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The Development of Permeable Paving in Britain
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Introduction

The following text is taken from a presentation given by Peter Hart, then Managing Director of Formpave Ltd., at the Bristol SUDS Conference in February 2005, in which he described how he came to be involved in developing the first British designed and manufactured Permeable Paving System using concrete blocks.

Further information on Permeable Paving Systems can be found on the Formpave Website.

More about SUDS on the SUDS Introduction page.

 

How it started:

Some 10 years ago, totally out of the blue, I had a phone call from a man called Bob Sargeant who was at that time the Chief Hydrologist of the Forth River Purification board, the ancestor of SEPA, the Scottish Environmental Protection Agency.

He wanted to know if I could make him some no-fines block pavers. He had been to most of the major manufacturers in the UK; all had promised to make blocks for him but had done nothing. I flew to Scotland the next day to meet him and was introduced to the concept of controlling storm water at source. Existing drainage techniques had resulted in uncontrolled run-off from hard surfaces leading to flooding and polluted rivers.

Research in Scotland had shown that 25% of the pollution in Scottish Rivers was caused by non-point-specific run-off. Earlier American research had found that if the first 20%, or "first flush", of run-off is caught, 90% of the pollution is also caught.

Through Bob I met Professor Chris Pratt, Dean of the School of the Built Environment at Coventry University, and a passionate and widely respected exponent of storm water source control. Chris had developed and patented a system of permeable paving blocks with a detention sub-base. His system not only cleaned exit water but also released it in a controlled manner, preventing flooding, but he could not interest any paving manufacturer in bringing it to market. I decided that we should work with Chris and his patent, and the Formpave system was born.

 

Gaining Acceptance:

Excluding the present assembly, of course, consulting Engineers in this country are a very conservative bunch. Because drainage had been undertaken in a time-honoured way, it was the only way. The fact that conventional drainage systems were causing flooding and pollution was largely ignored. I had a very difficult time trying to get the system specified. A typical conversation would go thus:

"It's new is it?
Where can I see it working?
What happens if it goes wrong?
What about my professional indemnity?
No thank you!"

Eventually the system was used on a 2000m² project in Scotland where there was no other option and the ball started rolling, if a little slowly. The next difficulty was in getting Local Authority highway engineers to adopt roads where the system had been used. There was also a very strong element of conservative thinking within local authorities. Comments ranged from "I've always done it this way" to "I am not having any **!*!*!!** water in my sub-base". My reply was to the effect that they were dinosaurs - and look what happened to the dinosaurs!

Eventually a road was adopted in Surrey and adoption is now common practice. However, it is still the case that SEPA are more concerned with quality issues and the Environment Agency in England and Wales with preventing flooding. To date, the system has been used on over 500 schemes and last year we sold approximately 300,000m² in the UK, and we have Licensees in 5 countries.

 

Ongoing Development:

Since the initial launch the system has continued to be developed, and research at Coventry University is going on at a pace as you will hear from Dr Steve Coupe a little later. The initial no-fines or porous block was superseded some 7 years ago by the Aquaflow block. This is stronger, easier to clean, and lets more water through than its predecessor. Subsequent developments have included a heavy duty ML block which can be machine laid. An Aquaslab and Aquasett all of which have the same vertical profile features as the rectangular Aquaflow block.

So much for developments in permeable blocks: what about the sub-base? Chris Pratt's initial designs used Type B drainage material as sub-base stone. It soon became clear, however, that a more or less single sized material could not provide the interlock required. After work at Nottingham University, it was decided to use a 63mm to 10mm crushed material for the sub-base stone. It was found that, when properly compacted, this type of sub-base was equally as strong at Type 1 and had a 30% reservoir capacity.

Further research and trials at TRL Crowthorn showed the benefit of geo-grids within the sub-base and our Dutch Licensees, Aquaflow BV, found that the structure could be further strengthened by substituting the top 100mm of 63-10 stone with 20-5mm. This also acts as a regulating layer and makes it easer for the geotextile to be laid.

Professor John Argue of the University of South Australia in Adelaide has undertaken work on the affects of silt on the ability of the surface to let water through. Paul Ramsey (of Formpave) will talk about this work in his presentation.

Inbitex - improving pollution control:

Dr. Steve Coupe (of Coventry University) will explain later that the use of a geotextile and the type of geotextile used in the system is critical if the exit water is to be properly cleaned. A new unique geotextile called "Inbitex" has been developed and is now specified in all Formpave systems.

Let me tell you why it is unique. The fabric is manufactured from polypropylene and polyethylene. During the manufacturing process, small dishes are created on the surface of the hairs from which the geotextile is manufactured. This is very important in the early establishment of a microbial biomass which is responsible for eating and degrading oils in the water. It is also important during dry periods when the microbial population can retreat into the dishes where moisture is to be found. Thus when it rains again the microbial biomass is quickly re-established.

What about the use of de-icing salts: won't the salt kill the microbes? Well yes and no - a large proportion of the microbes will be killed, but those which survive develop an immunity to salt and a salt-resistant community develops.

 

The future - water harvesting:

Depending on which figure you accept, some 3 million or so new houses are to be built in the UK during the next 20 years. Each person uses around 220 litres of water per day, where is the water to service those houses going to come from? Yorkshire Water imposed a hose pipe ban in October last year after the wettest summer for 15 years. We believe that water harvesting will become an increasingly important feature of our system. The system has already been used to harvest water for re-use in garden centres, schools and blocks of flats. Approximately 20-25% of the water entering the system is lost to evaporation and we have developed an Inbitex composite membrane to drastically reduce this loss. Having harvested the water we certainly do not want to lose it.

The composite membrane consists of a top layer of Inbitex, a plastic web and an impermeable bottom membrane. Water flows through the Inbitex laterally along the web and through slits cut at intervals into the impermeable membrane and into the sub-base. Early results from Coventry on reduction of evaporation and growth of microbes are very encouraging. Water is becoming increasingly important; more important than oil even. You can live without oil but not water, and we see great potential for our system in countries such as India and Africa where life itself is dependant on future supplies of water.

Together with our Dutch Licencee, Aquaflow B.V, we have commissioned work at Delft University on the effects of compaction on the strength of the Formpave sub-base and resistance to rutting. The results are quite startling and show the importance of the type of stone used and full compaction. A design guide will be published in due course covering this aspect.

Further research work will shortly be commissioned looking into the biological quality of the exit water. How close are we to drinking water standards and what happens to the biological quality of the water when it is, say, sprayed on crops in the form of an aerosol? Again this research will be undertaken at Coventry University via a knowledge transfer partnership.

Notes - In 2002 Formpave was awarded the Queen's Award for Enterprise and Innovation for their Storm Water Source Control System

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Inbitex® is available via...
Burdens Geo-Technics
WTB - Geo-Technics
UGS
UGS

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