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Permeable Pavement
As the name suggests, permeable pavement allows water to pass through rather than run off the surface. It typically consists of a gravel storage layer, with depths ranging from 10 to 45 cm (4 to 18 in.). However, it can be as deep as 120 cm (4 ft) when designed for extreme weather events. This gravel layer is covered by a permeable surface, such as permeable concrete, asphalt, or interlocking concrete pavers. Designers increasingly opt for large perforated pipes or modular crate systems instead of traditional gravel. These alternatives offer greater void space, allowing for a more compact bed design, which reduces the need for extensive excavation and materials. Depending on the underlying soil, an under-drainage system may be necessary in areas where complete infiltration isn't feasible. When an underdrain is used, it's recommended to implement an elevated or restricted outfall. Simple designs often include upturned elbows or drilled PVC caps on the underdrain, converting them into a slow-release mechanism.
All types of permeable pavement function similarly and typically force water flow to occur in a one-dimensional direction. Rainfall moves vertically through the permeable pavement (or travels a short horizontal distance in the case of pavers), partially filling the gravel storage layer below. This water is stored and later exfiltrates from the system. The water pathway—either through soil exfiltration or an underdrain—depends on rainfall intensity, volume, and soil permeability. If the design includes a storage volume, water accumulates until it reaches capacity. Once full, water will flow over the pavement surface or drain through a raised underdrain. Although permeable pavements exhibit high infiltration rates, intense rainfall can still generate runoff, particularly in concrete grid paver systems filled with sand. After the storm, the stored water gradually exfiltrates into the surrounding soils, with minor evaporation also occurring. Some permeable pavement designs can capture and infiltrate nearly 100% of rainfall events, including extreme events when engineered to meet this specification (Davis et al., 2022)
Five types of permeable pavements are commonly used, Permeable Asphalt (PA), Permeable Concrete (PC), Permeable Interlocking Concrete Pavers (PICP), Concrete Grid Pavers (CGP), and Plastic Grid Pavers (PG). General structural design considerations are discussed for each of the pavements below.
Permeable asphalt (PA) is composed of fine and coarse aggregate stones held together by a bituminous binder. The fine aggregate content is intentionally reduced to create a larger void space, typically ranging between 15–20%. The thickness of the asphalt layer is determined by the expected traffic load, generally varying from 7.5 to 18 cm (3 to 7 in.). Standards for permeable asphalt are provided by the National Asphalt Pavement Association. Additionally, an underlying base course is required to enhance storage capacity and provide the necessary structural strength (Ferguson, 2005; Davis et al., 2022).
Permeable concrete (PC) is composed of a mix of Portland cement, fly ash, washed gravel, and water. The water-to-cementitious material ratio typically ranges between 0.35 and 0.45. Fine, washed gravel, less than 1.3 cm (0.5 in.) in size (No. 8 or 89 stone), is incorporated into the mixture to increase the void space. Unlike traditional concrete, permeable concrete features a void content of 15–25%, allowing water to infiltrate the surface, percolate through the pavement, and often exfiltrate into the subsurface. An admixture is added to enhance bonding and strength. These pavements are usually poured at a thickness of 10–20 cm (4–8 in.) and often include a gravel base course for additional storage or infiltration. The
compressive strength of permeable concrete can vary from 2.8 to 28 MPa (400 to 4000 psi). For updated PC standards, refer to the National Ready-mix Concrete Association. The absence of fines is a distinctive characteristic of permeable concrete (Davis et al., 2022).
Permeable interlocking concrete pavements (PICP) come in a variety of shapes and sizes. When installed, the blocks form patterns that create openings, allowing rainfall to infiltrate. These openings, which typically constitute 5–20% of the surface area, are usually filled with pea gravel aggregate but can also be filled with topsoil and grass. According to ASTM C936 specifications, the pavers must be at least 6.0 cm (2.36 in.) thick with a compressive strength of 55 MPa (8,000 psi) or higher. A typical PICP installation includes the pavers and gravel fill, a 3.8 to 7.6 cm (1.5 to 3.0 in.) fine gravel bedding layer, and a gravel base course storage layer (ICPI, 2004). Bricks can also be used in place of Concrete blocks; these bricks are
designed with small tabs along their edges to ensure proper spacing during installation (Davis et al., 2022).
ASTM C 1319, the Standard Specification for Concrete Grid Paving Units (2021), outlines the properties and specifications for concrete grid pavers (CGP). Typically, CGP are 9.0 cm (3.5 in.) thick, with a maximum dimension of 60 × 60 cm (24 × 24 in.). The open area percentage ranges from 20% to 50%, with the void spaces filled with topsoil and grass, sand, or aggregate. The minimum average compressive strength for CGP is 35 MPa (5,000 psi). A standard installation involves placing the grid pavers with the selected fill media over 2.5 to 3.8 cm (1 to 1.5 in.) of bedding sand, followed by a gravel base course and a compacted soil subgrade (ICPI, 2004). Special attention is required in CGP applications due to their tendency to "rock" or
settle unevenly (Davis et al., 2022).
Plastic grid (PG) pavers, also known as geocells, are composed of flexible plastic interlocking units designed to facilitate infiltration through large gaps filled with gravel or topsoil planted with turfgrass. To enhance infiltration and storage, a sand bedding layer and gravel base course are often included in the installation. The empty grids offer 90–98% open space, with the actual void space depending on the type of fill media used (Ferguson, 2005). Although there are no uniform standards for PG pavers, one product specification lists the typical load-bearing capacity of empty grids at approximately 13.8 MPa (2000 psi), which can increase to 38 MPa (5500 psi) when filled with various materials (Davis et al., 2022).
This layer is positioned directly beneath the choker or bedding course and acts as a high-infiltration-rate transition layer between the bedding and subbase layers. It also provides additional storage and can offer some filtration. In certain cases, filter fabric may be needed at the base of this layer to minimize the migration of fines. Typically, this base reservoir is 3 to 4 inches thick and, depending on local design requirements, is composed of uniformly sized crushed stone (e.g., No. 57 stone) or bank run gravel. The aggregate used in this layer is usually of an intermediate size between bedding and subbase material, often ranging from ¾ to 3⁄16 of an inch in diameter (EPA, 2021).
The subbase layer, or reservoir, functions as the primary water storage and support layer. The stone used is uniformly graded, with sizes typically ranging from ¾ of an inch to 2½ inches in diameter. The thickness of the subbase layer varies based on project-specific factors such as water storage needs, traffic loads, subgrade soil conditions, and the requirement for frost heave protection. While this layer often has a minimum thickness of 4 inches, it can exceed 24 inches in some cases. In pedestrian or residential driveway applications, a subbase layer may not be necessary; instead, the base layer is made thicker to provide the necessary water storage and structural support (EPA, 2021).
An underdrain aids in the removal of water from the base and subbase layers. This system typically consists of a perforated pipe connected to an outlet structure. The need for an underdrain depends on the characteristics of the in situ soil and the overall design objectives. The storage layer must be capable of draining within 2–3 days to ensure adequate storage capacity for the next storm event. If the soil's infiltration rate falls below minimal thresholds, an underdrain becomes necessary and may also be preferred for construction or maintenance purposes. In contrast, highly pervious sandy soils do not require an underdrain. Soil testing is crucial, with estimations of hydraulic conductivity based on infiltration tests or soil particle size providing the most reliable information. Percolation or similar tests may differ significantly from actual in situ values—by factors of 10 to 20—especially if compaction protocols are not properly followed during construction. For instance, compaction can reduce the infiltration rate of a sandy clay loam from 1.3 cm/h (0.5 in./h) to as low as 0.06 to 0.13 cm/h (0.025 to 0.05 in./h), an order of magnitude lower. Even soils with low permeability will experience some infiltration due to the extended time available between storm events and the increased water head during storage. Additionally, some evapotranspiration may occur during dry periods between events. Significant reductions in peak flows and discharge volumes have been observed with permeable pavements installed over clayey soils (Braswell et al., 2018; Fassman & Blackbourn, 2010; Davis et al., 2022).
A geotextile layer can be employed to separate the subbase from the subgrade, effectively preventing the migration of soil into the aggregate layers of the subbase or base (EPA, 2021).
The subgrade layer of soil lies directly beneath the aggregate base or subbase. The infiltration capacity of the subgrade determines the extent to which water can move from the aggregate into the surrounding soils. It is important that construction staff avoid compacting the subgrade soil (EPA, 2021).
The primary maintenance concern for permeable pavements is clogging, which can significantly reduce infiltration rates. Fine particles that contribute to clogging may originate from vehicles, the atmosphere, or stormwater discharge from nearby land surfaces—the more frequent the source (e.g., vehicle traffic) or the larger the contributing area (e.g., drainage area), the quicker clogging will occur. While clogging tends to increase with age and usage, it typically does not result in complete impermeability (Davis et al., 2022).
Regenerative air street sweepers operate by using a blower to push air across the pavement surface, effectively dislodging debris, dust, and fine particles. The debris is then vacuumed up into a hopper for disposal. This type of sweeper is particularly effective for routine maintenance of permeable pavements as it helps prevent clogging by regularly removing surface sediments that can obstruct infiltration (Winston and Fassman-Beck, 2022).
Vacuum trucks are used for both routine and restorative maintenance of permeable pavements. These vehicles apply strong suction to the pavement surface, effectively removing debris, sediment, and other clogging materials from the pores. When used properly, vacuum trucks can restore surface infiltration rates to near-original levels, making them a vital tool for maintaining permeable interlocking concrete pavements and other permeable surfaces (Winston and Fassman-Beck, 2022).
Pressure washing is often used for spot maintenance on permeable pavements. High-pressure water is applied to the pavement surface to dislodge clogging materials and restore infiltration capacity. However, while pressure washing can improve infiltration rates, it is generally not sufficient as a standalone restorative measure, particularly for severely clogged surfaces (Winston and Fassman-Beck, 2022).
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Milling is a restorative maintenance technique where a portion of the pavement's surface is removed to restore permeability. This method is particularly effective when the clogging material has accumulated near the surface. The depth of milling required depends on the extent of clogging, but even shallow milling can significantly enhance the surface infiltration rate of permeable asphalt (Winston & Fassman-Beck, 2022).
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