PPP Notes - Site Improvements

SITE IMPROVEMENTS

(These notes are compiled from AGS) 

POROUS PAVEMENTS

 

• Porous pavements reduce or eliminate urban storm water problems at the source by changing the way urban structures are built and the way they operate hydrologically.
• They restore the landscape’s natural water-retaining function by bringing water back into contact with the underlying soil, or emulate it by filtering and storing water in the pavement structure.
• By combining pavement stormwater control functions into a single structure, they reduce costs, compared with dense pavements that require downstream storm water control facilities.
• Properly applied porous pavements can also enlarge urban tree rooting space, reduce the urban heat island effect, reduce traffic noise, increase driving safety, and improve appearance.
• Therefore, their selection and implementation are integral parts of the multifaceted concerns of urban design, and all of their effects are considered together in evaluations of benefits and costs.
• As porous paving materials become increasingly used, their potential cumulative effect is great, because pavements are the most ubiquitous man-made structures – they occupy two-thirds of the constructed surfaces in urban watersheds.
• This section briefly reviews the types of porous paving materials that are available and some provisions necessary in their installation.

 

POROUS PAVEMENT CONSTRUCTION

 

• To make a successful porous pavement, it must be selected right, designed right, installed right, and maintained right.
• Failures – clogging and structural degradation – result from neglecting one or more of these steps.
• Construction of porous pavements is not more difficult than that of dense pavements, but it is different, and its different specifications and procedures must be strictly adhered to.
• The dominant component in most porous paving materials is aggregate, such as crushed stone.
• It is crucial that this aggregate be “open-graded” – that is, have a narrow range of particle sized particles typically amounts to 30 to 40 % of the aggregate’s volume; the aggregate’s permeability is commonly over 1000 in. per hour.
• As long as the particles are angular, open-graded aggregate obtains structural stability from particle-to-particle interlock.
• To protect a pavement’s surface from sedimentary clogging, surface drainage should be away from the pavement edge in every possible direction, so that sediment is prevented from washing on and, conversely, being allowed to wash off.
• On the downhill side, large, numerous curb cuts should be added, if necessary.
• On the uphill side, a swale should be added, if necessary, to divert potentially sediment-laden runoff.
• These provisions limit most porous pavements to infiltrating the stormwater that falls directly upon the pavement, not runoff from surrounding earthen slopes.
• Each porous paving material has its own advantages and disadvantages for specific applications, and its own requirements for design, construction, and maintenance. 
• A development site should be analyzed in detail to distinguish pavement settings where different, optimally suited materials can be placed.
• In all pavements, areas can be distinguished with different needs for hydrology, appearance, subsurface tree rooting, and cost.

 

ALTERNATIVE POROUS PAVING MATERIALS

 

• There are a number of alternative porous paving materials, among them:
• Porous Aggregate is unbound gravel, crushed stone, crushed recycled brick, or decomposed granite.  In most regions, unbound aggregate is both the least expensive of all firm surfacing materials and the most permeable.  It is suitable for very light traffic such as that in residential driveways, lightly used portions of parking lots, and lightly used walkways.  Annual maintenance for weeding or to replace lost material may be necessary.
• Porous Turf makes a “green” open space where transpiration actively cools urban heat islands.  It is suitable for bearing very light traffic, such as that in parking spaces used once per week or during seasonal peak shopping periods.  The rooting-zone soil should be sandy, to resist compaction.  Porous turf must be regularly mowed, fertilized, and irrigated; and because maintenance must be scheduled, it should be used only where traffic can be controlled or predictably scheduled.
• Plastic Geocells are latticelike products that hold aggregate or topsoil in their cells, inhibiting displacement and compaction.  The surface permeability, temperature, and visual appearance are essentially that of the grass or aggregate fill.
• Open-Jointed Blocks and Open-Celled Grids are units of concrete, brick or stone, with open joints or cells.  Porous aggregate or turf in the openings gives the pavement its porosity and permeability.  Many block products can bear remarkably heavy traffic, but absorbs water intended for turf and holds heat in warm climates.
• Porous Concrete is Portland cement concrete made with single-sized aggregate.  A qualified installer is required.  Air entrainment and polymer fiber reinforcing can enhance the durability of porous concrete in cold climates.  Properly installed porous concrete can bear heavy traffic loads, and the surface is universally accessible by most measures.
• Porous Asphalt is bituminous concrete made with single-sized aggregate.  Polymer fibers and liquid polymer additive can reduce drainage of the binder down through the pores – without them, the binder would leave surface aggregate particles unbound while accumulating into a clogging layer inside the structure.  Properly installed porous asphalt can bear heavy traffic loads, and the surface is universally accessible by most measures.
• “Soft” Paving Materials include granular organic or recycled materials such as bark mulch, crushed shells, and rubber granules.  They are suitable for very light traffic such as that in pedestrian walkways, residential driveways, equestrian ways, and very lightly used parking stalls.  Materials with durable single-sized particles have the highest infiltration rates and are the least susceptible to displacement, crushing, and compaction.
• Decks are surrogates for pavements.  They are completely permeable to air and water as long as their decking components are perforated or spaced apart from each other.  Their footings leave the soil below almost entirely free for infiltration and tree rooting.  Decks are uniquely suited to sites with steep slopes or where native tree roots or ecosystem dynamics are to be very conscientiously protected.  They are made from a variety of natural, manufactured, and recycled materials; hence, their durability varies with the material and its preservative treatment.