What is the infiltration rate of permeable paving?

The required infiltration rate of permeable paving is 400mm/hr/m2. The infiltration rate of newly installed permeable paving can be over 4,000mm/hr/m2. The bedding course and sub base aggregate below the permeable paving will have even higher infiltration rates.  

However, the infiltration rate of the ground, known as the sub-grade, plays a key role in how permeable paving is designed. The build-up of permeable paving over ground where water can be easily absorbed will be very different from one where the ground has very low water permeability. 

This blog will explore the three different types of system used for permeable paving to ensure an adequate SuDS (Sustainable Drainage System) solution is achieved and that surface water can be managed and drained appropriately for any project scenario. 

What are the three types of permeable paving system needed for different sub-grade permeabilities? 

The first type of permeable paving system allows the water to drain quickly through the surface and sub base. This is used where the sub-grade has good permeability. This system is known as Type A – Total Infiltration.  

Where the sub-grade has very poor permeability, the water needs to be contained before being carefully released. This is called Type C – Attenuation.  

If the sub-grade tests show that the soil permeability is somewhere between the two, then a Type B – Partial Infiltration system is used. 

Each system will look no different on the surface, it is the build-up of the layers below the permeable paving that changes. The section below covers how each system works in more detail: 

Permeable paving system Type A – Infiltration  

This is specified when the sub-grade has been tested and shows sufficient permeability to allow water to drain through at the required rate. 

The water falls on the surface and drains through the joints between the permeable blocks. From there, it filters through the laying course of grit and into the coarse graded aggregate sub base. Finally, it drains through a permeable geotextile (shown in green in the diagram) and infiltrates directly into the ground.  

Water is not sent to drains so there is no need for drainage pipes or gullies. This makes Type A the simplest and most economical permeable paving option. 

Permeable paving system Type B – Partial infiltration  

This is specified when the ground is permeable enough to receive some water, but not to a level sufficient for Type A to be used. 

This system can be used in situations where the sub-grade may be capable of absorbing some water without losing its stability. Outlet pipes are used in the sub base to take excess water away to be collected and eventually discharged through a different part of the drainage system. The rate of that discharge will be controlled in line with regulatory guidance to meet local water management requirements.   

Permeable paving system Type C – Attenuation  

This is specified when the sub-grade has been tested and found to have insufficient permeability, or where introducing water could destabilise it. It can also be utilised where the water is required to be redirected for re-use or irrigation.  

An impermeable membrane (shown in blue on the diagram) lines the sub base and is wrapped up the sides to enable the complete capture of the water. This effectively forms a storage tank. Outlet pipes are constructed through the impermeable membrane at suitable locations to direct the water to secondary drainage systems such as watercourses, ponds or treatment systems. The diameter of the outflow pipe is site specific so it should be determined by the project engineer according to the requirements. 

The water can be held here before being redirected to another watercourse or drainage system. For this reason, System C can be used on contaminated sites. This is because it prevents pollutants from being washed down into the sub-grade where there is a danger they could eventually reach groundwater. 

System C is also used in areas with high water tables to prevent them from entering the sub base where they would compromise its ability to drain away surface water.  

Where can I learn how to select the most suitable permeable paving design for my project? 

Tobermore has extensive experience in supplying permeable paving. If you require assistance with your permeable solution, you can request a free permeable paving design from us here. 

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

“All the materials within a permeable paving system need to work together to ensure the infiltration rate matches the requirements of each project. We ensure that our customers get the correct design advice to deliver permeable paving that achieves the drainage specifications and meets the project’s aesthetic requirements.”  

If you would like to learn more about permeable paving, we have a dedicated CPD available that you can request. This can be delivered in person or online.