Guidance for Usage of Permeable Pavement at Airports (2017)

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3 1.1 Purpose The objective of ACRP Project 02-64 was “to develop practical guidance to educate airport practitioners on the advantages and disadvantages of the use of permeable pavement at a variety of types and sizes of airports” (from the ACRP Project 02-64 problem statement). The following guidance was developed in this project: • Potential installation locations for permeable pavements for both airside and landside applications. • Environmental, operational, and economic considerations in the use of permeable pavements. • Assessment tools to examine applicability of installations. Currently, there is no single reference that captures the appropriate features, practices, and procedures for incorporating permeable pavements into airport applications. In the absence of such a reference, potential and ideal locations for permeable pavements are underutilized in airport applications, and opportunities for using more efficient stormwater runoff and pavement systems are missed. This guide provides information on the selection, design, construction, and maintenance of permeable pavements at airport facilities, as well as case studies of existing applications. Note that the term “airport” encompasses areas with both landside and airside pavements. Landside pavements are those outside of secure areas where aircraft do not operate and include access roads, parking lots, and pedestrian walkways. Airside pavements are inside of secure areas where aircraft operate and include aircraft facilities (runways, taxiways, and aprons) as well as vehicular facilities (service roads, employee parking lots, and so on). 1.2 Background By design, airports have vast areas of paved surfaces, most of which consist of impermeable (or impervious) pavement. Paved runways in the United States alone constitute approximately 650 million square yards of impermeable pavement (FAA 2007). When taxiways, aprons, roadways, and parking lots are added in, the pavement area easily exceeds a billion square yards. As has long been recognized, impermeable pavements have the ability to increase both storm- water runoff and stormwater pollutants. Although many sustainability and low-impact devel- opment (LID) manuals include permeable pavement as an alternative to impermeable surfaces, available documentation indicates that its use is still limited at airports, particularly in aircraft movement areas. Consideration of the use of permeable pavements as a replacement for conventional imper- meable pavements has increased dramatically in the last 10 years. Permeable pavements are C h a p t e r 1 Introduction

4 Guidance for Usage of permeable pavement at airports an alternative to impermeable pavements because they encourage infiltration or filtration of stormwater runoff while still providing structural capacity (U.S. Army Corps of Engineers 2013). Infiltration and filtration eliminate or reduce stormwater runoff volumes discharged to storm drain or combined sewer systems and the pollutant loadings associated with the stormwater as it discharges to the final nearby water bodies or resource areas. Some of the common advantages and disadvantages associated with permeable pavements are summarized in Table 1. Permeable pavements generally consist of a permeable surface material underlain by an aggre- gate reservoir layer. Surface materials typically consist of porous asphalt, pervious concrete, permeable pavers, or reinforced turf or aggregate. While permeable pavement system designs are typically for stormwater storage and treatment and potential infiltration into the subsurface soils, the specific uses for the pavement and adjacent areas are critical factors that affect the selection and design of the system. Developing and applying appropriate design details are also necessary for success. The FAA currently provides little guidance to airport designers and operators on the design, construction, and maintenance of permeable pavements. The FAA does not currently have guidance on construction of permeable surface courses because testing to assess permeable pavement performance under high wheel loads and tire pressures associated with aircraft has not yet been completed. With the wide range of tire pressures encountered at public airports in the United States (wheel loads from 6,000 to 65,000 lbs, with tire pressures ranging from 50 to 254 psi), the FAA will not be recommending permeable pavements for surfaces on which aircraft operate until full-scale testing has been completed. Currently, the FAA suggests limiting permeable pavements to areas where aircraft do not operate. The FAA requires that the ability of permeable pavements to withstand anticipated loads be established prior to their introduction at airfields. Permeable pavements are not suitable for runways in particular. FAA Advisory Circular (AC) 150/5320-5D, Surface Drainage Design (FAA 2013), references permeable pavements in Table 11-2, Pollutant Removal Comparison for Various Urban BMP [Best Management Practices] Designs, but does not provide additional guidance on the use of permeable pavements as an option for surface drainage. AC 150/5320-6F, Airport Pavement Design and Evaluation (FAA 2016), provides guidance for the structural design of airport pave- ments, but does not include the structural design of permeable pavement systems. Additionally, AC 150/5370-10G, Standards for Specifying Construction of Airports (FAA 2014a), does not include standards for materials that would be considered for use in permeable pavement sys- tems, and AC 150/5380-6C, Guidelines and Procedures for Maintenance of Airport Pavements (FAA 2014b), does not discuss the maintenance of permeable pavement systems. Table 1. Summary of advantages and disadvantages of permeable pavements. Advantages Disadvantages Management of storm runoff (possibly reducing size of storm sewers and reducing the risk of flooding). Filtration of runoff, reducing contamination in waterways. Recharging groundwater supply. Reducing or eliminating need for retention/detention ponds or swales (also reduction in wildlife attractant). Reduced surface ponding (reduction in potential of hydroplaning). Reduced heat island effect. Limited use in conjunction with heavy vehicle loads. Potential lack of experience with design and construction (e.g., failure of pavement). Potential lack of experience with maintenance can lead to poor performance (e.g., clogging of surface). Possible groundwater contamination from fluid spills. Potential higher initial construction costs.

Introduction 5 Under Section 438 of the Energy Independence and Security Act of 2007, the U.S. Congress specifically indicates that federal developments that exceed a 5,000-ft2 footprint should main- tain or restore predevelopment hydrology. This does not directly apply to public-use airports at this time, but does apply to any federal development. The military provides some guidance on the use of permeable pavements at Department of Defense installations. The Army Low Impact Development Technical User Guide (U.S. Army Corps of Engineers 2013) discusses permeable pavements and provides illustrated examples of locations for their use. Additionally, the Unified Facilities Guide Specifications (UFGS) from the U.S. Department of Defense include example cross-sections for both porous asphalt and pervious concrete, including the underlying aggre- gate layers (U.S. Department of Defense 2016). Although the Army Low Impact Development Technical User Guide illustrates areas for the use of permeable pavement and includes specifications that can be used for construction, the guidance is still primarily related to the use of these pavements for residential and parking lot facilities. The guidance indicates that permeable pavement systems should be installed in areas with gently sloping or flat ground, light traffic, and limited use by heavy trucks, and where pavements will not receive snow and ice treatments (salt, sand, or chemicals). The available manuals and specifications for permeable pavements cover parking lots, light-duty roadways, and pedestrian areas, but there is little to no guidance for airside pavements associated with aircraft operations, nor is there a reference that is inclusive of the broad range of paved areas that fall within the jurisdiction of an airport. Because of the environmental and sustainability goals supported by the use of permeable pavements, there is a need to develop a concise practical document that airport designers and operators can turn to for guidance on the use of permeable pavement systems. This report provides information that airport agencies can use to better assess the option of incorporating a permeable pavement system at their facilities. 1.3 Research Approach There are numerous references and several guidelines that address the use, design, and con- struction of permeable pavements for application to roadways, parking lots, pathways, and so on; however, permeable pavement guidelines specific to aircraft facilities are essentially nonexistent. Nonetheless, permeable pavements have already been used for airport pavements, and there is interest in increasing that use. The research approach for developing permeable pavement guid- ance specific to airport applications built on existing experience on and off airports and included the following tasks: • Literature review. • Agency and industry experience. • Case studies. 1.3.1 Literature Review A comprehensive literature search and review was conducted to identify information on per- meable pavement practices and techniques that would be useful in developing practical guidance for applications at airport facilities. The search primarily targeted the practices, features, speci- fications, benefits, and challenges of using porous asphalt, pervious concrete, and permeable interlocking concrete pavement (PICP) on airside and landside facilities at airports. Because of the limited use of these pavements at airports and the fact that landside pavements at airports are similar to those used for highways and roads, the search was extended to include non-airport applications.

6 Guidance for Usage of permeable pavement at airports Over 250 documents were identified, obtained, and reviewed for possible use in this study. As expected, few had direct relevance to aircraft facilities; the vast majority pertained to pave- ments used for vehicle parking lots or low-traffic roads. Despite this outcome, much valuable information was obtained from the literature and carefully summarized to aid in the devel- opment of this guidance document. The information revealed that, while the FAA currently provides little guidance for airport designers and operators on the design, construction, and maintenance of permeable pavements, there are several national, regional, and local manuals and reports available that can be beneficially used for permeable pavements at landside facili- ties, and even limited-traffic airside facilities, at airports. The available information covers a wide range of activities, including project selection (suitable facility types and locations), system design (pavement, hydrological, environmental, and economic), construction (speci- fications, procedures, quality, and testing), maintenance, and performance. Additionally, the permeable pavement perspectives of a variety of stakeholders (owner agencies/operators, expert consultants, paving industries, and regulating authorities) are well represented in the literature. Key literature review documents and select permeable pavement project summaries are pre- sented in Appendix A. A complete bibliography is provided in Appendix C. (The appendices can be found at the summary web page for this report by searching for “ACRP Research Report 178” at www.TRB.org.) 1.3.2 Agency and Industry Experience An exploratory survey focused on identifying experience in planning/design and implemen- tation of permeable pavements was distributed to the aviation industry, including to airport owners, engineers/designers, and contractors. The survey also asked about respondents’ levels of interest in participating in a pilot project. There were 72 responses to the online survey, representing 41 airports or agencies and 23 con- sultants or industry groups. Very few airside permeable pavement projects were found. The projects that were identified included apron areas, roadways, and shoulders. Identified landside projects were primarily parking lots but also included roadways. There was moderate interest in participating in pilot projects (or interest in potential future applications of permeable pavements), with the most interest in parking lots or shoulders (areas with little to no aircraft traffic loading). These results likely reflect the lack of performance data in the airport environment and lack of concise guidance materials. 1.3.3 Case Studies Case studies were developed for four permeable pavement projects: two apron projects, a taxi- way shoulder project, and a roadway project. These projects represent frequent lightweight air- craft operations, infrequent commercial aircraft loadings, and vehicle use. Three of the projects (aprons and shoulders) have porous asphalt surfaces, while the roadway project incorporates a pervious concrete. The assembled case studies provide project data for the following: • Site selection and application, including funding and cost considerations. • Hydrological design. • Structural design. • Material selection and specifications. • Construction practices, plans, and specifications. • Maintenance activities. • Performance evaluation/monitoring.

Introduction 7 The case studies, including relevant specification and drawing examples, are provided in Appendix B (online at the summary web page for ACRP Research Report 178 at www.TRB.org). 1.4 Audience The primary users of this guidance document are expected to be designers, engineers, airport personnel, and others responsible for planning, design, construction, and maintenance of pavement and stormwater facilities at airports of all sizes, including commercial service, cargo service, general aviation, and military airports. 1.5 How to Use This Guide Ideally, those interested in knowing more about permeable pavements for airport applications will read this report in its entirety. Even though the documented experiences may not directly apply to a specific, planned project, they may provide insight into available techniques for dif- ferent (and possibly future) projects. However, this guide provides information in three broad categories related to permeable pavements, and readers may focus solely on the specific area of interest to them: • Planning. • Design. • Construction and maintenance. Chapters 2 and 3 are geared toward planning. Chapter 2 provides an overview of the current permeable pavement types and permeable pavement systems. Chapter 3 discusses the potential applications of permeable pavements at airports. Assessment tools are provided in Chapter 3 to facilitate the assessment of where permeable pavements may be applicable. Chapter 4 provides an overview of the hydrologic and structural design of permeable pave- ments. As current design procedures are primarily based on vehicular applications, discussion is provided on how these design methods can be applied to airport pavements. Chapter 5 discusses the key characteristics of permeable pavement materials and specifications that are currently available. Chapters 6 and 7 discuss aspects of construction and maintenance of permeable pavements. While construction of permeable pavements is similar to construction of conventional pavements, there are distinct differences, which are discussed. The maintenance of permeable pavements is also quite different from conventional pavements and is discussed. 1.6 Terminology The following definitions are for the primary terms used in this report. A more comprehensive glossary is provided at the end of the report. Conventional pavement: A pavement structure with low permeability and supporting pedes- trians and vehicles. Typical examples are Portland cement concrete (PCC), hot-mix asphalt (HMA), and interlocking concrete pavement placed on dense-graded bases. Drainage is accom- plished by surface flow over the pavement. Impermeable pavement: For the purpose of this document, a pavement with a dense surface that does not allow water to pass through. See also conventional pavement.

8 Guidance for Usage of permeable pavement at airports Permeable grid pavers: A cellular grid system filled with dirt, sand, or gravel. This system provides grass reinforcement, ground stabilization, and gravel retention. Permeable interlocking concrete pavement: A type of permeable pavement made of dis- crete, hand-sized paving units with rectangular or dentated shapes manufactured from concrete. These paving units are placed on a highly permeable bedding layer, and the joints are filled with a highly permeable aggregate. Permeable pavement: A pavement surface with penetrations capable of passing water and of supporting pedestrians and vehicles. Typical examples are pervious concrete, porous asphalt, permeable interlocking concrete pavement, and permeable grid pavers. Typically, the permeable pavement surfaces are underlain with variable depths of subbases that serve as both a structural layer and a reservoir layer where infiltrated water is temporarily stored in void spaces until it infiltrates into subsurface soils or is discharged via an underdrain. Pervious concrete: A type of permeable pavement made of PCC. Porous asphalt: A type of permeable pavement made of HMA.