Standard Block Paving can be used, without a planning application, when it is possible to provide rainwater run-off from a driveway into a border, soak away or rain garden contained within the boundary of the property.
Standard concrete block paving sits on a compacted layer of hard core or Type 1 MOT (crushed carboniferous limestone with particle size from 20 mm to dust), with a further layer of moist sand above, and once the blocks are installed the gaps between them are filled with kiln dried or silica sand to strengthen the surface and hold the blocks firm.
The sub-base does not allow water through and since SUDS legislation was introduced, surface water must be directed to an area within the boundary of the property to drain slowly into the ground – not to the domestic drainage system.
Surface water can be directed through either concrete dished channel blocks or aco / linear drainage towards a soak away, a rain garden or to be collected for re-use.
Permeable Block Paving should be used where it is not possible to control water run-off without it being directed by gravity on to the public highway and into the public sewer drainage system.
Permeable block paving relies on the porosity of the sub-base to allow surface water to infiltrate through to the sub-grade or soil and the type of soil can determine, in part, the design of the various layers of sub-base.
There are a number of different ways that the sub-base can be designed and these are determined by the type of soil and whether the surface water will be allowed to infiltrate into the sub-grade or soil, or be collected.
Permeable block paving should sit on Type 3 MOT sub-base which has aggregates ranging from 20 mm down to 4 mm. It is clean, crushed open-graded carboniferous limestone which allows water to be stored in the spaces between the aggregate.
Both the laying course and the joint filling material is clean, crushed gravel with a particle size from 6 mm to 2 mm, again to allow water to pass through. Conventional jointing sand and other fine materials are not used as they would clog up the pores between the open-graded aggregate and prevent water infiltrating into the sub-grade material.