Interpave Permeable Pavements Edition 6 2016 by Marshalls

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permeable pavements 7 specification and materials 7.1 paving blocks 7.2 laying course and jointing material 7.2.1 laying course and sub-base grading compactibility 7.2.2 jointing and voids material 7.3 coarse graded aggregate 7.4 hydraulically bound coarse graded aggregate 7.5 design bitumen macadam dbm 7.6 capping 7.7 impermeable membrane 7.8 geotextiles 7.8.1 characteristics 7.8.2 quality 7.8.3 installation procedure 8 detailing 8.1 edge restraints 8.2 outlets and conveyance 8.2.1 spacing of outflow pipes 8.3 inlets from roof drainage 8.4 sloping sites 8.5 landscaping 8.6 service corridors 9 construction 9.1 site precautions 9.2 concrete block paving 9.3 joint sealing 9.4 laying course and joint filling 9.5 protection from construction traffic 9.6 geotextile 9.7 permeable sub-base 9.8 impermeable membrane 9.9 preparation of subgrade 10.performance and maintenance 10.1 performance characteristics 10.1.1 surface infiltration rates and clogging 10.1.2 freezing 10.1.3 health safety and

3 permeable pavement principles figure 6 concrete block permeable pavements in a high density development permeable pavements are particularly suited to providing a hard surface within a sustainable drainage system suds framework although they are also effective in isolation suds is a design philosophy which uses a range of techniques to manage surface water by attenuation and filtration permeable pavements are particularly effective at the head of a suds management train as they have the capacity to mitigate pollution events before affected water passes to more sensitive environments unlike attenuation tanks they are also very useful in areas where sewers flow at capacity during storms owing to an increase in impervious cover from parking or buildings in these situations replacing existing pipes with larger ones is often not economical or even allowable because it merely transfers the additional runoff downstream where this may increase erosion and flooding problems unlike

5 selection of a pavement system 5 selection of a pavement system 5.1 subgrade permeability one of the key criteria in selecting a pavement system is subgrade permeability which is established from appropriate tests on site infiltration tests for traditional soakaways are usually carried out at depths greater than 1m below ground level permeable pavements infiltrate water into the ground at much shallower depths than traditional soakaways and therefore infiltration tests should be carried out close to the final formation level of the pavement this usually means that the tests are much shallower less than 1m depth and use a lower head of water to replicate the performance of the permeable pavement table 1 recommends appropriate pavement systems for a range of subgrade conditions including permeability derived from infiltration tests while table 2 gives guidance on soil classification system a system b total partial infiltration infiltration -6 -3 permeability of subgrade 10 to 10

6 structural and hydraulic design • allow for climate change concrete block permeable pavements are an ideal solution for achieving all the requirements listed above 6.2.1 climate change it is generally accepted that the earth’s climate is changing the most recent studies have predicted that • winters will become milder and wetter with more intense rainfall events • summers will be hotter and drier • heavy rainfall events will become more frequent the ‘foresight flooding future report’ evans et al 2004 concluded that effective land management including drainage must be put into place to protect urban areas from flooding in the future to allow for climate change the rainfall intensity should be increased the suds manual suggests a range of factors of between 5 and 30 but the environment agency and sepa often ask for an increase of 20 on the 1 in 100 year rainfall intensity 6.2.2 units one of the most common mistakes made when designing

6 structural and hydraulic design as an example if a site has a total area to be drained of 1500m2 then 1000m2 can be impermeable draining into 500m2 of permeable block paving to allow for the extra rainfall being collected by the permeable pavement the permeable sub-base thickness must be increased to give a larger storage volume for type c systems the thickness of sub-base can be increased using the equation below t t ai ap ap where t thickness of sub-base to store water from impermeable and permeable contributing areas t thickness of sub-base to store water from permeable area only from table 5 ai area of impermeable surfacing draining onto the permeable ap area of permeable paving for system a infiltration table 6 can be used rainfall data m5-60 20mm m5-60 17mm table 6 permeable sub-base thickness for infiltration system system a collecting impermeable area m5-60 14mm required permeable sub-base thickness mm r 1 in 30 year design event 1 in 100 year event 1 in 100 year

6 structural and hydraulic design 6.3.4 stage 3 – adjustment to pavement design for low cbr subgrades table 8 low subgrade cbr adjustment the design charts in figures 23 and 24 apply in the case of subgrade cbr 5 in the case of lower cbr values an adjustment must be made for system c pavements the adjustment will normally comprise either the provision of additional capping material or the provision of the coarse graded aggregate in the case of system a and system b infiltrating pavements because of the cascading water the additional strength is provided by increasing the thickness of unbound coarse graded aggregate – materials including fines ie capping materials cannot be used in the presence of water note that in many cases a subgrade cbr of less than 5 is an indication that the material may be too fine to act as an infiltration medium which means systems a and b cannot be used cbr of subgrade adjustment to thickness of coarse graded aggregate in the case of system a

7 specification and materials there are three categories of impermeable membrane as follows impermeable membrane category 1 where the consequences of localised failure of the impermeable membrane are minor 2000 gauge polythene can be used with overlapping joints impermeable membrane category 2 where it is important that there is no escape of water where for example contamination would be unacceptable a more durable material should be specified seek specialist advice impermeable membrane category 3 in the case of impermeable membranes installed over occupied buildings including car parks seek specialist advice 7.8 geotextiles geotextiles may be used in two locations within concrete block permeable pavements • an optional upper geotextile at the laying course/coarse graded aggregate interface may be included according to the paving block manufacturer’s recommendations • between the laying course and the permeable sub-base a report prepared by the environmental

9 construction 9 construction 9.1 site precautions preventing and diverting impermeable contaminants such as soil and mud from entering the base and pavement surface both during and after construction are imperative to ensure that the pavement remains permeable throughout its design life simple practices such as keeping muddy construction equipment well away from the area installing silt fences staged excavation and temporary drainage swales which divert runoff away from the area should be considered for other techniques to protect the pavement during construction while allowing site access see section 6.3.5 9.2 concrete block paving generally the concrete block layer should be constructed in accordance with bs 7533 part 3 2005 ‘code of practice for laying precast concrete paving blocks and clay pavers for flexible pavements.’ in accordance with good practice it is advisable that at the cessation of every workday the block surface layer is fully compacted and jointed to

10 performance and maintenance 10.3 adoption at the time of publication january 2010 there is no specific provision for the adoption of suds techniques such as permeable pavements although the situation will change as a result of new legislation existing legislation such as section 38 of the highways act 1980 and section 106 of the town and country planning act 1990 can provide a mechanism for their adoption in some cases whatever route is taken it is recommended that early consultation be undertaken with the relevant stakeholders to ensure responsibilities for long term maintenance are agreed the flood and water management act 2010 applies to any construction work that creates a building or other structure including ‘anything that covers land such as a patio or other surface’ that will affect the ability of land to absorb rainwater when the act has taken effect applicable construction works cannot start until drainage systems have been approved by ‘approving

appendix 1 total area 4217m2 so limiting discharge from site 4217 x 7/10000 2.95l/s in this case the water collected by the permeable pavement needs to be transferred into a conveyance system to take it to the outfall which is the sewer in the road to the top right of the plan this can be achieved in a number of ways but the conveyance system must be sized to provide sufficient capacity flow controls may also be required to ensure the water is stored in the relevant locations one possible layout is shown on the diagram but there are other equally acceptable solutions from table 5 the permeable sub-base depth required for attenuation is 120mm this does not allow for any impermeable contribution so increase thickness based on total of impermeable and permeable areas area of permeable paving 120 x 4217 1756 288mm therefore when rounded up 290mm of permeable sub-base is required conveyance sizing in a system c attenuation design the water will need to flow horizontally towards an

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