Why is a sub base important for paving blocks?

A sub base is important as it is responsible for providing a strong foundation below block paving and paving slabs. The sub base layer is positioned above the subgrade and, where the traffic category does not require an additional base layer, bears the weight of all the materials above it, including the extra loading imposed upon the surface of the paving blocks. 

A correctly specified and installed sub base delivers the following performance benefits: 

• Provides a long-term stable base to help prevent any movement of paving blocks during use. 

• Spreads and evenly distributes the loads associated with each traffic category. 

• Where a permeable sub base is specified, it provides an important drainage function. 

The thickness of the paving blocks on the surface course, the manner in which they are held in place, either bound or unbound, and the block paving laying pattern are all considered when specifying a paving solution to meet the requirements of the relevant traffic category of the block paving area. 

Traffic categories define the type of traffic an area of block paving will experience. They range from the lowest category 1 area, covering block paving that experiences only pedestrian and cyclist use, through to roads with heavy commercial vehicle traffic that have the highest category number. 

However, if equal attention is not paid to the sub base with respect to its depth, composition, and correct installation, the performance of the block paving surface course could be compromised and fail to meet the traffic category requirements. 

Failure to provide a suitable sub base can lead to the uneven settlement of paving blocks. This makes it more likely that damage to both the blocks and vehicles using the area can occur, as well as creating trip hazards in pedestrian zones. It increases the need for maintenance and repair of the block paving, shortening its working life before requiring complete replacement. 

Why is the MOT Type 1 sub base commonly used for block paving? 

MOT Type 1, often referred to simply as ‘Type 1’, is a graded crushed stone aggregate that ranges in size from 0mm to 40mm. The range of particle sizes creates an easily compacted aggregate with excellent load-bearing qualities, making it ideal for areas with high volumes of traffic. 

It is not just used for block paving, and the acronym ‘MOT’ derives from the old Ministry of Transport government department that developed the specification for use in the construction of roadways. 

The sub base material should be installed in layers to the specified depth. The depth needed will depend on the traffic category of the paved area and type of surface block paving course being used – either bound or unbound. The layers should be fully compacted using a vibrating plate compactor to provide a closed, textured surface. 

Why is a coarse graded aggregate sub base required for permeable paving blocks? 

Permeable paving works by allowing surface water to drain through permeable joints between the blocks. From there it filters through a laying course of grit and into the sub base. The surface water in the sub base then either permeates through a geotextile into the ground, or is held within the sub base by an impermeable membrane to be redirected through pipes to another watercourse. 

For the system to work well the water must flow freely through the sub base. The range of aggregate sizes in MOT Type 1 does not provide the required voids to store and allow the water to pass through. Therefore, a 4/20mm coarse graded aggregate to BS EN13242:2002 is used so there is no fine material present that could clog the system and reduce its efficiency.   

The bedding layer of grit above the sub base and underneath the permeable block paving also needs to be correctly specified to ensure that it will allow water to flow through, but is not too fine or it will risk blocking up paths for water flow through the sub base.  

Therefore, a 4/20mm coarse graded aggregate to BS EN13242:2002 must be used. This aggregate has approximately 30% voids within it to store the water and allow it to freely flow through. 

Where can I find out more about the most suitable sub base design for my block paving project? 

Tobermore has extensive experience in supplying block paving and permeable paving for a variety of projects and ensuring that the sub base design meets the required performance criteria.  

We offer a free sub base specifier tool where only a few project details need to be entered and then you instantly receive preliminary sub base design drawings to use for your initial design and project pack. This tool is not suitable for permeable paving – if you have a project where permeable paving is specified please request a permeable paving design from us. 

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

“The sub base specifier tool is very popular with our customers, and they get the reassurance that when they specify Tobermore we will go the extra mile to make sure that all aspects of the specification will work together to deliver a quality, robust block paving solution from the ground up.” 

If you have any questions about Tobermore block paving solutions and the sub bases that support them, please contact our sales team who will be happy to guide you through the choices available for your project.