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Appendix F Review of Applicability of Permeable Pavement in Urban Highway Environments (White Paper #4) 1 Introduction .................................................................................................................................... F-1 2 Permeable Pavement Technologies ............................................................................................... F-2 2.1 Types of Permeable Pavement Technologies .............................................................................. F-2 2.2 History of Permeable Pavements in the Highway Environment .................................................. F-5 2.3 Benefits Full Depth Permeable Pavements â Basis for Consideration ........................................ F-8 3 Evaluating the Applicability of Permeable Pavement in the Urban Highway Environment ................................................................................................................................. F-10 3.1 Common Considerations Associated with Full Depth Permeable Pavements in Urban Highway Applications ............................................................................................................... F-10 3.2 Summary of Potential Urban Highway Opportunities and Limitations ..................................... F-23 3.3 Permeable Shoulders versus Travel Lane Opportunities ........................................................... F-28 4 General Approach for Permeable Pavement Design Integration ............................................ F-29 4.1 Conceptual Steps for Design Integration ................................................................................... F-29 4.2 Overview of Structural Design Approach .................................................................................. F-30 4.3 Hydrologic Considerations ........................................................................................................ F-32 4.4 Balancing Structural and Hydrologic Considerations in Permeable Pavement System Design ........................................................................................................................................ F-32 5 Options for Further Research ..................................................................................................... F-34 6 Summary ....................................................................................................................................... F-35 7 References ..................................................................................................................................... F-36 F-i
1 Introduction Why are permeable pavements of specific interest in achieving volume reduction in the urban highway environment? What is the purpose of this white paper? Advancements in permeable pavement design represent a potential opportunity for achieving volume reduction of urban highway runoff. In particular, the reconstruction of existing roads and construction of new roads are potential opportunities for implementation of permeable surfaces with open-graded subbase. These pavement systems replace the surfaces that are the point of generation of runoff and therefore reduce the space requirements for standard end-of-pipe treatment applications. Applications are potentially feasible, from a space perspective, in highly constrained roadway cross sections, and permeable pavements are capable of volume reduction in a wide range of conditions. Additionally, because they take the place of conventional pavements, the incremental capital cost of permeable pavement systems can be relatively minor in new road construction or lane addition projects particularly when considering that treatment BMP sizing requirements may be less or eliminated. However, there has been limited usage of permeable pavements by road engineers in the urban highway environment due to a wide range of serious concerns regarding their use. Concerns including surface durability, implications of water in the road subbase (and lack of guidance for how this can be safely managed), construction phasing challenges and cost, maintenance requirements, snow management (particularly use of sands) and groundwater quality issues have all be cited as limitations for use, and indeed may be serious concerns in some cases. These issues do not necessarily rule out permeable pavement in all or part of the highway environment and associated right-of-way areas, but should be understood by project owners and project teams as part of making an informed decision about whether or not and where to use permeable pavement. Specifically, the following important limitations should be understood about permeable pavements: ⢠In serving dual purposes (as a reliable transportation surface and a stormwater control feature), the design process for permeable pavements is more complex and requires the coordination of pavement design disciplines with water quality, geotechnical, maintenance, etc. that are not traditionally all required to work as closely as they would need to. ⢠Permeable pavements designs rely on parameters related to infiltration rates (White Paper No. 1), water quality and/or water balance impacts (White Paper No. 2), and geotechnical design (White Paper No. 3), therefore may require a more complex design process. ⢠Limited information is available on the lifespan of full-depth permeable pavement in the highway environment, and based on available information from installations to date, it appears likely that permeable would have a shorter lifespan than traditional pavement (See Section 3.1.3). More frequent replacement of the top course may be required as compared to a traditional asphalt or concrete pavement. This has practical implications on project whole lifecycle costs. ⢠Permeable pavement requires more maintenance than traditional pavement and is essential to keeping the system working. In high traffic highway conditions, it may not be practical to provide the level of maintenance required, and permeable pavement may be infeasible. F-1
⢠In general, there is still limited information available to help make decisions about use of permeable pavement in the urban highway environment. The purpose of this white paper is to provide an up-to-date analysis of the potential applicability of permeable pavement technologies in the urban highway environment and to provide practical guidance and information to help owners, project managers, and designers navigate the decision process about whether permeable pavements should be considered for a specific project. This white paper is based on a synthesis of published literature, experiences of the research team, and selected stormwater guidance documents. This white paper is not intended to provide a comprehensive set of criteria for evaluating permeable pavements that are applicable in all cases, nor is it intended to replace permeable pavement design manuals that exist or are currently under development. Project planning and design professionals should exercise appropriate judgment in considering permeable pavements for infiltration of stormwater from highways, including important site- and agency-specific factors such as climate and maintenance capabilities. The remainder of this white paper is organized as follows: ⢠Section 2 introduces permeable pavements technologies. ⢠Section 3 provides discussion of the applicability of permeable pavement and provides guidance for evaluating the applicability of permeable pavement for a given project based on a number of factors. ⢠Section 4 introduces considerations for design of permeable pavement systems in the urban highway environment and introduces a conceptual design approach that can be followed to help coordinate efforts of different design disciplines. ⢠Section 5 discusses needs for future research related to pavement durability, quality assurance measures, and maintenance aspects of permeable pavements. Significant work is being conducted in parallel with this report. American Society of Civil Engineers (ASCE) (Permeable Pavements Technical Committee, Low Impact Development Standing Committee, Urban Water Resources Research Council, Environment and Water Resources Institute) in preparation of a Manual of Practice on Recommended Design Guidelines for Permeable Pavements. The ASCE Manual should serve as a valuable reference for more detailed design information when it is published. NCHRP Project 25-25/Task 82 posted a report online in October 2013 providing guidance on design, construction, and maintenance of permeable shoulders with stone reservoirs. 2 Permeable Pavement Technologies This section discusses the history of permeable pavement technologies, current permeable pavement technologies and the differences between flexible permeable pavements (i.e., permeable asphalt and pervious friction course) and rigid permeable pavements (i.e., permeable concrete). What permeable pavement technologies are available to the designer? How have permeable pavement technologies evolved with time? 2.1 Types of Permeable Pavement Technologies Permeable pavement is a universal term used to describe an assortment of materials that provide a hardened surface yet facilitate stormwater infiltration. Examples of these materials include flexible F-2
⥠Table o pavements (i.e., permeable asphalt and permeable friction course/open-graded friction course - PFC/OGFC), rigid pavements (i.e., permeable concrete), concrete pavers, polymer based grass pavers, plastic turf reinforcing grids and geocells. What are the key differences between flexible and rigid permeable pavement designs? How does this influence their applicability in the urban highway environment? Full depth permeable pavements are capable of substantial storage and infiltration. Friction courses (PFC/OGFC) that are overlain on pavement have minimal storage and volume reduction capabilities, therefore is not considered a volume reduction approach. However, the mix designs for these pavement types are nearly identical and experiences with installation and durability of PFC provide a reference for the potential applicability of full-depth permeable pavement systems and help address questions about the design life of permeable wearing courses that would be used in full depth permeable pavements. Full depth permeable pavements can be divided between flexible and rigid designs. These types of permeable pavement have similar applications but they vary in a number of ways. Some key differences between flexible and rigid permeable pavement are discussed in Table 1. Table 1. Comparison of Flexible vs. Rigid Permeable Pavement Attribute Permeable Asphalt Permeable Concrete Deformation Flexible Rigid Strength Typically lower strength; rely on combination of admixtures and subbase construction for strength. Typically high strength. Color Dark in color; absorbs heat. Facilitates faster melting in winter. Lighter in color; reflects sunlight. Can be more issues with ice formation. Mix Design More commonly used in northern climates; more suppliers and contractors familiar with product. More common in southern climates. Less common in general; mix is more susceptible to installation issues (water content must be exact). Installation No special tools/equipment required; good quality control in subbase preparation is critical to achieve sufficient compaction and infiltration. Pavement must be covered immediately after installation to retain moisture and allow concrete to cure; generally a slower construction process than permeable asphalt. An extended curing period is required prior to application of chloride deicers. 2.1.1 Full Depth Permeable Asphalt Permeable asphalt is an alternative to traditional warm mix asphalt (WMA) or hot mix asphalt (HMA) and is produced by eliminating the fine aggregate from the asphalt mix (Barrett and Stanard, 2008). Permeable asphalt typically consists of conventional WMA or HMA with significantly reduced fines resulting in an open-graded mixture that allows water to pass through. It is similar in appearance to conventional asphalt pavement, although generally coarser in texture. With the removal of aggregate fines there is a decline in pavement strength which is typically addressed by the use of higher grade binders and/or admixtures. Common admixtures include fibers and styrene butadiene rubber (SBR), a synthetic rubber. Permeable asphalt usage is greatest in the northern US where summer temperatures have less of a limiting impact for asphalt based flexible pavements. The permeable asphalt surface void space typically ranges from 16 to 25 percent. In comparison, voids for standard asphalt are typically 2 to 3 percent and are not interconnected. Permeable asphalt pavement is placed directly on an open-graded aggregate subbase to allow for infiltration. This subbase includes a F-3
choker course or an asphalt-treated permeable base (ATPB), an optional filter course (typically included for added water quality treatment), and a reservoir course for volume control, infiltration, and to act as a frost protection capillary barrier in northern climates. This subbase is used in replace of a conventional dense-graded subbase. Preparation of the subbase requires care to ensure that compaction requirements are met to both provide sufficient structural support while still maintaining sufficient infiltration capacity. This is achieved through construction quality controls. The balance of compaction and infiltration is one of the keys to achieving successful permeable pavement installations. Typical installations of permeable asphalt have included low to moderate traffic parking areas, sidewalks, pathways, recreational areas (e.g., basketball courts), driveways and low-speed/low-volume roadways. Higher volume roads and highways have been also constructed of permeable asphalt with success (Hossain et al 1992, MaineDOT 2010); however, installation of full-depth permeable asphalt in the urban highway environment is not a common practice. This is in large part due to the contradictory design elements of standard road construction with focus in part on keeping water out of the roadbase, in contrast with a well-drained permeable pavement subbase. Additional unique challenges exist for the construction phasing of a permeable subbase, in particular for redevelopment. This white paper will address each of these concerns in order to evaluate whether permeable asphalt is potentially a viable option for achieving volume reduction of highway runoff in the urban environment. 2.1.2 Full Depth Permeable Concrete Permeable concrete or permeable rigid pavement can be used in place of traditional rigid pavement where site conditions are suitable. Permeable concrete contains little or no sand content therefore creating a substantial void ratio (ACI 2008). The usage of permeable concrete is greatest in the southern US where the hot summer temperatures can limit the usage of flexible (asphalt) pavements although recent efforts have evaluated the application of permeable concrete for cold weather climates and generally made advancements in this area (Schaefer et al 2006). In climates where freeze-thaw is not a concern, permeable concrete has the substantial benefit of requiring little or no subbase for structural demands. Subbase construction may still be required to achieve volume control and infiltration requirements/goals. Strength requirements for permeable concrete are based on subgrade strength, pavement rigidity, and thickness of the concrete layer. In northern climates, substantial concerns exist for the usage of permeable concrete and durability issues associated with the application of chloride deicers. Lengthy curing periods of up to 12 months have been reported prior to chloride application (UNHSC 2012) The production and installation of permeable concrete involves substantial quality controls to ensure successful applications. The American Concrete Institute has developed national guidelines (ACI 2008) for the production, design, and construction of permeable concrete. Material testing standards have been developed from the American Society for Testing and Materials (ASTM) Subcommittee C09.49 on Permeable Concrete. The industry has developed a Permeable Concrete Contractor Training Course and Certification that is administered by the National Ready Mix Concrete Association. The state of the practice in production and installation has improved via these national level efforts. 2.1.3 Potential Alternative Configuration for Stormwater Storage and Infiltration below Traditional Asphalt WSDOT is conducting research on an alternative configuration for permeable pavement that would provide storage and infiltration surface area below a traditional pavement, by providing a system of inlets and pipes to convey water into this storage/infiltration area rather than through a permeable pavement top course (Personal Communication, Mark Maurer). This configuration could help avoid potential issues F-4
with the strength, durability, constructability, and maintenance of the permeable top course, while still making use of storage capacity and infiltration surface area below the road or shoulder. In such a configuration, the designer would need to be conscious of the potential sediment load that would be transported directly into the subbase layers and include provisions for removing sediment in the flows prior to routing flows to the subbase reservoir to avoid premature clogging of the infiltrating surface (below the subbase reservoir). In contrast, a permeable top course provides both a conveyance and filtering function; hence the need for vacuum maintenance of the top course to remove captured sediment before it clogs the surface or migrates to the subsurface. The tradeoffs between these two approaches for delivery of stormwater to the subbase reservoir should be considered in evaluating this alternative configuration. 2.1.4 Other Permeable Pavement Technologies Permeable friction course (PFC) overlays are the most commonly used method of permeable pavement in urban highway environments. PFC overlays, also known as OGFC have been installed on roadways in an effort to make existing roadways quieter and safer. The pervious course leads to shorter safe stopping distances for cars, quicker surface drying periods and less splash and spray during precipitation events as well as more muffled sounds in general. Permeable friction course overlays can be cost effective and appropriate in the urban highway environment (TxDOT 2012, Younger 1994, NAPA 2002a). They have been shown to provide water quality benefits in terms of concentration reductions. However, because PFC does not appear to provide significant volume reductions (some more minor evaporation losses), it is not considered a significant volume reduction approach (VRA). Permeable concrete pavers are concrete units installed with open, permeable spaces typically filled with aggregate. Concrete pavers provide an architectural appearance while also providing structural load bearing capabilities. Concrete pavers are most commonly used for suitable for walkways, patios, driveways, and parking lots. The design of permeable paver systems is analogous to permeable asphalt (flexible design), however the cost of permeable paver systems tends to be substantially higher than permeable asphalt or permeable concrete due to the substantial handwork required and may be less applicable to the urban highway environment. Methods of placing pavers in blocks with specialized equipment may improve the efficiency of installation and associated costs to some extent. Polymer based grass pavers, plastic turf reinforcing grids and geocells provide load bearing reinforcement for unpaved surfaces of gravel or turf. These systems are comprised of a series of flexible plastic rings in a grid filled with aggregate laid on top of a base course. The flexible plastic grid is engineered to withstand structural loads, reduce compaction of the aggregate, and provides stability, flexibility and continuity over large surface areas. This permeable pavement type has been used for parking aisles and bays, automobile or truck storage yards, loading dock areas, recreational trails, boat ramps, infiltration basins and residential sidewalks and driveways. It is not applicable for direct highway use in travel lanes. However, it could be considered in shoulder applications. 2.2 History of Permeable Pavements in the Highway Environment 2.2.1 Early Experiences In the late 1960s, permeable pavement was produced to promote infiltration, reduce storm sewer loads, reduce floods, raise water tables, and replenish aquifers (NAPA, 2002b). Throughout the 1970s, the concept was discussed and refined to a point where the Environmental Protection Agency (EPA) sponsored research to determine the capabilities of several types of permeable pavements for urban runoff F-5
control (Thelen, 1978). Many permeable pavement sites have been constructed since the late 1970s, with both failures and successful applications. Failures tended to be associated with clogging of the surface from silt, raveling (i.e., individual aggregate particles dislodge from the pavement surface) and degrading of underlying layers (NAPA, 2002b). As failures occurred, lessons were learned that have been applied to manufacture, design, and installation of permeable pavements. A substantial body of research has emerged as a result in part for the development of PFC/OGFC and for full depth permeable pavements. 2.2.2 Introduction of Permeable Friction Course In 1974, the Federal Highway Administration (FHWA) developed a mix design procedure for permeable friction course (PFC) which is also known as open-graded friction course (OGFC), plant mix seal, popcorn mix, or asphalt concrete friction course (Mallick, et.al., 2000; NAPA, 2002a). The drainage consists of rainwater infiltrating vertically through the PFC to an impermeable underlying layer of standard hot mix asphalt and then runoff moving laterally from the crowned point to the day-lighted edge of the PFC (NAPA, 2002a). PFC was found to reduce hydroplaning, provide higher frictional resistance in wet conditions, reduce spray, enhance visibility, reduce nighttime glare, and reduce pavement noise (NAPA, 2002a). Early issues arose concerning the durability of these open-graded mixtures resulting in delamination and raveling of surface overlays (Younger et al, 1994). These issues were particularly severe in northern climates. These problems have been resolved, to some extent, through the use of modified asphalt binders with polymers and fibers, open aggregate gradations, and quality control assurances (Younger et al. 1994, Kandhal and Mallick 1999). In the United States, Oregon, Washington, California, Nevada, Arizona, Florida, Vermont, and Georgia have used OGFC extensively. Some other DOTs have discontinued the use of PFC due to concerns about durability and service life. In Europe, it has been widely implemented since the 1980s in Germany, Netherlands, France, Italy, United Kingdom, Belgium, Spain, Switzerland, and Austria. The Oregon Department of Transportation (DOT) has been using PFC on its highway systems since the late 1970s (Younger et al. 1994, NAPA, 2002a), both in the more temperate western portions of the state and the higher elevation eastern portions of the state. PFC became Oregon DOTâs preferred choice for surface course and has applied it to more than 2000 lane miles of Oregon highways (NAPA, 2002a). However, in light of recent ODOT experiences with premature failures corresponding to increased snow conditions in 2008 and 2009. ODOT has updated its Pavement Design Guide to recommend that PFC not be installed where there is frequent snowplow activity, significant landslide activity, or underlying hot mix asphalt is experiencing damage from moisture. Meunch et al. (2011) additionally recommended that ODOT consider the limiting use of PFC to lower traffic volumes and considering the use of studded tires, particularly on busses and trucks, in evaluating PFC applicability. Consistent with the ODOT experience, PFC has had less success in more northern climates in part due to problems resulting from use of studded tires, freeze-thaw cycles, and subsequent pavement delamination. There is potential for water to become trapped in void spaces at the boundary between the PFC and the underlying impermeable roadway and this water can then freeze and become dangerous as black ice and exacerbates deicing challenges. 2.2.3 Full Depth Permeable Pavement Systems One of the earliest installations of full depth permeable asphalt was a 1975 demonstration project at the Walden Pond State Reservation in Concord, Massachusetts (Wei 1986, Ferguson 2005). The parking lot is nearly 40 years old. A 2007 inspection indicated the pavement had modest infiltration capacity however the pavement surface was observed to be in good condition, with no visible cracks or evidence of heaving F-6
(Briggs et al 2008). There have been tremendous advances in the technology of permeable asphalt mix design since this time. This heavily visited park has no winter sanding operations, good underlying soils, street sweeping included in the maintenance program, and no fine soils or silts being tracked on or deposited on the pavement. The principal cause of breakdown of asphalt parking lots in northern climate is cracking due to freeze-thaw. However, a deep and well-drained subbase typical of full depth permeable pavements appears to be less susceptible to free-thaw. Full depth permeable pavement systems represent a distinctly different concept in stormwater control with emerging applicability in the urban highway environment. Unlike PFC, full depth permeable pavement consists of a permeable wearing coarse, underlain by an open-graded permeable subbase, such that water is conveyed directly into the subbase reservoir where it then infiltrates into the subgrade and/or is discharged via an underdrain system. The wearing course of full depth permeable pavements is nearly identical with respect to the mix design and production of PFC, full depth systems however deviate from traditional pavements in terms of the design and construction of the subbase. The 2003 publication by NAPA presents the essential design elements for both permeable asphalt mix design and subbase construction (2003, 2008 NAPA). Guidelines for permeable concrete design have been developed by the American Concrete Institute (ACI 2008). Full depth permeable pavement systems have been widely applied in parking lots and walkways, and are seeing more widespread use in low volume roadways. For example, in Pelham, New Hampshire, a 1,300 foot permeable asphalt road was installed for a new residential adult community to provide increased groundwater recharge (UNHSC, 2009). This project sits atop a large sand deposit, ideal for the infiltration of stormwater runoff. The permeable asphalt mix design included a combination of asphalt admixtures to boost strength and durability. Although the cost of materials for a permeable asphalt road was 25 percent higher than that of a traditional asphalt roadway, the developers saw overall project cost savings of 6 percent (compared to the total project costs with a traditional road) by avoiding substantial stormwater management infrastructure like curbs, catch basins, and retention ponds (UNHSC, 2009). 2.2.4 Moving Towards Urban Highway Applications of Full Depth Permeable Pavements Research and development in PFC for highway applications has led to additional durability improvements to the permeable pavement wearing surface through the use of additives and higher performance-grade binders. These additives have the potential to broaden application of permeable surfaces to highways with increased ESAL load/traffic demands. Additionally, recent project experiences and research have established empirical evidence for the potential suitability of permeable surfaces within high volume roadways with heavy loadings. Finally, some of the issues identified with durability of PFC overlays in northern climates are addressed in part by the use of a full depth permeable system that does not trap water in surface pores where freeze-thaw cycles can be damaging. St. John and Horner (1997) were sponsored by WSDOT to evaluate permeable asphalt highway shoulders, gravel shoulders, and conventional asphalt shoulders on a highway near Redmond, Washington. They found that permeable asphalt shoulders reduced runoff by 85 percent and solids and pollutants by more than 90 percent as compared to a conventional asphalt shoulder; permeable asphalt shoulders also significantly out-performed gravel shoulders. After one year, the permeable asphalt showed no signs of clogging and had an infiltration rate of 1750 in/hr. Recent information regarding the long-term performance of this system could not be located, however an unpublished research proposal on the WSDOT website (WSDOT 2013) indicates that the agency is proceeding with plans to study permeable shoulders, including potentially conducting a retrospective analysis of the retrofit originally studied by St. John and Horner (1997). WSDOT is also considering use of permeable shoulders on SR-18 F-7
in Federal Way, WA (Personal Communication, Mark Maurer). WSDOT has policy limitations related to the use of permeable pavements in travel lanes, however this does not prohibit permeable shoulder installations (WSDOT 2011). In 2009, the Maine Department of Transportation constructed the first state permeable asphalt roadway in the northeast. The project includes a full depth permeable pavement for 1,500 feet of a 4 lane highway reconstruction for the Maine Mall Road in South Portland. In 2008 the average annual daily traffic for 2008 was 16,750 vehicles, including significant heavy truck traffic volumes. Four years after construction, the durability has been noted as exceptional. MaineDOT is conducting ongoing monitoring and information about long-term performance for volume reduction and durability (MaineDOT 2010). A cost assessment was conducted by MaineDOT and the full depth construction was found to be about 2 percent more than standard new road construction. In this case, however, the road was reconstructed, so costs were greater than would have been seen in a traditional reconstruction (for reasons discussed later in this white paper). The University of California Pavement Research Center (UCPRC), in cooperation with Caltrans, has recently completed laboratory and modeling investigations to evaluate the structural and hydraulic performance of permeable pavement as highway shoulders that can be subject to heavy loads (Wang et. al. 2010, Jones et al., 2010; Chai et al., 2012). This study included in-depth analysis of subgrade properties and influence of compaction on permeability as well as strength. The studies conducted by UCPRC have determined that the retrofit of roadways is technically feasible and is economically advantageous in freeways when compared to conventional stormwater structure installation (Chai et al. 2010, summarized by Kayhanian, 2012). Further, this research suggested that permeable shoulders could be effective for subgrade permeability as low as 10-5 cm/s (0.014 in/hr). However, field scale evaluation was not conducted, and as noted by Kayhanian, âany simulated design must be constructed and tested before [wide scale] implementation in highway or road environments.â Permeable concrete applications are also being advanced through research. FHWA-sponsored research in progress is evaluating a full depth permeable concrete shoulder with photo-catalytic cement binder for water quality as well as air quality benefits in a 46,000 AADT urban freeway in St. Louis (MO) (Cackler et al., 2012). This pilot included development of full depth permeable concrete shoulder designs capable of withstanding highway loadings. Monitoring, which includes hydrology and water quality as well as pavement integrity, is expected to conclude in 2013 or 2014, with a follow up report prepared after monitoring is completed. Other FHWA-sponsored research in progress is evaluating permeable concrete overlays for a low volume highway in Minnesota (Schaefer et al., 2011). While issues have been encountered with durability at joints, the researchers suggest that these issues could be addressed with modifications to design and construction, and overall found significant potential for widespread application of permeable concrete wearing course even in these more northern climates. 2.3 Benefits Full Depth Permeable Pavements â Basis for Consideration Why should full depth permeable pavements be considered in the urban highway environment? When compared to other highway surfaces, permeable pavement surfaces (including full depth permeable pavement and PFC) have demonstrated the following advantages (FHWA, 1990): â Provide and maintain excellent high speed, frictional qualities (the frictional characteristics are relatively constant over the normal operating speeds); F-8
â Reduce the potential for hydroplaning; â Reduce the amount of splash and spray; â Are generally quieter, often providing a 3 to 5 decibel reduction in tire noise when new; â Improve the wet weather, night visibility of painted pavement markings; â Filtering and capture of pollutants. Benefits that may be associated with full depth permeable pavements, that are not associated with PFC, include: ⢠High levels of runoff volume reductions can be achieved annually, in many some instances greater than 90 percent of the potential runoff is infiltrated; ⢠Reduction in magnitudes and durations of peak flow rates; ⢠Reduction in temperature of runoff during summer months; ⢠Reduction in the usage of costly drainage infrastructure including catch basins, pipe, ponds, hydraulic control structures, and/or stormwater treatment controls; ⢠Reduction in the use of land area required for stormwater management including limited right of way and adjacent areas; ⢠Improved safety in cold weather climates (reduction in black ice), when subgrade drainage is adequate; ⢠The use of quality control measures are well established in the transportation environment for materials production and construction that can be adapted for permeable pavements; ⢠Relatively minor cost difference compared to traditional pavements for new road construction. For these reasons, full depth permeable pavements warrant consideration in the urban highway environment, and indeed is being considered by DOTs in both warmer and colder climates. F-9
3 Evaluating the Applicability of Permeable Pavement in the Urban Highway Environment While permeable pavements have been used successfully in many applications, including some experience in the urban highway environment, a number of questions remain regarding whether permeable pavements are applicable in a high traffic, high loading, and/or maintenance-constrained environment. This section provides guidance for evaluating these questions for urban highway projects. These questions are critical for project owners to evaluate on a case-by-case basis to determine if permeable pavement is appropriate for a given project. Section 3.1 begins with a summary of the state of the practice related to common questions that are raised about permeable pavement in the context of the urban highway environment. Based on this review of issues and summary of the state of the practice, Section 3.2 then provides a summary of factors that influence permeable pavement applicability and the conditions that are more suitable and less suitable for permeable pavement. 3.1 Common Considerations Associated with Full Depth Permeable Pavements in Urban Highway Applications There are a range of perceptions about the applicability of permeable pavements in the urban highway environment. While some of these are consistent with current state of knowledge or lack thereof, some may be based on historic pavement technologies, improper siting, or issues with quality control, which may be possible to address in modern pavement designs and proper construction with good guidance and implementation. This section identifies key perceptions about permeable pavements in the urban highway environment and discusses implications for incorporating these technologies into the urban highway environment. It is important to note that some concerns may not be possible to cost-effectively address and may render permeable pavement infeasible for a given project or region. What are key issues that are commonly identified as limiting the applicability of permeable pavement in the urban highway environment? Where is the current state of the practice relative to these issues? What are the implications for use of permeable pavement in the urban highway environment? 3.1.1 Design Complexity Issue: Permeable pavements can be more complex to design and specify than traditional pavements. Permeable pavements are required to serve dual purposes (as a transportation surface and a stormwater control feature), therefore the design process for permeable pavements is inherently more complex and crosses over multiple design disciplines. However, detailed design guidelines and specifications, for permeable pavements in general, can be found from research and engineering institutions (e.g., ASCE 2013, UNHSC 2009)), industry organizations (NAPA 2008, ACI 2003), and increasingly in many state stormwater manuals, and department of transportation specifications. Specifications have become increasingly available in recent years; many state asphalt associations provide additional guidance and specifications, so it is important to check local requirements. All specification guidelines and information should be evaluated on a case-by-case basis and will vary due to local materials/practices and the F-10
specifics of the project (size, traffic, climate, etc.). There is an increasing familiarity with these mix designs by the major suppliers whom are accustomed to the production of PFC. Standard mix designs are becoming more commonplace in many areas and thus easier to specify and produce. Nonetheless, standardized design guidance specifically tailored for urban highway applications remains relatively limited. Table 2summarizes some of the major design considerations applicable to installation of permeable pavements in the urban highway environment and discusses how this guidance can be adapted to urban highway applications. Table 2. Summary of Design Considerations in the Urban Highway Environment Design Element Typical Applications in Non-Highway Environment Considerations Specific to the Urban Highway Environment Permeable Asphalt Wearing Course ⢠Designed as flexible pavement per AASHTO (1993) or applicable local guidance. ⢠Typically 10 to 15 cm (4 to 6 inches) thick. ⢠Common admixtures include cellulose or mineral fibers for added strength and Rubber Solids (SBR) ⢠Two-layer wearing course typically required for higher loadings, including asphalt-treated permeable base (ATPB) directly beneath permeable asphalt ⢠Admixtures typically needed for strength Permeable Concrete Wearing Course ⢠Designed as ârigid pavementâ per ACI (2008). ⢠Typically 15 to 20 cm (6 to 8 inches) thick. ⢠Design of wearing course should account for lower modulus of elasticity, flexural strength, and subgrade strength compared to traditional concrete pavement. ⢠Thickness of wearing course may need to be considerably greater to accommodate highway loadings and provide reliability consistent with highway applications. Asphalt-Treated Permeable Base (ATPB) ⢠Course permeable layer underlying surface course ⢠Typically 7.5 to 15 cm (3 to 6 in.) in depth ⢠Provides a more stable paving platform for permeable asphalt wearing course ⢠ATPB adds structural strength to the pavement needed for the highway environment Choker Course (Optional) ⢠Clean-washed, uniformly graded aggregate course (typically AASHTO No. 57) ⢠Typically 2.5 to 5 cm (1 to 2 in.) in depth ⢠Provides an even surface for paving and prevents excessive pavement loss into the reservoir coarse ⢠When an ATPB is used, a choker layer may not be necessary. Filter Course (Optional) ⢠Poorly graded (i.e., open graded) sand filter course between choker and reservoir course ⢠Typically 20 to 30 cm (8 to 12 in.) in depth ⢠Must be underlain by intermediate setting bed of 8 cm (3 in.) of pea gravel. ⢠Provides additional water quality benefits, but may be vulnerable to clogging where sediment loads are higher. Reservoir Course (i.e., Subbase) ⢠Clean-washed, uniformly graded aggregate (often AASHTO No. 2 or 3) with 40 percent voids ⢠Typically 0.2 to 0.9 m (8 to 36 in.) in depth ⢠Depth determined from analysis of requirements for volume/peak control, frost depth, structural, depth to bedrock, seasonal high water table and site grading ⢠In permeable asphalt, subbase thickness is a key design variable for structural loading requirements as well as stormwater requirements. ⢠In permeable concrete applications, subbase is not necessarily required; typically controlled by stormwater requirements. Geotextile Liners ⢠Recommended on vertical sides of excavation to prevent inward migration of fines and soil piping and ⢠No specific additional considerations. F-11
Design Element Typical Applications in Non-Highway Environment Considerations Specific to the Urban Highway Environment short-circuiting ⢠Required bottom of excavation for soils with poor load bearing capacity or high fines content Subgrade Compaction and Preparation ⢠Subgrade compaction by heavy equipment traffic should be minimized where feasible by construction phasing ⢠Where operation of heavy equipment on subgrade is required, bed bottom should be prepared by scarifying or tilling the subgrade to no less than 20 cm (8 in.) ⢠Avoiding subgrade compaction may be more challenging in roadway construction and/or may be required for strength requirements; a factor of safety in infiltration rates may be needed to account for unavoidable/necessary compaction. Pavement and Subgrade Slope ⢠Pavement surface should be as level as feasible to promote uniform infiltration. ⢠Bed bottom should be as level as feasible to promote uniform infiltration. ⢠Level surface is not common in highway installations for pavement or bed bottom. ⢠Segments with significant longitudinal grade may significantly complicate design and construction of permeable pavements. ⢠For sloping systems, earthen berms or fabric barriers should be constructed prevent bed bottom erosion and provide runoff storage capacity. Underdrain and Outlet Control ⢠Underdrains should be used for soils with lower infiltration capacity or uncertain long-term infiltration capacity (i.e., lower than 0.05 in/hr). ⢠Elevated underdrains can be used to provide infiltration of the water quality volume at minimum prior to discharge. ⢠Manhole inlets may be designed with outlet control features similar to typical detention systems to manage factors such as peak discharge rate and volume. ⢠Positive overflow from the underdrain/overflow system to a location that will not create negative impacts to slope stability or highway flooding is imperative. System Fail Safe ⢠Typically a backup method for water to enter the reservoir course is incorporated into design ⢠This ensures system function in the event that the pavement becomes clogged or is sealed or repaved with standard impermeable asphalt. ⢠Overflow pathways to a supplemental drainage system are critical in the highway environment to help ensure that lanes are not flooded in the event of pavement failure or highly intense, sustained precipitation. Tributary Area to Permeable Area Ratio ⢠Contributing drainage area to the permeable asphalt surface typically limited to two to four times the area of permeable pavement ⢠High sheet flow loading (run-on) to permeable asphalt may lead to premature clogging or could require excessive maintenance ⢠Additional hydraulic loading (i.e., adjacent rooftops) is best routed to pavement subbase through drywells or infiltration trenches with access for maintenance ⢠In cases where permeable pavement is only installed on road shoulders, ratios greater than 4:1 may be unavoidable. ⢠Increased ratios associated with permeable shoulders will tend to increased maintenance demands. F-12
Design Element Typical Applications in Non-Highway Environment Considerations Specific to the Urban Highway Environment Quality Control ⢠Quality control needs to be documented and implemented during mix production, subbase construction, and materials placement ⢠Application of QC controls in highway environment can leverage the usage of standard QC elements adapted to evaluate infiltration and compaction requirements. Sources: ASCE (2013 in draft), NAPA (2008), ACI (2008), unpublished professional experience. 3.1.2 Strength Limitations Issue: Is it possible to design a permeable pavement with enough strength for urban freeways? Strength needed for urban highways can be achieved in most conditions through top course mix design and subbase design, as discussed in Section 3.1.1, above. Asphalt. Contemporary guidance for permeable asphalt mix designs specifies the use of an asphalt binder two grades stiffer than the local standard and/or admixtures to boost strength and durability. Most examples of poor durability pavements have not included stiffer binders and/or admixtures. Admixtures typically include fibers and/or synthetic rubbers such as styrene butadiene rubber (SBR) or styrene butadiene styrene (SBS), which allow the pavement to stand up to the shear stress of high speed traffic on urban freeways (NAPA 2003). Strength and durability of the wearing course can be evaluated through a number of standardized QC tests including: Air Void Content (ASTM D6752), Binder Draindown (ASTM D6390), Retained Tensile Strength (AASHTO 283), Cantabro Abrasion Test on un-aged and 7 day aged samples (ASTM D7064-04). The overall strength of a flexible pavement is also a function of the subgrade modulus of resilience (Mr) and the subbase design. The strength of permeable pavement systems can be augmented through the use of an ATPB layer, a thicker subbase, and/or higher compaction of the subgrade material. Chai et al., (2012) found that permeable pavement designs were feasible for highway shoulders, considering a detailed evaluation of loading, pavement design, and subbase/subgrade properties. In some soils, saturated subgrade strength is very limited, however the researchers found that this lack of strength could be compensated using a thicker subbase layer. Concrete. For permeable concrete designs, the thickness of permeable concrete can be increased to meet additional structural requirements and compensate for lower modulus of elasticity, flexural strength, and subgrade strength (Delatte 2007) associated with permeable concrete applications. ASTM, Standard C-1754/1754M provides a standard test method for density and void content of hardened permeable concrete. Improvements in cement binders and admixtures Kevern and Farney (2012) show promise for improving surface durability and curing times. 3.1.3 Lifespan and Longevity Issue: Are permeable pavements subject to premature failure and rapid raveling? Is information available about the life span of permeable pavements in the urban highway environment? While notable experiences with pavement failure are relevant and important to consider, technologies and quality control have improved considerably. Earlier applications of open-graded friction course were considered dry (relatively low liquid asphalt content) and subject to premature failure and rapid raveling (McGee et al., 2009). Mixes with higher asphalt content were shown to perform very well; however, F-13
would ravel more quickly once the mix oxidized (McGee et al., 2009). Additions of polymers to mixes have reduced the potential for materials to oxidize, thereby theoretically making the pavement more flexible and durable for a longer period of time. Relatively extensive experience with PFC overlays in the urban highway environment provides empirical evidence of lifespan of permeable asphalt top course and has supported enhancements in permeable asphalt durability. According to a 1998 National Center for Asphalt Technology survey, which included responses from 43 state highway agencies, more than 73 percent of respondents reported greater than 8 years average service life (Table 3). Table 3. Average Service Life of PFC Overlays Average Service Life Percentage of 43 States Less than 6 years 17% 6 to 8 years 10% 8 to 10 years 30% 10 to 12 years 33% 12 years or more 10% Source: Kandhal and Mallick, 1998 However, PFC wearing surfaces do tend to have a shorter life span than the standard dense-graded hot mix asphalts due to the occurrence of a frost lens at the interface of the PFC and the impermeable binder course. Liu, et al., (2010) reported reduced performance associated with reduced durability and functionality (i.e., permeability and noise reduction effectiveness) and raveling and clogging. Full depth permeable asphalts are expected to avoid these issues to some extent as water is not trapped near the surface and susceptible to freeze-thaw cycles, however more research will be needed to assess the structural durability in northern climates were frost heave damage is commonplace for roadways. In addition, in some states allowed seasonal studded tire use, which has also been noted to be a significant factor in pavement design life, for both PFC and dense mix asphalt (Meunch et al. 2011). A review of the effect of traffic volume on pavement lifespan (Meunch et al. 2011) suggests that permeable shoulders, rather than travel lanes, may help improve lifespan of permeable pavement relative to vehicular wear due to their lower traffic volume. Wang et al., (2010) assumed a replacement time of 10 years in their lifecycle cost assessment of permeable shoulders, and found them to be economically advantageous compared to other stormwater controls, indicating that a long lifespan is not necessarily required to make permeable pavement economically viable. A key consideration related to longevity of permeable concrete has been the freeze-thaw durability of permeable concrete and durability around joints. Schaefer (2011) found that some issues remain with durability around permeable concrete joints, however this FHWA-sponsored research concluded that âwell-designed permeable concrete mixes can achieve strength, permeability, and freeze-thaw resistance to allow use in cold weather climates.â Kevern and Farney (2012) have noted significant improvements in binders and admixtures. 3.1.4 Infiltration Limitations Issue: Can permeable pavements be constructed on low permeability soils? Does the necessary compaction of roadway subgrade for structural reasons eliminate the potential for infiltration? F-14
It is rare that a site would not be suitable for use of permeable pavements solely due to low permeability soils. Permeable pavements typically have a much larger footprint to drainage area ratio than other types of volume reduction approaches, which means that a given storm depth results in a smaller depth of ponding on an infiltrating surface and therefore lower infiltration rates can still infiltrate this runoff in a reasonable time period. Similarly, as a traditional road base design for poor load bearing soils requires construction of a sufficiently deep structural base, a sufficiently deep reservoir course would be needed in combination with an appropriate underdrain design. Chai et al. (2012) found that permeable shoulders were feasible in typical California soils (silts and clays) with limited saturated bearing strengths to a lower effective permeability of approximately 10-5 cm/s (0.014 in/hr). With any design project is it important to first determine that site conditions are suitable for the intended design. For permeable pavement projects one of the first steps is to determine if full or partial infiltration is a viable option at the site. Obstacles preventing or limiting infiltration could include very low subsurface soil infiltration rate, utility conflicts, high groundwater table, contaminated subsurface soils and/or groundwater plumes down gradient, proximity to water supply and bedrock, swelling soils, and other factors. If infiltration is limited by one or more of these obstacles, partial infiltration can be achieved by providing an underdrain (or raised underdrain) in a reservoir course beneath the pavement surface. If infiltration is not allowable or desired then the systems can be lined with a compacted soil liner. While there is negligible potential for volume reduction in a lined system, these designs can provide stormwater management in the form of filtration, detention and slow release of collected stormwater off- site. In addition to subgrade, the subbase layers also need to be tested for infiltration capacity during installation. Once the reservoir course aggregate is installed, the filter course is installed typically to both a minimum 95 percent standard Proctor density (ASTM D698 or AASHTO T99) and a minimum infiltration rate typically no less than 10 ft/day determined by ASTM D3385 is recommended. 3.1.5 Pavement and Subbase Slope Limitations Issue: Should installations be limited to flat or low slope surfaces? What challenges are posed by sloping installations? Ideally infiltration systems are constructed with low or no slope bed bottoms as this allows a level pool to form within the reservoir. However in a highway environment, level longitudinal grades are not common and it is infeasible to adjust road grades simply to accommodate permeable pavement. If slope is not considered in the design of permeable pavements, the effectiveness of permeable pavements can be reduced (i.e., less effective storage volume) and erosion of subbase/subgrade material can occur as water flows longitudinally below the surface along the soil interface. To allow for level pools and to control longitudinal flow, subbases can be constructed in a tiered stair-step manner to maintain level bottoms, however this approach is challenging to construct. A more common approach for use in roadways involves the use of low transmissivity geotextiles along isoelevation contours at elevation intervals of 6 to 12 inches to create internal barriers within the reservoir course. This helps prevent erosion within the reservoir layer on slope and provide runoff storage capacity. This method is relatively inexpensive and simple to construct, and has been used successfully on slopes up to 9 percent in low volume roadways (UNHSC, 2009). However it does have the potential to slow construction and increase complexity, especially where slopes exceed 2 to 5 percent (at 12 inch elevation spacing, this would correspond to a fabric wall every 20 to 50 feet). Therefore, permeable pavements are more ideally used on slopes as flat as possible, and not greater than 2 to 5 percent. F-15
3.1.6 Geotechnical and Pavement Design Concerns Issue: Does permeable pavement introduce geotechnical and pavement design issues associated with saturated subbase/subgrade that cannot be mitigated? Geotechnical and associated pavement design issues are a key site-specific factor in assessing the feasibility of permeable pavements. In general, this question can be divided into two elements: ⢠Would infiltration cause geotechnical issues associated with subgrade strength, slope stability, soil volume change, utility damage, or other factors, either immediate below the roadway or in adjacent areas? This is a site-specific question. Guidance for evaluating this issue based on site-specific information is provided in White Paper #3. In summary, conditions exist where geotechnical issues can be mitigated, however some complex or challenging geotechnical conditions provide a valid technical basis for limiting or preventing infiltration from the road base. ⢠Does infiltration into subgrade (and resulting saturation) result in geotechnical properties of subgrade materials that cannot be accommodated in pavement structural design? This is also a site- specific question that requires knowledge of the response of subgrade materials to saturation. However, generally low strength subgrade can be accommodated by adding greater strength (i.e., depth) to the subbase and wearing course profile (Chai et al., 2012). In fact, design guidelines for standard pavements generally assume some degree of saturation and provide guidance for designing pavement structure when subgrade materials have limited strength. Subgrade materials with the potential for swelling upon saturation are generally problematic in the highway environment, regardless of whether intentional infiltration is proposed. In summary, these factors do not necessarily preclude the use of permeable pavements, and must be addressed in traditional roadway design as well as permeable pavement. However these factors must be considered in planning and design, and, in some cases, the introduction of additional water into the subbase can introduce issues that cannot be reasonable mitigated. See White Paper #3 for more information. 3.1.7 Asphalt Draindown Concerns Issue: Is there potential for hot asphalt mix to drain down through the permeable pavement surface layer, reducing permeability and causing a clogging layer within the pavement? Typical permeability rates for new, properly constructed permeable asphalt layer have been measured from 1000 to more than 3,800 cm/hr (400 to more than 1500 in/hr) (summarized from multiple sources in Ferguson 2005), although the permeability of the pavement surface is expected to decrease over time. Clogging of several permeable asphalt systems has been anecdotally attributed to long- term draindown of asphalt binder in the pavement layer, however this would be expected largely for older mix designs without the admixtures (Ferguson 2005). Draindown may occur when the asphalt binder slowly liquefies during summer heat and drains down off the aggregate during hot periods, mixing with sediment, dust and other materials to form a clogging layer deeper in the pavement. According to the National Asphalt Pavement Association (NAPA) (2002b), this can also be observed during construction when the asphalt mix is at its highest temperature. As noted by Kandhal and Mallick (1999), this issue can be addressed directly by the usage of admixtures. Contemporary mix designs are easily achieving this draindown requirement with the usage of fibers which increase the asphalt coating thickness and with liquid admixtures to increase stiffness. F-16
Careful mix design (especially the selection of the asphalt binder grade, mix production temperatures and the use of additives) is recommended to reduce of the risk of draindown. The addition of fibers to the permeable asphalt mix has been shown to effectively manage draindown while adding material strength. Potential for draindown can also be detected as part of QC during installation. Excessive draindown evidenced by pooling of asphalt in the trucks is a key visual measure of QC concerns. 3.1.8 Groundwater Contamination Potential Issue: Do highway applications of permeable pavement have a higher risk of groundwater contamination than other applications of permeable pavement? Infiltration systems including, permeable pavements, pose new potential risks to groundwater quality compared to traditional pavements (Pitt et al. 1999). Careful siting of infiltration systems is an important element of design in general and must be observed in the installation of permeable pavements in the urban highway. White Paper #2 addresses the potential for groundwater contamination as a result of infiltration of highway runoff, and provides guidelines for evaluating the relative risk posed by site. Key factors include the depth of groundwater (higher risk associated with shallower depths), the nature of subsurface soils (permeability, organic versus inert, etc.), and the design of VRAs. In general, White Paper #2 concludes that infiltration of stormwater runoff can be safely done where conditions are suitable. Key factors that differentiate permeable pavements from other infiltration systems include: (1) net influence of permeable pavements on chloride loading, and (2) remediation issues in the event of contamination spills. Perhaps the most significant groundwater contamination issue faced in high use transportation corridors is salts associated with winter maintenance activities on impermeable roadways. As a result, numerous chloride TMDLs are in process or in place in the United States. This presents a unique management challenge to both balance public safety and environmental protection because chloride concentrations are not reduced with standard best management practices, leaving source control as the primary strategy. Permeable asphalt has been shown to require substantially less chloride for winter maintenance top achieve and equivalent level of service (Roseen et al 2013). As such permeable pavements represent a management opportunity with reduced risk for both public safety and environmental protection. In this respect, the use of permeable pavements may have a net benefit to surface water quality and potentially groundwater quality in cold climates. However, careful consideration of salt management strategies is recommended in determining whether permeable pavements would pose an incremental risk to groundwater quality as a result of providing a more direct pathway for salts to migrate to groundwater. Where routine handling of hazardous materials and risk of spill exists, minimizing the use of infiltration systems may be warranted (Pitt et al. 1999). Typically, full-depth permeable asphalt installations have been sited in locations where the risk to groundwater contaminations is low (i.e., parking lots and sidewalks). The urban highway environment present risks associated with potential for spills from large commercial vehicles. If a spill were to occur on a highway paved with permeable pavement the roadway section would need to be closed for longer remedial activities to occur on the pavement surface and in the subsurface aggregate storage bed and soils. The need for longer road closures to remediate spilled contaminants is unique to the use of permeable pavements. However, given the relative infrequence of significant spills, it may not be appropriate to exclude the use of permeable pavements on this basis alone. Such systems would help to contain spills prior to surface discharges which in some cases could be much more complex to clean up. F-17
3.1.9 Construction Considerations Issue: Are permeable pavements slower to construct than traditional pavement? Do they pose major issues in the construction process? Qualified engineering and construction oversight are essential elements of permeable pavement construction because installation differs significantly from conventional pavements. Quality control measures, in general, are routine and well established in roadway construction and material production. In fact, performance payments based on QC targets are standard for large projects. These QC measures are easily being adapted for the use of permeable pavements. General construction sequencing and QA/QC considerations for full depth permeable pavements are included in Table 4. Table 4. Summary of Construction, Installation and QA/QC Considerations Element Typical Applications in Non-Highway Environment Considerations Specific to the Urban Highway Environment Mobilization and site preparation ⢠Installation and maintenance of erosion and sediment control measures. ⢠Installation of permeable pavement systems should occur after site stabilization for erosion control is in place. ⢠Site grading in the vicinity of the roadway should be conducted and areas stabilized prior to initiation of pavement installation. Preparation of bed bottoms (i.e., subgrade) ⢠Ensure existing subgrade is not over-compacted; where subgrade compaction is unavoidable scarify subgrade to restore infiltration capacity. ⢠Inspection of subgrade infiltration is a standard QC element prior to placement of constructed materials. ⢠Ensure all bed bottoms are level or for sloping systems, ensure earthen berm or fabric barriers are constructed. ⢠Obtain owner/engineer approval of the prepared subgrade. ⢠In general, designs in the highway environment should design for a moderate to high degree of unavoidable/necessary compaction of the subgrade below travel lanes and shoulders. Installation of geotextile ⢠Install geotextile (if used) in accordance with the manufacturerâs standards and recommendations. ⢠Adjacent strips of geotextile should overlap a minimum of 40 cm (16 in.). ⢠Secure geotextile at least 1.2 m (4 ft.) outside the bed to prevent runoff or sediment from entering reservoir course (remove excess once site is fully stabilized). ⢠Geotextiles may be an important element to control erosion in the subgrade where longitudinal slopes exist. Aggregate placement and compaction and infiltration ⢠Aggregate shall be placed in 20 to 30 cm (8 to 12 in.) lifts and compacted until there is no visible movement. ⢠Construction equipment shall be kept off the bed bottom and traffic on the aggregate should be minimized. ⢠Once the reservoir course aggregate is installed to the desired grade, either the filter course (if included in design) and/or choker course (if included in design) is installed. ⢠If used, the filter course should be tested at compaction to maintain a minimum infiltration rate typically no less than 10 ft/day determined by ASTM D3385. Permeable asphalt top course application ⢠The permeable pavement is installed directly over the aggregate base layers. ⢠Placement temperatures and compactive effort should be per engineerâs specifications. ⢠Rollers should move slowly and uniformly to prevent ⢠When admixtures are added for strength, compaction may need to occur at higher temperatures. F-18
Element Typical Applications in Non-Highway Environment Considerations Specific to the Urban Highway Environment displacement of the mix, and rollers should not be stopped or parked on the freshly placed mat. ⢠Permeable asphalt should not be installed on wet aggregate or wet treated bases or when the ambient air temperature is below 13°C (55°F). Two lift application ⢠Permeable pavement surface shall be conducted in two lifts ⢠Permeable asphalt surface course shall be installed over the ATPB where required for high traffic loads and over choker course where lighter traffic loads allow. ⢠ATPB is typically needed for strength in higher load applications and may also be used to temporarily accommodate construction traffic prior to surface paving (Hansen 2008) ⢠ATPB must be covered with geotextile and protected from sediment and must be vacuumed and flushed with water prior to the final lift of permeable asphalt paving Curing of permeable asphalt ⢠No vehicular traffic is permitted on the pavement until cooling and hardening (curing) ⢠Typical curing time: 48 hours minimum ⢠To ensure adequate curing time lane opening must be sequenced to ensure flow of traffic Concrete top course installation and curing ⢠Typically formed and leveled by hand in traditional parking lot and sidewalk applications ⢠Permeable concrete must not be âfloatedâ, as this can seal the surface ⢠Curing typically involves covering with sheet plastic for a curing period of at least 7 days. ⢠Faster installation methods may help improve viability in the highway environment. ⢠Schaefer et al. (2011) successfully tested a self-consolidating and slip- formable mix in a MNDOT right of way. ⢠Kevern and Farney (2012) have studied admixtures to improve curing time. 3.1.10 Capital Costs Issue: Is permeable pavement is cost prohibitive? Permeable pavements are commonly thought to add substantial cost to a project and are therefore thought to be cost prohibitive. In fact, for new development, total project costs for permeable pavements can result in significant cost reductions when the result is avoidance or downsizing of stormwater infrastructure (e.g., BMPs, ponds, piping, control structures) (Roseen 2013, Gunderson 2010, MEDOT, 2010, Wang et al. 2010). Permeable asphalt surfaces are generally 20 to 50 percent higher in costs than conventional asphalt on a unit area basis. This is due to the use of additives (e.g., fibers, polymers) to control draindown and increase stability. Costs for permeable asphalt (not including the reservoir course) range from about $21 to $38 per m2 ($2 to $3.50 per sf). As a complete system (inclusive of subsurface layers), the estimated cost is $65 to $130 per m2 ($6 to $12 per sf). In 2009 costs for the Maine Mall Road were $180/ton for F-19
permeable asphalt and $105/ton for the ATPB (Hodgman 2012). In 2013, costs were reported to be approximately an additional $14/ton for an 800 ton placement in Provincetown, MA (Roseen 2013). Additional costs were associated for one-time costs for usage of temporary equipment to handle specialty binder. Much of the increased unit cost of permeable asphalt pavement systems comes from the stone storage reservoir, which in some instances is thicker than the aggregate base for conventional pavement. However for high use roadways, structural requirements for subbase thickness will often exceed needs for runoff storage and frost design. Additionally, costs can be offset by the significant reduction in the required conventional stormwater management elements (inlets, pipe, basins, right of way, curb and gutter, etc.). Furthermore, permeable pavements with reservoir layers often avoid the need for end-of-pipe detention basins or other, similar stormwater management systems. When these factors are considered, permeable pavement with infiltration can be approximately the same cost or often even less expensive than conventional pavement with its associated stormwater management facilities. For example, a 6 percent project savings was realized for a permeable asphalt residential road application in New Hampshire by avoiding the use of curbing, pipe, catch basins, detention basins and outlet control structures (Gunderson 2010). MaineDOT conducted a costing analysis and found the permeable asphalt roadway to be 2 percent more than a conventional roadway for new construction. Similarly, Wang et al. (2010) found that for California highways, permeable shoulders would have an economic advantage over traditional pavement with associated stormwater controls, even when a top course lifespan of only 10 years was assumed. It is important to note that findings of potential cost savings applicable only to new construction or lane additions. Costs for including permeable pavement in refinishing and retrofit projects can be prohibitive because of the need for reconstruction of the subbase (i.e., the need to remove the traditional subbase layer and replace it with an open graded subbase), and the associated import and export hauling and disposal required. Refinishing or retrofit projects would also not recognize savings from reduced drainage and treatment infrastructure since these types of infrastructure are already in place; therefore, costs tend to be higher for redevelopment and resurfacing projects done with permeable pavement rather than conventional pavement. Additionally, where a supplemental drainage pathway is needed in critical highway segments (such as âsagâ and âdepressedâ sections) to provide a failsafe against flooding, the economics of permeable pavement can be less favorable. Project-specific analysis of costs, considering costs that are incurred as well as avoided, is recommended. 3.1.11 Maintenance Costs Issue: Are maintenance activities are cost prohibitive and/or impracticable for highway applications? The success of permeable pavement systems relies not only on proper design and construction but also on effective maintenance. The primary goal of permeable asphalt system maintenance is to prevent the pavement surface and underlying storage/infiltration bed (if applicable) from becoming clogged by sediment and debris. Maintenance is often viewed as a barrier to implementation of low impact development (LID) technologies due to the minimal documentation of frequency, intensity and cost of maintenance for these practices. However, proponents of permeable pavements regard these practices as lower in maintenance when compared to other conventional stormwater controls (MacMullen, 2007; Powell et. al., 2005; EPA, 2000). In 2012, the University of New Hampshire (Houle, et. al., 2012) compiled the results of a 6-year study (2004-2010) on maintenance. The primary maintenance activity indicated for the permeable asphalt F-20
parking lot in this study was pavement vacuuming. In this study the permeable asphalt system was found to have the lowest maintenance burden in terms of personnel hours and second lowest annual maintenance costs when compared to the seven other stormwater control measures included in the study. The lowest annualized maintenance costs expressed as a percentage of capital costs were permeable asphalt (4%) followed by the vegetated swale (6%), the subsurface gravel wetland (8%), and the bioretention systems (8%). At this rate, annual LID system maintenance expenditures equal total upfront capital costs after 24.6 years for the permeable asphalt system. Additionally, researchers have found that vacuum sweeping of PFCs does not appear to be necessary as a result of the pressure/suction effects of high speed vehicle traffic (Stanard 2007). However, it has not been established whether similar effects would be observed in full depth permeable pavements since drainage conditions below the permeable top course are different. Stormwater managers in the Pacific Northwest have (Personal Communication, Mark Maurer) have noted that they have observed problems with moss growth in permeable pavements at multiple sites that significantly reduced the infiltration rate and caused slip hazards. They have not found any method other than moss poisons that will remove the moss. When moss is pressure washed to remove the surface, the roots stay in the pores and regrow quickly. They are having to pressure wash to clean some sections 3 to 4 times a year, which is a significant maintenance burden. While this suggests that maintenance should not pose a barrier for permeable pavements in general, there are specific maintenance issues exist with respect to the urban highway environment that may represent significant barriers in some cases. Table 5 summarizes the typical maintenance requirements for permeable pavement installations and considerations of implementation in the urban highway environment. Table 5. Summary of Maintenance for Permeable Pavement Maintenance Activity Summary of Available Guidance Considerations Specific to the Urban Highway Environment Pavement Vacuuming ⢠Typical practice is to specify vacuuming the pavement surface twice per year in the late fall and early spring ⢠Vacuuming typically is performed at low speeds and therefore may require special provisions (e.g., traffic control) in a highway setting ⢠Twice per year is recommended for typical applicationsâin areas that receive unusually high amounts of sediment, vacuuming should be done more frequently ⢠Self-cleaning aspects of PFC notes by researchers under high speed traffic may apply to some degree for full depth pavements located in travel lanes Pressure Washing ⢠Studies using PFC have shown good results from vehicles that contain both pressure washing and vacuum equipment to remove accumulated particles in the pavement surface ⢠Pressure washing is not likely practical in the urban highway environment due to the area required to be treated and the time required to implement treatment Moss Removal ⢠In wet climates, moss can develop on permeable pavement and severely limit its permeability. Moss removal would require frequent pressure washing and vacuuming, which may still not be effective. ⢠Moss removal activities may be cost prohibitive and in the urban highway environment. ⢠While this phenomenon may occur in only limited geographic regions and locations within a project (e.g., shaded spot, with poor exposure), this could be a fatal flaw for the use F-21
Maintenance Activity Summary of Available Guidance Considerations Specific to the Urban Highway Environment and upkeep of permeable pavements. Maintenance Costs ⢠In 2012, the University of New Hampshire (Houle, et. al., 2012) compiled the results of a 6-year study (2004-2010) for seven different types of stormwater control measures, including a permeable asphalt pavement system, logging all inspection hours and maintenance activities. They calculated a cost of $2700/ha/yr for permeable pavements, the lowest of the annual maintenance costs for LID systems and lower than wet and dry ponds by more than 50 percent ⢠Others (e.g., Narayanan and Pitt, 2006) have found relatively minor incremental cost of permeable pavement maintenance compared to traditional pavement. ⢠Maintenance costs in the urban highway environment would be dependent on: o Need to close lanes for sweeping o Specialized equipment and operators compared to standard DOT equipment o Need for lane closure for remedial actions in the event of a contaminant spill ⢠Given that maintenance is critical for long-term operation, in some conditions, maintenance costs and practicality could be a significant limitation for use of permeable pavements in urban highway environments. Asset Tracking and Management ⢠Like components of other stormwater management systems, permeable pavements need to be tracked and provisions need to be in place to ensure that activities on top of or adjacent to these systems do not compromise their function (e.g., slurry seal over permeable pavement) ⢠DOTs typically have well established asset management systems, however permeable pavement have special considerations that may not fit within these systems. Systems should be adapted to permeable pavement before it is used on a widespread basis 3.1.12 Cold Climate Performance and Winter Maintenance Issue: Will freeze/thaw crack and damage permeable pavements? Does permeable pavement function in colder climates? Does winter sanding and deicing clog the pavement surface? Do winter snow removal activities (plowing) destroy the permeable pavement surface? Winter performance and maintenance is a common concern raised in in the context of permeable pavements and has been relatively well studied. Permeable pavements have been found to be more resistant to freezing than standard pavements due largely to the rapid disconnection from subsurface moisture and from rapid thawing due to rapid infiltration of meltwater (Backstrom, 2000). Roseen and Ballestero (2008) state that if designed properly, permeable pavements have demonstrated cold-climate performance, including drainage under freezing conditions, exceeding conventional practices by measures of both water quality and hydraulics. Abrasives such as sand or cinders for winter traction must not be applied on or near the permeable pavement. While sanding is not recommended on PFC or full depth pavements, sanding is believed to occur on PFC applications in some states. However, the potential impacts of sanding PFC are different than full depth permeable pavements. First, in the event of clogging of PFC with sand, surficial drainage still occurs and many of the intended benefits of PFC are maintained. However, in full depth permeable pavement applications, sand has the potential to clog the surface course and/or migrated into the subbase and cause clogging there, either of which would significantly alter the intended hydrologic performance. As such, it is recommended that abrasives not be used at all on sites with permeable pavement to minimize sand being tracked onto the pavement by vehicles and to prevent accidental applications to the permeable asphalt surfaces. As with any pavement, frequent snow plowing is an essential component of maintenance in cold climates but should be done carefully to avoid damaging the surface with heavy F-22
equipment (Roseen, et.al., 2013). Additionally, in areas where studded tires are allowed and heavily used, frequency of maintenance of both traditional and pervious pavements is expected to increase. Permeable pavement is less likely to form black ice and often require less plowing and fewer deicing chemicals. University of New Hampshire Stormwater Center found that a permeable asphalt parking lot reduced the salt needed for winter maintenance by approximately 50 to 75 percent that of a standard impermeable asphalt parking lot (Roseen et al 2013; UNHSC, 2010). Permeable asphalt pavements can be more challenging to de-ice if compacted snow and ice develop. The reduction in the amount of deicing materials is primarily observed with respect to the reduced development of black ice. Success will vary by application and sun exposure. Ultimately, if reduced winter deicing practices are acknowledged (UNHSC 2009b), permeable asphalt could have a winter maintenance cost benefit. Permeable concrete has not been commonly used in cold climates because of concerns related to the effects of chloride deicers on curing and strength. Recent research (Schaefer et al. 2011) concluded that these concerns can be addressed in part. However, this topic requires further research and should be considered in selecting between permeable asphalt and concrete. 3.2 Summary of Potential Urban Highway Opportunities and Limitations What type(s) of project conditions present high opportunity for permeable pavement applications? What site information is needed to help determine whether permeable pavement is applicable? As introduced in Section 3.1, the applicability of full depth permeable pavement for urban highways depends on a wide range of factors. While permeable pavement technologies and design/QC approaches have evolved and will continue to evolve, there are certain factors that apply in some cases that cannot be overcome. As a result, permeable pavements may be prohibitive for some project conditions and applications, but favorable for others. The purpose of this section is to synthesize the topics discussed in Section 3.1 to provide concise guidance for evaluating the potential suitability of permeable pavement for site-specific conditions. Table 6 summarizes the factors that influence permeable pavement applicability and identifies data that can be relevant to help decide whether permeable pavements are a suitable and favorable VRA. Table 6 also summarizes the information that needs to be collected to support this evaluation. Table 7 provides a checklist for documenting the results of site-specific screening of permeable pavements. Table 6. Summary of Opportunities and Limitations for Full Depth Permeable Pavement Factors Influencing Applicability Site Assessment and Project Characterization Needs Favorable Conditions Unfavorable/Challenging Conditions F-23
Factors Influencing Applicability Site Assessment and Project Characterization Needs Favorable Conditions Unfavorable/Challenging Conditions Soil and Geotechnical Factors (strength, permeability, and hazards) ⢠Soil texture ⢠Infiltration rates at various degrees of compaction ⢠Assessment of strength under wetted conditions ⢠Granular soils tend to be more favorable ⢠Soils that display limited loss in strength when wet ⢠Soils that maintain some permeability when compacted ⢠Fine grained and plastic soils are more challenging ⢠Soils that exhibit shrink/swell properties ⢠Soils that exhibit substantial loss of strength when wet may require major ⢠Soils that exhibit substantial loss of permeability when compacted (less than approximately 0.01 inches/hour Project Strength and Durability Requirements ⢠What are the pavement system strength and durability requirements for the intended application? ⢠What are is the strength of the underlying soil when saturated? ⢠Lower strength requirements, such that thickness of subbase required for hydrologic goals would meet strength goals ⢠Shoulder applications ⢠Higher strength requirements that would require expensive additives and design features (subbase thickness) or cannot be achieved with currently available technologies ⢠Travel lane applications Groundwater Conditions/ Contamination Potential ⢠Depth to seasonably high groundwater table ⢠Existing soil contamination ⢠Soil texture, organic content, permeability ⢠Use of deicing chemicals ⢠Greater than 2-10 feet between reservoir bottom and seasonally high groundwater table (varies by jurisdiction) ⢠Steeper local groundwater gradient helps dissipate mounding ⢠Soils provide natural pollutant attenuation via organic content and cation exchange ⢠Aquifer recharge areas where infiltration could impact groundwater quality ⢠Natural or anthropogenic soil contamination or plumes ⢠Shallow seasonally high groundwater table and coarse inert soils ⢠Flatter groundwater gradient; higher mounding potential Hazardous Material Spill Liability ⢠Potential for hazardous spills based on surrounding land uses that could require removal of pavement and subbase ⢠Typical urban highway projects without elevated risk factors ⢠High use sites with elevated volume of truck traffic and/or industrial activity ⢠Where the risk of concentrated pollutant spills is more likely such as gas stations, truck stops, and industrial chemical storage sites Pavement Surface Drainage Requirements ⢠What are the applicable drainage design requirements? ⢠Can supplemental drainage pathways be provided? ⢠Design drainage flowrates (for peak events) are much less than capacity of permeable pavement (allowing margin of safety for clogging) ⢠Supplemental overflow pathways are possible ⢠Any condition where clogging of permeable pavement could create a condition where travel lanes do not drain to meet applicable drainage design criteria F-24
Factors Influencing Applicability Site Assessment and Project Characterization Needs Favorable Conditions Unfavorable/Challenging Conditions Cold Climate ⢠Freeze-thaw potential ⢠Winter maintenance practices ⢠Agency policies on level of service during cold weather ⢠Temperate climates ⢠If cold climate, then sanding should not be used ⢠If cold climate, Agency would consider permeable pavement as a traction improvement to reduce salt application ⢠Cold climates with sand usage and/or institutional barriers to alternative maintenance practices ⢠Extensive use of studded traction tires Moss Growth Potential ⢠Observation of moss growth on roadways in the project vicinity/region ⢠Exposure of site ⢠Any region where moss growth is not a concern ⢠Sunny sites, with good wintertime exposure and limited potential for tree canopy growth to change exposure conditions ⢠Regions that experience problems with moss growth on shoulders ⢠Sites with poor exposure, particularly with potential for additional shading as trees grow ⢠Roadway segments below grade that maximizes winter shading Type of Highway Segment ⢠Width of shoulder/median ⢠Adjacent slopes ⢠Is back-up drainage critical? ⢠Wider shoulder/median helps provide greater area for permeable shoulders and low- speed sweeping practices ⢠Relatively mild cross slope helps reduce geotechnical issues ⢠Areas where back-up drainage is not needed ⢠Segments on berms, or in highly constrained sections ⢠Segments on steeper cross slopes ⢠Sag segments or constrained segments where supplemental drainage must be provided as a failsafe for peak flow conveyance Type of Construction ⢠New construction or refinishing / retrofit ⢠Export and import needed to rebuild subbase ⢠Existing drainage/treatment infrastructure already in place ⢠New construction or lane additions where use of permeable pavement can avoid traditional asphalt and conveyance/treatment costs or retrofit construction where subbase is already being re- built ⢠Rebuilding subbase would be a major additional cost ⢠Drainage infrastructure is already in place and would not otherwise need to be upgraded ⢠Treatment already provided and would not otherwise need to be upgraded Longitudinal Grade ⢠Review site plans ⢠Flatter segments (less than 1 to 2 percent) where internal berms/fabric cutoff walls could have a greater spacing ⢠Steeper segments greater than 2 percent pose more challenges for construction, but may be feasible. ⢠In some cases, the compounding influence of decreased infiltration potential and increased frequency of berms could render permeable pavement cost prohibitive as a result of longitudinal grade F-25
Factors Influencing Applicability Site Assessment and Project Characterization Needs Favorable Conditions Unfavorable/Challenging Conditions Adjacent Ground Cover ⢠Site plans ⢠Characterization of area draining to permeable pavement ⢠Permeable pavement accepts runoff from only impervious areas ⢠Vehicle tack-on from adjacent areas is limited ⢠Landscaped/pervious areas drain toward permeable pavement (e.g., depressed segment) ⢠Areas with disturbed soils drain toward permeable pavement ⢠Road experiences long-term track-on from adjacent streets/land uses Department Acceptance/ Experience ⢠Evaluate local case studies and research ⢠Review local specifications and design guidance ⢠Local experience and familiarity with permeable pavement design and maintenance ⢠Prior successful installations ⢠Interest in permeable pavement as a pilot project ⢠Negative perceptions about permeable pavement technologies ⢠Lack of experience and/or prior installations Local Contractor Experience and Owner QA/QC Experience ⢠Evaluate past experience/performance with permeable pavement ⢠Visit previous installations and evaluate current performance ⢠Evaluate owner construction QA/QC protocols ⢠Contractor has sufficient past experience (1+ successful installations) ⢠Contractorâs past sites appear to be functioning properly ⢠Owner has experience with permeable pavement testing and acceptance (or can obtain it) ⢠Contractor has no experience installing permeable pavement ⢠Past installations of permeable pavement conducted by Contractor have failed or are exhibiting attributes of failure and contractor has not changed its methods to correct the issues ⢠Owner does not have processes in place to control quality of construction Local Supplier Experience ⢠Evaluate potential local sources of specialized permeable asphalt and concrete mixes ⢠Suppliers are familiar with mixes and willing to develop custom batches ⢠Existing installations have been successful ⢠Suppliers are not familiar with specialized mixes and/or are resistant to developing specialized mixes ⢠No history of successful implementations F-26
Factors Influencing Applicability Site Assessment and Project Characterization Needs Favorable Conditions Unfavorable/Challenging Conditions Owner Maintenance Equipment, Capabilities, and Asset Management ⢠Evaluate personnel resources available for pavement maintenance ⢠Evaluate financial resources available for pavement maintenance ⢠Evaluate current asset management and tracking framework ⢠Budget available to purchase a vacuum street sweeper for department use and staff available to operate equipment according to maintenance schedule OR budget available to hire an outside company to conduct regularly scheduled maintenance ⢠Systems in place to track installations and maintenance and ensure that they are not compromised by other road treatments ⢠Budget not available to purchase vacuum street sweeper or to hire an outside company to conduct required maintenance ⢠Systems not in place to track installations and maintenance and/or significant justified concerns that installations would be compromised by other road maintenance DOT and/or Local Contractor Experience with Maintenance ⢠Are maintenance contractors available in the location with experience maintaining permeable pavement? ⢠Can the DOT self preform the maintenance? ⢠Contractors and/or DOT staff with experience ⢠No contractors or DOT staff with experience Table 7. Checklist of Permeable Pavement Selection and Design Considerations Permeable Pavement Selection and Design Considerations Results of Project-Specific Screening (check the box that applies) Evaluated - not an issue Potential issues must be addressed in design, spec, and construction Prohibitive Soil and Geotechnical Factors Project Strength and Durability Requirements Groundwater Conditions/ Contamination Potential Hazardous Material Spill Liability Pavement Surface Drainage Requirements Cold Climate Moss Growth Potential Type of Highway Segment Type of Construction Longitudinal Grade Adjacent Ground Cover Department Acceptance/ Experience Local Contractor Experience F-27
Permeable Pavement Selection and Design Considerations Results of Project-Specific Screening (check the box that applies) Evaluated - not an issue Potential issues must be addressed in design, spec, and construction Prohibitive Owner QA/QC Experience/Protocols Local Supplier Experience Owner Maintenance Equipment, Capabilities, and Asset Management/Tracking DOT and/or Local Contractor Experience with Maintenance 3.3 Permeable Shoulders versus Travel Lane Opportunities Generally, DOT-related practice and research tends to be emphasizing opportunities for permeable shoulders rather than full width permeable pavement across travel lanes. Shoulders tend to have lower traffic load and are more accessible for maintenance, which may be compelling reasons to emphasize shoulders. Shoulders may provide adequate area for stormwater management. The compendium of factors introduced above suggests that shoulders likely provide a better balance between benefits and constraints. However, some advantages of permeable pavement in travel lanes should also be considered in deciding where to implement permeable pavements. Table 8 provides a summary of advantages and disadvantages associated with permeable shoulders and permeable travel lanes. Table 8. Comparison of Opportunities for Permeable Shoulders and Permeable Travel Lanes Advantages Disadvantages Permeable Shoulders ⢠More widely accepted and studied ⢠Tend to have lower traffic load ⢠May support lower levels of compaction in some cases ⢠Tends to provide better access for maintenance ⢠Typically adequate area to capture and manage a water quality design volume ⢠Failure less likely to have impact to travel lanes ⢠Permeable shoulders may require higher infiltration rates to achieve the same level of volume reduction due to smaller footprint and run-on from adjacent areas ⢠Can be prone to clogging, particularly if maintenance is limited or high sediment production from adjacent travel lanes or landscaping Permeable Travel Lanes ⢠Can achieve higher levels of volume reduction in lower infiltration rate environments ⢠Experience suggests that water pressure from high speed traffic may help keep pores open ⢠Can realize other benefits such as reduction in splash, reduction in noise, safer cold weather conditions, and better ride quality ⢠Less widely accepted/studied ⢠Must design for higher loads and higher traffic volumes ⢠Design lives expected to be shorter, particularly in areas with significant studded tire use ⢠Maintenance with vacuum sweeper would require lane shut down ⢠Remediation of spilled contaminants would require lane(s) to be shut down F-28
4 General Approach for Permeable Pavement Design Integration What is a recommended work flow to help reduce complexity and foster common understanding? Based on the considerations described in Section 3, permeable pavements may be applicable in some cases in the urban highway environment. In cases where project conditions appear favorable for permeable pavement and the design team has selected permeable pavement as a project VRA, how should the design team approach the challenges associated with designing of these systems? How can barriers associated with design complexity be overcome? This section is intended to introduce a general stepwise process for helping design teams incorporate permeable pavements into urban highway designs. This process relies heavily on the project teamâs familiarity with existing design guidance, such as AASHTO publications, and primarily discussed how this guidance can be adapted to accommodate permeable designs. 4.1 Conceptual Steps for Design Integration As introduced above, the design permeable pavement is fundamentally different from other VRAs, as permeable pavement serves a dual role â as a structural surface for vehicular traffic and as a hydrologic/hydraulic control. This can complicate the design process. Once the design team has decided to utilize a permeable pavement design or at least developed a detailed design for evaluation, the following steps have been developed to help provide guidance for approaching this process as a project design team. Step 1 â Site Characterization and Screening of Permeable Pavement Suitability. Successful permeable pavement design and installation begins with good site characterization and screening as described in Section 3. Other alternatives should be considered as part of the overall screening and prioritization process described in the Guidance Manual. The project team should arrive at permeable pavement as a potential option after conducting an initial site suitability and feasibility screening process. Implementing permeable pavement also requires a commitment to coordination between multiple design disciplines as well as a commitment to maintenance of the finished product. When permeable pavement is being considered, it is recommended that a design workshop be conducted at the outset of the design process to obtain multiple discipline and department buy-in for the coordination needed to implement permeable pavement. While each agency may approach this step differently, this step should not be overlooked. Step 2 â Develop Initial Hydrologic and Structural Designs. Hydrologic and structural designs are based on fundamentally different considerations; and inherently require iterations to balance these considerations. As an initial step, these disciplines may work independently to develop designs that meet the respective functional purpose. As a design team gains familiarity with respective criteria that apply to each discipline, the need for iterations may decrease. Sections 4.2 and 4.3, below, provide guidance for conducting each of these analyses. Step 3 â Compare Structural and Hydrologic Designs and Develop Design Modifications to Balance Structural and Hydrologic Considerations. Differences between the design requirements for the respective functional purposes must be compared and reconciled to yield a design that effectively and efficiently serves both design functions. A number of adjustments, such as increasing subgrade compaction, including different structural elements, and other factors may help balance these F-29
considerations. Section 4.4, below, provides guidance for adapting design features to balance these considerations. Figure 1 illustrates this conceptual design process. Figure 1. Conceptual Process for Permeable Pavement Design 4.2 Overview of Structural Design Approach Structural design approaches differ between flexible pavements (i.e., asphalt and paver blocks) and rigid pavement (i.e., concrete). Fortunately, well proven methods used for the design of traditional pavements can be adapted to the design of their permeable counterparts through the use of different input Develop Initial Hydrologic Design Establish Design Infiltration and/or Discharge Rates Calculate Storage Requirements Calculate Reservoir Thickness Characterize Site Conditions Develop Project Layout VRA Suitability Screening, Feasibility Screening, and Consideration of VRA Options Evaluate Other Factors Develop Initial Structural Design Establish Subgrade Strength when Saturated Calculate Subbase/Pavement Strength Requirements Subbase Layer Thicknesses/ Profile Establish Hydrologic Design Criteria Establish Structural Design Criteria Compare Pavement Profile Required for Hydrologic to Pavement Profile Required for Structural Considerations Develop Modified Design to Balance Hydrologic and Structural Considerations Criteria Satisfied Efficiently? No Yes Develop Plans and Specifications Step 3 Step 2 Step 1 Tentative Decision to Use Permeable Pavement based on Input from Multiple Design Disciplines and Maintenance Department F-30
parameters. In general, permeable materials are not as strong as their traditional counterparts, and a higher degree of saturation in the subgrade material must be considered, which can reduce bearing strength in some types of soil. Finally, because permeable materials have not been as well tested as their counter parts, designs typically include factors of safety, and/or are designed for higher reliability. Design guidance and tools for flexible and rigid permeable pavement design can be found from the respective trade organizations: www.asphaltpavement.org/index.php?option=com_content&view=article&id=359&Itemid=863 http://acpa.org/PerviousPave/ These design approaches, and adaptations for permeable pavement design, are introduced below. Flexible Pavement Many local transportation agencies use the empirically-based AASHTO 1993 Guide for Design of Pavement Structures whose underlying concepts emerged from test pavements (many with dense-graded bases) in the 1950s repeatedly trafficked by trucks that established relationships among materials types, loads and serviceability. The AASHTO equation in the 1993 Guide calculates a required Structural Number or SN given traffic loads, soil type, degree of compaction, climatic and soil moisture conditions, and associated assumptions about subbase strength (ASCE, 2013 DRAFT). The designer then finds the appropriate combination of pavement surfacing and base materials whose strengths are characterized with layer coefficients. When these layer coefficients are added together, the sum product of the coefficients (ai) multiplied by the pavement material thicknesses (di) should meet or exceed the SN required for a design (where SN = Σai à di). This empirical design approach is applicable to permeable pavement with consideration given to layer coefficients that represent permeable pavement system components (ASCE, 2013 DRAFT). Table 9 provides recommended AASHTO layer coefficients for structural evaluation of permeable asphalt. Based upon the recommended SNs the recommended minimum thicknesses for the permeable asphalt surface for roadway traffic loadings ranges from a minimum of 4.0 inches for a residential street (some truck traffic) to 6.0 inches for heavy truck traffic. Table 9. AASHTO Layer Coefficients for Permeable Asphalt Systems Material Layer Coefficients in Traditional Design Layer Coefficients in Permeable Design Top Course Asphalt 0.40 â 0.44 (Dense mix) 0.2 - 0.3 (permeable mix, possibly higher with admixtures)1 Asphalt Treated Permeable Base (ATPB) 0.30 â 0.35 0.30 â 0.35 Open-Graded Aggregate Base Layers 0.12 â 0.14 (Dense Graded) 0.06 â 0.09 (Open Graded) Source: Hansen, 2008; Hein, 2006; Hein, et al. 2013. 1The Oregon Department of Transportation uses the same structural coefficient for PFC as dense-graded HMA (NAPA, 2002). Rigid Pavement AASHTO, ACI, or PCA design procedures for rigid pavements are appropriate for permeable concrete design, however strength and durability relationships have not been as well established or field verified, F-31
so designs should provide a higher design conservatism (Delatte, 2007). Pavement designers should assume lower flexural strength, lower modulus of elasticity, and should characterize subgrade design strength assuming saturated conditions. Cackler et al. (2012) summarized that results reported in the literature show a linear decrease in modulus of elasticity values for permeable concrete with increased void content, with a similar to relationships for strength. Reported modulus of elasticity for permeable concretes with 25% voids was around 3.6 million psi (24,800 MPa) and decreased to 1.5 million psi (10,300 MPa) at 40% voids, for mixtures using limestone aggregate (Crouch et al., 2007). Consistent with Cacklerâs summary, Hein et al. (2013) states that the flexural strength of permeable concrete typically ranges from about 2 to 3 MPa (290 to 435 psi) in comparison to about 4.5 to 6.5 MPa (650 to 945 psi) for traditional concrete. 4.3 Hydrologic Considerations In addition to the structural considerations, the pavement system must be sized to meet the hydrologic/stormwater management design requirements required by site-specific regulations. To determine the storage required for stormwater management, all system components including infiltration rate of the underlying soils and underdrain system outflow must be taken into consideration. Similar to the other VRAs described in these guidelines, permeable pavement can be considered to consist of a storage volume and an associated time and pattern by with that storage volume is discharged. Using the âVolume Performance Toolâ developed as part of these Guidelines, hydrologic analysis can be conducted to estimate long-term volume reduction as a function of local climate and various design parameters. Other hydrologic design approaches, such as continuous simulation or event-based simulation, using locally-approved methods can be used to evaluate the storage and discharge characteristics needed to meet hydrologic design goals/requirements. This will be expressed in terms of a stone reservoir layer and outlet elevations and sizes. However, for evaluation of long-term performance of surface runoff volume reduction, a method that utilizes long-term simulations of precipitation and runoff/infiltration is required. For design purposes, an appropriate factor of safety should be applied to the measured site soil infiltration rate to account for compaction of subgrade and long-term clogging effects (see White Paper #1). The design infiltration rate may be influenced by the type of soils encountered, the degree of compaction required, and the amount of anticipated sediment loading from run-on from traditional pavement. In some hydrologic design processes (such as a continuous simulation performance-based design process, the infiltration rate may be a factor in determining the required storage volume. 4.4 Balancing Structural and Hydrologic Considerations in Permeable Pavement System Design The result of the structural design process will be layer thicknesses, material types, and associated assumptions about the degree of compaction of subbase materials. Similarly, hydrologic design requirements will be expressed in terms of a thickness of subbase and assumed compaction/permeability of subgrade materials. The hydrologic design process also includes assumptions about the slope of the bed, outlet controls/underdrains, permeability of top course and filter materials, if used, and the porosity of subbase reservoir. As such, the overlapping design parameters primarily include: subbase thickness, subbase strength (which are a function of porosity), and subgrade strength (which may be a function of degree of compaction and saturation). A âbalancedâ design is found when both structural and hydrologic criteria are met, and the overlapping parameters converge such that the design achieves efficiency. An imbalanced design would occur when F-32
one design function results in a depth much greater than needed to serve the other design function. If the base/subbase thickness required for hydrological design is significantly thicker than required for structural capacity, the designer may modify some of the design parameters to make the design more cost effective. This may include: ⢠Modify the base/subbase materials to increase porosity to provide greater storage (but sacrificing strength). ⢠Modify the base/subbase materials to increase permeability (e.g., less compaction) to permit more rapid drainage of water from the pavement section (but sacrificing strength). ⢠Increase the frequency, diameter or slope of outlet pipes to increase supplemental water outflow. ⢠Increase the area of permeable pavement to non-permeable pavement if a shoulder application or otherwise includes significant run-on from up-slope non-permeable pavements. If the base/subbase thickness required for structural design is significantly thicker than required for hydrological design, this may result in excess capacity for hydrologic functions. The designer can choose to accept this surplus as a factor of safety against long-term clogging, or could pursue one of the following options to increase strength and reduce hydrologic function as part of balancing the design: ⢠Increase the thickness of the surface layer. These layers have a higher structural capacity than the base/subbase layers. Minor increases in the thickness of these layers may allow a reduction in the thickness of the subbase and associated storage volume. ⢠Add an ATBP layer, if designing permeable asphalt, to help reduce the required thickness of the based and associated storage volume. ⢠Improve the quality of the base/subbase layers to provide a higher layer coefficient, while potentially reducing the porosity (thus sacrificing some volume). ⢠Providing additional compaction of the subgrade, thus potentially increasing strength by potentially reducing the infiltration rate. A number of other factors, such as interface with adjacent pavement structures, may also control design. Ultimately, a balanced design is not essential, as long as both structural and hydrologic design criteria are met at a cost that is determined to be feasible and affordable. F-33
5 Options for Further Research What are high priority needs and opportunities for further research? Areas of future research can generally be categorized as: ⢠Water quality and quantity performance ⢠System design ⢠Construction methodologies ⢠Quality control testing ⢠Maintenance needs Many studies have been conducted and continue to be conducted on the design and installation of permeable asphalt systems. Most studies focus on full-depth permeable asphalt designed to manage stormwater from low to moderate traffic parking areas, sidewalks, pathways and driveways. Studies and implementation of full-depth permeable asphalt systems on low-speed/low volume roadways and highways is lacking. Further research is needed on roadway and highway installations of full-depth permeable asphalt to obtain a greater depth of knowledge on the cost and operation and maintenance required for these systems long term. Additionally, the water quality benefits of full-depth permeable asphalt require further research; specifically water quality information from underdrained permeable asphalt systems. These systems act as underground detention systems but preliminary research indicates greater water quality benefits than many traditional stormwater detention systems. Potential additional research topics include: ⢠Evaluation of permeable asphalt mix designs and admixtures ⢠Development and adaptation of a field QC standard for compaction testing of permeable asphalt ⢠Development and adaptation of a laboratory QC standard for pavement durability testing ⢠Evaluation of compaction methods for permeable asphalt pavements ⢠Development of vacuum sweepers specifically for the high volume maintenance of permeable pavements ⢠Recommended curing time for permeable asphalt as a function of pavement strength and temperature ⢠Study of road sanding and permeable pavements F-34
6 Summary Permeable pavements have a number of potentially compelling benefits in the urban highway environment. However, they also have a number of key limitations that may preclude their use in many conditions. As with all stormwater controls, the long-term success of permeable pavements depends on (1) using permeable pavements only in locations that are suitable for use, (2) careful design, specification, and construction QA/QC of permeable pavements to meet structural and hydrologic objectives, and (3) effective ongoing maintenance. If project circumstances do not allow for these requirements to be met, then permeable pavements are not a good choice for the project. However, where these barriers do not exist or can be overcome, permeable pavements may provide cost and effectiveness benefits (compared to traditional stormwater controls) that justify their usage. F-35
7 References AASHTO (1993). Guide for Design of Pavement Structures, Washington, D.C. American Concrete Institute (ACI) (2008). "Specification for Permeable Concrete Pavements." 522.1-08, Committee 522. American Concrete Paving Association (ACPA) (2012). Pervious Pave â Background, Purpose, Assumptions and Equations, Washington, D.C. Alvarez, A.E., and A.E. Martin. (2009). âPermeable Friction Course Mixtures are Different.â Rocky Mountain Asphalt Conference & Equipment Show, Colorado State University, Fort Collins, CO. ASCE (2013 in draft). Recommended Design Guidelines for Permeable Pavements. Manual of Practice on Recommended Design Guidelines for Permeable Pavements, B. Eisenberg, K. Lindow, and D. Smith, eds., American Society of Civil Engineers, The Permeable Pavements Technical Committee, Low Impact Development Standing Committee, Urban Water Resources Research Council, Environment and Water Resources Institute. Co-authors: Bean, E. Z., Braga, A. M., Clar, M. L., Lindow, K. C., Early, K., Eisenberg, B. E., Fassman, E. A., Gwilym, K. M., Haselbach, L. M., III, W. F. H., Karimipour, S., Kevern, J. T., Kunkler, L., Lowry, T. P., Perry, S. D., Potts, A., Putz, R., Roseen, R., Rowe, A., and Smith, D. R. ASTM (2012). Standard C-1754/1754M, âStandard Test Method for Density and Void Content of Hardened Permeable Concrete,â Annual Book of ASTM Standards Vol. 4(2), ASTM International, ASTM International, West Conshohocken, PA. Backström, M. (2009). Permeable Pavement in a Cold Climate, Licentiate Thesis, Lulea University of Technology, Lulea, Sweden. http://epubl.luth.se. Barrett, M. E. (2008). âEffects of Permeable Friction Course (PFC) on Highway Runoff.â 11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008. Barrett, M.E. (2006). âStormwater Quality Benefits of a Permeable Friction Course.â World Environmental and Water Resources Congress 2006: Examining the Confluence of Environmental and Water Concerns, American Society of Civil Engineers, Reston, VA. Barrett, M.E. and C.B. Shaw (2007). âBenefits of Porous Asphalt Overlay on Storm Water Quality.â Transportation Research Record: Journal of the Transportation Research Board, No. 2025, Transportation Research Board of the National Academies, Washington, D.C. Bean, E., W. Hunt, and D. Bidelspach (2007). âField Survey of Permeable Pavement Surface Infiltration Rates.â Journal of Irrigation and Drainage Engineering. May/June 2007 Briggs, J., A. Gong, E. Hadley, R. Roseen, and T. Ballestero (2008). âPermeable Pavement Sites In New England: 2007 Assessment Of Infiltration Capacity & Condition.â University of New Hampshire Stormwater Center, Durham, NH. Cackler, T., J. Alleman, J. Kevern, and J. Sikkema (2012). Technology Demonstrations Project: Environmental Impact Benefits with âTX Activeâ Concrete Pavement in Missouri DOT Two-Lift Highway Construction Demonstration. Final Report I, October 2012. Federal Highway Administration (DTFH-61-06-H-00011 (Work Plan 22)). http://www.intrans.iastate.edu/research/documents/research- reports/TX_Active_for_FHWA_w_cvr.pdf F-36
Chai, L., M. Kayhanian, B. Givens, J. Harvey, and D. Jones (2012). âHydraulic Performance of Fully Permeable Highway Shoulder for Storm Water Runoff Management.â J. Environ. Eng., Vol. 138 Issue 7, pages 711â722. http://dx.doi.org/10.1061/(ASCE)EE.1943-7870.0000523. Cooley, L.A., J.W. Brumfield, R.B. Mallick, W.S. Mogawer, M. Partl, L. Poulikakos, and G. Hicks (2009). NCHRP Report 640: Construction and Maintenance Practices for Permeable Friction Courses, Transportation Research Board of the National Academies, Washington, D.C. Crouch, L., J. Pitt, and A. Hewitt (2007). âAggregate Effects on Pervious Portland Cement Concrete Static Modulus of Elasticity,â Journal of Materials in Civil Engineering, Vol. 19, No. 7. Delatte, N. (2007). âStructural Design of Permeable Concrete Pavements,â Presented at 86th Annual Meeting of the Transportation Research Board Annual Meeting, Washington, D.C., 16 pages. Estakhri, C.K., A.E. Alvarez, A.E. Martin (2008). âGuidelines and Construction and Maintenance of Porous Friction Courses in Texas.â Report 0-5262-1, Texas Transportation Institute, Texas A&M University. Ferguson, B. (2005). Permeable Pavements, CRC Press, Boca Raton, FL. Federal Highway Administration (FHWA) (1990). Open Graded Friction Courses. FHWA Technical Advisory T5040.31, Washington D.C. Gunderson J. (2010). âBoulder Hills LID Economic Case Study.â Forging the Link Between Research- Based Institutions, Watershed Assistance Groups, and Municipal Land Use Decisions, UNH Stormwater Center, Durham. Hansen K. (2008). Permeable Asphalt Pavements for Stormwater Management; Design, Construction and Maintenance Guide. National Asphalt Pavement Association, pages 19â21. Hein D. (2006). Resilient Modulus Testing of Open Graded Drainage Layer Aggregates for Interlocking Concrete Block Pavement, 8th International Conference on Concrete Block Paving, San Francisco, CA. Hein, D., E. Strecker, A Poresky, R. Roseen and M. Venner (2013) Permeable Shoulders with Stone Reservoirs, Draft Report, Project Number NCHRP 25-25/Task 82. http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=3315 Hodgman, R. (2012). "Meeting notes regarding costs of 2009 installation of the Maine Mall Road, South Portland, ME." R.M. Roseen, ed., Maine DOT Transportation Research Program, Augusta, ME. Hossain, M., L.A. Scofield, and R.W. Meier Jr., (1992). âPorous Pavement For Control Of Highway Runoff In Arizona: Performance To Date.â Transportation Research Record 1354, TRB, National Research Council, Washington, D.C., pages 45â54. Houle, J.J., R.M. Roseen, T.P. Ballestero, T. Puls, and J. Sherrard, J. (2013 in press). âA Comparison of Maintenance Cost, Labor Demands, and System Performance for LID and Conventional Stormwater Management.â Journal of Environmental Engineering, American Society of Civil Engineers. Jones, D., J. Harvey, H. Li, and B. Campbell (2010). Summary of Laboratory Tests to Assess Mechanical Properties of Permeable Pavement Materials. Caltrans Document No.: CTSW-TM-10-249.01, November 30, 2010. http://www.ucprc.ucdavis.edu/PDF/UCPRC-TM-2009-05.pdf. Kandhal, P. and R. Mallick (1999). Design of new-generation open-graded friction courses. NCAT Rep. No. 99-3, National Center for Asphalt Technology, Auburn, AL. F-37
Kayhanian, M. (2012). âAdapting Full-Depth Permeable Pavement for Highway Shoulders and Urban Roads for Stormwater Runoff Management.â Stormwater: The Journal for Surface Water Quality Professionals, http://www.stormh2o.com/SW/Articles/Adapting_FullDepth_Permeable_Pavement_for_Highway_18 309.aspx, August 13, 2012, accessed May 2013. Kevern, J.T. and C. Farney (2012). âReducing Curing Requirements for Permeable Concrete Using a Superabsorbent Polymer for Internal Curing.â Transportation Research Record: Journal of the Transportation Research Board, No. 2290, Transportation Research Board of the National Academies, Washington D.C., page 115â121 Liu, K., A. Alvarez, A.E. Martin, T. Dossey, A. Smit, and C. Estakhri (2010). Synthesis of Current Research on Permeable Friction Courses: Performance, Design, Construction, and Maintenance. Report 0-5836-1, Texas Transportation Institute, Texas A&M University. MacMullan, E. (2007). âEconomics of LID.â EcoNorthwest, Eugene, OR. MaineDOT. (2010). Technical Report 10-05: Performance of a Permeable Pavement System on the Maine Mall Road in South Portland. Maine Department of Transportation, Transportation Research Division, Augusta, ME. McGhee, K., T. Clark, and C. Hemp (2009). Research Report: A Functionally Optimized Hot-Mix Asphalt Wearing Course: Part 1: Preliminary Results. Virginia Transportation Research Council, Final Report VTRC 09-R20, Charlottesville, VA. http://www.virginiadot.org/vtrc/main/online_reports/pdf/09-r20.pdf Muench, S.T. and J.P. Mahoney (2002). âWashington Asphalt Pavement Association Asphalt Pavement Guide: Pavement Types.â Washington Asphalt Pavement Association, Inc., http://www.asphaltwa.com/wapa_web/ index.htm (December 16, 2009). Muench, S., C. Weiland, J. Hatfield, and L. Wallace (2011). Open-Graded Wearing Courses in the Pacific Northwest, Final Report, SPR 680. Sponsored by Oregon DOT. http://www.wsdot.wa.gov/NR/rdonlyres/3EB8E603-819F-476F-A54B- 70F71D517EFF/0/OregonDOTClassFReportJune2011.pdf National Asphalt Pavement Association (NAPA) (2002). âDesign, Construction, and Maintenance of Open-Graded Friction Courses.â NAPA. (2003). âDesign, Construction, and Maintenance Guide for Permeable Asphalt Pavements.â Information Series 131. NAPA. (2008). âDesign, Construction, and Maintenance Guide for Permeable Asphalt Pavements.â Information Series 131. Pagotto, C., M. Legret, and P. Le Cloirec (2000). âComparison of the Hydraulic Behaviour and the Quality of Highway Runoff Water According to the Type of Pavement.â Water Resources, Vol. 34, No. 18, pages 4446â4454. Pitt, R., S. Clark, and R. Field (1999). âGroundwater contamination potential from stormwater infiltration practices.â Urban Water, Vol. 1, Issue 3, pages 217â236. Powell, L.M., E.S. Rohr, M.E. Canes, J.L. Cornet, E.J. Dzuray, and L.M. McDougle (2005). âLow- Impact Development Strategies and Tools for Local Governments.â LMI Governmental Consulting. F-38
Rasmussen, R.O., R.J. Bernhard, U. Sandberg, and E.P. Mun (2007). âThe Little Book of Quieter Pavements,â FHWA-IF-08-004, U.S. Department of Transportation, Federal Highway Administration, Washington, D.C. Roque, R., C. Koh, Y. Chen, X. Sun, and G. Lopp (2009). âIntroduction of Fracture Resistance to the Design and Evaluation of Open Graded Friction Courses in Florida.â Florida Department of Transportation and University of Florida, Tallahassee, FL. Roseen, R.M. (2013). âProject meeting notes with materials costs from Aggregate Industries for placement of 800 tons of PG76-22 for Commercial Street, Provincetown, MA,â Stratham, NH. Roseen, R.M. and T.P. Ballestero (2008). âPermeable Asphalt Pavements for Stormwater Management in Cold Climates.â HMAT, Vol. 13, Issue 3, National Asphalt Pavement Association. Roseen, R.M., T.P. Ballestero, J.J. Houle, J.F. Briggs, and J.P. Houle (2011) âWater Quality and Hydrologic Performance of a Permeable Asphalt Pavement as a Stormwater Treatment Strategy in a Cold Climate,â ASCE Journal of Environmental Engineering. Roseen, R.M., T.P. Ballestero, K.M. Houle, D. Heath, and J.J. Houle (2014). âAssessment of Winter Maintenance of Permeable Asphalt and Its Function for Chloride Source Control,â Journal of Transportation Engineering, Vol. 140, Issue 2. San Diego County (2008). Personal communication from Dane Clingan, San Diego County, California. February. Schaefer, V.R., J.T. Kevern and K. Wang. (2011). An Integrated Study of Permeable Concrete Mixture Design for Wearing Course Applications, Final Report, sponsored through Federal Highway Administration DTFH61-06-H-00011 (Work Plan 10) and the Ready Mixed Concrete (RMC) Research & Education Foundation. Schaefer, V., K. Wang, M. Suleiman, and J. Kevern (2006). Mix Design Development for Pervious Concrete In Cold Weather Climates, Sponsored by: Iowa Department of Transportation. http://www.ctre.iastate.edu/reports/mix_design_pervious.pdf Smith D.R. (2011). Permeable Interlocking Concrete Pavements: Selection, Design, Construction, Maintenance, 4th ed., Interlocking Concrete Pavement Institute, Herndon, VA. St. John, M. and R. Horner (1997): Effect of Road Shoulder Treatments on Highway Runoff Quality and Quantity. http://www.wsdot.wa.gov/research/reports/fullreports/429.1.pdf Stanard, C., R. Candaele, R. Charbeneau, and M. Barrett (2007). State of the Practice: Permeable Friction Courses, Report No: FHWA/TX-08/0-5220-1, sponsored by Texas DOT and FHWA. http://www.utexas.edu/research/ctr/pdf_reports/0_5220_1.pdf Texas Department of Transportation. (2004). Standard Specifications for Construction and Maintenance of Highways, Streets, and Bridges, Item 342, 312-329. Retrieved March 20, 2005 from TxDOT Web Site: http://www.dot.state.tx.us/business/specifications.htm Thelen, E. and L.F. Howe (1978). Permeable Pavement, Franklin Institute Press, Philadelphia, PA. University of New Hampshire Stormwater Center (UNHSC) (2009). University of New Hampshire Stormwater Center 2009 Annual Report, Durham, N.H. (http://www.unh.edu/erg/cstev/2009_stormwater_annual_report.pdf ). F-39
UNHSC (2009). âFact Sheet: Permeable Asphalt Pavement for Stormwater Management.â http://www.unh.edu/civil-engineering/research/erg/cstev/pubs_specs_info/ porous_ashpalt_fact_sheet.pdf UNHSC, J. Houle, R. Roseen, and T. Ballestero (2012). âUNH Stormwater Center 2012 Biennial Report.â Cooperative Institute for Coastal and Estuarine Environmental Technology, Durham, NH. United States Environmental Protection Agency (U.S. EPA) (2000). âLow Impact Development, a Literature Reviewâ, EPA-841-B-00-005, Office of Water (4203). Environmental Protection Agency Washington, D.C. Wang, T., J. Harvey, and D. Jones (2010). A Framework for Life-Cycle Cost Analyses and Environmental Life-Cycle Assessments for Fully Permeable Pavements Technical Memorandum, Caltrans Document No.: CTSW-TM-09-249.03, UCPRC Document No.: UCPRC-TM-2010-05, June 30, 2010 http://www.ucprc.ucdavis.edu/PDF/UCPRC-TM-2010-05.pdf Wei, I.W. (1986). âInstallation and evaluation of permeable pavement at Walden Pond State Reservation: Final Report to Commonwealth of Massachusetts.â Department of Civil Engineering, Northeastern University, Commonwealth of Massachusetts, Division of Water Pollution Control Research Project 77-12 & 80-22, Boston, MA. Washington State Department of Transportation (WSDOT) (2011). WSDOT Pavement Policy. http://www.wsdot.wa.gov/nr/rdonlyres/d7971b81-5443-45b9-8b9b- bfc0d721f5a1/0/wsdotpavementpolicyfinal71211.pdf WSDOT (unpublished). Unpublished research proposal for study of permeable pavement shoulders within state highways. http://www.ecy.wa.gov/programs/wq/psmonitoring/effectiveness/PermeableShouldersGuttersv3.docx Younger K., R. Hicks, and M. Partl, (1994). Evaluation of Permeable Pavement for Road Surfaces, Report No. FHWA-OR-RD-95-03, sponsored by Oregon DOT and FHWA. F-40
