Permeable paving and water management

Clay Technology magazine
26 Sep 2019

Rising rainfall and urban growth are contributing to greater water management challenges and increased need for sustainable drainage systems to help alleviate the pressure. 

The effects of climate change are evermore noticeable, and continuously growing cities with buildings and paving covering green spaces means there is less land available for efficient water drainage. One way to tackle the problem is installing porous structures that limit run-off and help water seep into the earth and away from the surface.

Sustainable Drainage Systems (SuDS) is a concept introduced to the UK under official government guidance published in 2005. Since then, social and environmental pressures have deepened the need for more diverse and sophisticated methods of implementing smart water management. Marshalls SuDS marketing development manager, Chris Griffiths, works with concrete block permeable paving (CBPP) and development of the company’s water management range.

‘SuDS have been recognised in the UK for nearly 25 years. Formpave from Hanson was the UK’s first permeable paving system and that was introduced over 20 years ago,’ Griffiths told Clay Technology. ‘We’re currently seeing a move over towards SuDS that provide multiple benefits, in terms of biodiversity and amenity, as well as dealing with water volume and quality. This trend is manifesting itself most visibly in the fast-growing number of raingardens we are involved in designing. These are pockets of attractive, biodiverse planting built into hard landscapes that add real sensory appeal to urban schemes, but also provide water storage and infiltration in storm events.’

While SuDS encompasses a whole arsenal of tools, including soakaways and attenuation tanks, one of the most common is permeable paving, which is deemed ‘extremely important’ by Scott Parnell Water Management Operations Manager, Wendi O’Toole. ‘As we experience climate change – long, hot summers and heavy, sudden rainfall – the possibility of flooding becomes more likely. Ensuring we manage this responsibly in construction is essential. Permeable paving will reduce surface run-off, and therefore pressure on drainage and the risk of stormwater flooding,’ O’Toole told Clay Technology.

‘As our climate becomes more unpredictable and the possibility of flooding increases, it is more and more important that planners and engineers work together closely to ensure the best possible solutions to tackle and prevent flooding are being developed, used and installed.’

Sticking issues

For domestic installations, permeable paving may be sufficient to manage low levels of surface water, but when it comes to large areas with high degrees of exposure and rainfall, some believe there is a tendency for saturation and reduced effectiveness. As such, there are several investigations underway into cementitious mixes that prevent clogging, but as the pavers act as an inlet is this necessary?

‘Clogging really isn’t an issue. It’s one of the more frustrating myths about CBPP that we spend a lot of time dispelling,’ Griffiths explained. ‘We have been selling permeable systems for nearly 20 years and our experience is that permeable paving clogs very rarely. What usually happens is that during normal use, a crust of dirt, sand and debris settles into the joints, which makes them appear blocked. This “crust” is usually about 5–10mm thick and is as permeable as untreated ground.

‘This slows down the rate at which water infiltrates but that still leaves the surface permeable enough to cope with any UK rainfall event. This is supported by Soenke Borgwardt’s independent 2006 study titled Long-term in-situ infiltration performance of permeable concrete block pavement, which concludes that after 10 years with zero maintenance, CBPP will plateau at about 10% of its original permeability. In the case of our Priora system, this is about 1,750 litres per second per hectare (l/s/h), versus an average UK rain shower of 180l/s/h.

‘Of course, if someone has dumped a tonne of fine sand on a CBPP surface it might well end up clogging, but brushing it with a stiff brush or sweeper is usually enough to agitate the dirt in the joints and restore the pavement to its original permeability.’

Water storage

When permeable paving is insufficient, Scott Parnell recommends installing attenuation tanks to store excess stormwater. O’Toole explained how the systems work.

‘We use modular crates, an impermeable membrane, geotextile and top hats which are sealed to pipe adaptors that form a fully welded system,’ she said. ‘The impermeable membrane is prefabricated, unrolled and cut to the required length in strips and placed together, overlapping by at least 100mm. The overlap is welded using a 110v Leister Twinny welding machine, which combines hot air with roller pressure to create a homogenous single or twin-welded seam. Additional strips are added to create a base panel. The base panel incorporates the side of the tank. The panel is folded and rolled and then delivered to site to be installed.

‘During installation, geotextile will be unrolled to form the base and cut to size until the whole base of the tank has been fully covered. The prefabricated membrane base panel is then placed in position. The panel is unrolled and unfolded by hand by a technician and once the membrane and geotextile is in place, the assigned modular crates can be installed to the agreed size and shape of the tank. The modules are then clipped together using shear connectors to create multiple layers. At module installation, top hat sections for inlet and outlet pipe penetrations are then sealed to the membrane at the agreed levels. Top hats are sealed to the membrane using a 110v Leister Triac hand weld gun and silicon pressure roller. When the top hats and modules are in place, the sides of the tank are pulled up and corners are sealed. The tank is then stable to walk on and edge barriers will be provided by the principal contractor if required.

‘The excavation can be backfilled as the work progresses, if this is more suitable. The lid panel is placed into position and sealed to the top of the tank, with top hats added for vent pipe work where required. Geotextile is placed to the sides and top of the tank to help protect the membrane during the backfill process, thus completing the installation.’

Research and development

Another area of research and development is the choice and blend of materials for new bricks and paving. An array of options is crucial as O’Toole explained the performance could change depending on the local conditions, including ‘location, existing infrastructure, flood conditions, topography, geology. For example, how permeable paving is applied on impermeable soil types, like clay, could differ from how it is installed on soil that drains more easily, such as sandy soils’.

While some companies are investing in geosynthetic material combinations, others are keen to incorporate plastic-based ‘bio bricks’ into their projects to remove low-grade plastics from the waste stream. This is largely being driven by the public perception of concrete as a carbon-intensive material, and the need to be seen to be taking a stand. Regardless of the reason, there is always a risk when shifting to new materials, and manufacturers will have to weigh up the credentials of the whole value chain and not solely focus on concrete.

‘While concrete is carbon-intensive to manufacture, it has been proven to absorb a significant proportion of that carbon back during its use,’ Griffiths said. ‘It is also extremely durable and robust, meaning that cradle-to-grave, carbon can be lower than less resilient materials.

‘Plastic can be recycled into some really useful and adaptable construction materials, and its use in some new kinds of road surfaces seem to have produced positive results. I’ve yet to see any plastic blocks that perform in the same way as concrete though. While we’re open to (and constantly investigating) new materials and technologies, using the right material for the right job is paramount to a successful project.’