Chris Hodson of Interpave discusses why concrete block permeable paving (CBPP) is a key sustainable drainage system (SuDS) technique
Extensive new tree planting with provision for long-term nurturing is now a requirement for developments, with new government policies taking effect. Concrete block permeable paving offers an important opportunity to help satisfy these requirements with minimal maintenance requirements over the long-term.
The 2021 ‘National Planning Policy Framework’ (NPPF) states: “Planning policies and decisions should ensure that new streets are tree-lined and that opportunities are taken to incorporate trees elsewhere in developments.” However, measures are needed to nurture and allow trees to mature and deliver their real potential – including net carbon storage, urban cooling through shading and evapotranspiration, biodiversity and public wellbeing. So, the NPPF also requires that: “Appropriate measures are in place to secure the long-term maintenance of newly-planted trees.”
Paving and trees in harmony
Urban trees and paving have traditionally been seen as in conflict, creating maintenance problems. But this is not the case with concrete block permeable paving (CBPP), a key sustainable drainage (SuDS) technique designed to reduce flood risk and make cities more liveable. CBPP offers unique opportunities to collect, attenuate and convey rainwater runoff, removing pollutants before irrigating green infrastructure and supporting biodiversity.
Unlike conventional impermeable hard landscape materials, CBPP allows the same pattern of run-off transfer to the ground as natural vegetation, allowing water – and importantly air – to reach tree and shrub roots, still providing an attractive, safe surface above. Long-term tree maintenance measures will now be included in planning consents and a straightforward spatial solution – such as CBPP providing irrigation – offers a multifunctional solution over the long-term.
The benefits of CBPP for trees while retaining accessibility are recognised by the current Code of Practice BS 8300-1:2018, which states that: “Tree grilles should be avoided. Smooth or paved permeable surfaces should be used wherever practicable.” Permeable paving can be laid level and still avoids puddles, as well as potholes, without the need for drainage gullies. It provides a safe surface for everyone – including wheelchair users and people pushing prams. Two decades of experience in the UK demonstrate the long-term performance of CBPP with minimal, if any, maintenance.
Essentially, CBPP technology shares the same impressive performance as conventional modular concrete paving, being slip resistant, durable, strong and sustainable. The difference with CBPP is enlarged joints, filled with a permeable aggregate, and the materials used below the blocks, which are specifically selected to accommodate water. It’s important to remember that CBPP is unlike – and not to be confused with – permeable materials, which behave very differently.
Demonstrating minimal maintenance requirements
Probably the most persistent misconception with CBPP is that – without regular maintenance – the joints soon become clogged and impermeable, preventing water from draining into the paving. In fact, over time detritus and silt do collect in the joint material between blocks, where a surface ‘crust’ is formed, protecting the laying course and sub-base material but still allowing water infiltration. CBPP is not significantly affected by moss or weeds in the joints, or by leaves collecting on the surface. Even if occasional localised blockages do occur, in a storm excess water simply flows to more permeable, adjacent areas.
Recent trials of concrete block permeable paving following long-term use, endorse current minimal maintenance recommendations, with important implications for whole-life-costs on all types of housing. One of the projects – Martlesham Park and Ride, Suffolk – is the subject of a video of the trials and an illuminating interview with the project designer about long-term performance, and also the benefits of CBPP for trees.
The 13,000 m2 CBPP project has delivered problem-free performance over 17 years. Extensive mature trees around and within the site were simply retained as part of the scheme. Maintenance has effectively been cosmetic, occasionally sweeping tree debris and vegetation from the surface and leaf fall in the autumn. Tree roots have continued to grow within the permeable pavement sub-base but – importantly – there have been no visible signs of block rutting or lifting, or roots coming through the surface – and there is plenty of international research supporting this benefit.
Regeneration and re-use
As well as new construction, concrete block permeable paving can also be used as an overlay, regenerating and re-using existing, conventional road bases or other hard surfaces, as well as their embedded carbon. These thin overlays create attractive, safe and sustainable shared-surfaces that can supply a gradual flow of clean water horizontally into rain gardens or bioretention areas with trees and other green infrastructure, to store water for irrigation, biodiversity and SuDS.
This approach is exemplified in Bridget Joyce Square, London, an important, award-winning regeneration project. Here, a typical, adopted asphalt street and adjacent parking areas were transformed for community use with CBPP overlay shared surfaces and tree-planted rain garden basins – an exemplar for future urban landscapes. The basins provide water storage for SuDS to reduce overloading existing drains, as well as for irrigation. Interpave revisited the project in August 2021, around 5 years after completion, and noted that trees and other green infrastructure were healthy, substantial and particularly well-established. The permeable paving is also performing well and, it is understood from local sources, experienced no problems during this year’s extreme summer storms, despite extensive flooding nearby.
Chris Hodson is consultant to the trade association Interpave.