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18 3.1 Introduction Airports, especially commercial airports, can have vast networks of paved surfaces, as sum- marized in Figure 8. As previously discussed, the paved surfaces at these facilities are generally classified as either landside or airside, with landside referring to those areas outside of secured locations, and airside being those areas within secured areas where aircraft operate. Traffic on landside pavements consists of both vehicles and pedestrians; on airside pavements, those may also be present (e.g., traffic on service roads, security roads, facility access roads), but the primary traffic is aircraft. Within this network of airport pavements, permeable pavements can help to fill specific stormwater management needs. This chapter discusses the typical applications of permeable pavements for both landside (vehicular areas) and airside areas. The benefits and risks of these applications are discussed, and methods are presented for determining the suitability of permeable pavement for a project, including assessment tools. 3.2 Airport Permeable Pavement Projects One of the initial goals of this research was to identify permeable pavements specifically serving aircraft traffic and generally associated with airports. Figure 9 summarizes the airport proj- ects identified through the literature review and industry outreach. Many of the applications of permeable pavements at airports have been for landside (vehicular) areas. However, there are projects that have been implemented on airside areas, including pavements supporting aircraft loadings. As identified in Figure 9, the airside applications exposed to aircraft traffic include runway shoulders, taxiway shoulders, and aprons. Permeable pavement shoulders have been designed for heavier aircraft, but these areas are designed for infrequent load applications. (Shoulders need to be designed in accordance with FAA AC 150/5320-6F.) It is also not known whether these shoulders have ever actually been trafficked by aircraft. The permeable pavement apron projects were designed for lighter loads. The Culpeper apron project also included taxiing lanes, which suggests light aircraft taxiways are potential candidates. Airside vehicular applications include roadway shoulders (Wittman Field) and an access road (SeattleâTacoma International Airport). The shoulders at Wittman Field are for roadways access- ing general aviation facilities. SeattleâTacoma constructed a permeable concrete grid pavement for an airfield facility access road. It is located mid-field and has infrequent traffic. Landside applications have primarily been parking lots. Only Paine Field was found to have used pervious concrete for a roadway project. C h a p t e r 3 Permeable Pavement Applications
permeable pavement applications 19 FOD potential needs to be considered when selecting materials for airside pavements. While some of the vehicular applications have included PICP and grids, these surfaces are not ideal for aircraft areas because of their FOD potential. The grids and PICP joints are generally filled with aggregate, which can come loose and be a FOD concern. Therefore, the primary surfaces considered for aircraft applications are porous asphalt and pervious concrete. The use of PICP and grids in airside applications needs to be carefully considered since any loose aggregate may be tracked onto aircraft facilities. Based on the applications already implemented, other airside areas (such as overruns) could also be candidates for permeable pavements. Although porous friction courses have been used on runways, permeable pavements should not be used for runways, at least at this time. Even though permeable pavement has been designed for shoulders serving heavy aircraft, there are no data currently available to determine how these have stood up to heavy aircraft loadings. Therefore, applications should be limited to shoulders, at most, for heavy aircraft facilities. A general assessment of possible permeable pavement locations is provided in Tables 2 and 3. Figure 10 provides a flowchart to illustrate the process of possible selection. Using permeable pavement assumes use of high-durability mixes and appropriate hydrologic and structural design (discussed more in Chapters 4 and 5). There are many considerations to make in determining the suitability of using permeable pavement. These considerations are discussed in the following. 3.3 Project Selection Considerations The suitability of permeable pavement for a particular project depends on many variables. The selection of permeable pavement begins by establishing the overall design objective: retention/ infiltration, delaying time of peak discharge, and so on. These requirements are set by the storm- water management or environmental regulations that need to be met; these vary widely by locale Landside â¢ Vehicular â¢Roadways â¢Parking Lots â¢Service Areas â¢ Public/Pedestrian â¢Sidewalks â¢Plazas Airside â¢ Aircraft â¢Runways â¢Taxiways â¢Aprons â¢Maintenance Areas â¢ Vehicular â¢Service Roads â¢Parking Lots â¢Service Areas â¢Employee/Pedestrian â¢Sidewalks Figure 8. Typical airport pavement facilities.
20 Guidance for Usage of permeable pavement at airports Figure 9. Summary of identified airport permeable pavement locations.
permeable pavement applications 21 Aircraft Facilities Comments Runways No Not recommended. Taxiways No Possibly for light aircraft. Aprons Yes Primarily for light aircraft. Maintenance areas No Risk of spills is a primary concern. Shoulders/overruns Yes Designed for appropriate traffic. Vehicular/Pedestrian Facilities Service roads Yes Generally not for high-volume, heavy vehicle roads. Parking lots Yes Designed for appropriate traffic. Service areas No Risk of spills. Heavy wheel loads turning can abrade the surface. Sidewalks Yes Must be Americans with Disabilities Act (ADA) compliant. Table 2. Potential airside applications of permeable pavement. Vehicular/Pedestrian Facilities Comments Roadways Yes Generally not for high-volume, heavy truck roads. Parking lots Yes Designed for appropriate traffic. Service areas No Risk of spills. Heavy wheel loads turning can abrade the surface. Sidewalks Yes Must be Americans with Disabilities Act (ADA) compliant. Table 3. Potential landside application of permeable pavement. Type of Traffic Airport Location Stormwater Management Paving Project Airside Aircraft Continued A-1 Vehicular/ Pedestrian Continued V-1 Landside Vehicular/ Pedestrian Continued V-2 Figure 10. Potential permeable pavement locations. (continued on next page)
22 Guidance for Usage of permeable pavement at airports Potential Application Risk of Spills Locked-Wheel Turns Weight Facility Type Aircraft Aprons > 60,000 lbs Yes Not Recommended No Shoulders < 60,000 lbs Yes No Yes No Mainline/ Shoulders Maintenance Areas A-2 Potential Application Locked-Wheel Turns, High-Speed Braking Weight Facility Type Aircraft A-1 Runway Shoulders/ Overruns ONLY Taxiway > 60,000 lbs Shoulders < 60,000 lbs Yes Shoulders No Mainline/ Shoulders Apron Maintenance Areas Continued A-2 Continued A-2 Not Recommended Not Recommended Not Recommended Figure 10. (Continued).
permeable pavement applications 23 and may be more stringent than FAA stormwater management requirements. Therefore, local regulations should be consulted. As examples: â¢ Paine Fieldâs pervious concrete roadway project was one part of an overall design to reduce peak flow, delay time of discharge, and provide water filtration (water quality). â¢ The Paine Field apron project was intended to provide no net increase in impervious sur- faces to mitigate the otherwise needed drainage system improvements for providing a paved surface. â¢ Culpeperâs apron and Richmondâs shoulders were primarily designed to delay the time of peak discharge. In some cases, existing conditions may eliminate the option of designing one type of system or another, such as full- or partial-infiltration systems. Potential Application Fueling/Maintenance Traffic Volume Facility Type Vehicles/Pedestrians (Landside) V-2 Roadways High Shoulders Low Mainline1/ Shoulders Parking Lots Yes Not Recommended No Mainline1/ Shoulders Service Areas Not Recommended Pedestrian Sidewalks2 1Ensure adequate structural design for heavy vehicles. 2Must be Americans with Disabilities Act (ADA) compliant. Potential Application Fueling/Maintenance Traffic Volume Facility Type Vehicles/Pedestrians (Airside) V-1 Service Roads High Shoulders Low Mainline1/ Shoulders Parking Lots Yes Not Recommended No Mainline1/ Shoulders Service Areas Not Recommended Pedestrian Sidewalks2 Figure 10. (Continued).
24 Guidance for Usage of permeable pavement at airports 3.3.1 Traffic Demands Traffic on airport pavement is divided into vehicular traffic and aircraft traffic. Aircraft traffic data need to conform to the FAAâs design procedure in AC 150/5320-6F, Airport Pavement Design and Evaluation (FAA 2016). For aircraft facilities being considered for permeable pavement, it is sug- gested that facilities with frequent load applications be limited to loads of no more than 60,000 lbs, which corresponds to FAAâs aircraft weight limit for nonprimary airports that may apply for the use of state standards or materials. For infrequent applications, like shoulders, a permeable pavement system could be designed for heavier loads, as in projects discussed previously. However, as far as is currently known, these projects have not carried heavy aircraft loads. Other aircraft traffic con- siderations are whether stacking (lining up), turning, braking, or high-speed uses are anticipated. Roadway and parking lot pavements are generally limited to vehicular traffic and medium- weight truck applications. Guidance in available design manuals by the National Asphalt Pavement Association (NAPA), the American Concrete Institute (ACI), and the Interlocking Concrete Pavement Institute (ICPI) should be followed for vehicle recommendations. 3.3.2 Soil Conditions A common response to the survey for this study was that subgrade soils did not have adequate infiltration rates to allow the use of permeable pavement. However, that would only be a require- ment for a system designed for full infiltration. Many of the permeable pavement applications were found to be designed to delay the time of peak discharge and not for full infiltration. However, the designers found that the peak volumes also decreased because there is some infiltration even with low-permeability soils. Soil conditions, particularly infiltration rates, influence the required thickness of the base/ subbase reservoir layer and system outflow configuration. The Natural Resources Conservation Services (NRCS) hydrologic soil groups of the site should be identified, and soil permeability should be tested. A geotechnical engineer should be involved to determine the suitability of the soils on site for permeable pavements (ASCE 2015). Soil conditions that should be identified during a preliminary analysis include: â¢ NRCS hydrologic soil groups. â¢ History of fill soil or previous disturbances or compaction of soils. â¢ Current and future land uses with drainage onto the site (ASCE 2015). Certain conditions preclude the use of infiltration systems, and in such instances no-infiltration systems should be used: â¢ The site is directly over solid rock or an impermeable rock/soil layer, such as compacted glacial till with no loose permeable rock layer above it. â¢ The site is over fill soils that have unacceptable stability when exposed to infiltrating water, such as expansive soils or poorly compacted fill soils. â¢ The site is adjacent to fill or natural slopes where soil conditions may result in lateral breakout of the stormwater on the slope. â¢ The site is in an area with karst geology with limestone deposits subject to sinkhole develop- ment due to underground artesian water movement. â¢ The site is in an area with soils that have high shrink/swell potential (ASCE 2015). 3.3.3 Stormwater Capacity The base/subbase reservoir of the permeable pavement needs to provide sufficient storage for the design storm and drainage area. This may not be possible in all situations. Shoulder pavement
permeable pavement applications 25 may provide adequate capacity for a rainfall event on a taxiway, but shoulder pavement may not provide sufficient capacity for a rainfall event on a large apron area. Wittman Airfield constructed a hybrid system in which the taxiway pavement was a conventional HMA (impervious) surface, and the shoulders consisted of a porous asphalt (Givens and Eggen 2012). The porous asphalt shoulder provided for infiltration of rainwater from the pavement into the aggregate reservoir. While the main trafficked surface was a conventional HMA on a dense-graded base, the aggregate reservoir layer was carried across the entire width of the taxi- way (as shown in Figure 11), which greatly increased the holding capacity of the system. 3.3.4 Topography The topography and drainage patterns of the site and surrounding area should be evaluated. Slope should be limited to 5% (Hansen 2008). Stepped (or terraced) subgrade design may be used on sloped areas (Hansen 2008). The stepped design provides for required infiltration or storage capacity. While topography generally needs to be relatively flat, sufficient grade still needs to exist to provide for outflow. Permeable pavements should not be built in floodplains since sediment transported during flood events can clog the pavement. 3.3.5 Subsurface Constraints During a field investigation, the locations of all subsurface and surface structures on site should be identified, and the locations of any utility lines should be confirmed. Permeable pavement should not be used on sites where infiltration into the subgrade will affect existing structures, such as basements subject to flooding, building foundations, septic systems, wells, and embankments in risk of horizontal permeability. Infiltration systems should not be used within 100 ft of source wells (ASCE 2015). It is not recommended to have the reservoir course within 10 ft of basements or building foundations (ASCE 2015). It is also not recommended to have utility lines in the reservoir course unless they are adequately protected and approved by the utility owner. Permeable pavement should not be used in areas with soil contamination. 3.3.6 Groundwater Conditions Groundwater conditions need to be investigated, and any evidence of high water tables should be noted during a field analysis. The bottom layer of the permeable pavement system should not Source: Givens and Eggen (2012). Figure 11. Hybrid permeable pavement design.
26 Guidance for Usage of permeable pavement at airports intersect with groundwater. A minimum of 2 ft between the elevation of seasonally high ground- water and the bottom of the permeable pavement is recommended to ensure adequate filtering of stormwater before it enters groundwater (ASCE 2015). Additional depth or other require- ments may be mandated by local regulations or if the site is located near drinking-water aquifers or in a water resources protection area, recharge zone, or wellhead protection zone (ASCE 2015). 3.3.7 Additional Selection Considerations Several other considerations need to be made in determining the suitability of any location for permeable pavement: â¢ Ability to control or limit, as required, sources of run-on, such as adjacent pavements or vegetated areas. â¢ Treatment of potential sediment sources, such as roof drains. â¢ Equipment availability for future maintenance. â¢ Availability of sufficient funding. â¢ Experience level of local contractors and suppliers. â¢ Risk of chemical spills. 3.3.8 Assessment Tool Table 4 presents a preliminary framework for assessing some of the key variables that need to be considered when determining whether to use permeable pavements at airports. Similar to Consideration Feasible Possible Not Likely Stormwater management regulations Stringent Moderate None Stormwater quality regulations Stringent Moderate None Stormwater storage capacity Infrequent or low-intensity storms/large permeable pavement area Moderate frequency and/or intensity storms/moderate permeable pavement area Frequent and intense storms/small permeable pavement area Seasonal groundwater depth below reservoir Greater than 5 ft Between 2 and 5 ft Less than 2 ft Depth to bedrock Greater than 5 ft Between 2 and 5 ft Less than 2 ft Risk of flooding None Infrequent Frequent Source wells Greater than 150 ft away 100 to 150 ft away Less than 100 ft away Utilities None Non-critical Critical Distance to building foundation Greater than 10 ft Between 5 and 10 ft Less than 5 ft Grades Less than 2% Between 2% and 5% Greater than 5% Subgrade infiltration Greater than 0.5 in./h (or full infiltration not required) 0.1 to 0.5 in./h Less than 0.1 in./h Control of run-on No adjacent run-on. Run-on can be controlledduring design Significant run-on; cannot be diverted Sediment point sources No sediment sources. Sediment sources can bepretreated Significant sediment sources Risk of spills Low Moderate High Contractor experience Certified/experienced Some experience No experience Material producer experience Certified/experienced Some experience No experience Designer/engineer experience Experienced Some experience No experience Owner/agency interest Strong champion Moderate None Funding source Funding secured Low level of justification required High level of justification required (Modification of Standards) Maintenance equipment Available Able to obtain Not available Total items checked per column 7 11 2 Multiply by weighting factor 5 3 1 Weighted score per column 35 33 2 Overall Score: 70 81 to 100 46 to 80 20 to 45 Table 4. Selection criteria for use of permeable pavement.
permeable pavement applications 27 the feasibility matrix developed by Hein et al. (2013) for highway shoulder pavements, this tool rates variables as they apply to a specific project and weights responses to assess the level of feasibility. This decision tool provides the user with practical guidance on the types of projects that are feasible for a variety of facilities. However, even though a project may be feasible, it may not necessarily be the right selection. A complete engineering study is necessary to fully determine the applicability of permeable pavements. This tool was developed based on lessons learned from case studies of successful projects and available literature but is based on limited data because there have not been many permeable pavement implementations for aircraft use. Table 4 includes a list of considerations, discussed previously, that need to be addressed in selecting permeable pavement, and those are rated according to three levels: feasible, possible (items that need more analysis), and not likely. Each rating level selected is then summed and multiplied by a weighting factor, and then the weighted scores are added for the total score. An example project assessment is shown in the table for a potential general aviation apron project. The apron example is based on the following project variables: â¢ County stormwater management requirements require the airport to control stormwater for a 100-year storm event. Land is not available for a traditional stormwater basin, and the cur- rent stormwater infrastructure cannot support additional development (feasible). â¢ The countyâs stormwater management does not currently have set quality control require- ments (not likely). â¢ The overall area of the apron should provide sufficient surface area for required surface infil- tration (possible). â¢ Seasonal groundwater depth and depth to bedrock are both greater than 5 ft, and there are no source wells in the area. There is very little risk of flooding (feasible). â¢ Utilities are within the project area but are not critical utilities and can be moved (possible). â¢ Distance to building foundation will be between 5 and 10 ft. Approach aprons will be used between the apron and hangar (possible). â¢ Grades in the area are generally less than 2% (feasible). â¢ Subgrade infiltration is poor because of high clay content (not likely). â¢ Control of run-on from other areas is more than likely possible, but some areas may remain (possible). â¢ Sediment point sources can be routed into the underdrain system, if needed (possible). â¢ Risk of spills is a possibility since it is an apron, but anticipated leases will require any maintenance be performed in the hangars (possible). â¢ The contractor, material producer, and designer have had some experience with roadway permeable pavements but have not had experience with permeable pavements intended for aircraft (possible). â¢ The owner has a strong interest in low-impact design alternatives (feasible). â¢ The funding source has not been secured, but the owner is aware of several state grants that may be available (possible). â¢ The airport does not currently own a vacuum sweeper for maintenance but anticipates that the equipment can be obtained based on the following yearâs budget (possible). Based on the preliminary considerations, the number of checked items in each column are summed. For the example in Table 4, there are 7 feasible, 11 possible, and 2 not likely responses. Each column total is multiplied by the weighting factor, and the total is summed. The overall score is 70, which indicates that a permeable pavement project may be possible but that additional study needs to be performed.