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Left to soak

Drainage - Paving

Hard paving that absorbs rainfall is catching on, reports Daniel Rea.

'Just after Christmas it poured down in sheets for two days. It would be normal to have had a flood on our hands, and we weren't sure about this new paving, ' says contractor Buxted's gang leader Paul Partridge, gesturing towards an expanse of pristine blockwork. 'But it soaked away as fast as it landed.'

Buxted is in the throes of laying 2ha of permeable paving at a new housing and retail park in Deepcut, Surrey, for developer Barratt. Sustainable drainage, or SuDS - involving soakaway and water retention solutions in place of conventional piped drains - have been regarded warily by most UK clients, which have concerns about long term performance and the cost of maintenance. But at Deepcut there is not a drain in sight.

Barratt was persuaded to opt for the sustainable drainage option by the paving system's phenomenal technical performance: It will absorb surface water at 1.25litres/sec/m 2- a flow rate that exceeds UK drainage regulations 90-fold.

Water is temporarily retained in a high void-ratio sub-base where contaminants are trapped.

Organic pollutants and hydrocarbons degrade over time while heavy metals are immobilised.

An amazing 20% of retained water evaporates. Of the remaining 80%, if an impermeable membrane is laid under the sub-base water can be collected for recycling or piped off long after the initial runoff peak has passed.

Alternatively, it can be allowed simply to soak into the ground.

Discharge rates are typically 2.5l/sec/ha - around 5,000 times slower than the absorbtion rate.

Interest in permeable paving is growing, reports Arup associate director Clive Onions. At the recently completed Bristol Business Park, attenuation ponds or tanks would have been required to slow surface water run-off into the sewer had piped drainage been used. Permeable paving significantly increased the land available for building, and hence the profitability of the site, Onions claims.

Minimising land 'lost' to infrastructure will become increasingly important, he believes.

With growing scarcity of land in the south east and the government pushing to double the number of new homes built every year to 220,000, there is need to rethink drainage strategies.

In Somerset, garden centre Sanders Garden World opted for permeable paving for entirely different reasons: Rain landing on the centre's 6,500m 2of car park and roof is recycled for watering plants. As it filters through the sub-base the water's pH is moderated from an acidic 4.5 to a plant-friendly 7.5. Avon Somerset & Gloucestershire Fire Service has ordered permeable paving to deal with the huge volumes of water it will be sluicing around at its new Avonmouth training ground.

As an indication of changing attitudes to drainage, blockwork supplier Formpave has seen business grow rapidly, with orders for almost 1M. m 2permeable paving on its books.

The principle of Formpave's paving design is simple, but the detail is important. Blocks have bevelled corners. When bedded side by side, these create Vshaped channels along which water flows to vertical drainage slots at the blocks' ends. The 50mm laying course is made up of clean stone, crushed to a uniform 5mm, underlaid with a permeable geotextile membrane that is critical to the capture of pollutants. Beneath this is a 350mm sub-base, laid in two layers of increasing particle size.

Void ratio in the sub-base is around 30%, giving a 1m 3storage capacity for every 10m 2of paving laid. And below the sub-base is a second membrane. This can be permeable to allow infiltration, or non-permeable to 'tank' the water, which can then be recycled or piped off.

From a sustainability point of view, despite permeable paving's benefits to the water regime, the sub-base requires a large amount of virgin aggregate. However, permeable paving pioneer Professor Chris Pratt, formerly of Nottingham Trent and now at Coventry University, says there is no reason crushed concrete or brick cannot be used instead. He is testing the materials right now.

The Environment Agency is keen to see more sustainable drainage schemes brought forward as a way of replenishing ground water and mitigating flood risk. And Agency diffuse pollution research project manager Phil Chatfield reports that 'oil drips, even the loss of a whole sump-full of oil from a car, can be removed. These trapped hydrocarbons are naturally broken down by microbes in the aggregate. Silts adsorb heavy metals and other runoff pollutants, and hold them in the top layer.'

Chatfield thinks areas where water is scarce will see such systems implemented more widely.

Meanwhile, one of Formpave's central sales pitches is that permeable paving is competitive on cost - around £40/m 2all-in for geotextile membranes, aggregate, blocks and groundworks.

One of the big questions over permeable paving is maintenance. 'People talk as though traditional systems needed little or no maintenance. Paving solutions like Formpave's also need maintenance, but it's simply different, rather than more expensive or awkward, ' comments Chatfield.

The recommended routine is twice yearly suction cleaning, and replacement of the top 50mm laying course every 20 years. Performance reducing clogging is highly unlikely before that, he says.


For more on SuDS visit www. www. ciria. org. uk/suds www.

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