The SuDS Manual - Marshalls · block porous paving. These are summarised in Sections 20.1.1 and 20.1.8. 20.1.1 Modular permeable paving ... Figures 20.4 and 20.5 provide examples

This guidance covers the planning, design, construction and maintenance of Sustainable DrainageSystems (SuDS) to assist with their effective implementation within both new and existing developments.It looks at how to maximise amenity and biodiversity benefits, and deliver the key objectives of managingflood risk and water quality. There is also supporting information covering topics such as materials,landscape design, maintenance, community engagement and costs and benefits.

The information presented in this publication is a compendium of good practice, based on existingguidance and research both in the UK and internationally, and the practical experience of the authors,project steering group and industry.

This guidance provides the framework for designing SuDS with confidence and to maximise benefits. Itscontents are relevant for a wide-range of professions and roles and it highlights that through engagementand collaboration SuDS can be integrated into the design of urban areas, to create high quality places forfuture generations.

The key message is that SuDS should be designed to maximise the opportunities and benefits that can besecured from surface water management.

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Highways England

Hydro International

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Stormwater Management

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387Chapter 20: Pervious pavements

CIRIA SuDS Manual 2015

20.1 GENERAL DESCRIPTION

Pervious pavements provide a pavement suitable for pedestrian and/or vehicular traffic,whileallowingrainwatertoinfiltratethroughthesurfaceandintotheunderlyingstructurallayers.Thewateristemporarilystoredbeneaththeoverlyingsurfacebeforeuse,infiltrationtotheground,orcontrolleddischargedownstream(Section 20.1.9).

Pervioussurfaces,togetherwiththeirassociatedsubstructures,areanefficientmeansofmanagingsurfacewaterrunoffclosetoitssource–interceptingrunoff,reducingthevolumeandfrequencyofrunoff,andprovidingatreatmentmedium.Treatmentprocessesthatoccurwithinthesurfacestructure,thesubsurfacematrix(includingsoillayerswhereinfiltrationisallowed)andthegeotextilelayersinclude:

▪ filtration

▪ adsorption

▪ biodegradation

▪ sedimentation.

Therearetwotypesofperviouspavementsthataredefinedonthebasisofthesurfacingmaterials:

Porous pavementsinfiltratewateracrosstheirentiresurfacematerial,forexamplereinforcedgrassorgravelsurfaces,resinboundgravel,porousconcreteand porous asphalt.

Permeable pavements have a surface that is formed of material that is itself impervioustowater.Thematerialsarelaidtoprovidevoidspacethroughthesurfacetothesub-base(egstandardconcreteblockpavingisspecificallydesignedtoallowrainwaterfallingontothesurfaceorrunoffdischargedoverthesurfacetoinfiltratethroughthejointsorvoidsbetweentheblocksintotheunderlyingpavementstructure).

Themaintypesofsurfacesusedaspartofperviouspavementconstructionare:

▪ modular permeable paving

▪ porous asphalt

▪ grass reinforcement

▪ resin bound gravel

▪ porous concrete

20Chapter Pervious pavements

This chapter provides guidance on the design of pervious pavements – pavements that are suitable for pedestrian and/or vehicular traffic, while allowing rainwater to infiltrate through the surface and into underlying structural and foundation layers.

Appendix C, Section C.5.1 demonstrates how to design an infiltrating pervious pavement for a residential area.

Appendix C, Section C.5.3 demonstrates how to design a lined pervious pavement for a supermarket.

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388 Part D: Technical detail

▪ macro pervious

▪ sports surfaces

▪ block porous paving.

These are summarised in Sections 20.1.1 and 20.1.8.

20.1.1 Modular permeable paving

Themostcommonsurfaceisconcreteblockpermeablepaving,butothermodularsurfacingmaterialscanalsobeused(claypavers,naturalstoneetc).

Alltypesofsurfacehavewidenedjointsfilledwithgrittoallowwaterintotheunderlyingbeddinglayerandsub-base.

Potentialusesinclude:

▪ pedestrian areas

▪ privatedriveways

▪ car parks

▪ lightlytoheavilytraffickedroads

▪ ports.

The common layout is to use modular permeable pavement for car park spaces and normal asphalt lanes between(Figure 20.1). This is to reduce costs and also because the asphalt can tolerate turning forces moreeffectively.Thesub-basestoragelayerextendsbelowtheasphalt.

Therearealsospacersystemsavailablethatallowtheuseofnormalpavingslabsaspermeablesurfaces(withappropriatefreedrainingjoint,beddingandsub-basematerial).Thesearebestsuitedtoareaswithonlypedestriantraffic.

20.1.2 Porous asphalt

Porous asphalt can be used as an independent surface or to provide a stronger base to concrete block permeablepavementswhereitistobetraffickedfrequentlybytrucks.Porousasphaltsurfacingreducestrafficnoise.

Figure20.2 Privatedrivewayusingnaturalstoneaspermeable paving (courtesy The Ethical Stone Company/SteinTec)

Figure20.1 Parkandrideschemeontheoutskirtsof Cambridge using concrete block permeable paving (courtesy EPG Limited)

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▪ car parks


▪ lightlytraffickedroads

▪ playgrounds

▪ schools.

Figure 20.3 showsporousasphaltsurfacingbeingused for a car park at East Midlands Airport. The storagebelowthecarparkhasbeenincreasedusing150 mm thick geocellular sub-base replacement units thatcomplywithBS7533-13:2009.

20.1.3 Grass reinforcement

Grass reinforcement uses plastic or concrete grids infilledwithgrassorgravel.

Thistypeofpavementismostsuitableforlightlytraffickedlocations,especiallywhereitonlyhasseasonaluse,sothatthegrasshastimetorecover.


▪ overflowcarparkstoleisurefacilities

▪ schools


▪ hotelandofficecarparks

▪ fireaccessorotherinfrequentHGVtraffic.

Itisimportantthatthesesystemsarewellconstructedtoensurethatthesoilsarenotcompacted.Thetype of grass needs to suit the local climate.

Figures 20.4 and 20.5 provideexamplesofplasticgridsandconcretegridsrespectively.

Figure20.3 CarparkatEastMidlandsAirportwithporous asphalt surfacing (courtesy EPG Limited)

Figure20.4 PlasticgridsatLakeGarda,Italy(courtesyEPG Limited)

Figure20.5 Concretegridsinaparkandrideoverflowcarpark,Gwynedd(courtesyEPGLimited)

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20.1.4 Resin bound gravel

Resinboundgravelprovidesawiderangeoffinishcolours,whichmakesitattractiveforuseinpublic,recreational spaces (Figure 20.6).

Thistypeofpavementismostsuitableforlightlytraffickedlocations.


▪ schools

▪ pedestrian areas around buildings or precincts

▪ privatedriveways.

20.1.5 Porous concrete

Porous concrete can be used as a surfacing material or to provide improved structural stability to the baseofconcreteblockpermeablepavementswhereitistobetraffickedfrequentlybytrucks.


▪ car parks

▪ lightlytraffickedroads.

Figure 20.7showsalarge(2800m2)parkingareaconstructedwithporousconcretetomeetthesustainabledrainageplanningrequirementsforanewretaildevelopmentsite.Theporousconcretesurfacehasbeenusedintheparkingbays,trafficaislesandtheaccessrouteintothisareaofparking.Inotherpartsofthesiteithasbeenusedonlyinparkingbayswiththeimpermeableasphaltaislesdrainingonto the porous concrete.

20.1.6 Macro pervious paving

Macropervioussystemsarewherenormallyimpermeablesurfacesaredrainedtochannelsorothercollection systems designed to trap oil and silt.

Thisapproachallowswaterstorageinthesub-basebelowimpermeablesurfacesinareaswheretherearehightrafficloadsand/orhighshearforcesfromturningvehicles(ChaddockandNunn,2010).

TheexampleofamacroperviouspavementshowninFigures 20.8 and 20.9 uses a treatment channel tocollectrunoff,andthisdischargestoanopen-gradedblanketofsub-base(thesamematerialsasusedbelowconcreteblockpermeablepaving)orgeocellularsub-basereplacementbelowthe

Figure20.6 Constructionofpavementusingresinbound post consumer recycled glass aggregate (courtesy Filterpave Limited)

Figure20.7 Retaildevelopmentcarparkwithporousconcrete,HighWycombe(courtesyEPGLimited)

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impermeable surfacing. Other options for discharge to the drainage layer could be considered such as viaabioretentionsystem.Whateverapproachisused,themainrequirementisthatrobusttreatmentandremoval of silt is required before discharge into the sub-base.

Thistypeofpavingserveforalltypesofuses,aslongasthesiteisnotsubjecttoveryhighsiltloadsandwhereregularmaintenancecanbeassured.

20.1.7 Sports surfaces

Eitheraggregatesub-baseorplasticsub-basereplacementunitscanbeusedbelowturforporousartificialsurfacestomanagesurfacewaterrunoffformulti-usegamesareas,sportspitchesandplayareas.

Thesesurfacescanbeusedaspartofawatermanagementsystemwherethewaterisstoredforirrigationorotheruse.Somesystemsincludepassiveirrigationwherewaterisliftedupfromthestoragelayer into the overlying surface by capillary action.

Figure20.8 MacroperviouspavementunderconstructionintheMidlands–kerbdraincollector/pollutiontrapwithconnectorsbeforeplacingdiffusersandpermeable sub-base to the left (courtesy Phil Tomlinson)

Figure20.9 Macroperviouspavementinwarehouseyardinnorth-westEnglandwithchannelcollection/pollution trap and concrete pavement construction (courtesy EPG Limited)

Figure20.10 ConstructionofsportsdrainageandattenuationlayerbelowschoolsportspitchinHull(courtesy Phil Tomlinson)

Figure20.11 Constructionofattenuationandirrigationsystemusedbelowequestriansurfacing(courtesyAndrewBowen)

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20.1.8 Block porous paving

Concrete(orotherrecycledmaterialssuchasglass)blockporouspavingreliesonwaterpermeatingthroughtheporouspavingunitmaterialratherthanthroughwidenedjoints.ExperienceintheUKindicatesthattheyaremuchmorepronetocloggingthananyoftheothertypesofsystem,duetoverysmallsizeofthevoidsinthesurfaceofthepavingunit.Therefore,theirpotentialuseislimitedduetothisrisk of clogging.

20.1.9 Systems of water management

Therearethreeprincipalsystemsofwatermanagementbelowthesurfaceofperviouspavementsthatare described in Figures 20.12 to 20.14.

Type A (Figure 20.12)reflectsasystemwherealltherainfallpassesintothesubstructure(whereitmaybestoredtemporarily)fromwhereitinfiltratesintothesoilbeneath.Normally,therewillbenodischargefromthesystemtoasewerorwatercourse.However,anemergencyoverflowmayberequiredtocaterforeventsinexcessofthedesigneventortoallowforthesystembecominglessefficient(ieinfiltrationratesreducing) over its design life.

In a Type B system (Figure 20.13),theproportionoftherainfallthatexceedstheinfiltrationcapacityofthesubsoilsflowstothereceivingdrainagesystem.Thiscanoccurbydirectdrainagethroughthesub-baseorbyconveyanceviaperforatedpipeswithinorbelowit.Geocompositeblanketscanalsobeusedtocollectandconveywaterbelowthesub-baselayerorcanbeplacedverticallyattheedgesoftheconstructiontoallowconnectiontoapipe.Bypreventingthebuild-upofwaterabovethesubgrade,therisks to soil stability are reduced.

ThereisnoinfiltrationwithaTypeCsystem(Figure 20.14).Thesystemisgenerallywrappedinanimpermeable,flexiblemembraneplacedabovethesubgrade(formationlevel).Oncethewaterhasfilteredthroughthesub-base,itisconveyedtotheoutfallviaperforatedpipesorfindrains.Thiscanbeusedforsituationswhere:

▪ soilshavelowpermeabilityorlowstrength(andcouldthereforebedamagedbytheintroductionofinfiltratingwater)

▪ thewateristobeharvestedandused

▪ theunderlyinggroundwaterissensitiveandrequiresprotection

▪ thewatertableiswithin1mofthesub-base

▪ the site is contaminated and the risks of mobilising contaminants must be minimised.

Figure20.12 Perviouspavementsystemtypes:TypeA–totalinfiltration

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Variationsofthesethreebasictypesofperviouspavementconstructionincludethefollowing:

▪ Grass reinforcement systems can be used over standard pavement materials (eg Type 1 sub-base). ThesesystemswillprovideInterception,butattenuationandtreatmentoftheresidualrunofffromthesurfacewillstillberequired,astheydonotprovideforanystorageofwaterinthesub-base.

▪ Impermeableasphaltorconcretesurfacingusedoverpermeablesub-base(knownasmacropervioussurfaces[MPPS],orreservoirpavements)wherethewaterisintroducedintothestorageinthesub-baseviaaseriesofdistinctentrypoints–fastenoughtopreventfloodingduringthedesignstormbutwithoutallowingsiltanddebristoenterthesub-base.Thesystemofferstheopportunitytoaccruethebenefitsofaperviouspavementwhentheuseoftraditionalpavingsurfacesisthepreferredoptionduetotrafficconsiderations.Theperformanceofthesilttrappingdevicesiscrucialinthisapplicationasitisimpossibletosubsequentlyremovesiltfromthesub-basewithoutcompletesystem reinstatement. Simple catch pits or normal channels are not suitable.

20.2 GENERAL DESIGN CONSIDERATIONS

Thereisarangeofsurfacingmaterialsthatcanbeusedtoallowwatertosoakintotheunderlyingsub-base. The choice of the most appropriate surfacing for a given location is crucial to the successful use of

Figure20.13 Perviouspavementsystemtypes:TypeB–partialinfiltration

Figure20.14 Perviouspavementsystemtypes:TypeC–noinfiltration

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perviouspavementstomanagesurfacewater.Thiswillmainlybebasedontheexpectedtrafficloadingsand the visual appearance that is required.

Type C system (Section 20.1.9)designsmaybemodifiedtoallowaproportionofrunofftobestoredandusedforvariousnon-potableapplicationssuchasirrigation,toiletflushingetc(Figure 20.15 and Beecham et al,2010).Becauseoftheevaporationoccurring,theproportionofrunoffcapturedbyaperviouspavementsystemislowerthanfromanimpermeablesurface,anditisrecommendedthatrunoffcoefficientsof40%areusedforrainwaterharvestingdesign(Interpave,2010).

Forfurtherinformationonopportunitiesforusingrainwater,seeChapter 11 and Leggett et al (2001a,2001b).

Theaggregatesub-baseinperviouspavementscansometimesbereplacedwithgeocellularsub-basereplacement systems (Chapter 21).Thesewillprovideahigherstoragecapacity(with>90%porosity),butconsiderationwillneedtobegiventotheuseofgeotextilelayerstoensureadequacyoftreatmentof the runoff (Section 20.6).Theuseofgeocellularstructuresbeneathpavingsystemsexposesthemtovery high loads. Module elastic deformation and the strengthofjointsbetweenmodulesarecriticaltothe performance of the overlying layers of blocks or asphalt,andcarefuldesignwillberequired.

Further advice on using geocellular structures is provided in Chapter 21.

Pervious pavements are generally used to managerainfalllandingdirectlyonthesurface,but their capacity is such that they are often also used to manage runoff draining from adjacent areas,suchasroofsoradjacentimpermeableareas of car parks. If an adjacent impermeable area is draining onto the surface of the pervious pavement,themaximumratioshouldbe2:1(impermeable:pervious)tominimisetheriskofsilt

Figure20.15 Exampleofrainwaterharvestingsystem(fromInterpave,2010)

Figure20.16 Concreteblockpermeablepavingwithgeocellular sub-base replacement system (from Interpave,2013)

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completelyblockingthepavementsurface.Wherepavementsaredrainingadjacentimpermeableareas,cloggingwillinitiallydevelopclosetotheimpermeablepavementandacloggingfrontwillgraduallymigrate across the pervious pavement.

Roofdrainagecandirectlargevolumesofwaterintothepavementveryquickly,andinletdiffusersmayberequiredtoregulatetheflowvelocities(Section 20.10.1).Theserequireverycarefuldesign,especiallywheresyphonicdrainageisdischargingintothepavement.Wherewaterfromroofsisdirectedviacatchpitsdirectlyintothesub-basetheratioofimpermeable:perviousabovedoesnotapply.Forsmallerareasofroofitispossibletodischargethedownpipedirectlyontothepavement,inwhichcasethemaximumratioof2:1shouldstillapply.

Health and safety risk management design guidance is presented in Chapter 36.

20.3 SELECTION AND SITING OF PERVIOUS PAVEMENTS

Perviouspavementscanbeusedonmostsites,buttheyneedtobeusedinappropriatelocations.Theycanoftenbecombinedwithothersolutionssuchasdetentionbasins,pondsandwetlandsallowingthesesubsequentattenuationandtreatmentfeaturestobeshallowerandsmaller.Theuseofperviouspavementshouldbeavoidedwherethereisahighriskofsiltloadsonthesurface.

Pervious pavements are typically built as an alternative to impermeable surfaces and therefore require no extradevelopmentspacefortheirconstruction.Theyrequireonlyasmallheaddifferencefromtherunoffsurfacetotheiroutfallandcanthereforebeemployedonveryflatterrain.

Constructedperviouspavementstendtobeusedtodrainhighwayswithlowtrafficvolumesandspeeds(lessthan30mph),carparkingareasandotherlightlytraffickedornon-traffickedsurfaces.However,theyarecapableofsupportingHGVtraffic(ChaddockandNunn,2010,BS7533-13:2009)and,intheUK,specifictypesofperviouspavementshavebeenusedsuccessfullyforsurfaceswithheavyaxleloadtraffic.IntheUSA,thereareisolatedexamplesofsuccessfuluseofperviouspavementsonstatehighways,andtheyarecurrentlylookingattheuseofperviousconcretedesignforheavilytraffickedpavements(WanielistaandChopra,2007).Suchpavementsshouldbedesignedonanindividualbasisandinconjunctionwithmanufacturersandexperiencedgeotechnicalandpavementengineers.Theyshouldhaveastifflayerofasphalt,asphaltconcrete,concreteorhydraulicallyboundcoarse-gradedaggregatebelowthebeddinglayer.Themainconcernsarethefrequentvehiclebrakingandturningactionsthatcancausethesurfacestorut,concreteblockstospreadandporousasphalttospall.

Theacceptabilityofinfiltrationfromapermeablepavementshouldbedeterminedbyfollowingtheguidance provided in Section 25.2,complyingwithallrelevantrequirementsforinfiltrationsystemswithrespecttogroundstability,depthtowatertableetcandSection 26.7fortheprotectionofgroundwater.

Unlinedpavementsshouldnotbeusedonbrownfieldsitesunlessithasbeendemonstratedthattheriskposedbyleachingofcontaminantsismanagedtoacceptablelevels.Unlinedpavementsshouldnotbeusedtotreatrunofffromareaswithhighcontaminantloadsiftheriskofgroundwaterpollutionduetoinfiltrationisunacceptablyhigh.Whereinfiltrationisprevented,theseasonallyhighgroundwaterlevelshouldalwaysbebelowthebaseofthepavementformation.

Perviouspavementscanbeusedinmostgroundconditionsandcanbesitedonwaste,uncontrolledornon-engineeredfill,ifnecessarywithaliner,wherethedesignallowsfordifferentialsettlement.Unlinedpavementsshouldnotbeusedinlocationswhereinfiltratingwatermaycauseslopeinstabilityorfoundationproblems,forexampleareasoflandslides,atthetopofcuttingorembankmentslopesorclosetobuildingfoundationsunlessafullassessmentoftheriskshasbeencarriedoutbyasuitablyqualifiedgeotechnical engineer or engineering geologist.

Forinformationonallowinginfiltrationclosetobuildings,seeChapter 25.

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Figure20.17 Soilclassificationguide(afterInterpave,2010)

Theeffectsofwaterstorageonthestructuralcapacityoftheunderlyingsoilsshouldalsobecarefullyassessedandslopesandcollectionsystemsusedtomanagetheserisks.Thereshouldalwaysbeanominal fall on the pavement formation level.

Figure 20.17givesguidanceonsoilclassification,andTable 20.1 recommends appropriate pavement systemsforarangeofsubgradeconditions.BotharetakenfromInterpave(2010),butcanbeappliedtoanysurfacingsystem,notjustconcreteblockpermeablepaving(CBPP).

The location of buried services should be taken into account in any design to ensure the long-term successofperviouspavementprojects.Shallowservicesshould,whereverpossible,belocatedbeneathareasofconventionalimpermeablesurfacing(whichdraintoadjacentperviousareas),orwithinservicecorridorsorverges,thusavoidingtheperviousconstruction.Deepersurfaceandfoulsewerscanoftenpassbelowthesub-baseformationlayer.Thisapproachwillminimisetheneedtoexcavatethroughthepervious construction to access services.

Usinganappropriatemixofpermeableandimpermeablesurfacingcanprovidestructuretotheoveralldesignlayout–bothvisuallyandtechnically,helpingdesignersrealiseaspirationspromotedbyDCLG(2007).Forexample,animpermeablecentralcarriagewaymightbeemployedtocontainservices,visuallydifferentiatedfromperviousparkingbays.Alternatively,impermeableservicecrossingscouldalsobeusedaspedestrianways,clearlydifferentiatedfromperviousareasintendedforvehicles.

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TABLE20.1

Guidance on selection of a pavement system type (after Interpave, 2010)

Ground characteristics Type A:total infiltration

Type B:partial infiltration

Type C:no infiltration

Permeability of subgrade definedbycoefficientofpermeability k (m/s)

1 × 10−6 to 1 × 10−3

1 × 10−8 to 1 × 10−6

1 × 10−10 to 1 × 10−8 (1)

Highestexpectedwaterlevelwithin1000mmofformation level

Pollutants present in subgrade

Groundconditionssuchthatinfiltrationofwaterisnotrecommended(solutionfeatures,oldmineworkingetc,Chapter 8)

BOX20.1

Units used for infiltration

TheSIunitofreportingsoilpermeabilityism/s.Therefore,soilinfiltrationratesareusuallyalsoreportedinm/s.Thereisageneralunderstandingwithintheindustryofwhatconstitutesahighorlowvaluequotedintheseunits.

Theinfiltrationrateofrainwaterintothetopsurfaceofaperviouspavementisoftencomparedtorainfallintensity.Rainfallintensityisreportedinmm/h,andthereforetheinfiltrationofwaterintothepervious pavement is reported in these units.

Note1 Partialinfiltrationsystemsmaybeusedinsoilswithpermeabilitylessthan10−8m/sbuttheinfiltrationofwaterisnotallowedforin

thestoragedesign.ThishelpswiththeprovisionofInterception.

Thereisanextensivebodyofevidencedemonstratingthatperviouspavementsperformadequatelyincoldclimates.Theytendtowithstandfreeze–thawconditionswellandtendtobelessaffectedbyfrostheavethanstandardpavementsurfacing(LakeCountyForestPreserves,2003;Kevernet al,2009)duetotheairintheaggregatebaseactingasaninsulatinglayerlimitingfrostpenetrationintothepavement,coupledwiththehigherinternallatentheatassociatedwiththehighersoilmoisturecontent.Perviouspavementsdonottendtoiceonthesurfacebecausewaterandmeltingsnowdrainstraightintothepavementratherthanpondingbeforerunoff.Perviouspavementsalsotendtothawfasterthannormalpavementsandthusrequirelowerthanaveragesaltapplications.Studieshavealsoshownlittlelossinthetreatmentperformanceofperviouspavementsduringcoldweather.However,theycandevelopahoarfrost on the surface more frequently than normal pavement construction.

20.4 OVERALL DESIGN REQUIREMENTS

Perviouspavementsprovidetwofunctions.

1 They need to be able to effectively capture the design storm event and discharge it in a controlled manner to the subgrade or drainage system.

2 Theyneedtoprovidesufficientstructuralresistancetowithstandtheloadingsimposedbyvehiclestravelling on the surface.

Thereforetherearetwosetsofcalculationsrequired,andthegreaterthicknessofpermeablesub-basefromthetwocalculationsisusedasthedesignthickness.Perviouspavementsgenerallyrequireflowcontrols at the outlets to ensure effective use of the storage in the sub-base. A recommended design flowchartisprovidedinFigure 20.19.

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Figure20.18 Examplesofdifferentblockpavingfinishes(courtesyInterpave)

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Figure20.19 Perviouspavementdesignflowchart(afterInterpave,2010)

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20.5 HYDRAULIC DESIGN

20.5.1 General

Therearefouraspectstothehydraulicdesignofperviouspavements:

a) confirmationoftheadequacyoftherateofinfiltrationofrainwaterthroughthepavementsurface

b) calculation of the storage volume required for design storm event management

c) calculationoftheoutfallcapacityrequiredtoconveyandcontrolthedischargeofwaterfromthepavement structure

d)exceedancedesign

ExceedancedesignisdiscussedinSection 20.5.5,while(a),(b)and(c)arediscussedhere.

a) Infiltration of rainwater through the pervious surface

Thesurfaceinfiltrationrateshouldbesignificantlygreaterthanthedesignrainfallintensitytoavoidsurfacewaterponding,andthecalculationoftheinflowrateshouldincludeallanticipatedrunofffromadjacentareas.Typically,infiltrationcapacitiesofpervioussurfacesaresignificantlygreaterthandesignrainfall intensities and are not generally limiting factors for the use of a pervious pavement. Surface pondingofexceedanceeventsshouldbeplannedforinthedesign,takingaccountofthelikelywaterdepthonthesurfaceandthetimeforwhichitislikelytoremain.Notethatthesurfaceinfiltrationcapacityhasnorelationshiptotheinfiltrationcapacityofthesoilsbelowthepavementconstruction.

Aminimumvalueof2500mm/h(fornewpavements)isconsideredreasonableforapavementsurfacetobeconsideredperviousinrespectofsurfacewatermanagement(whentestedinaccordancewithstandardtestmethods).Theinfiltrationcapacityofthesurfacematerialsisnormallystatedbythesupplierormanufacturer.ThereisnostandardUKorEuropeantestprocedureformeasuringthesurfaceinfiltrationrateofpervioussurfaces.However,ASTMC1781M-13hasbeendevelopedforconcreteblockpermeable paving and ASTM C1701M-09 for pervious concrete (it could also be applied to other porous materialssuchasporousasphaltandgravel-orgrass-filledreinforcementsystems).ItisrecommendedthatmanufacturersshouldprovidesurfaceinfiltrationratesmeasuredusingtestmethodsandthattheyareadoptedasastandardmethodintheUKwiththefollowingamendments:

1 The results should be stated in both mm/h and m/s.

2 Sealingtheinfiltrationringtothesurfacetobetestedshouldbeachievedusingmasticsealant,rapidset mortar or other suitable sealant material.

Thereisnodoubtthattherateofinfiltrationthroughporousandpermeablesurfacesreducesovertime.Themainwaysthatthesurfacesbecomeblockedare:

▪ washingoftopsoilandconstructionmaterialsontothesurface–theserisksshouldbereducedthrough construction best practice and appropriate detailed design

▪ accumulation of silt and debris in the joints or pore spaces at or close to the surface

▪ theapplicationofgrittingsandtocarparksurfaces(notcommonpracticeintheUK)andtheuseofsandasajointingmaterialinconcreteblockpermeablepaving(notUKpractice)

▪ binderslumpingfromtheaggregatematrixinporousasphaltovertime,whichthendrainsintothevoids–theriskofthisoccurringshouldbereducedbytheuseofmodernbindertechnologytopromote adhesion of the binder.

However,itisveryrarethatthecloggingcausescompletesealingofthewholesurface,andnormallyitwillcontinuetoprovidesufficientdrainagecapacity.Itisrecommendedthatafactorofsafetyof10isappliedtothesurfaceinfiltrationrateofallsurfacetypes,toallowforcloggingtoaffectaproportionofthesurfaceareaoverthepavementdesignlife(iethelong-termsurfaceinfiltrationratewillbeaminimumof250mm/h).InformationonrehabilitatingpavementsthatsufferfromcloggingisprovidedinSection 20.14.

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Despitethereductioninsurfaceinfiltrationrateovertime,theavailableevidence(Table 20.2) indicates thatthelong-termreducedrateismorethansufficientinmostcasestodealwithanyrainfallintensitieslikelytooccurintheUK.Evenifthepavementsbecomecompletelyclogged,theevidenceshowsthattheycanberehabilitatedusingsweeperscombinedwithre-grittingofthejoints.

Figure 20.20showsanexampleofconcreteblockpermeablepavingaftera50mmdepthrainfallevent.Thispavementwasaboutsixyearsoldatthetimeandhadnotbeenmaintainedinthattime.

TABLE20.2

Evidence of durability and clogging of the surfaces of pervious pavements

Pervious pavement type

Clogging mechanism

Evidence of likely clogging rates/extents

Rehabilitation mechanisms

Grass reinforcement (concrete grids)

Sand-filledvoidswithgrassovergrowthactlikesandfiltersandtrap sediment close to the surface

Cloggingdepthsof6–12mm(UrbanWaterways,2011);lossof60–75%oftheinitialsurfaceinfiltrationrateduring a simulated 35-year life (Jayasuriya et al,2007)

Clogged sand can be removed andreplacedwithmechanicalsweepers,althoughthegrasswillalsohavetobereseeded

Porous asphaltDustandsedimenttrapped in surface pores

Clogginginthetop25–75mmcanoccurrapidlywithoutgooddesignandmaintenance,wheresiltloadsaresignificant.EvidenceintheUKisthat pavements are still serviceable after about eight years

Rotatingsweeperandjetwash;useasurfacelayerwithfinerpores(iesmalleraggregate) and increasing aggregatesizewithdepth(BeeldensandHerrier,2006)

Porous concrete Dustandsedimenttrapped in surface pores

Clogginginthetop25–75mmcanoccurrapidlywithoutgooddesignandmaintenance,wheresiltloadsaresignificant

Useasurfacelayerwithfinerpores (ie smaller aggregate) and increasing aggregate sizewithdepth(BeeldensandHerrier,2006);specialistrotating and oscillating sweeper(thetypeusedtoremove tyre residue from airportrunways

Concrete block permeable paving

Dustandsedimentistrapped in the joints betweentheblocks

Penetrationto50mm(oversixyears)(UrbanWaterways,2011);lossof70–90%ofas-newsurfaceinfiltrationrateoverthefirstfewyearsofuseafterwhichinfiltrationrate levels off and remain effectively constant(Borgwardt,2006);inheavilytraffickedpavementsthewheeltracksmaybecomecompletelycloggedinafewyears(ChaddockandNunn,2010)

Brushing and suction sweepingofthesurface,replacementoftop20mmofjointingmaterial,herbicideapplicationandweedremovalprogrammes

b) Pavement subsurface storage capacity

Therequiredcapacityofthesub-basedependsonrainfallcharacteristics,designreturnperiod,infiltrationpotentialintothesubgrade,dischargeconstraints,andtheimpermeableareadrainingtothe pervious pavement.

Thethicknessofthesub-baserequiredcanbeobtainedbysimplecalculation(seeInterpave,2010)orbydetailed hydrological and hydraulic modelling. It should be noted that the Interpave procedure assumes notimeofconcentration,whichislikelytooutputaconservativedesigndepthrequirement.Proprietarydrainagesoftwarenowexiststhatcanpredicthydraulicprofilesacrossapavementandcomputescapacitiesfordesigneventsinmoredetail.However,manyofthealgorithmsinthemodelsarestillverysimpleandonlyapproximatetoactualperformance.

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Analysis(Kellagher,2013)showsthattypically,atleasttwicetheareaofthepavement surface can be served by thesub-basewhenverytightthrottlecontrolsareapplied,andnearlythreetimestheareawhenthethrottlerateisgreaterthan5l/s/ha.However,whereadjacentareasdrainintothesurface,the ratio of impermeable to pervious shouldbelimitedto2:1topreventclogging.Ifroofwaterdischargesintothesub-baseviacatchpits,theratiocanbe increased.

Wherepartialortotalinfiltrationsystemsareused,theinfiltrationwillmeanthatthe system can therefore serve even greaterareas.However,careshouldbetakennottohydraulicallyoverloadthepavement,asthiscancause soil stability risks due to saturation.

Calculations for a range of rainfall durations should be carried out to verify the performance of the available storage volume.

Theavailablestorageinthebase/sub-baselayerisdeterminedbythevolumeofthesub-base,theslopeofthepavementandtheusablevoids(ievoidsthatarefreelydraining)withintheaggregate.Acommonlyusedvalueofporosityis30%fortheaggregatesthatmeettherequirementsforcoarse-gradedaggregatesinBS7533-13:2009orType3sub-baseinaccordancewithDfT(1998).Careshouldbetakenifusingvalueshigherthanthis,thatallthevoidsinamaterialarefreedraining(egclaysoilsmayhaveaporositybutthevoidsareverysmallandnotsuitableforstoringwater).Ifaporositygreaterthan30%isusedinthedesign,thematerialshouldbetestedonsitetoconfirmcompliance.

Onslopingsites,thevolumeofavailablestoragewithinthesub-basewillbereducedwhencomparedtoaflatsurface.ICPI(2011)suggeststhatwhereslopesare3%orgreater,designersshouldconsiderterracingorinternalcheckdamsinthesub-basetoprovideaseriesofcompartments(BS7533-13:2009)Wherethewaterinfiltratestothesoilbelow,thissolutioniseasytodesign,asthedifferentcompartmentsdonotneedtobeinterconnectedwithpipes.

ForTypeCsystems,waterstillhastobeallowedtoflowoutofthesub-basethatisconfinedbythecheckdamintothelowercompartmentviaapipeorotherstructure.Flowcanbefromonecompartmenttoanotherifitispossibletoprovideasufficientlylowflowcontrol(verysmallorificesmayberequired).Theminimumsizeoforificeshouldbe20mm.Solutionstothisissueincludecombiningareassothatalargerflowcontrolcanbeused,ordischargingseveraldammedareastoasinglelargerflowcontroloutsidethepermeablepavementarea(iflevelspermit).Thedesignoftheflowcontrolandinterconnectingpipeshouldminimisetheriskofblockage.Whereverpossible,thedesignshouldallowaccesstoeithersideoftheflowcontrolincaseitneedstobeunblocked(althoughtheriskofthisoccurringisverylow).

Othersolutionstostorageonslopingsitesincludeterracingthesiteintoaseriesofflatareas,makingtheformationasaseriesofhorizontalterraceswiththepavementsurfaceslopingabovethem,makingthesub-basethickersothatthewateratthelowendofthepavementremainswithinthesub-baseorprovidingextrastorageinatrenchatthetoeofaslope.

Researchintothepotentialimpactofsurfaceslopeoninfiltrationratesintothepavementsurfacehasdemonstratedthatbelowslopesofapproximately20%,thisshouldnotbeasignificantissue.TheimpactofslopeonstorageandpotentialdesignsolutionsareshowninFigure 20.21.

Figure20.20 Concreteblockpermeablepavingaftera50mmrainfallevent (courtesy EPG Limited)

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c) Outflow from the pavement structure

Forthesub-basestoragetooperateeffectively,thesystemrequiresflowcontrols(unlesstheonlydischargemechanismisviainfiltration).Thesearegenerallysmallorificeplatesinacontrolchamberandcanbeverysmall(minimum20mm)becausetheriskofblockageislow,becausethewaterhasbeenfiltered.Wherethesub-baseisdividedintodiscreteareas(separatedbyimpermeableconstruction),carefulconsiderationisrequiredofflowcontrollocationsandcharacteristicstoensurethattheuseofthestorageisoptimisedandthatthereisnoriskofinappropriateconstrictionandpotentialflooding.Theoutflowcanalsobetoarainwaterharvestingsystemviaasumpandpumpchamber.

Figure20.21 Controlofwateronslopingsites(fromInterpave,2013)

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The spacing of the outlet pipes or collector pipes for sealed systems can be determined in an approximatemannerusingguidanceprovidedbyCedergren(1974).Themaximumsurfacerunoffratethatcanberemovedbyaflatpermeablesub-basecanbeestimatedusingEquation 20.1.

EQ.20.1

Equation to estimate outfall pipe spacing

q = k (h/b)2

q = maximumintensityofrainfallentering into the pavement sub-base that can bedrainedbypipesatspacingof2bandsub-base thickness of h (m/s)

k = coefficientofpermeabilityofsub-base(m/s)(minimumvalueisspecifiedinSection 20.11)

h = maximumdepthofwaterstoredinsub-base (and base if appropriate) above impermeable formation or membrane (m)

2b = distancebetweenpipes(m)

Figure20.22Outfallpipespacing(afterInterpave,2010)

EQ.20.2

Darcy’s law to calculate sub-base flow

Q = A.k.i

Where

Q = flowcapacityofsub-base(m3/s)

A = cross-sectionalflowarea,ieheight×widthofsub-basethroughwhichwaterisflowing(m2)

k = coefficientofpermeabilityofsub-base(m/s)(minimumvalueisspecifiedin Section 20.11.)

i = hydraulicgradient(m/m)(Thehydraulicgradientistheheadofwaterdrivingtheflow.Forthispurpose,itisassumedtobetheslopeofthesubgradetowardstheoutlet.Thisisnotthetruehydraulichead,butisasimpleapproximationwhichisgenerallyconservative.)

Outflowfromthesub-baseshouldbeviaasystemofperforatedpipesorfindrainsthatprovidealargesurfaceareaforwatertoflowinto.Outletsthatcomprisesimplytheopenendofapipe(wrappedingeotextile)arepronetocloggingandarenotsuitable.Perforatedpipesshouldextendatleast1mintothesub-base,andthepipesshouldbeslottedorhavecircularholesformedaspartofthemanufacturingprocess. Perforations should not be made in pipes by site operatives. The perforated section of pipeshouldhavesufficientflowcapacitythroughthewallstomanagetheanticipatedflows,andtheperforationsshouldbecompatiblewiththeaggregatesize,suchthatmigrationofaggregateparticlesintothepipeisprevented.Thecapacityofthepipetoconveywatershouldalsobesufficienttomanageanticipatedflows.Theopenendsofanypipesthatendincontactwithgravelshouldbecapped.

Watershouldflowhorizontallythroughthesub-basetoreachtheoutletcollectionsystemsandthereshouldbesufficientcapacityinanyaggregatetoconveytheratesofflowrequired.HorizontalwaterflowscanverycrudelybeestimatedusingDarcy’slaw(ICPI,2011)–Equation 20.2.

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20.5.2 Interception design

Studieshaveshownthatthefrequencyofrunofffromalltypesofperviouspavementsissignificantlyreducedwhencomparedtogulleyandpipesystemsdrainingimpermeablesurfaces.Kellagher(2013)foundthatveryhighlevelsofcompliancewithInterceptioncriteriaareachievablethroughtheuseofperviouspavements,providingthereisanominallevelofinfiltrationavailable.Thisisbecause,duringsmallevents,thewatersoaksintothepervioussurface,layingcourseandsub-base,andisreleasedback into the atmosphere through evaporation once the rainfall has stopped. In unlined systems (Type A andBpavements),infiltrationcanalsodeliverInterception.Theresultsofvariousstudiesdemonstratingthe ability of pervious pavements to provide Interception storage are summarised in Table 20.3. These showthatrunofftypicallydoesnotoccurfromperviouspavementsforrainfalleventsupto5mm.

Permeablepavementscanbecombinedwithrainwaterharvestingsystems,whichisanotherapproachtoproviding Interception (Chapter 11).

20.5.3 Peak flow control design

Permeablepavementshelpreduceflowratesfromasitebyprovidingattenuationstorage.Theavailablestoragevolumeisprovidedbythevoidspaceinthesub-base:

Available attenuation storage in sub-base = Volume of sub-base × porosity in the soil/aggregate/ geocellular layer designed to be the storage volume

Onslopingsites,thevolumeofstoragewillbereducedcomparedtothesameareaonalevelsite.Thevolumeofstorageinaslopingsiteisgivenby:

Available attenuation storage in sub-base = 0.5 × L × B × T × porosity in the soil/aggregate/geocellular layer designed to be the storage volume

WhereL=lengthofsub-basewherewatercanbestored=T/tanβT = thickness of sub-base measured verticallyB=widthofsub-basewherewatercanbestoredβ=slopeangle

TABLE20.3

Interception storage provided by pervious pavements

Site Reference Type of pervious pavement

Interception storage (rainfall required to initiate runoff – mm)

Maximum Minimum Average

NationalAirTrafficControlServices,Edinburgh

Pratt et al(2001) CBPP 17.2 2.6 7.3

Kinston,NorthCarolina Collins et al(2008) CBPP >5 n/a n/a

Sydney,AustraliaRankin and Ball (2004)

CBPP 16 2.5 51

NorthCarolina Collins et al(2008) Concrete grass grid — — 6

Toronto Drakeet al(2012)CBPP and porous concrete

— — 7

Note1 Typicalfromcurvefitofresults

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Aflowcontrolstructureisrequiredtoconstraintherateofwaterdischargedfromthesub-baseviaanoutletpipe.Wheredesignsareaccommodatingsmallareasofpavement(egfordrivewaysandaccessroutes),it may be appropriate to link adjacent pavements together so that the control system can be larger. The requiredstoragevolumeforpeakflowcontrolshouldbeassessedinaccordancewithSection 24.9.

20.5.4 Volume control design

Contribution of permeable pavement systems to volume control should be evaluated using standard methods,basedonexpectedinfiltrationratesand/oravailableattenuationstorageandspecifiedflowcontrols.AssessmentofvolumetriccontrolshouldfollowthemethoddescribedinChapter 3.

Toachievesuitablevolumetriccontrol,overflowstodifferentareasofthedrainagesystemmayberequiredoralternativelytheflowcontrolattheoutletcanbedesignedtoprovideavariabledischarge.Theuseofrainwaterharvesting(usingthepermeablepavementasthestorage)canalsobeusedtohelptoachieve a volumetric reduction in runoff.

20.5.5 Exceedance flow design

Perviouspavementsystemsshouldincludeexceedanceeventmanagementasanintegratedpartofthesystemdesign.Oneoptionistousegulliessetslightlyabovetheelevationofthepavement.Thisallowsforsomepondingabovethepavementsurfacetobeusedforextrastorage.

Temporarystorageofrunofffromextremeeventsabovethepavementsurfaceshouldnotbepermittedwherethereisariskofsurfacecloggingfromdepositedsedimentsandotherdebris.

FurtherguidanceonexceedancedesignisprovidedinSection 24.12.

20.6 TREATMENT DESIGN

Permeablepavementdrainagehasbeenshowntohavedecreasedconcentrationsofarangeofsurfacewaterpollutantswhencomparedtoimpermeablesurfacedrainage,includingheavymetals,oilandgrease,sedimentandsomenutrients(Prattet al,1995and1999,JamesandShahin,1998,BratteboandBooth,2003,Beanet al,2007,Drakeet al,2012).Allbutnutrientremovalhasbeenrepeatedlydemonstratedinmanyresearchlocations.Evidenceoftheremovalefficienciesofpermeablepavementsare included in Chapter 26, Annex 3.

Becausemostpermeablepavementssubstantiallyreducethevolumeofrunoffandoutflow,itislogicalthattheywillalsosignificantlyreducepollutantloadingstoreceivingsurfacewaters.Theacceptabilityofallowinginfiltrationfromthepavementwilldependontheextentofthelikelyrunoffcontaminationandsitecharacteristics (see Chapter 4, Table 4.3).

Severalstudiesconfirmthatpermeablepavementsdemonstratesignificantlylowertotalpollutionloadingsthanstandardpavements(Dayet al,1981,Rushton,2001,Beanet al,2007,Drakeet al,2012).

Treatmentprocessesoccurringwithinperviouspavementsinclude:

▪ filtrationofsiltandtheattachedpollutants–themajorityofsiltistrappedwithinthetop30mmofthejointingmaterialbetweentheblocks

▪ biodegradationoforganicpollutants,suchaspetrolanddieselwithinthepavementconstruction

▪ adsorptionofpollutants(pollutantsattachorbindtosurfaceswithintheconstruction)whichdependsonfactorssuchastexture,aggregatestructureandmoisturecontent

▪ settlement and retention of solids.

Enhancedsoilscanalsobeusedtoimprovetreatmentwithintheperviouspavementsystem.Thiscanbeachievedusingeitherproprietarysystemsorbytheadditionofsmallamountsofsubstrate,ormaterials

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withahigheradsorptioncapacitythanconventionalaggregates(sawdust,peat,claysoils,granularactivated carbon can all increase adsorption). The added materials should not reduce the structural or hydraulic performance of the aggregates. The required microbes are usually already present in the ground and further applications of microbes is not required.

Thepollutantsaretrappedwithintheconstructionatvariouslocationsaccordingtothetypeofperviousconstruction. It has also been found that oils held in some types of pervious construction may be degradedbymicroorganisms(Pratt,1999).Henceoilsaturationofthepavementisunlikelywheresupplyisevenlyspreadovertime.Amajoroilspillcouldoverwhelmthesystem,butthisriskcanbemitigatedbyusingspecialistoiladsorbinggeotextileswithintheconstruction(aheavierweightgeotextilewillbemoreeffective,especiallyifithasbeenspecificallydevelopedtoattractoil).Itisthoughtlikelythatnutrientsoccurringintheenvironmentnearperviouspavements,suchasingrasscuttings,leavesandanimaldroppings,maywellprovidetherequiredstimulusforindigenousmicrobialcommunitydevelopment.Forsiteswithalowriskofoilspillage,thereisnoneedtousegeotextilebetweenthebeddinglayerandsub-baseforpollutionremovalperformance,becausethegeotextilemakeslittleifanydifferencetoremovalofother pollutants.

Ifgeocellularstorageisusedinsteadofaggregatesub-base,thebenefitsoftreatmentwithinthesub-basegravelswillbelost.However,theuseofahorizontalgeotextileabovethegeocellularunitscanhelpmitigatethisloss(PuehmeierandNewman,2008),andhasbeendemonstratedtoprovidecomparableperformance.Also,asignificantproportionofthepollutionremovalhasbeendemonstratedtooccurinthetopofthejointingvoidsinconcreteblockpermeablepaving,thetoplayerofporousasphalt(ifthesurface layer has a smaller grading) and in the grass/rootzone layer of grass systems.

Ifincreasedconfidenceintheremovalofnitrogenhastobeachievedthenwaterwouldhavetobefedfromthesub-basematerialbelowtheperviouspavementstothenextstageoftheManagementTrain,specificallydesignedtooptimisenutrientremoval.Thiscouldbeachievedbylinkingupthepavementstoapondorseriesofpondsorbioretentionsystemwithananaerobiczoneasthesehavebetterremovalratesforphosphorousandnitrogen.Concretegrassgridpaversfilledwithsandhavebeenfoundtobemoreeffectiveatremovingtotalnitrogenthanothertypesofpervioussurface(UrbanWaterways,2008).

Drakeet al (2012)foundcleardifferencesinwaterqualityissuingfromCBPPandporousconcrete.Thetwosurfacesappeartocapturedifferentpollutants.ThismaybetheresultofthehigherpHconditionswithintheporousconcreteaffectingmetaladsorption.Therewasalsoinitialleachingofsomecontaminants from the concrete (phosphate and high pH). Porous concrete also takes time to stabilise and,inthelongerterm(1year+),performanceseemstoapproachthatofCBPP.

Thetreatmentdesignshouldensurethatthesurfacelayerhassufficientlysmallvoidstotrapsiltwithin30mmofthesurfacebutstillbepermeableenoughtoallowwatertoflowintothesub-base.Porousasphalt,porousconcrete,reinforcedgrass,resinboundgravelandconcreteblockpermeablepavingwith2/6.3jointingmaterialshouldallmeetthisrequirement.

20.7 AMENITY DESIGN

Perviouspavementscanprovideamenityintheformofboththeusefulness(ietheyaffordflexibleandmultipleuseofspaceforawiderangeofactivities)andthevisualaspectsofthesurfacematerials(especiallygrasssystems).However,therearenospecificdesignrequirementstoachieveamenityoverandabovethechoiceofsurfaceaspartoftheoverallplanning,architecturalorlandscapedesign.

20.8 BIODIVERSITY DESIGN

Perviouspavementsdonotprovideanydirectbiodiversitybenefits,althoughtheyareveryusefulfortreatingandcontrollingwatertomaximisethebiodiversityinanydownstreampondsorwetlands.Therearenospecificdesignrequirementsorapproachesforbiodiversity.

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20.9 STRUCTURAL DESIGN (PAVEMENT ENGINEERING)

20.9.1 Introduction to structural pavement design

ThepavementdesignphilosophyintroducedbyPowellet al(1984)isstillthebasisforflexiblepavementdesignintheUK.Thesoilbelowaroadpavementisusuallymuchweakerthantheroadpavementmaterialsandcannotsupportdirectwheelloads.Themainprincipleofroadpavementdesignisthattheconstructedlayersdistributetheconcentratedloadsfromwheelstoalevelthatthesoilbelowtheroad(referredtoasthesubgrade)cansupportwithoutfailureorexcessivedeformation.Atthesurfaceoftheroad,thepressurefromwheelsisthehighest,andsostrong,highqualitymaterialsareusedintheupperlayers(egconcrete,asphalt,blockpaving).Thepressurereduceswithdepthallowingweakermaterialstobeusedlowerinthepavement(assub-baseandcappinglayers).Inthelongerterm,thecappingand/orsub-basepreventgroundwaterreachingtheboundupperlayers.

ThemainlayersthatareplacedtoformaroadpavementareshowninFigure 20.23.

Thecappingandsub-baselayerareknownasthefoundationandshouldgivesufficientload-spreadingtoprovideanadequateconstructionplatformandbasefortheoverlyingpavementlayers.Theasphalt,asphaltconcrete,concrete,blocksorotherpavementmaterialsarereferredtoasstructuralorsurfacelayersandshouldnotcrackorsufferexcessiveruttingundertheinfluenceoftraffic.Oneofthemainstructurallayersisthebase,whichwillusuallycompriseeitherporousasphalt,asphaltconcreteorhydraulically bound material. The base layer is of particular importance in concrete block paving designed tocarryregularHGVtraffic.

20.9.2 Pervious pavement structural design principles

AlthoughnoapprovedstructuraldesignmethodsforperviouspavementsexistintheUK,thereareanumberofgeneralprinciplesthatshouldbefollowedwhenperviouspavementsaredesigned.Guidanceby Pratt et al(2001)shouldbereferencedforsupportingdetailonperviouspavementdesignmethodsand materials.

Normalroadpavementmaterialsarenotintendedtoallowwaterintotheconstruction.Perviouspavementsdoallowwaterintotheconstruction,andthismeansthatthepavementshouldbedesignedandthematerialsspecified,sothatitcansupporttrafficwhensaturatedwhileallowingwatertoflowfreelythrough it. The materials used for pervious pavement construction should be graded to give the right balancebetweenachievinggoodstructuralperformanceandprovidingsufficientpermeabilityandvoidspaceforwaterstorage.Careshouldbetakentoensurethatlossoffinerparticlesbetweenunboundlayersdoesnotoccur,asthiscanreducethestrengthofgranularlayers.Geotextilecanbelaidbetween

Figure20.23 Layersinaroadpavementconstruction

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unboundlayerstopreventthisfromoccurring.Whereageotextileisnotprovidedbetweenlayingcourseandsub-base,theaggregatesshouldmeetstandardgeotechnicalfiltercriteria(Section 30.5.3).

Theotheroverridingconsiderationisthatthesurfacingwillbesufficientlydurableandcanwithstandthelikelyturningandimpactforcesfromtrafficwithdamage(egspreadingofblocksunderexcessivetrafficshear forces).

Ingeneral,theapproachtakentoallsurfacesisasfollows,followingKnaptonet al(2012):

1 Forlightlytraffickedpavements,theloadsappliedbywheelsarethecriticalfactor,andtheguidanceforthosepavementsisbaseduponwheelloads.

2 Formoreheavilytraffickedhighwaypavements,thepavementsaredesignedonthebasisofthecumulativenumberofstandard8000kgaxles.

20.9.3 Determination of the CBR value for design of pervious pavements

Design CBR value

Californianbearingratio(CBR)variesinverselywithmoisturecontent(asthelatterincreasestheCBRvalue decreases). The equilibrium CBR value is the long-term value that occurs once the pavement is constructedandthemoisturecontentofthesubgradesoilcomesintoequilibriumwiththesuctionforceswithinsubgradeporeairspaces.Suctionforcescanoccurasaresultofunloadingduetoexcavation.Changesingroundwaterlevelsorwettingasaresultofwaterstorageinthesub-basewillalsoaffecttheequilibrium CBR value. Equilibrium CBR values should be used for permeable pavement design. This can bedeterminedbycarryingoutlaboratoryCBRtestsinaccordancewithBS1377-4:1990attheequilibriummoisturecontentasdescribedbyPowellet al (1984). For Type A and B pavements the CBR should be testedaftersaturation.TheICPI(2011)recommendsa96-hoursaturationperiod.

Alternatively,thevalueshouldbeestimatedbasedonthetypeofsoil(plasticityindexandgrading)followingtheguidancebyPowellet al(1984)andBS7533-13:2009,asprovidedinTable 20.4.

TABLE20.4

Equilibrium subgrade CBR estimation

Soil type Plasticity index Guideline equilibrium CBR Value for pervious surface design1, 3 (%)

Heavy clay

70 2

60 2

50 2

40 2.5

Silty clay 30 3

Sandy clay20 4

10 3

Silt2 – 1

Sand (poorly graded) – 7

Sand(wellgraded) – 10

Sandygravel(wellgraded) – 15

Note1 Assumesthinconstruction.Ifpavementthickness(fromsurfacetosubgrade)isgreaterthan1200mm(seeHA,2009).2 Estimatedassumingsomeprobabilityofmaterialsaturating.3 TheseCBRvaluesassumeahighwatertableandthatthesubgrademaybewettedduringthelifeofthepavement.

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Thehighestvaluesofplasticityindexmeasuredonasiteshouldbeused(togivethelowestCBRvalue)forthedesign,unlessthereareasubstantialnumberofresultsavailabletoallowthemeanorotherstatisticalvaluetobeusedwithconfidence,butifthemeanisused,therewillbeanincreasedriskofpavementfailureinsomeareas.ItmaybepossibletoremovesoftspotsandthereforeignorethoselowCBRvalueswhichrelatetotheremovedmaterial.

OnsiteswheretheCBRvariesfromplacetoplace,appropriatedesignsmaybeprovidedfordifferentpartsofthesiteusingthelowestCBRrecordedineachpart.

Oncethesubgradeisexposedduringconstruction,theCBRvalueofthesoilshouldbeconfirmedbylaboratorytestingofCBRsamples(BS1377-4:1990)orusingin situmethods(BS1377-9:1990).Thisistheshort-termCBRvalueatthetimeofconstruction.IfitisfoundtobelessthanthedesignCBR,thesubgradeshouldeitherbeimprovedtoachievethedesignCBR,orthefoundationthicknessshouldberedesigned.ThereasonforthisisbecauseconstructionduringverywetweathercanadverselyaffectthesoilstrengthandleadtolowerequilibriumCBRvalues.

InsummarythefinaldesignCBRvalueshouldbethelowerof:

1 the equilibrium CBR value obtained from CBR tests at equilibrium moisture content (saturated for 96 hours for Type A and B) or based on plasticity and grading results using the correlations above

2 theshorttermCBRvalueobtainedfromCBRtestsonthesubgrade,takenonceitisexposedforconstruction.

Subgrade with low CBR (CBR < 2.5%)

BritishStandardBS7533-101:2015specifiesthattheminimumpermitteddesignCBRis2.5%fornormalpavementsandthisalsoappliestoperviouspavements.SubgradeswithalowerdesignCBRareconsideredunsuitabletosupportapavementfoundation.Inthesecases,asubgradeimprovementlayer should be provided to permanently improve the load-bearing capacity of the subgrade. This can be achievedbyremovingtheweakmaterialtosufficientdepthandreplacingitwithsuitablefillmaterial.Thethicknessremovedmaytypicallybe0.5–1.0m.Althoughthenewmaterialmaybeofbetterquality,thenewdesignCBRshouldbeassumedtobeequivalentto2.5%,inordertoallowforeffectsofanysofterunderlying material and the potential reduction in the strength of the replacement material to its long-term CBR value.

Theexistingsubgradematerialsmayalsobeimprovedbytheadditionoflimeand/orcementtogiveanacceptablelong-termCBRvalueiftheareaswithalowCBRareextensive.ThiswillonlybepossiblewithTypeCpavements(noinfiltration).Theimpactofwateronthestabilisedmaterialsshouldbecarefullyconsidered.

The incorporation of a geosynthetic material into the foundation design may also overcome the issue of aweaksubgrade.SpecialistadviceshouldbesoughttoadoptanalternativedesignCBRvaluethatmaybenecessary,basedontestingorpreviousexperiencewiththespecificgeosyntheticandthematerialsbeing used on the scheme.

20.9.4 Traffic categories

Perviouspavementscanbedesignedtocarryanyvolumeoftrafficloads.Table 20.5definestrafficloadsintermsoftrafficcategories.Thesetrafficcategoriescanbeusedforthedesignofanytypeofsurfacingincluding porous asphalt and other materials and are used as the basis for the structural design of all types of surface discussed in this manual.

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Note1 Basedon1.7standardaxlespervehicle.

TABLE20.5

Traffic loading categories for pervious pavement designTr

affic

cat

egor

y (B

S 75

33)

Stan

dard

axl

es

per d

ay

Life

time

traf

fic(m

sa)

NR

SWA

road

type

Max

imum

an

ticip

ated

axl

e lo

ad (k

g)

Example number of commercial vehicles per day1

Typical application

11AreaswithaxleloadsgreaterthanpermittedbytheRoadVehicles(ConstructionandUse)Regulations1986 as amended are not included in this document

10 ≤4,000 ≤60 0Sitespecific(seeKnapton,2007) Adoptedhighwaysandcommercial/industrial

developments used by a high number of commercial vehicles

Ports and airport landside

Bus stops and bus lanes

9 ≤2,000 ≤30 1Sitespecific(seeKnapton,2007)

8 ≤700 < 10 2 8000 Approx420

7 ≤275 <2.5 3 8000 Approx170

6 ≤60 < 0.5 4 8000 Approx35

Adoptedhighwaysandotherroadsusedbyamoderate number of commercial vehicles

Pedestrian areas subjected to regular overrun of commercial vehicles

Industrial premises

Petrol station forecourts

5 ≤5 < 0.05 n/a 8000 Approx3

Pedestrian areas subjected to occasional overrun of commercial vehicles and maintenance/cleaning machines

Car parks receiving occasional commercial vehiculartraffic

Railwayplatformsexcludingedge

4 1 n/a n/a 8000

Mainly car or pedestriantrafficwithemergencyHGVvehiclesonly

Urbanfootwayswithnoplannedvehicularoverrun

Pedestrian areas or car parks used by light commercial vehicles emergency vehicles and by maintenance vehicles

3 0 n/a n/a 2,000 NoHGVSmallcarparkssubjecttocar,lightvanandmotorcycle access

2 0 n/a n/a 1,000 NoHGVPedestrianandcycleareas,domesticdriveways

1 0 n/a n/a 1,000 NoHGVPedestrian-onlyareas,includingdomesticapplications

0 0 n/a n/a 0 Novehiculartraffic Norequirement(decoration)

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No commercial vehicle traffic loading

Sitecategories0,1,2and3shouldonlybeselectedwhereitcanbeensuredthatnocommercialvehiclesusethepavement,forexamplewherebollardsorheightbarriershavebeeninstalled.

Indeterminingthesitecategory,theuseofthesurfaceofthepavementbyanyconstructiontrafficshouldbeassessedandallowedfor.

Pedestrian loading and maintenance vehicles

Openareasarenowbeingincreasinglymaintainedbymechanicalsweepersandothercollectionvehiclesthatcanhavesurprisinglyhighwheelloadingsandotherdetrimentaleffects(egsuctionsweepers)onpavedareas.Theiruseshouldbeassessedindeterminingthetrafficcategoryappropriatetoadoptforthedesign.

Amenity areas

Inareaswherethereisnopossibilityofvehicularaccess(egpatiosandprivategardenpaths),acategory0 or 1 design may be adopted

Site traffic

Wherethesiteistobeusedforconstructiontraffic,thelayertobeusedbythattrafficshouldbeofadequatestrengthfortheuse.Normallythecompletedbaseshouldbeadequate,butincaseofdoubt(egforalarge-scaledevelopment),anestimateofthetrafficshouldbemadeandapavementdesigncarriedout using current design guidance for bound or unbound pavements.

Inareasnotsubjecttocommercialvehicletraffic,thedesignshouldconsiderloadingduringconstructionandmaintenanceofadjacentareas,andothervehiclesthatmightaccessthearea,includingemergencyvehicles.Theiruseshouldbeallowedforinthedesign.

Design life

A20-yeardesignlifeshouldbegenerallyapplicable(structural)unlessaccessforpossiblemaintenanceofthebaseislikelytobedifficultorexpensive,inwhichcasealongerdesignlifemaybeadvisable.Wherethepavementservesafinitearea,zerogrowthintrafficislikelytobeapplicable.Ifcalculatedgrowthfiguresareavailable,theseshouldbeusedtoascertainthenumberofstandardaxles.

20.9.5 Structural design approaches

The philosophy for conventional (ie non-pervious) pavement design is that the sub-base and/or capping layerisonlyinfluencedbythestrengthofthesubgrade,withthethicknessoftheupperstructurallayers(baseandsurfacecourses)influencedbytrafficloadings.ThedesignmethodusedinBS7533-13:2009forconcreteblockpermeablepavingisdifferent,inthatthetrafficloaddoeshaveaninfluenceonthethicknessofthesub-baseand/orcappinglayer.Knaptonet al (2012)havecombinedthetwoapproachesforperviouspavementsandusedmaximumwheelloaddesignforlightlytraffickedareasofperviouspavingandaconventionalaxlefatigueapproachformoreheavilytraffickedpavements.TheyalsoextendedthescopeforperviouspavementstoheavydutyindustrialpavementsbyapplyingthedesignapproachproposedbyInterpave(Knapton,2007).

IntheUK,theestablisheddesignmethodologiesforflexiblepavementswitheitheranasphaltorhydraulicboundbasecanalsobeusedtodesignporousasphaltpavementsforheavilytraffickedroads(ChaddockandNunn,2010).StructuraldesignofpavementsismostimportantforthosesurfacesusedinareassubjecttoheavierormorefrequentHGVtraffic.

Analyticaldesignisbecomingmorewidelyacceptedinpavementengineeringandcanbeappliedtoperviouspavements.Analyticaldesignisaveryusefulapproach,whichallowstheuseofdifferentporousorpermeablematerials,anditshouldbeencouraged.However,itisvitalthatthematerialpropertiesassumed in the design are achieved during construction (eg the stiffness of concrete block permeable

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pavingshouldreflectnormalconstructionpracticeandnotrelyonveryhighqualityworkmanship)andanappropriatesitetestingregimewillberequired.Also,thematerialsusedinconstructionshouldprovideallthe other necessary attributes (eg durability and providing suitable skid resistance). Most analysis uses asimplifiedmulti-layerlinearelasticmodel,althoughfiniteelementanalysiscanalsobeused(Knaptonet al,2012).Anelementofjudgementwillstillberequiredinthedesign,basedonexperienceoftheperformanceofdifferentmaterials.Thisapproachcanbeusedforawiderangeoftrafficconditions.IntheUSA,theICPI(2011)haspublishedadesignprogramforconcreteblockpermeablepavingcalledPermeableDesignPro.AlthoughthisisbasedonUSAdesignmethods,thereisnoreasonwhyitcannotbeappliedtoUKdesignofconcreteblockpermeablepaving.Oneofthekeydesignparametersinanalyticalapproachesisthestiffnessofthevariousmaterials,andvaluesareprovidedinTable 20.6.

TABLE20.6

Stiffness of various materials used in pervious surface construction

Material Stiffness Source

Porous asphalt2GPa ChaddockandNunn(2010)

3.2–7.1GPa Chopra et al (2011)

Porous concrete Range25–45GPa

Typical 38 GPa

Dynamicmodulus,ChaddockandNunn(2010)

Permeablesub-baseforusebelowall types of surfacing

93–138MPa

250–400MPa(butthesewouldneedreducingtoallowforsaturationby40–70%,whichinworstcasewouldgive100–160MPa)

ICPI(2011)

Shackel(2006)

Concrete block permeable pavers 1000–4500MPadependingontypeofblock Shackel et al(2000)

Resilientmodulusisameasureofstiffnessunderloadsthatareappliedquickly(suchastrafficloads).Shackel et al (2000)foundthattherewaslittledifferenceinresilientmodulusbetweenpermeableandnon-permeableversionsofblockpavingandalsobetweendifferentlayingpatterns.Pavershapewasfoundtohaveasignificantimpactofresilientmodulus.

Notethatwhenmeasuringthestiffnessofpermeablesub-basematerialsonsite,theyarelikelytoreturnlowervaluesthanwhenunconfinedonthesurface.Experience,analysis(Interpave,2007)andalsotesting(ChaddockandNunn,2010)haveshownthatonceconfinedbytheoverlyingpavementconstructionthestiffnesswillincrease.

20.9.6 Structural design considerations for different surface types

Thefollowingsectionsprovidespecificinformationonstructuraldesignforvarioussurfacingmaterials.

Grass reinforcement and resin bound materials

Thissectioncoversreinforcementgridsystemsthatareplasticorconcreteandinfilledwithgrassorgravel.Italsocoversresinboundmaterials.ThereisnorecognisedUKdesignstandardforthesetypesofperviouspavements.Oftendesignersrelyonrecommendationsmadebymanufacturers.However,the surfacing provides very little contribution to the load-bearing capacity of the pavement structure and therefore the sub-base thicknesses used for asphalt or CBPP can be applied to these types of surface (ICPI,2011).

ThesystemsareoftenusedwithnormalType1sub-basebelow(ienotforwaterstorage),inwhichcasestandardpavementdesignapproachesshouldbeused.Ifcoarse-gradedaggregateisusedbelowthesurfacestostorewater,thesub-basedepthsinTable 20.7 can be used. A capping layer or increased thicknessofcoarse-gradedaggregatemayberequiredwheretheCBRvaluesarelessthan5%.Whereusedwithcoarse-gradedaggregatesub-base,ageotextilewillberequiredbetweenthesandbedding/growinglayerandthesub-base,otherwisethesandwillbewasheddownintothesub-base.Thesesurface types are not recommended for load classes above site category 4.

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The sections in Table 20.7applyinthecaseofsubgradesof5%CBRorgreater.ForpavementsoverlowerCBRvaluesthataretraffickedbyvehicles,thefollowingshouldbeprovided:

▪ 1%CBRsubgradeimprovementrequired(Section 20.9.3)

▪ 2%CBRsubgradeimprovementlayerrequired(maybeincorporatedintocappinglayertoprovideatotal layer thickness of 350 mm)

▪ 2.5%CBR300mmcapping

▪ 3%CBR225mmcapping


Thecappinglayerdesigncanalsoincorporategeogrid(s),whichmayreducetherequiredthicknessofmaterial.However,thesub-baseandcappinglayerthicknessmayneedincreasingtoallowforusebyconstruction vehicles.

Whenusedoveropen-gradedsub-basesuchasType3,ageotextileseparationlayerwillberequiredtopreventthesandinfill/beddinglayerfrombeingwashedintotheunderlyingsub-base.TrialsbyChaddockandJones(2007)haveshownthatgreatcareisneededinlappingthegeotextiletoensurethatwashoutdoesnotoccur.Itisrecommendedthatanoverlapofatleast500mmisprovided,andthatcareistakenattheedgestoensurethatlocalisedwashoutcannotoccur(lapthegeotextileupwardsattheedgesandsuitable folds at corners to contain the bedding sand).

Porous asphalt

Porous asphalt can be designed using analytical pavement design procedures using appropriate values ofresilientmodulusforthematerialsspecifiedinthepavement.Alternatively,theadviceinChaddockandNunn(2010)maybeusedtodesigntheporousasphaltlayers.Anotherapproachistocombinethesub-baseandotherlayerthicknessesspecifiedbyKnaptonet al(2012)butreplacetheconcreteblocksandbeddinglayerswithaporousasphaltlayer.ThislatterapproachisthebasisforTable 20.8. Further advice ontheuseofporousasphaltincarparksandprivatedrivesisprovidedbyMPA(2009).Detailedguidanceonsuitablemixturesshouldalwaysbeobtainedfromthesupplier.







TABLE20.7

Typical construction thickness for grass reinforcement and resin bound materials over subgrade of 5% CBR or greaterTraffic category(BS 7533)

Grid Bedding layer Sub-baseCGA

4 Varies 50 mm 300 mm

3 Varies 50 mm 225mm



0 Varies 50 mmSufficienttoprovidesuitableconstruction base

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Notethatporousasphaltmaynotbesuitableasasurfacinginsomelocations(egpetrolforecourts,portsor bus stops) due to either the risk of degradation resulting from fuel spills or the nature of the risk of surfacedeformationresultingfromthepossiblerangeoftrafficforces.

Porous concrete

PorousconcreteisnotwidelyusedatpresentintheUK.However,itisusedintheUSA,wherecomprehensivedesignandconstructionguidanceisavailable(egACPA,2011,CRMCA,2009,ACI,2010).Astructuraldesign programme for pervious concrete pavement design is available from the ACPA (called PerviousPave)

TABLE20.8

Typical construction thickness for porous asphalt over subgrade with 5% CBR or greater

Traffic category(BS 7533)

Porous asphalt BaseHBCGA1

Sub-baseCGA2

11AreaswithaxleloadsgreaterthanpermittedbytheRoadVehicles(ConstructionandUse)Regulations 1986 as amended are not included in this document3

10Asphalt requires specialist consideration and specification

SitespecificusingInterpaveguide for heavy duty pavements(Knapton,2007)

150 mm

9Asphalt requires specialist consideration and specification

SitespecificusingInterpaveguide for heavy duty pavements(Knapton,2007)

150 mm

8DesignfollowingChaddockandNunn(2010)4

300 mm HBCGA 150 mm

7DesignfollowingChaddockandNunn(2010)

200mmHBCGA 150 mm

6180 mm

80 mm

–

125mmHBCGA

150 mm

150 mm

5160 mm

80 mm

–

100 mm HBCGA

150 mm

150 mm

4 150 mm – 300 mm

3 120mm – 225mm

2 70 mm (assumes hand lay) – 150 mm

1 70 mm (assumes hand lay) – 100 mm

0 70 mm (assumes hand lay)Sufficienttoprovidesuitable construction base

Note1 HBCGAreferstohydraulicallyboundcoarse-gradedaggregate(conformingtoBSEN14227-1:2013),minimumcementcontent

3%,strengthclassC5/6asdefinedinBSEN14227-1andminimumpermeability10,000mm/hrwhentestedinaccordancewithASTM C1701M-09 or other suitable test).

2 Thesub-baseCGAdepthsareminimumvaluesthatcorrespondwiththeequivalentthicknessesprovidedinTable 20.10 for modular surfacing.Thesub-baseCGAandanycappinglayercanalsobedesignedtoFoundationClass2inaccordancewithHA(2009).

3 Specialvehicles(SV)falloutsidetheRoadVehicles(ConstructionandUse)Regulations1986.SVvehiclescomplywiththeRoadVehicles(AuthorisationofSpecialTypes)(General)Order2003ortheIndividualVehicleSpecialOrders.Theyhavehigheraxleloadsandweightsandarecommonlyknownasabnormalloads.

4 TheChaddockandNunn(2010)reportwasbasedonapilotstudy.Thetablesinthereportshowingpavementdesignsforupto80msa(millionstandardaxels)areanextrapolationoftestdataandhavenotbeenvalidatedinfullsizeschemes.Ifdesignsarerequiredfortrafficcategory7andabove,specialistadviceshouldbeobtainedfromsuppliersaboutwhetherporousasphaltissuitableandtoprovideanappropriatespecification.

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andthiscanbeadaptedtodesignporousconcretepavingforUKconditions.TheequationsusedintheprogrammeandthebackgroundinformationisprovidedbytheACPA(2011).Obla(2007)indicatesthatnumerousapplicationshaveuseda125–150mmthickperviousconcretelayerover150mmsub-base.FieldperformanceoftheseprojectshasshownthattheyareadequatetohandlethetrafficloadsexpectedincarparkswithmainlypassengercarswithveryoccasionalHGVs(trashtrucks).Whereheavierloadsandhighertrafficareexpected,thenUSexperiencesuggestsusingathickerlayerofporousconcrete(200–300mm).

Porousconcreteisanear-zero-slump,open-gradedmaterialconsistingofportlandcement,coarseaggregate,admixturesandwater.Ithaslittleornofineaggregate(ACI,2010).Thecombinationoftheseingredientswillproduceahardenedmaterialwithconnectedpores,2–8mminsize,whichshouldallowwatertopassthrougheasily.Theporositycanbe15–35%,withtypicalcompressivestrengthsof(2.8–28MPa).Porousconcreteislaidasaplainconcreteslabwithoutreinforcement.

Recommended concrete and sub-base CGA thicknesses are provided in Table 20.9.

TABLE20.9

Typical construction thickness for porous concrete over subgrade with 5% CBR or greater

Traffic category(BS 7533)

Porous concrete (plain slab) Sub-baseCGA

11AreaswithaxleloadsgreaterthanpermittedbytheRoadVehicles(ConstructionandUse)Regulations 1986 as amended are not included in this document

10 Sitespecificdesign





5 150 mm 300 mm

4 135 mm 300 mm

3 125mm 225mm

2 125mm 150 mm

1 100 mm 100 mm

0 100 mmSufficienttoprovidesuitableconstruction base








The designs in Table 20.10havebeenassessedusingACPA(2011)assuminga40-yeardesignlife,0%growth,resilientmodulusofsub-baseCGAis110MPaand28-dayflexuralstrengthofporousconcreteis2.8MPa.ForCBRlessthan5%usethefactorsabovetoincreasecappinglayerthicknessordesignusingACPA(2011).

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BecausethereiscurrentlynotrackrecordofusingporousconcreteintheUK,itisrecommendedthatexpertadviceisobtainedtoundertakesitespecificdesignsforsitecategories6–10.

Modular surfacing (including concrete block permeable paving)

Thiscanbeusedforawiderangeoftrafficconditionsfromlighttoveryheavydutypavements.BS7533-13:2009providesstandardthicknessesforthepavementlayers,althoughthesecanbeadjustedifmaterialswithadifferentstiffnessareusedfollowingtheapproachdescribedbyKnaptonet al(2012)orusingtheICPI(2011)designapproach.

The structural design of CBPP suggested in Table 20.10 has been developed from the approach of Knaptonet al(2012).Theirapproachisbasedonacombinationoffiniteelementanalysisofstaticwheelloadsandanalysisoffull-scaletestresults.ThemaindifferencebetweenTable 20.12andtheKnaptonet al(2012)approachisthatthetableprovidesthesameconstructionthicknessforallofTypesA,BandCpavements.ThisisbecauseanyeffectofinfiltratingwatershouldhavebeentakenaccountofwhenchoosingthedesignCBRvalue.Ingeneral,TypesAandBpavementswillresultinlowerdesignCBRvaluesbecauseofthepresenceofwaterincontactwiththesubgrade.Thewaterproofinglayerandsandprotectionlayer(orgeotextileprotection)providedinTypeCsystemsdonothaveanyinfluenceonthestructural design and therefore are not included in the table for structural design.

ThedesignlayerthicknesshasbeencheckedusingtheguidanceprovidedbytheICPI(2011).Thisapproachmakesassumptionsaboutthedefaultpropertiesofthematerialsusedineachlayer,whicharesummarised in Table 20.11.

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The sections in Table 20.10applyinthecaseofsubgradesof5%CBRorgreater.ForpavementsoverlowerCBRvaluesthataretraffickedbyvehicles,thefollowingshouldbeprovided(fromKnaptonet al,2012):




TABLE20.10

Typical construction thickness for modular paving over subgrade with 5% CBR or greater

Traffic category

Type of surface – minimum thickness

Bedding layer nominal thickness

Base HBCGA1

(porous) or AC (cored)

Sub-baseCGA2

Design basis

Concrete/clay

blocks

Natural stone slab

Concrete flag

Setts

11AreaswithaxleloadsgreaterthanpermittedbytheRoadVehicles(ConstructionandUse)Regulations 1986 as amended are not included in this document3

10SitespecificusingInterpaveguideforheavydutypavements(Knapton,2007)

Knapton(2007)

9SitespecificusingInterpaveguideforheavydutypavements(Knapton,2007)

Knapton(2007)

8 80 mm

Seek advice from supplier

50 mm300 mm HBCGA or 220mmAC32

150 mm

ICPI (2011)

7 80 mm 50 mm200mmHBCGA or 130 mmAC32

150 mm

6 80 mm 50 mm125mmHBCGA or 90 mmAC32

150 mm

5 80 mm 50 mm100 mm HBCGA or 70 mmAC32

150 mm

4 80 mm 50 mm – 300 mm Knapton et al (2012)and ICPI (2011)

3 60 mm 50 mm – 225mm

2 60 mm 50 mm – 150 mm

1 60 mm 50 mm – 100 mm

0 60 mm 50 mm

Sufficientto provide suitable construction base

Note1 HBCGAreferstohydraulicallyboundcoarse-gradedaggregate(conformingtoBSEN14227-1:2013),minimumcementcontent

3%,strengthclassC5/6asdefinedinBSEN14227andminimumpermeability10,000mm/hrwhentestedinaccordancewithASTM C1701M-09 or other suitable test).

2 Thesub-baseCGAdepthsareminimumvaluesthatcorrespondwiththethicknessgivenbyKnaptonet al(2012)orfromcalculationsusingICPI(2011).Thesub-baseCGAandanycappinglayercanalsobedesignedtoFoundationClass2inaccordancewithHA(2009).

3 Specialvehicles(SV)falloutsidetheRoadVehicles(ConstructionandUse)Regulations1986.SVvehiclescomplywiththeRoadVehicles(AuthorisationofSpecialTypes)(General)Order2003ortheIndividualVehicleSpecialOrders.Theyhavehigheraxleloadsandweightsandarecommonlyknownasabnormalloads.

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TABLE20.11

Properties assumed in generic concrete block permeable paving design in Table 20.10

Material Elastic modulus (MPa) Structural layer coefficient for use in ICPI (2011)

Poisson’s ratio

Permeable pavers on a 50 mm bedding layer that meets requirements ofBS7533-13:2009

1000 (based on Shackel et al,2000).

NotethatevidencefromKnapton(2008)is that the modulus of the surface layer has very little effect on the predicted stress on the subgrade and performance of the pavement.

Increasing elastic modulus values of the surface layer to justify a reduction in pavement depth is not normally recommended,asitrequiresextremelyhighconstructionquality,andthereisnoguaranteethatallthejointswillbecompletely full of jointing material for the life of the pavement to maintain an elevated elastic modulus.

0.3 0.40

AC32 6000 0.3–0.44 0.30

HBCGA (hydraulically-bound CGA) meets requirements of cement bound material category 3 (CBGM3) clause 800 series(DfT,1998)

4000 0.24 0.25

Sub-base–coarsegraded aggregate in accordancewithBS7533-13:2009

1000

Notethatthemainfactorthataffectsstiffness is not the Los Angeles test (LA test) value but the grading and angular nature of the particles

0.09 0.35

5%CBRsubgrade 50 n/a 0.45



Thecappinglayerdesigncanalsoincorporategeogrid(s)whichmayreducetherequiredthicknessofmaterial.However,thesub-baseandcappinglayerthicknessmayneedincreasingtoallowforusebyconstruction vehicles.

ACreferstoasphaltconcrete(AC32dense40/60designedinaccordancewithBSEN13108-1:2006.

Forloadclass5andabove,concreteblockpermeablepavingshouldonlybelaidinaherringbonepattern.

NoteforinfiltrationTypeAsystemsthecappinglayermaterialshouldbesufficientlypermeabletoallowwatertopercolatethroughit,withoutitlosingstrength.Itshouldalsohaveaninfiltrationratethatisgreaterthanthematerialbelowit.Itshouldalsobesufficientlydurableandwear-resistant.Alternatively,anincreasedthicknessofcoarse-gradedaggregatecanbeused.Thegradingfor6F2capping(DfT,1998)canbemodifiedtoreducetheamountoffinesandmakeitmorepermeable(ielessthan5%passingthe63micronssieveand0–25%bymasspassingthe600micronssieve).Thishasbeenusedsuccessfullybelowinfiltratingpavements.

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Porous sports surfaces

Poroussportssurfacesareusuallynotheavilytraffickedbyvehicles.Animportantissueforsurfacingand sub-base layers used under sports pitches and games areas is that they meet the requirements oftherelevantsportingfederationsforballbounceetc.Mostsuppliersofartificialorturfsurfacingwillhavethisdataorwilltestcompletedinstallationstodemonstratecompliance.Theyalsoneedtomeetstricttolerancesonsurfacelevels.Usuallythesub-baseconstructionrequiredtoachievetheseotherrequirementswillbesufficienttosupportthelikelyvehicleloads(egfrommaintenancevehicles).

20.10 PHYSICAL SPECIFICATIONS

20.10.1 Pre-treatment and inlets

Rainfallnormallyfindsitswaythroughthepervioussurfaceviadirectinfiltration.However,wherethepavementhassufficienthydrauliccapacity,additionalrunofffromadjacentimpermeableareascanbedirectedontothepervioussurface.Theflowofwaterfromthesurfaceofadjacentpavedareasshouldbedistributedalongtheedgeofthepermeablearea;itshouldnotbechannelledtoadiscretepointasthiswillcausecloggingofthesurface.

Runofffromadjacentroofareascanbedraineddirectlyintothesub-base,whereitislikelytohaveverylowlevelsofsilt.However,suchflowsshouldbedischargedviaasilt/debristraptopreventanyrisksofcloggingofthepavementconstructionbelowthesurface.Theflowshouldbedistributedintothesub-base using a diffuser such as the one in Figure 20.24.

Wheresyphonicdrainageisdischargedintothesub-base,thesiphonbreakshouldbebeforeentryintothe permeable sub-base via a ventilated manhole. The siphon break should be designed to provide sufficientcapacityandtoreduceflowvelocitieswithinthesub-basetopreventsurchargingofthesystem.Thesiphonbreakandsubsequentdiffuserstodistributetheflowintothesub-baseshouldbedesignedinconjunctionwiththesyphonicdrainagedesigner.

20.10.2 Outlets

IfthepavementisaTypeAsystemthatisdesignedtoallowallwatertoinfiltrateintotheground,thereisnoneedforanyspecificoutlet.IfthesystemisaTypeBorTypeC,wherewaterleavesthesub-basetoflowtothenextpartofthedrainagesystem,anoutletisrequiredfromthesub-base.Thisisusuallyachievedusingeitheraseriesofperforatedpipes(whichcanbewithinthesub-baseorintrenchesbelow(Figure 20.25),dependingonthethicknessofthesub-baseandtrafficloadsandthestrengthofthepipes),orwithalengthoffindrainalongoneedgeofthesub-baseconnectedtotheoutletpipe(Figure 20.26).

Figure20.24 Flowdiffusertodistributeroofrunoffintopermeablesub-base(fromInterpave,2013)

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Awell-protectedobservationwellconsistingofa150mmperforatedpipe,orequivalent,shouldbeplacedatthedownstreamendofthefacility.Thewellcanbeusedtomeasuretheactualemptyingtimesofthepavementsystemandtokeeparecordperformancechangeswithtime.

Figure20.25 Perforatedpipeoutletsbelowsub-base

Figure20.26 Findrainoutletfrompervioussurfacesub-base(fromInterpave,2010)

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20.11 MATERIALS

The sub-base beneath pervious surfacing systems has a large proportion of interconnected voids throughwhichwatercanmovefreelyandcanalsobestored.Thismaterialisdifferentfromastandardpavementsub-baseandhastobespecifiedsothatithassufficientpermeability,porosity,strengthandresistance to abrasion.

Thissectionprovidesspecificationsforsomeofthekeyelementsofpervioussurfacessuchasbeddingandjointingmaterialandsub-base.Itdoesnotprovideacomprehensivespecificationforallelementsthat are required to meet recognised standards (eg the concrete blocks in concrete block permeable paving should meet all the same performance standards as normal concrete blocks).

20.11.1 Bedding layer and jointing material for concrete block permeable paving

Beddingandjointingmaterialneedstobefree-drainingandhavesufficientdurabilitytoresistwearfromsmallmovementsbetweenblocks.AtypicalgradingspecificationisgiveninTable 20.12,butadviceshouldalwaysbesoughtfromtheperviouspavementmanufacturerwithregardtotheexactmaterialtypethatissuitableforeachsystem.Thejointingmaterialinsomesystemsmayhavesmaller,3mmparticlesifthejointsbetweenblocksaresmaller.

ThematerialshouldalsomeetthedurabilityrequirementsinBS7533-13:2009(Section 20.11.4).

20.11.2 Sand infill and bedding layer for grass reinforcement

Sandinfilltoreinforcedgrasssystemsneedstobefree-drainingbutwithsufficientorganiccontentto support plants. A root zone material (Chapter 30)issuitable,andtherearealsomanyothermixesrecommendedbysuppliersofthegrids.Normaltopsoilisnotsuitable.

20.11.3 Geotextile filter characteristics

Geotextilesthatactasfiltersshouldallowfreeflowofwater,thatiswithzerobreakthroughhead.Theyshouldbemanufacturedfrompolyethylene,polypropyleneorothersuitablemonofilamentthatcanwithstandtheloadsappliedduringconstructionandshouldhaveadesignlifeequivalenttothepavementdesignlife.Theyshouldnotbeadverselyaffectedbypollutants,alkalineoracidicgroundwater.

Geotextilesplacedhighinthepavementconstruction(egbetweenthebeddinglayerandsub-baseofCBPP) are subject to higher stresses than those deeper in the construction (eg at the bottom of the sub-base). This needs to be evaluated and considered in the design.

FurtherguidanceongeotextilesisgiveninChapter 30.

20.11.4 Sub-base aggregate characteristics

Thesub-baseshouldhaveaminimumporositythatisconsistentwiththedesigncalculations(normallyatleast30%).Thesub-baseshouldalsohaveaminimumpermeabilityof6×10−2m/swhentestedinaccordancewithHA(1990).

TABLE20.12

Bedding and jointing layer specification (2/6.3 to BS 7533-13:2009)

BS sieve size (mm) Percentage passing

14 100

10 90–100

6.3 80–99

2.0 0–20

1.0 0–5

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TABLE20.13

Typical grading requirements for sub-base aggregates (after BS 7533-13:2009 and DfT, 1998)

Sieve size (mm) Percent passing

Coarse aggregate4–40 mm (4/40)

(BS 7533-13:2009)

Coarse aggregate4–20 mm (4/20)

(BS 7533-13:2009)

Type 3 sub-base0–40 mm (0/40)

(DfT, 1998)

80 100 – 100

63 98–100 – 80 -99

40 90–99 100 50–78

31.5 – 98–100 31–60

20 25–70 90–99 18–46

10 – 25–70 10–35

4 0–15 0–15 6–26

2 0–5 0–5 0–20

1 – – 0–5

Therequirementforlowfinescontentmeansthatthesurfaceloadingwillessentiallybecarriedbypoint-to-pointcontactbetweenaggregateparticlesinthesub-base.Inordertomaximisethefrictionbetweenparticlesandthusincreasestrength,theparticlesshouldberoughandangulartogivegoodinterlock.Crushedrock(granite,basalt,gabbro)orconcretewith>90%fracturefaces,orblastfurnaceslagisrequiredtoachievethis.Sandandgravelwithroundedparticlesshouldnotbeusedinperviouspavementsub-baseconstruction.AggregatesshouldcomplywithBSEN13242:2002+A1:2007orBSEN12620:2002+A1:2008.Thechoiceisacompromisebetweenstiffness,permeabilityandstoragecapacity.Typical gradings for sub-base aggregates are provided in Table 20.13. The material types are from BS 7533-13:2009andfromDfT(1998)Clause805Type3basematerial.However,thereisnoreasonwhyothergradingscannotbeusediftheyaremorereadilyavailableandmeetallthenecessaryrequirements,andprovidedthebasematerialissufficientlydurable.

Asthesub-basewillbeincontactwithwaterforalargepartofthetime,thestrengthanddurabilityofaggregateparticleswhensaturatedandsubjecttowettinganddryingshouldbeassessed.Thematerialsshouldalsonotcrushordegrade,eitherduringconstructionorinservice.ThespecificationofLAtestvalues,microdevaltestsandflakinesstestswilladdresstheseissues.Sub-baseaggregatespecificationrequirements are summarised in Table 20.14.ThistableisfromBS7533-13:2009forCBPPbutshouldbeappliedtosub-basematerialsusedbelowalltypesofsurfacingandalsoforType3materialandforrecycledmaterials.Notethatthesedurabilityrequirementsareas,ifnotmore,importantthanthegradingand should not be ignored.

Recycledmaterialcanbeusedwhereasourceisconvenientlyavailablebutcareshouldbetakenthatthisisofconsistentquality,hasanappropriategradingandisfreeofunacceptablematerialssuchasorganic matter or steel scrap. Leachate from crushed concrete sub-base material is likely to have a high pHvalue,whichcouldimpedevegetationgrowthandthusleadtosoilerosionatthedrainoutletand/orcausethegrowthofprecipitatesatthedrainoutlet.Therefore,outletsfromrecycledconcretesub-basesbelowpervioussurfacesshouldbedesignedtominimiseblockagebyhavingalargesurfaceareathroughwhichwateriscollected,andtheoutletsshouldbeaccessibletoremovebuild-uporprecipitates.

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Notethatboththeresistancetowearandresistancetofragmentationareimportant.TheLAtestisanindicationoftheresistancetofragmentationandcanonlybecarriedoutondryaggregate.TheMicro-Devaltest(MDtest)measurestheresistancetoabrasionwheninterlockingparticlesaresubjecttorepeatedloadinginthepresenceofwaterwhichisanimportantpropertyforsub-basesbelowperviouspavements.

Impermeable membrane characteristics

Thesearetypicallymanufacturedfromhighdensitypolyethylene(HDPE),polypropyleneorethylenepropylenedienemonomerrubber(EPDM)andshouldbe:

▪ durable,robustandabletowithstandconstructionandoperationalloads

▪ resistanttopuncture,multi-axialstressesandstrainsassociatedwithmovementandenvironmentalstresscracking(orprotectedbygeotextileorsandlayersaboveandbelowasrequired–thegreatestrisk of puncture is often from the sub-base material laid on top of the membranes)

▪ unaffected by potential pollutants

▪ installedwithfullywatertightjointsanddischargeoutlets.Weldedjointsshouldbetestedtoensurethe integrity of the system and provide a more robust jointing method. The membrane should be able to resist the punching stresses caused by sharp points of contact from the aggregate sub-base.Itshouldalsohavesufficientstrengthtoresisttheimposedtensileforcesfromtrafficorotherloading.Wheretherisksassociatedwithpunctureareparticularlyhigh,considerationcanbegiventoprotectingmembraneswithgeotextilefleeces.

FurtherguidanceongeomembranesandgeotextilesisgiveninChapter 30.

TABLE20.14

Sub-base aggregate specification requirements (after BS 7533-13:2009)

Properties Category toBS EN 13242:2002 or BS 12620:2002

Grading Grading4/40,Gc85-15,GTc20/17.5

Fines content f4

Shape FI20

Resistance to fragmentation LA30

Durability:

▪ waterabsorptiontoBSEN1097-6:2000,Clause7

▪ forWA.2%,magnesiumsulphatesoundness

WA242

MS18

Resistancetowear MDE20

Acid-solublesulphatecontent:

▪ aggregates other than air-cooled blast-furnace slag

▪ air-cooled blast-furnace slag

AS0.2

AS1.0

Totalsulphur:

▪ aggregates other than air-cooled blast-furnace slag


≤1%bymass

≤2%bymass

Volumestabilityofblast-furnaceandsteelslags:


▪ steel slag

Free from dicalcium silicate and iron disintegration (BSEN13242:2002,6.4.2.2)

V5

Leaching of contaminants

Blast furnace slag and other recycled materials should meet the requirements of the Environment Agency WasteAcceptanceCriteria(WAC)forinertwastewhenleachatetestedinaccordancewithBSEN12457-3:2002

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Porous asphalt

ExamplespecificationrequirementsforporousasphaltareprovidedbyKorkealaakso(2014).

Thesurfaceinfiltrationrate(orpermeability)forporousasphaltquotedbyKorkealaakso(2014)arebasedonmeasurementusingthetestmethoddescribedinSeries900ofDfT(1998).However,itisrecommendedthatforconsistencyinfuture,thesurfacepermeabilityofporousasphaltismeasuredusingthesamemethodasporousconcretefromASTMC1701M-09,althoughthespecificationlimitswouldrequireamendingtoreflectthedifferenttestmethod.

Porous concrete

AspecificationforporousconcreteisprovidedbyCRMCA(2009).

Thekeyrequirementsare:

▪ compressivestrength–specifiedbypavementdesigner,typicallybetween3MPaand27MPa(FHA,2012)

▪ cementcontent267–326kg/m3

▪ porosity15–25%

▪ water:cementratio0.26–0.35.

AlsototherequirementsintheCRMCAspecificationitisrecommendedthatthesurfaceinfiltrationrateisaminimumof250mm/hwhenmeasuredinaccordancewithASTMC1701M-09.

20.12 LANDSCAPE DESIGN AND PLANTING

Permeablepavementsdonotoftensupportvegetationaspartofthesurfacing(exceptforgrassreinforcedpavements),butthelandscapecanbedesignedandintegratedeitheraroundtheedgeofpavementsystems,orinzoneswithinthepavementsurfacelayout.

Iftreesorwoodyshrubsaredesired,theyshouldbecarefullyselected.Iftreesandshrubsareplantedclosetopermeablepavingitmayrequiremoreregularsweepingtomaintainthesurfaceinfiltrationrate,althoughthisisnotlikelytobeexcessive.

Permeablepavementsareanexcellentformofconstructionneartrees,becausetheyallowairandwatertoenterthesoil,whichisbeneficialtotreegrowth.Iftreerootshavesufficientwaterandairinthesoil,they are unlikely to damage the permeable pavement construction.

Wheregrassisanintrinsicpartoftheporoussurface(egFigure 20.8),itshouldbeestablishedbeforetrafficking,andthesurfaceshouldbekeptfreeofsedimentuntilthegrassisestablished.Thechoiceofgrassisimportant,anditshouldhaveahightolerancetowearanddrought,andalowtendencytothatchbuild-up(forplantingguidance,seeChapter 29).Wherereinforcedgrassisused,itisimportanttomakesurethattheinfillsoilislowerthanthegrids.

Whereverpossible,itissuggestedthatareasinandaroundperviouspavementsshouldhaveatopsoillevelthatisatleast50mmbelowthetopofthekerbadjacenttotheperviouspavement.Preferably,theareasshouldslopeawayfromtheperviouspavement(Figure 20.27).Whereareasdrainontoaperviouspavement,theirsurfacesshouldbestabilisedsothatthemobilisationofsiltandotherfinedebrisisminimised.

Ifthiscannotbeachieved(buttherearemanyexamplesofpermeablepavingworkingsatisfactorilynexttostandardlandscapes),theriskofcloggingshouldbeminimisedthroughmorefrequentsweepingregimes.Therequiredfrequencyofsweepingshouldbeestablishedthroughvisualmonitoringofthesurface,particularlyfollowingintenserainfall.

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Figure20.27 Landscapedetailforperviouspavement

Figure20.28 Hotelcarparkwithgrass-reinforcedconcreteblendingintothesurroundinglandscape,Cambridgeshire(courtesy Peterborough City Council)

20.13 CONSTRUCTION REQUIREMENTS

Theprinciplesofgoodconstructionthatapplytoasphalt,standardconcreteblockpavingandotherimpermeablesurfacesgenerallyalsoapplytoperviouspavements.Thefollowingguidanceshouldalsobeconsideredwhenconstructingaperviouspavementstructure.Thelistisinorderofconstruction–fromthesubgradeupwards.

20.13.1 Subgrade

ProofrollingoftheformationbelowTypeAandBperviouspavementsisnotrecommendedasitcanreducetheinfiltrationrateofthesoil.Subgradesoftspotscanbeidentifiedusingahand-heldMEXEconeor similar (ie an instrument to measure in situCBRvalues).Ifsoftspotsareidentified,theyshouldbeexcavatedandbackfilledwithsuitablewell-compactedmaterialand,forTypeApavements,thematerialsshould be of similar permeability to the surrounding subgrade.

TheformationshouldbepreparedbytrimmingtolevelinaccordancewithDfT(1998),toatoleranceof+20to−30mm.Ifsubgradeimprovementisemployed,testingwillbeneededtodemonstratethatthedesignCBRvalueshavebeenconsistentlyachievedand,forTypeApavements,thattheinfiltrationrateof the soil is suitable.

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TheformationbelowTypeApavementsshouldbeprotectedfromanyoperationsthatcouldreducetheinfiltrationrateofthesoil(egheavyconstructiontraffic,stockpilingfinematerials,mixingconcreteonit).

20.13.2 Geomembrane and/or geotextile

Anyimpermeablemembraneshouldbecorrectlyspecified,installedandtreatedwithcaretoensurethatit is not damaged during construction (Chapter 30).

Geotextilesshouldbelaidinaccordancewithmanufacturers’instructionsandwithoverlapsbetweenadjacentstripsof300mmwithoutanyfoldsorcreases.Itisrecommendedthatspecialistadvicebesoughtfromthemanufacturerorsupplierofthegeosyntheticfilter(Section 20.11.3).

20.13.3 Capping layer and sub-base

Thefinesinaconventionalimpermeablematerialhelptobindthedifferentsizeparticlestogether,andacttorestrictthepassageofwater.Inthecaseofperviouspavementmaterials(whichlackfines),thereispotential for segregation of materials during the transportation and construction process. Care should be takentoavoidsegregation,butthematerialshouldbeturnedoverbyanexcavatorifthisoccurs.Theriskofsegregationcanbeminimisedbyusinganangular,crushedmaterialwithhighsurfacefriction.

Thelackoffinesandtheopenmatrixstructurecanresultinsurfacemovementwhenconstructiontrafficpassesoverthesub-base,anditcanbemoredifficulttoformtotherequiredgrades.Thismovementcanbeminimisedbyblindingthesurfacewithalayingcourseorothersmallersizeaggregatetofillinthevoidsat the surface and stabilise it. The depth of the sub-base should be adjusted to include this blinding layer.

Thesub-baseshouldbelaidin100–150mmlayersandcompactedtoensurethatthemaximumdensityisachievedfortheparticularmaterialtypeandgrading,withoutcrushingtheindividualparticles,orreducingtheporositybelowthedesignvalue.Thereshouldbeatoleranceof+20to−15mmonthedesignsurfacelevelofthesub-baselayer.Compactionwithvibratingrollerscanbedifficult,asitresultsinmovementofthesurfaceandoftendead-weightrollersaremoreeffective.Sitetrialsarethebestwayto determine the appropriate compaction method.

Oncelaid,thesub-baseshouldnotbetrafficked.This is to prevent it rutting and also to prevent it cloggingwithmudandotherconstructionmaterials.Ifithastobetrafficked,itshouldbeprotectedusingoneofthefollowingmethods:

▪ alayerofDBMthatcanthenbeallowedforas part of the structural design of pavement (Section 20.9)–thisshouldbepuncturedwith75mmdiameterholes,onanorthogonalgridof 750 mm (Figure 20.29)

▪ ageotextileandsacrificialaggregatelayer–removedbeforelayingfinalblockwork

▪ forTypeCpavementsonly,anormalcappinglayer for construction and then construct the pervious surface once its use as a construction surface is no longer required.

Base

HBCGAshouldnotbemixedinconcretemixerson site. It needs to be produced in batching plants ormixedatthequarry.Carefulmixingisrequiredbecauseofthelowcementcontent,andthereisaneed to ensure that it is evenly spread throughout

Figure20.29 LayerofprotectiveDBMoversub-baseafter coring and before laying concrete block permeable paving (courtesy Peterborough City Council)

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thematerialandnotsegregated.LayingHBCGAshouldnotbecarriedoutinweatherthatistoohot,orinheavyrain.TheguidanceprovidedbyACPA(2011)onlayingporousconcretesurfacingshouldbefollowedasitisapplicabletoHBCGA.

AsphaltbasematerialsshouldbelaidinaccordancewithDfT(1998).

Surfacing

Generally,concreteblockpavementsshouldbeconstructedinaccordancewithcurrentindustryguidancesuchasthatprovidedbyUK-basedmanufacturerandcontractorassociations(egInterpaveandInterlay).Adviceshouldbesoughtfromthespecificmanufactureronanyproduct-specificrequirements,layingandjointingmaterials,blockpatternsandblocklayingprocedures.Inaccordancewithgoodpractice,theblocksurfacelayershouldbefullycompactedandjointedtowithin1mofthelayingfaceattheendofeach day. Other pavement surfaces should be constructed according to the relevant British Standards and/orthemanufacturer’sguidance.

Once concrete blocks are laid on the screeded bedding layer they should be vibrated into the bedding layer.Thiscausesgrittofillthelowerpartofthejoints.Gritshouldbebrushedintothetopofthejointsandtheblocksvibratedagain.Theremaybesettlementofgritinthejointsoverthefirstfewmonthsofuseanditiswisetoallowfortheblockstobegrittedagainafterafewmonthsinservice.

Porous concrete used as a surfacing may need joints forming in it. The ACPA recommends that the joints arenotformedbysawcutting,asthisleadstodust-blockingoftheadjacentareasofsurfacing.Thejointsshould be formed using a “pizza cutter” roller before the concrete has set. Compaction of the porous concrete (and HBCGA) should be undertaken using rollers and not vibrating plate compactors.

PorousasphaltshouldbelaidinaccordancewithDfT(1998).

Resin-boundgravelshouldbelaidinaccordancewiththemanufacturer’srecommendations.

Forgrassreinforcementsystems,thebeddingsandthicknessshouldbekepttoaminimum.Amaximumthicknessof20mmisrecommended.Itisdifficulttocutgrassreinforcementsystemsofanykindtofitcomplicatedshapeswithoutlossofintegrity.Thisshouldbeconsideredinthedesignandconstruction.Thegrassgridsshouldnotbeoverfilledwithsoilbecauseitleadstocompaction,andthegrasswillnotgrow.Atleast25mmdepthshouldbeleftbetweenthetopofthegridandtheinfillsoil.Foroptimumvegetationcoveragethepaverorgridneedstohaveinexcessof30%ofitsareaavailableforgrassgrowth.Concretepaverscanbeheavyandmayneedtobemachinelaid.Unlessproduct-specificskidresistancedataisavailable,theyshouldonlybeusedinlow-speedsituations.Sometimesconcretegrassgridswillcrackduetounevensupportinthebeddingsandorsub-base,butonceitiscracked,thepaverwillbedintothesandandwillusuallycontinuetoprovidesupporttotraffic.Plasticgridsneedexpansionjointsorallowanceformovementintheconstructionastheycanexpandandbuckleinhotweather.

Preventingsoilandmudandothercontaminantsfromenteringthepavementsurface,sub-baseandsubgrade,bothduringandafterconstruction,isimperativetoensurethatthepavementremainspermeablethroughoutitsdesignlife.Constructionequipmentshouldbekeptawayfromthearea,andsiltfences,stagedexcavationworksandtemporarydrainageswales(whichdivertrunoffawayfromthearea)should all be considered to manage these risks. Landscaping activities should be carefully designed andcarriedouttopreventdepositionoftopsoil,turfandothermaterialsonthesurfaceofthepavement.Infiltrationsurfacesshouldnotbecompactedandshouldbeprotectedatalltimes.

Further detail on construction activities and the programming of construction activities is provided in Chapter 31.

AconstructionphasehealthandsafetyplanisrequiredundertheConstruction(DesignandManagement)Regulations(CDM)2015.Thisshouldensurethatallconstructionriskshavebeenidentified,eliminated,reducedand/orcontrolledwhereappropriate.

Generic health and safety considerations are presented in Chapter 36.

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20.14 OPERATION AND MAINTENANCE REQUIREMENTS

Regular inspection and maintenance is important for the effective operation of pervious pavements. Maintenanceresponsibilityforaperviouspavementanditssurroundingareashouldbeplacedwithanappropriateresponsibleorganisation.Beforehandingoverthepavementtotheclient,itshouldbeinspectedforclogging,litter,weedsandwaterponding,andallfailuresshouldberectified.Afterhandover,thepavementshouldbeinspectedregularly,preferablyduringandafterheavyrainfalltocheckeffective operation and to identify any areas of ponding.

Perviouspavementsneedtoberegularlycleanedofsiltandothersedimentstopreservetheirinfiltrationcapacity.Extensiveexperiencesuggeststhatsweepingonceperyearshouldbesufficienttomaintainanacceptableinfiltrationrateonmostsites.However,insomeinstances,moreorlesssweepingmayberequiredandthefrequencyshouldbeadjustedtosuitsite-specificcircumstancesandshouldbeinformedby inspection reports.

Abrushandsuctioncleaner(whichcanbealorry-mounteddeviceorasmallerprecinctsweeper)shouldbeusedforregularsweeping.Careshouldbetakeninadjustingvacuumingequipmenttoavoid removal of jointing material. Any lost material should be replaced. It is also possible to clean the surfaceusinglightweightrotatingbrushcleanerscombinedwithpowersprayingusinghotwater,asshowninFigure 20.30.Thisisdoneeverytwoyearsatthesiteshown.

If the surface has clogged then a more specialist sweeperwithwaterjettingandoscillatingandrotatingbrushesmayberequired,especiallyforporousasphaltsurfaces,torestorethesurfaceinfiltrationratetoanacceptablelevel.Thespecialistequipment should be adjusted so that it does not strip binder from the aggregate in the asphalt.

Thelikelydesignlifeofgrassreinforcementwillbedictatedbytraffickingandislikelytobeabout20yearsifdesignedcorrectly.Forconcreteblockpermeable paving the design life should be no differentfromstandardpaving,assumingthataneffective maintenance regime is in place to minimise risksofinfiltrationclogging.Porousasphaltwilllosestrengthandbegintofatigueduetooxidationofthebinder. This is likely to occur slightly faster in porous asphaltthannormalasphalt,sothedesignlifewillbereduced slightly. Porous concrete should have a similar design life to a normal concrete slab.

The reconstruction of failed areas of concrete block pavement should be less costly and disruptive than the rehabilitation of continuous concrete or asphalt porous surfaces due to the reduced area that is likely tobeaffected.Materialsremovedfromthevoidsorthelayersbelowthesurfacemaycontainheavymetalsandhydrocarbonsandmayneedtobedisposedofascontrolledwaste.Sedimenttestingshouldbecarriedoutbeforedisposaltoconfirmitsclassificationandappropriatedisposalmethods.

GuidanceonwastemanagementisprovidedinChapter 33.

Table 20.15 provides guidance on the type of operational and maintenance requirements that may be appropriate.Thelistofactionsisnotexhaustiveandsomeactionsmaynotalwaysberequired.

MaintenancePlansandschedulesshouldbepreparedduringthedesignphase.Specificmaintenanceneedsoftheperviouspavementshouldbemonitored,andmaintenanceschedulesadjustedtosuitrequirements.

FurtherdetailonthepreparationofmaintenancespecificationsandschedulesofworkisgiveninChapter 32.

Figure20.30 Deepcleaningasupermarketcarpark,Dundee(courtesyAbertayUniversity)

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Manyofthespecificmaintenanceactivitiesforperviouspavementscanbeundertakenaspartofageneralsitecleaningcontract(manycarparksorroadsareswepttoremovelitterandforvisualreasonstokeepthemtidy)andtherefore,iflittermanagementisalreadyrequiredatsite,thisshouldhavemarginalcostimplications.

Generally,perviouspavementsrequirelessfrequentgrittinginwintertopreventiceformation.Thereisalsolessriskoficeformationaftersnowmelt,asthemeltwaterdrainsdirectlyintotheunderlyingsub-base and does not have chance to refreeze. A slight frost may occur more frequently on the surface of perviouspavementscomparedtoadjacentimpermeablesurfaces,butthisisonlylikelytolastforafewhours.Itdoesnothappeninallinstallationsand,ifnecessary,thiscanbedealtwithbyapplicationofsalt.It is not likely to pose a hazard to vehicle movements.

Generic health and safety guidance is presented in Chapter 36.

CDM2015requiresdesignerstoensurethatallmaintenanceriskshavebeenidentified,eliminated,reducedand/orcontrolledwhereappropriate.Thisinformationwillberequiredaspartofthehealthandsafetyfile.

TABLE20.15

Operation and maintenance requirements for pervious pavements

Maintenance schedule Required action Typical frequency

Regular maintenanceBrushing and vacuuming (standard cosmeticsweepoverwholesurface)

Onceayear,afterautumnleaffall,orreducedfrequencyasrequired,basedonsite-specificobservationsofcloggingormanufacturer’srecommendations–payparticularattentiontoareaswherewaterruns onto pervious surface from adjacent impermeable areas as this area is most likely to collect the most sediment

Occasional maintenance

Stabiliseandmowcontributingandadjacent areas

As required

Removalofweedsormanagementusingglyphospateapplieddirectlyintotheweedsby an applicator rather than spraying

Asrequired–onceperyearonlessfrequently used pavements

Remedial Actions

Remediateanylandscapingwhich,through vegetation maintenance or soil slip,hasbeenraisedtowithin50mmofthe level of the paving

As required

Remedialworktoanydepressions,rutting and cracked or broken blocks considered detrimental to the structural performanceorahazardtousers,andreplace lost jointing material

As required

Rehabilitation of surface and upper substructurebyremedialsweeping

Every 10 to 15 years or as required (if infiltrationperformanceisreducedduetosignificantclogging)

Monitoring

Initial inspection Monthly for three months after installation

Inspect for evidence of poor operation and/orweedgrowth–ifrequired,takeremedial action

Three-monthly,48hafterlargestormsinfirstsixmonths

Inspect silt accumulation rates and establish appropriate brushing frequencies

Annually

Monitor inspection chambers Annually

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20.15 REFERENCES

ACI(2010)Report on pervious concrete,ACI522R-10,ACICommittee522,AmericanConcreteInstitute,FarmingtonHills,MI,USA(ISBN:978-0-87031-364-6)

ACPA(2011)PerviousPave, background, purpose, assumptions and equations, American Concrete Pavement Association,Rosemont,IL,USA.Goto:www.acpa.org/PerviousPave/About%20PerviousPave.pdf

BEAN,EZ,HUNT,WFandBIDELSPACH,DA(2007)“EvaluationoffourpermeablepavementsitesineasternNorthCarolinaforrunoffreductionandwaterqualityimpacts”Journal of Irrigation and Drainage Engineering,vol133,6,AmericanSocietyofCivilEngineers,Reston,VA,USA,pp583–592

BEECHAM,S,LUCKE,TandMYERS,B(2010)“Designingporousandpermeablepavementsforstormwaterharvestingandre-use”.In:Proc first int European congress of the International Association for Hydro-Environment Engineering and Research,4–6May2010,Edinburgh,Scotland

BEELDENS,AandHERRIER,G(2006)“Waterperviouspavementblocks:theBelgianexperience”.In:Proc eighth int conf on concrete block paving,6–8November2006,SanFrancisco,CaliforniaUSA.Goto:www.sept.org/techpapers/1293.pdf

BORGWARDT,S(2006)“Long-termin-situinfiltrationperformanceofpermeableconcreteblockpavement”.In:Proc of the eighth int conf on concrete block paving,6–8November2006,SanFrancisco,CaliforniaUSA.Goto:www.icpi.org/sites/default/files/techpapers/1303.pdf

BRATTEBO,BOandBOOTH,DB(2003)“Longtermstormwaterquantityandqualityperformanceofpermeable pavement systems” Water Research,vol37,18,ElsevierBV,UK,pp4369–4376

CEDERGREN,HR(1974)Drainage of highway and airfield pavements,JohnWiley&Sons,London,UK(ISBN:978-0-47114-181-5)

CHADDOCK,BandJONES,CR(2007)Structural design and pilot-scale trials of modular cellular paving for greened safe havens and emergency access routes,TransportResearchLaboratory,Berkshire,UK(ISBN:978-1-84608-850-0).Goto:http://tinyurl.com/p6h93xl

CHADDOCK,BandNUNN,B(2010)A pilot scale trial of reservoir pavements for drainage attenuation,PPR482,TransportResearchLaboratory,Berkshire,UK(ISBN:978-1-84608-952-7).Goto:http://tinyurl.com/ovhmbgo

CHOPRA,M,WANIELISTA,M,STUART,E,HARDIN,MandJUJ,I(2011)Pervious pavements – installation, operation and strength. Part 2: porous asphalt systems,FloridaDepartmentofTransportation,Florida,USA

COLLINS,KA,HUNT,WFandHATHAWAY,JM(2008)“HydrologicandwaterqualityevaluationoffourpermeablepavementsinNorthCarolina,USA”.In:Proc 11th int conf on urban drainage,31Augustto5September2008,Edinburgh,Scotland,UK

CRMCA(2009)Specifiers guide for pervious concrete pavement design, version 1.2,ColoradoReadyMixedConcreteAssociation,CO,USA.Goto:http://tinyurl.com/koy9879

DAY,GE,SMITH,DRandBOWERS,J(1981)Runoff and pollution abatement characteristics of concrete grid pavements,Bulletin135,VirginiaPolytechnicInstituteandStateUniversity,USA

DCLG(2007)Manual for streets,ThomasTelfordpublishing,ThomasTelfordLtd,London(ISBN:978-0-72773-501-0).Goto: http://tinyurl.com/qd7qbgn

DRAKE,J,BRADFORD,AANDandSETERS,T(2012)Evaluation of permeable pavements in cold climates. Kortright Centre, Vaughan. Final report,TorontoandRegionConservationAuthority,Ontario,Canada.Goto:http://tinyurl.com/pw8aagc

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FHA(2012)Pervious concrete, TechBrief,FHWA-HIF-13-006,USDepartmentofTransport,FederalHighwayAdministration,WashingtonDC,USA.Goto:http://tinyurl.com/qh3lc4u

HA(1990)“Apermeameterforroaddrainagelayers”.In:Design manual for roads and bridges (DMRB),vol4,Section2,HA41/90,HighwaysAgency,London,UK.Goto:http://tinyurl.com/nfdu66h

HA(2009)Design guidance for road pavement foundations, DraftHD25,InterimAdviceNote73/06Revision1,HighwaysAgency,London,UK.Goto:http://tinyurl.com/p3yq6lv

DfT(1998)Manual of contract documents for highway works, Volume 1 Specification for highway works,HMSO,London,UK(ISBN:978-0-11552-705-0).Goto:www.standardsforhighways.co.uk/mchw/vol1/

ICPI(2011)Permeable interlocking concrete pavements, fourth edition,InterlockingConcretePavementInstitute,Virginia,USA(ISBN:978-1-45078-440-5).Goto:www.icpi.org/node/2962

INTERPAVE(2007)The performance of geotextiles in concrete block permeable pavements, literature review,Interpave,Leicester,UK.Goto:www.paving.org.uk/commercial/research.php

INTERPAVE(2010)Permeable pavements. Guide to the design, construction and maintenance of concrete block permeable pavements, sixth edition,Interpave,Leicester,UK.Goto:http://tinyurl.com/pozgxzc

INTERPAVE(2013)Understanding permeable paving. Guidance for designers, developers, planners and local authorities, fifth edition,Interpave,Leicester,UK.Goto:http://tinyurl.com/pbzz2o4

JAMES,WandSHAHIN,R(1998)“Pollutantsleachedfrompavementsbyacidrain”.InWJames(ed)Advances in modeling the management of stormwater impacts, volume 6,ComputationalHydraulicsInt.,Guelph,Canada,pp321–349

JAYASURIYA,LNN,KADURUPOKUNE,N,OTHMAN,MandJESSE,K(2007)“Contributingtothesustainableuseofstormwater:theroleofperviouspavements”Water Science and Technology,vol56,12,IWAPublishing,London,UK,pp69–75

KELLAGHER,R(2013)SuDS study summary. Interception of 5 mm of rainfall,projectreportBE-030-R2,HRWallingford,Wallingford,Oxon,UK.Goto: http://tinyurl.com/qcyzceu

KEVERN,JT,SCHAEFER,VRandWANG,K(2009)“Temperaturebehaviorofperviousconcretesystems” Transportation Research Record: Journal of the Transportation Research Board,vol2098,TransportationResearchBoardoftheNationalAcademies,Washington,DC,USA,pp94–101

KORKEALAAKSO,J,KUOSA,H,NIEMELÄINEN,EandTIKANMÄKI,M(2014)Review of pervious pavement dimensioning, hydrological models and their parameter needs. State of the Art,researchreportVTT-R-08227-13,VTTTechnicalResearchCentreofFinland,Finland.Goto:http://tinyurl.com/oszsyth

KNAPTON,J(2007)Heavy duty pavements. The structural design of heavy duty pavements for ports and other industries, edition 4,Interpave,Leicester,UK.Goto:http://tinyurl.com/nbr4gxx

KNAPTON,J,MORRELL,DandSIMEUNOVICH,M(2012)“Structuraldesignsolutionsforpermeablepavements”.In:Proc 10th int conf on concrete block paving,24–26November2012,Shanghai,People’sRepublicofChina,pp1–23

LAKECOUNTYFORESTPRESERVES(2003)Permeable paver research summary. Lake County Forest Preserves,Chicago,USA.Goto:http://atfiles.org/files/pdf/PermPavers.PDF

LEGGETT,D,BROWN,R,BREWER,D,STANFIELD,GANDHOLLIDAY,E(2001a)Rainwater and greywater use in buildings. Best practice guidance,C539,CIRIA,London,UK(ISBN:978-0-86017-539-1).Goto:www.ciria.org

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LEGGETT,D,BROWN,R,BREWER,D,STANFIELD,GandHOLLIDAY,E(2001b)Rainwater and greywater use in buildings – decision making for water conservation,PR80,CIRIA,London,UK(ISBN:978-0-86017-880-4).Goto:www.ciria.org

MPA(2009)Constructing and surfacing of parking areas including private drives,AsphaltApplications,AsphaltInformationService,MineralProductsAssociation,London,UK.Goto:http://tinyurl.com/pwy6avw

OBLA,KH(2007)“Perviousconcreteforsustainabledevelopment.Recentadvancesinconcretetechnology”.In:Proc first int conf on recent advances in concrete technology,September2007,NationalReadyMixedConcreteAssociation,WashingtonDC,USA.Goto:http://tinyurl.com/puygb2s

PRATT,CJ(1999)“Useofpermeable,reservoirpavementconstructionsforstormwatertreatmentandstorage for reuse” Water Science and Technology,vol39,5,ElsevierBV,UK,pp145–151

PRATT,CJ,MANTLE,DGandSCHOFIELD,PA(1995)“UKresearchintotheperformanceofpermeablepavement,reservoirstructuresincontrollingstormwaterdischargequantityandquality”Water Science and Technology, vol32,1,ElsevierBV,UK,pp63–69

PRATT,CJ,NEWMAN,APandBOND,PC(1999)“Mineraloilbiodegradationwithinapermeablepavement:longtermobservations”Water Science Technology,vol39,2,ElsevierBV,UK,pp103–109

PRATT,C,WILSON,SandCOOPER,P(2001)Source control using constructed pervious surfaces.Hydraulic, structural and water quality performance issues,C582,CIRIA,London(ISBN:978-0-86017-582-7).Goto:www.ciria.org

POWELL,WD,POTTER,JF,MAYHEW,HCandNUNN,ME(1984)The structural design of bituminous roads,LR1132,TransportResearchLaboratory,Berkshire,UK.Goto:http://tinyurl.com/q35mzjk

PUEHMEIER,TandNEWMAN,AP(2008)“Oilretainingandtreatinggeotextileforpavementapplications”.In:Proc 11th int conf on urban drainage,31Augustto5September2008,Edinburgh,Scotland,UK

RANKIN,KandBALL,JE(2004)“Areviewoftheperformanceofpermeablepavers”.In: Proc 2004 int conf on water sensitive urban design, WSUD2004: Cities as Catchments,Adelaide,SouthAustralia,pp366–377

RUSHTON,BT(2001)“Low-impactparkinglotdesignreducesrunoffandpollutantloads”Journal of Water Resources Planning and Management,vol127,3,specialissue,AmericanSocietyofCivilEngineers,RestonVA,USA,pp172–179

SHACKEL,B(2006)“Designofpermeablepavingsubjecttotraffic”.In:Proc eighth int conf on concrete block paving,6–8November2006,SanFrancisco,CaliforniaUSA.Goto:www.sept.org/techpapers/1293.pdf

SHACKEL,B,LITZKA,JandZIEGER,M(2000)“Loadingtestsofconventionalandecologicalconcreteblockpaving”.In:Proc sixth int conf on concrete block paving: JIPEA World Congress 2000,17–21September2000,Keio-PlazaIntercontinentalHotel,Tokyo,Japan,pp81–89

URBANWATERWAYS(2008)Permeable pavement research. Research update and design implications, NCCooperativeExtension,StateUniversityandA&TStateUniversity,NorthCarolina,USA.Goto:http://tinyurl.com/bk8rvt9

URBANWATERWAYS(2011)Maintaining permeable pavements,NCCooperativeExtension,StateUniversityandA&TStateUniversity,NorthCarolina,USA.Goto:http://tinyurl.com/navuult

WANIELISTA,MandCHOPRA,M(2007)Performance assessment of Portland Cement pervious pavement. Report 4 of 4: Performance assessment of a pervious concrete pavement used as a shoulder for an Interstate rest area parking lot, final report,FDOTProjectBD521-02,FloridaDepartmentofTransportation,Florida,USA.Goto:http://tinyurl.com/pf7lsgu

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STATUTES

Regulations/Orders

Construction(DesignandManagement)Regulations(CDM)2015

TheRoadVehicles(ConstructionandUse)Regulations1986(No.1078)

TheRoadVehicles(AuthorisationofSpecialTypes)(General)Order2003(No.1998)

British Standards

BS1377-4:1990Methods of tests for soils for civil engineering purposes. Compaction-related tests

BS1377-9:1990Methods of tests for soils for civil engineering purposes. In-situ tests

BS7533-13:2009Pavements constructed with clay, natural stone or concrete pavers. Guide for the design of permeable pavements constructed with concrete paving blocks and flags, natural stone slabs and setts and clay pavers

BS7533-101:2015Pavements constructed with clay, natural stone or concrete paving units. Part 101: Code of practice for the design of pavements using modular paving units (in press)

European Standards

BSEN1097-6:2000 Tests for mechanical and physical properties of aggregates. Determination of particle density and water absorption(withdrawn)

BSEN12457-3:2002Characterisation of waste. Leaching. Compliance test for leaching of granular waste materials and sludges. Two stage batch test at a liquid to solid ratio of 2 l/kg and 8 l/kg for materials with a high solid content and with a particle size below 4 mm (without or with size reduction)

BSEN13108-1:2006Bituminous mixtures. Material specifications. Asphalt concrete

BSEN13242:2002+A1:2007Aggregates for unbound and hydraulically bound materials for use in civil engineering work and road construction

BSEN12620:2002+A1:2008Aggregates for concrete

BSEN14227-1:2013Hydraulically bound mixtures. Specifications. Cement bound granular mixtures

USA Standards

ASTM C1701M-09 Standard test method for infiltration rate of in place pervious concrete

ASTM C1781M-13 Standard test method for surface infiltration rate of permeable unit pavement systems

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Documents

The SuDS Manual - Marshalls · block porous paving. These are summarised in Sections 20.1.1 and 20.1.8. 20.1.1 Modular permeable paving ... Figures 20.4 and 20.5 provide examples