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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 and then explain how to use the free sub base specifier tool to obtain suitable permeable drawings for each category.
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:
| Category / Application | Msa | Road Category | Application |
|---|---|---|---|
| 9 | ≤30 | 1 | • Adopted highways and commercial/industrial developments used by a high number of commercial vehicles |
| 8 | ≤10 | 2 | |
| 7 | ≤2.5 | 3 | |
| 6 | ≤0.5 | 4 | • Adopted highways and other roads used by a moderate number of commercial vehicles • Pedestrian areas subjected to regular overrun of commercial vehicles • Industrial premises • Petrol station forecourts |
| 5 | ≤0.05 | N/A | • Pedestrian areas subjected to occasional overrun of commercial vehicles • Car parks receiving occasional commercial vehicular traffic |
| 4 | ≤0.007 | N/A | • Urban footways with no planned vehicular overrun • Pedestrian areas used by light goods vehicles, emergency vehicles and maintenance vehicles |
| 3 | N/A | N/A | • Footways, pedestrian and cycle areas, domestic driveways or access routes subject to manoeuvring of cars and light goods vehicles • No access for other commercial vehicles |
| 2 | N/A | N/A | • Footways, pedestrian and cycle areas, parking spaces subject to cars and light goods vehicles • No access for other commercial vehicles |
| 1 | N/A | N/A | • Footpaths, pedestrian and cycle areas, excluding all vehicles |
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 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 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 BS7533-103 in terms of installation.
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.
Every permeable paving project has its own requirements, with the appropriate pavement build-up influenced by factors such as the expected loading category and the Tobermore paving product specified.
With Tobermore’s free Sub Base Specifier Tool, you can quickly generate a preliminary sub-base design drawing tailored to your project.
Simply enter a few key project details, select Permeable Paving as your product type, choose the loading category that best represents your project and select your preferred Tobermore product. The tool will then generate a preliminary sub-base specification and drawing for infiltration, partial infiltration or attenuation systems, helping to support your initial design and project planning.
Whether you’re working on a car park, residential development, commercial scheme or other permeable paving project, the tool provides a quick and easy starting point for planning the pavement build-up.
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.”
Simply enter a few project details and you’ll instantly receive preliminary sub base design drawings for infiltration, partial infiltration and attenuation systems.
