Design and specification of permeable paving by loading categories

Loading categories describe the type of traffic and loading that an area of permeable block paving is expected to withstand. These range from light domestic applications through to heavy traffic areas with up to 1000 large HGVs per week.   

The permeable paving specification must provide sufficient support to meet the requirements of each loading category. This requires particular attention to be paid to the sub base and capping layers that support the permeable paving.  

Most permeable pavement designs require a full engineering approach. However, at the project appraisal stage, having standard solutions available is usually suitable. In this blog, we will first look at the loading categories for permeable paving. Then we will describe some standard solutions of sub base and capping layers for different loading categories. 

What are the loading categories used for permeable paving? 

The loading categories used for permeable paving designs are listed in the table below. They range from Category A, the lowest loading category, through to Category F, describing the highest: 

What factors affect the design of the permeable paving sub base and capping layers? 

Two key factors affect permeable paving specification, particularly the design of the sub base and capping layers. One is the strength of the soil, and the other is the loading category that the permeable paving is required to support. 

The influence of CBR on permeable paving sub base and capping layers  

The strength of the soil and its stiffness can be measured by its CBR (California Bearing Ratio). Prior to project commencement, it is advised that all sites are independently inspected and assessed by a suitable qualified engineer to establish the CBR of the subgrade present. 

If the CBR figure is 5% or greater, capping layers – these are layers of material introduced to improve the supporting strength of the permeable paving design – are not required.  

The influence of loading categories on permeable paving sub bases and capping layers 

The loading categories we covered in the earlier table may require capping thickness to be adjusted upwards to ensure that a firm working platform can be achieved, capable of sustaining loads from construction traffic without deforming excessively. It should also offer sufficient reaction to allow the overlying materials to be fully compacted. All pavements, including permeable solutions, must meet the requirements of BS7533 Part 3 in terms of installation. 

What are examples of standard designs of sub bases and capping layers for permeable paving? 

We will now illustrate some standard permeable paving designs to show how CBR and loading category influence the specification of permeable paving, particularly the thickness of the capping and sub base layers. 

The thicknesses of coarse graded aggregate in the examples will be suitable for the traffic conditions shown. In the case of attenuation designs, where water is contained to be released later into drainage systems, the aggregate should be thick enough to meet the attenuation requirements. However, a site-specific hydraulic analysis should be carried out. 

In the case of infiltration designs, where water slowly drains from the sub base into the ground, there is usually no need to check hydraulic capacity. However, a check must be made on the infiltration capacity of the ground. The ground must be proven to accept 20mm of water in one hour. This figure will have to be scaled up if the pavement is infiltrating water draining towards it from impermeable paving or from downpipes. 

Where traffic weights exceed those commonly encountered on public highways, specialised design advice should be sought. 

Permeable paving example of loading category A: 1-5% CBR (Attenuation design)

Permeable paving example of loading category B: 1-5% CBR (Attenuation design)

Permeable paving example of loading category C: 1-5% CBR (Attenuation design)

Permeable paving example of loading category D: 1-5% CBR (Attenuation design)

Permeable paving example of loading category E: 1-5% CBR (Attenuation design)

Permeable paving example of loading category F: 1-5% CBR (Attenuation design) 

Permeable paving example of loading category A: 3 and 4% CBR (Infiltration design)

Permeable paving example of loading category B: 5% CBR (Infiltration design)

Permeable paving example of loading category C: 5% CBR (Infiltration design)

Permeable paving example of loading category D: 3 and 4% CBR (Infiltration design) 

Permeable paving example of loading category D: 5% CBR (Infiltration design) 

Permeable paving example of loading category E: 1-5% CBR (Infiltration design)

Permeable paving example of loading category F: 3 & 4% CBR (Infiltration design) 

Permeable paving example of loading category F: 5% CBR (Infiltration design) 

Where can I get advice on the design and specification of permeable paving by loading category?  

You can request our complimentary permeable paving and SuDS design service from the Tobermore design team. 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: 

“It is important that permeable paving systems are designed to offer the correct support for the project loading categories. Understanding the factors at play, such as the strength of the soil as well as the loads that will be encountered by the permeable paving, is key to getting the design right. We have a wealth of experience and can also call upon specialist engineering and design advice when required. In this way, we ensure our customers continue to get the best support possible to match the high quality and durability of Tobermore’s permeable paving systems.”