Permeable Paving Overview
Flooding & Pollution
As urban and industrial areas throughout the country have continued to expand, the total area of impermeable surfaces such as roofs, roads, pavements and car parks has increased. This means that rainfall cannot infiltrate naturally into the ground and puts our already overloaded drainage systems to an even greater test. With global warming leading to increased levels of rainfall, our ageing drainage systems cannot cope with high volumes of storm water run off. This leads to flash flooding and increased pollution.
During periods of dry weather, heavy metals, hydrocarbons, oil, rubber and other pollutants are deposited on impermeable surfaces. When it rains these pollutants are washed into drainage systems and end up further downstream in rivers where they damage wildlife habitats.
As flooding has become more common across the UK in recent years, new developments must be carefully located and designed to minimise any increased surface run-off. Where appropriate, storm water source control measures, which also improve water quality, should be incorporated into the development proposal.
The solution to flooding problems is the use of more sustainable methods of storm water drainage which take into account quantity, quality and social issues. These are referred to as SUDS (Sustainable Urban Drainage Systems). SUDS are made up of a number of structures, which include permeable pavements, swales and filter strips, basins and wetlands, and infiltration devices.
They are more sustainable because they:
- deal with run off close to where it falls;
- manage potential flooding at its source;
- protect or enhance water quality;
- provide a habitat for wildlife in urban areas;
- protect water resources from accidental spills and pollution;
- allow new development in areas where existing sewerage systems are at full capacity, enabling new development within existing areas;
- are sympathetic to the environmental setting and the
- needs of the local community;
- encourage natural groundwater recharge
The inclusion of a permeable paving surface as part of a SUDS solution will not only increase the likelihood of planning proposals being approved but may also offer the developer a 10-15% cost saving over traditional methods due to a significant reduction in drainage costs (pipes, manholes etc). Machine laying the paving blocks also reduces costs.
Water quality improvement
Permeable paving is very effective at removing pollution from runoff. The pollutants may remain on the surface or may be flushed into the underlying pavement layers where a very high percentage of the pollutants are filtered, trapped or degrade over time. For more detailed information on water quality please refer to the Guide to the design, construction and maintenance of< concrete block permeable pavements - Edition 5 produced by Interpave available for download on www.paving.org.uk
Four Elements of Permeable Paving
There are four elements which make up permeable paving:
The Hydropave range of products have permeability rates of 1800 litres/sec/hectare, providing a significant safety factor on the required permeability rates of 180 litres/sec/hectare.
The sub-base thickness should be designed to detain rain falling throughout 24 hours and must provide at least 30% void space. A 4/20mm coarse open graded crushed rock to BS EN13242:2002, value of 100kN or more must be used. If the designer opts to specify alternative materials, a site trial must be undertaken to ensure that the laying course material does not invade the sub-base material.
The laying course and jointing material must be a 6.3-2mm grit to BS EN13242:2002. This has been proven in tests to perform best and will not invade the sub-base material. In ground conditions where water cannot infiltrate into the subgrade, an impermeable membrane must be laid between the subgrade and sub-base and wrapped up the sides to detain the water.
As water flows slowly into the sub-base it collides with individual aggregate components and deposits pollutants on their surface. The large surface area of the aggregates will ensure effective filtering of the pollutants which are then broken down by natural microbial action or eventually rest at the bottom.
The specification of a permeable paving structure depends upon the hydraulic and traffic loading characteristics and upon the properties of the subgrade. Most pavements in the UK will be required to have a water detention capability rather than infiltration because of the subgrade types. It is recommended to design a permeable paving system to store rainfall occurring during 24 hours. For information relating to the BREEAM ratings of concrete and natural stone products please refer to ‘The Green Guide to Specification’ 3rd Edition 2002. Or alternatively visit www.breeam.org
There are two common types of permeable paving systems, Attenuation and Infiltration.
The Attenuation system temporarily stores the water in the crushed stone beneath the paving before being slowly released back into the drainage systems. This therefore reduces the peak downstream flow from a result of heavy rainfall.
The Infiltration system allows the water to flow slowly through the crushed stone beneath the paving before being slowly released back into the ground through a geotextile.
What system should I use?
Tobermore can introduce you to specialist engineers, who can provide design advice for your permeable paving project.
What are the key components of a permeable paving system?
The key components of the system are the sub-base, the bedding layer, the paving blocks and the jointing grit. As long as the system has been adequately designed for the site the most important next step is to get these components right.
How do I design a permeable paving system?
Please refer to the following source of information:
BS7533-13:2009 Pavements constructed with clay, natural stone or concrete pavers. Guide for the structural design of permeable pavements.
If you require specialist advice Tobermore can put you in contact with experienced design consultants.
An upper geotextile may be used between the sub-base and the bedding layer. Please refer to BS7533-13:2009 for further information.
What if the ground is poor or there will be significant vehicles on the paving?
Various improvement layers can be installed below the permeable paving to ensure the system can meet the structural load on the project. The engineer on the project will design these into it based on the loading category required. The two most common types of improvement layers are Dense Bitumen Macadam (DBM) and Cement Stabilised Coarse Graded Aggregate (CSCGA).
Specifications for Dense Bitumen Macadam and Cement Stabilised Coarse Graded Aggregate can be found in BS7533- 13:2009. Method statements on how to install these layers can be supplied on request.
What is the ratio required of permeable area (Hydropave) to impermeable areas?
Sometimes, water from building roofs or nearby impermeable areas is fed into permeable paving. It is best to do this only in the case of attenuation systems when a typical ratio of 2:1 can be used between impermeable and permeable zones.
Note: A permeable paving design relies heavily on using the correct aggregates. Prior to installation, we would ask you to test both the 4/20mm coarse graded aggregate and also the 6.3-2mm bedding and jointing grit as per the relevant British Standard specification (BS EN 13242:2002). In particular, the material should be categorised as LA20 according to Table 9, SZ18 according to Table 10 and MDE15 according to Table 11 within this standard. The grit should be insoluble in dilute hydrochloric acid and should be naturally occurring material. In our experience, incorrect use of aggregates is one of the most common reasons for failure of a permeable paving design.
How to Install a Permeable Paving System
Please refer to the following sources of information - BS 7533-3:2005 Pavements constructed with clay, natural stone or concrete pavers. Part 3: Code of practice for laying precast concrete paving blocks and clay pavers for flexible pavements.
Sub-Base: 4/20mm aggregate
After putting the required depth of stone in position, it needs to be re-orientated using either a tamdem roller or a plate vibrator.
Bedding Layer: 6.3-2mm laying course material
The bedding layer should be screeded over the 4/20mm.
When the blocks are placed to the chosen pattern they should be plate vibrated, before the 6.3-2mm grit is brushed into the joints until they are completely filled.
All joints must be filled to the top with 6.3-2mm grit. Joints which are not fully filled can lead to possible movement of the blocks after use. We recommend that after a few weeks use that any joints, which have settled and are not full, are topped up with grit. Joints should be kept filled at all times. You will need approximately 1 ton of grit for every 100m2 of 80mm paving. Care should be taken that the permeable joints do not become contaminated as work on the scheme is completed. Special care needs to be taken when soft landscaping is carried out so that soil does not enter the joints.
Due to the superior drainage capabilities of Hydropave, it is possible to install the product in more adverse weather conditions in comparison to standard paving.
For optimum performance we recommend that paving is cleaned twice a year.
Fill the joints
When permeable paving is first laid we often find that there is some settlement of the jointing material and so it is important to top up these joints with 6.3 –2 mm grit after a few weeks. Indeed, it is also important that, at any time during the life of the pavement, the joints are kept full of grit. Please check carefully that you are using the correct grade of joining grit.
WARNING – do not replace the jointing grit with sand as the sand will block the joints and stop the permeable system from working.
Please do not use vacuum sweepers as they can suck the grip out of the joints.
The surface layer (Hydropave blocks)
Initial Cleaning - When an area has just been paved, please allow it to settle for a few weeks. Then, you may wish to lightly hose down the paving to remove any excess dirt. The area should then be treated with a weed killer suppressant, two or three times a year as required.
General Dirt - Regular sweeping to prevent the build up of detritus is recommended. Light coloured blocks, emphasise tyre marks and oil spills on the pavement. It must be accepted that these products will need more maintenance if the overall appearance is to be maintained. A light power hose at medium pressure is generally all that is required to clean general dirt and grime. Any jointing grit which has been removed must be replaced. Do not use high pressure power washers as aggressive power-washing can damage the product surface and dislodge the jointing grit.
The joints between the Hydropave blocks - It is important to ensure that the grit filled joints do not become blocked, as the water will not be able to flow into the sub-base. Experience has shown that joints have good permeability for many years. However, it is good practice to brush the joints with a stiff brush, to break the crust of detritus, which inevitably forms at the top of a joint. This simple exercise improves the permeability significantly.
Care should be taken that the permeable joints do not become contaminated at the construction phase and special care needs to be taken with soft landscaping to ensure that soil does not enter the joints.
How to clean clogged joints
If the joints on a paved surface have been badly contaminated and clogged it may be necessary to clean them thoroughly. This can be done as follows:
- Remove existing jointing grit and detritus by high pressure water jet.
- Reset the pavers, which will have become displaced
- Replace any damaged pavers at this time since they will have been loosened.
- Apply 6.3-2mm single size grit to the joints, using a stiff brush to sweep the material in ensuring joints are filled.
- WARNING – do not replace the jointing grit with sand as this will stop the permeable system from working.
The sub-base below the Hydropave blocks
We would strongly advise not to disrupt the sub-base below the Hydropave blocks. This sub-base is specially designed for each particular site. If you have no alternative but to disrupt the sub-base please speak to the contractor prior to disruption so you can receive clear guidance on how to reinstall the material in the correct formation.
In the event of any material being lifted and reinstated, please use the following Material Specification:
* In particular, the material should be categorised as LA20 according to
Table 9, SZ18 according to Table 10 and MDE15 according to Table 11 within this standard. The grit should be insoluble in dilute hydrochloric acid and should be naturally occurring material.
** You must ensure the grit and coarse graded aggregate are compatible. For example, if you use a finer grit it may perculate into the sub-base and may destabilise the pavement. Please refer to the following source of information – BS7533-13:2009 Pavements constructed with clay, natural stone or concrete pavers. Guide for the structural design of permeable pavements.
Click here to download the Hydropave Maintenance Document
Hydropave Permeable Paving Range
Tobermore offer an extensive range of Hydropave Permeable Paving Products. With all the versatility of our standard and most popluar paving products, the Hydropave permeable paving range allows you to create spectacular hard landscaping features with environmentally friendly products that reduce the risk of flooding and pollution.
Many of these quality products can be machine laid thus reducing labour costs.