Landscape & Urban Design Issue 55 2022 | Page 35

CBPP upper layer is simply applied as an overlay replacing planed-off old asphalt or other road surface , onto the original structural road base . In many cases , this can simply run kerb-to-kerb to form a shared surface level with the footway and flush kerb top . This approach also enables low-cost improvement works – perhaps linked to ‘ Healthy Streets ’, new Highway Code measures , or as part of creating traffic calming , Low Traffic Neighbourhoods or Homezones .
The same blocks and 2 / 6.3mm grit bedding layer and jointing material as for permeable pavements generally are used . Water is attenuated , treated and conveyed within the laying course , which is key to this technology , enabling filtration of silt and retention / treatment of pollutants without clogging . A gradual flow of clean water can then be released near the surface , meeting SuDS requirements .
Permeable Paving and Trees in Harmony CBPP and trees have been proven to work together in synergy . CBPP – whether full-construction or in overlay form – can collect rainfall away from the canopy and convey it to the tree . It can then simply discharge horizontally into a raingarden , perhaps with overflow into an existing adapted gulleys . The raingarden stores water during heavy rain for SuDS , retains soil moisture during dry weather and provides additional water quality ‘ polishing ’, as well as irrigation .
Alternatively , CBPP can be used over standard tree pits , proprietary tree planters , Stockholm System or other structural soil installations , enabling irrigation and simple gas ( oxygen / carbon dioxide ) exchange essential to trees – without
additional reservoirs or pipes . CBPP also avoids tree root disruption common with other paved surfaces . An exemplar of this technology is the award-winning , adopted highway regeneration project Bridget Joyce Square in Australia Road , White City , London . Interpave revisited the project in August 2021 , around 5 years after completion , and noted that the permeable paving was performing well and experienced no problems during recent extreme summer storms , despite extensive flooding nearby .
Tests were also carried out demonstrating that all the water infiltration rates of the permeable paving were more than double those recommended by ‘ The SuDS Manual ’ ( CIRIA , 2015 ). The trees and other green infrastructure were healthy , substantial and particularly wellestablished . A case study exploring Bridget Joyce Square is available on Interpave ’ s website and other innovative project case studies will be published shortly .
This approach provides numerous benefits including :
• Interception losses , managing runoff during regular rainfall events
• Attractive , popular surfaces with no puddles or potholes , for user safety
• Maximising re-use of existing road-base and its embodied carbon
• Optimising the original drainage regime , but below the surface
• Low-intervention , low carbon , noheat retrofit installation
• Long-life with minimal maintenance and low whole-oflife cost
• No open gulleys – wildlife-safe
• Integral water filtration , with debris / litter remaining on the surface
• Straightforward access to belowground services
• Sustainable re-use of blocks for reinstatement or changed layouts . www . paving . org . uk
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