What are permeable paving installation guidelines and maintenance principles?

The installation guidelines for permeable paving will depend on how the water draining through the 6mm joints between the blocks or slabs is managed by the sub base. The maintenance principles of the permeable paving slabs or permeable block paving will remain the same, regardless of the composition of the sub base. 

In this blog, we will show the two common ways in which surface water is handled by permeable paving: 

1. Water can be stored within the sub base before being redirected elsewhere, known as attenuation or System C. 
2. An infiltration approach can be taken, referred to as System A, where water is allowed to slowly flow through the sub base and into the ground below.  

The key components of a permeable paving system are the sub base, the bedding layer, the paving blocks and the jointing grit. The permeable paving specification most suited to your project can be determined by specialist engineers or architects. We at Tobermore can help with site specific technical design of permeable paving and offer insight in this blog. 

We will first look at the design considerations that help determine the best specification for permeable paving. This will be followed by installation and maintenance guidelines.  

How should a typical attenuation system (System C) be designed? 

An attenuation system temporarily stores the surface water in the crushed stone of the sub base beneath. An impermeable membrane lines the sub base to effectively form a storage tank. Outlet pipes are placed at suitable locations, and these allow the stored water to be slowly released back into the drainage system. This reduces the peak downstream flow resulting from heavy rainfall. The water can also be directed into holding tanks for later treatment. Therefore, this type of attenuation system can be used on sites where contaminated water needs to be prevented from reaching the ground or where the water flow needs to be directed elsewhere. 

The composition of a typical attenuation permeable paving system 

The image below details the typical attenuation, or System C, permeable paving design that uses Hydropave permeable paving. 

How should a typical infiltration system (System A) be designed? 

The infiltration system allows the surface water that drains through the paving joints to flow through the grit of the bedding layer and into the coarse graded aggregate of the sub base. From here, the water is slowly released back into the ground through a geotextile layer. 

The composition of a typical infiltration permeable paving system 

The composition of a typical infiltration permeable paving sub base is detailed here. The drawing below shows a system that uses Hydropave permeable paving blocks. 

What are the design considerations for the jointing and bedding layer, and sub base aggregate? 

The size and type of jointing and bedding layers, and how they interact with the sub base aggregate, should be assessed with reference to the relevant standards. For construction phase, we can provide support with value-engineering of a sub base for your project. Below are examples of the considerations for jointing and bedding layers: 

Jointing and bedding material for permeable paving 

Jointing and bedding material should be a 6.3-2mm grit. This should be categorised as LA30, Fl20 and MDE20 according to table A.3 within standard BS EN7533-13:2009. The grit should be insoluble in dilute hydrochloric acid and should be a naturally occurring material.  

Sub base material for permeable paving 

A clean 4/20mm coarse graded aggregate to BS EN13242:2002 must be used for this component. A crushed rock angular stone works best.  

Compatibility between grit and sub base 

The grit and coarse graded aggregate must be compatible. If a grit is used that is too fine, it may percolate into the sub base and may destabilise the permeable paving. The standard BS7533-13:2009 ‘Pavements constructed with clay, natural stone or concrete pavers – Guide for the design of permeable pavements constructed with concrete paving blocks and flags, natural stone slabs and setts and clay pavers’ should be used as a source of information. It is also a good document to use as an overall guide when designing permeable paving specifications.  

How is a permeable paving system installed? 

A permeable paving system should be installed in line with BS 7533-3:2005 ‘Pavements constructed with clay, natural stone or concrete pavers – Code of practice for laying precast concrete paving blocks and clay pavers for flexible pavements’. Here are some installation guidelines for key components of a permeable paving system: 

Installation of the sub base in a permeable paving system 

After putting the required depth of 4/20mm aggregate in position, it needs to be reoriented using either a tandem roller or a plate vibrator. 

Installation of the bedding layer in a permeable paving system 

The bedding layer of 6.3-2mm laying course material should be screeded over the 4/20mm aggregate of the sub-base. An upper geotextile may be used between the sub base and the bedding layer. BS7533-13:2009 should be used for further information if this is being installed as part of the permeable paving specification. 

Installation of paving blocks in a permeable paving system 

When the blocks are placed in the chosen pattern, they should be plate vibrated before the 6.3-2mm grit is brushed into the joints. 

Joint filling in a permeable paving system 

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. After a few weeks, it is recommended that any joints which have settled and are not full be topped up with grit. Joints should be kept filled at all times. Approximately one tonne of grit will be required for every 100 m2 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. When this type of work is being carried out, the surface of the permeable paving should be protected by an appropriate cover to protect the joints from being contaminated. 

Suitable weather conditions for the installation of permeable paving 

Due to the superior drainage capabilities of permeable paving, it is possible to install the product in more adverse weather conditions compared to standard paving. 

What maintenance is required for permeable paving systems? 

The maintenance requirements for permeable paving are minimal, and the ability of permeable block paving to continue to provide drainage over many years has been proven. Even after allowing for clogging, studies have shown that the long-term infiltration capability of permeable paving will normally substantially exceed UK hydrological requirements. The infiltration rates of permeable paving do decrease, but they stabilise over time. Even allowing for clogging, long term infiltration rates substantially exceed UK rainfall requirements. 

To maintain its optimum performance, it is recommended that permeable paving be cleaned twice a year. 

How to undertake the initial cleaning of newly installed permeable paving 

When an area has just been paved, it should be allowed to settle for a few weeks. After this, the paving may be lightly hosed down to remove any excess dirt. The area should then be treated with a weed killer suppressant, two or three times a year as required. 

How to remove general dirt from permeable paving 

Regular sweeping to prevent the build-up of detritus is recommended. Light coloured blocks emphasise tyre marks and oil spills on the pavement. 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 is removed must be replaced. High pressure power-washers must not be used as aggressive power-washing can damage the product surface. A trial area should be tested before large-scale power-washing takes place. Do not use vacuum sweepers as jointing grit may be removed. 

How to clean clogged joints in permeable paving  

After several years of use, joints may become clogged with detritus, and it may be necessary to clean them. 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.
• Finish off as above.

Where can I get advice on specifying, installing and maintaining permeable paving systems?  

If you would like expert advice on how to correctly design, install and maintain permeable paving, please contact Tobermore’s technical team who will be happy to help. They have in-depth knowledge built up from years of successfully specifying Tobermore permeable paving solutions for a wide variety of projects.  

Craig McBride, Tobermore’s Technical and Product Training Manager, comments: 

“Our technical team understand the value of looking at permeable paving specification as more than just the selection of the permeable block paving itself. We ensure that the whole design is right from the ground up and offer installation and maintenance guidance. In this way, we help our customers get a permeable paving solution from Tobermore that will stand the test of time.”