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PRACTICAL GUIDE TO PERMEABLE PAVING
Introduction
This is a quick guide for the installation of Permeable Paving products. It is meant as an aid for the layman to understand what can be expected of Permeable Paving and the scope of work involved. Remember that best practices for preparing for and lying of Concrete Block Paving in general must always be observed.
Important links:
- To find out more about Vanstone's Permeable Paving Products
- Design Considerations for Permeable Paving
- Introduction for Engineers / Landscape Architects
What is Permeable Paving?
Permeable Paving is paving that allows rain water to penetrate through the paving into the underground. Once through the paving it can either be allowed to infiltrate the ground straight away or be kept there for reuse or to be lead away by means of drainage pipes or allowed to slowly seep away.
If your ground is sandy, natural infiltration will happen quickly and efficiently. Clay or dense soil will retard infiltration and may cause damming-up after big storms. You can reduce damming by installing underground drainage pipes to lead the water away.
It is important to think of the layer to be installed underneath your paving. This layer is called the road foundation. It should be as porous as the paving otherwise it will simply block the water. Use a no-fines gravel like crushed stone that can be bought from your nearest quarry or sand and stone supplier.
The sand on which your paving blocks are bedded and the filler sand with which they are fixed must also be completely porous. Use a clean crusher sand or riversand with grain sizes ideally between 2mm and 5mm. Sizes between 1mm and 6mmm will also work. Make sure that all fine material smaller than 1mm is completely sifted out. Small size (like 6mm) cruched stone only does not work either. This is because the joints between blocks are not filled properly and interlock, which is essential for load carrying ability, will be compromidsed.
The interface between the layers of bedding sand and crushed stone and also between crushed stone and the natural ground, should not mix. Bedding sand may filter down into and in between chrushed stones. This will leave cavities underneath paving blocks, causing local weakness. If the natural ground is fine or sandy, it may press up inbetween chrused stones and that may cause settlement. If such mixing is a risk, use membranes of geofabric between layers.
Step-by-Step:
- Decide what you would like to do with the rain water collected by the paving? Let it infiltrate the underground, collect it for later use or lead it to a pond, channel or stormwater system.
- Establish the permeability of the natural ground. Is it fast draining sand or is it dense clay? It may be something in-between that drains somewhat slowly. If you have infiltration in mind, the natural soil must be at least partially permeable. If it is completely dense, pipes will have to be installed to lead the water away. Underwater storage in this case becomes a good idea. A collection sump can be constructed from where the water can be pumped for irrigation or other uses.
- Determine the nature of the traffic that is going to use your paving:
- For pedestrians and occasional light vehicles, typical of domestic driveways, Aqua Random is suitable but Aqualock may also be used. For the road foundation: remove soil 100mm to 150mm thick, make sure the sub-grade is strong enough, lay road foundation 100-150mm thick crushed stone, lay paving bed 2-5mm no-fines sand 25-30mm thick and level, lay Permeable Paving Blocks, fill with 2-5mm no-fines sand and compact. If mixing of the crushed stone layer by fine material from above or below is a risk, a layer of geofabric may be used in-between.
- For semi-public places like central driveways in townhouse complexes or low frequency roads as within school grounds, Aqualock 60mm is suitable. If you are not absolutely sure about the strength of the road foundation, consult a qualified civil engineer for advice. Construct the road foundation bed, and sublayers according to the engineer's specification. Remember that the foundation layer under the paving as well as the bed for the paving blocks and filler sand need all be porous as mentioned above.
- For large public paved areas with huge stormwater runoffs, Permeable Paving is eminently suitable. Aqualock 80mm will handle the water as well as traffic on a properly constructed road foundation. It is essential that a professional engineer be consulted to properly plan and design such paved area.