas. Break the trenches with high points leaving a concrete separation where required to meet this requirement.
- Trench drains at the hangar doors are typically placed just inside the track for the rolling doors to contain the fire suppression liquids. In some cases there is also a trench drain just outside the hangar doors to capture storm water. Remember that if you are going to connect into the sanitary system with a small diameter pipe, the trench capacity should match up with the capacity of the outlet piping. If you oversize the trench drain it will make the cost of the trench drain unnecessarily expensive.