Concrete Block Permeable Paving in Domestic Applications

Concrete Block Permeable Pavements are one of the most effective source controls and Sustainable Drainage Systems (SuDS) available to manage surface water runoff and comply with the Government guidance on the permeable surfacing of front gardens.  

Regulatory Background.

Since October 2008 the permitted development right which allowed homeowners to pave more than 5m2 of their front garden without planning permission changed. This was to reduce the impact that hard landscaping had on surface water flooding and the pollution of watercourses. Following the change, if a homeowner wants to pave more than 5m2 of their front garden, and isn’t using a permeable solution, then they will have to apply for planning permission from their Local Authority.

To create a permeable surface that qualifies as permitted development, homeowners have three main options:

1. Direct surface water runoff to a rain garden or soakaway from a conventionally constructed impermeable surface.
2. Install a loose gravel surface or a reinforced grass area.
3. Use concrete block permeable paving, porous asphalt, or permeable resin-bound surfaces.

How Concrete Block Permeable Pavements Work

Concrete block permeable pavements work by allowing surface water to pass through the wider aggregate filled joint, into the permeable laying course and through to the open graded angular sub-base. 

The sub-base then stores, or attenuates, the surface water within the 30% voids of the sub-base, absorbing some of the water, then allowing the remaining water to either infiltrate back into the ground or be directed to an outlet where it is restricted and passed into another SuDS component or into a sewer. However, connecting the pavement to a sewer should be the final option as this may not be classed as permitted development.

Suitability Of The System for Domestic applications.

When looking to construct a concrete block permeable pavement, the first step is to assess the ground conditions to determine the appropriate system. 

Generally, if the subgrade consists of well graded sand and well graded sandy gravel the water will be able to infiltrate back to ground. This is classed as a Type A system.

If the subgrade is sandy clay, silty clay or heavy clay the pavement will need to be lined and the water conveyed to another form of SuDS, or to a sewer if permission has been granted. This is classed as a Type C system.  

To be able to confirm if the subgrade at the property can allow the surface water to infiltrate back into the ground, then a simple test can be carried out.

Testing Ground Permeability

Select an area within the garden which is representative of the position of the pavement. 

Step 1: Dig a hole 300x300mm to the level at which the pavement will be built from and at the same level at which the water will drain away from. The base of the hole should be representative of the type of ground which will accept the majority of the water from the pavement. 

Top Tip! Beware of any made ground and buried rubble, which may have been left over from the house construction, as this will affect the results of the test and will not provide a result which is indicative of the rest of the ground conditions.

Step 2: Fill the hole with water to a level of 300mm Allow it to completely drain away, saturating the soil

Top Tip! This pre-wetting may take several hours to complete.

Step 3: Once the water has drained away repeat step 2, and record the amount of time it takes for the water in the hole to drain away. 

• <30 seconds: Excellent permeability.
• 30 seconds to 13 minutes: Good permeability.
• 13 minutes to 11 hours: Fair permeability.
• >11 hours: Poor permeability.

Now the permeability level has been ascertained, the correct system can be constructed.

System Design.

1. Type A systems are encased, around the base and four sides, in a non-woven geotextile which stops the migration of the sub-base into the subgrade but still allows the passage of water back to ground.
2. Type C systems are wrapped in a durable 1mm thick liner, to the base and 4 sides, and has a shallow trench to its low side which has a perforated drainage pipe, wrapped in a non-woven geotextile, which collects the water and conveys it though to the restricted outlet.

If the grounds permeability is classed as excellent, good or fair then the surface water can be infiltrated back into the ground with a Type A system. 

If the permeability is poor, then the sub-base must be lined to collect the surface water, and this can be put back into another SuDS component or a connection to a drain may be required with a Type C system.

Type A systems are encased, around the base and four sides, in a non-woven geotextile which stops the migration of the sub-base into the subgrade but still allows the passage of water back to ground.

BS 7533-13: 2009 Guide for the design of permeable pavements constructed with concrete paving block & flags, natural stone slabs & setts and clay pavers, provides details on the compliant materials to be used within the pavements. The MPA Precast, or Interpave, documentation Design & Construction of Concrete Block Permeable Pavements also provides information in compliance to the British Standards as well as best practice for construction of the pavements. 

We will now look at the information from these documents to design and construct a concrete block permeable pavement which is suitable for use in a domestic residential application as a driveway or path/patio which will only deal with the surface water which lands upon it and does not take into consideration any adjacent hard standing areas.

Best practice installation

Sub-Grade Preparation:

Unlike traditional impermeable paving Concrete block, permeable pavements can be installed without a fall as the water falling upon it will find a joint to infiltrate through, usually within 100mm of where it falls. However, the sub grade should always be tapered to a low point for the collection of surface water if a Type C pavement is to be used and the outlet placed in a trench at the bottom of the system to minimise standing water at the bottom of the pavement.

The pavements will work best up to a gradient of around 1 in 20, and for steeper pavements then consideration must be given to restrict the flow of water through their sub-base using internal dams. An alternative solution is to terrace the pavement to provide separate level areas which will compartmentalise the storage capacity of the sub-base aggregate. 

Check the sub grade for any soft spots. These are usually found in areas of silty, sandy & heavy clays, and remediated by removing the weak material to a sufficient depth of between 0.5m and 1.0m. Replace weak material with a suitable durable capping that has sufficient permeability to allow water to infiltrate through it and does not reduce the permeability of the existing material?. If levels are required within the pavement then they should be created at this point within the sub-grade.

Select the type of liner to be used around the outside of the pavement, dependant on the results from the infiltration tests.

Non Woven Geotextile:

Will allow for the infiltration of water through into the subgrade.

Impermeable Liner:

Will not allow the passage of water though it.  It needs to be lapped by 300mm and double taped, to impede the ingress of water and convey through to the outlet. 

Please note: Woven geotextiles should not be used on a Type A system as they do not provide enough porosity to allow infiltration through them.

Sub-Base Coarse Graded Aggregate (CGA):

Permeable pavements do not use an MOT Type 1 material within their construction, they use a coarse graded crushed rock within the sub-base.  This means the thickness of the sub-base needs to be increased to provide suitable structural integrity. 

BS 7533-13: 2009 requires the use of a 4-20mm crushed rock with a grading in compliance to Table A1. 

The details provided within  the standard shown above are the minimum structural requirements for a domestic application, with no HGV overrun suitable for a patio, private driveway, enclosed play area or a footpath with no vehicular overrun. The minimum structural thickness of the sub-base for this type of application is stipulated as 250mm; however if the pavement has a slope to it, is in an area of high rainfall, has restrictions to the outlet or has adjacent areas falling into the permeable pavement, then the hydraulic storage requirements of the pavement may need the depth of the sub-base to be increased and specialist design support should be sought from an engineer.

The sub-base should be installed in maximum 150mm layers and rolled to create the correct level of interlock between the sharded particles whilst still retaining the minimum 30% porosity required to store the water within the voids of the aggregate. This is shown on the detail below as the area denoted in blue.

If dams are required in the sub-base, to restrict the flow of water due to a slope, these are constructed at this point. Lay the 1mm thick impermeable liner 300mm along the full width of the base of the pavement, then wrap this up through the sub-base backfilling it with the compliant sub-base on either side as you go. This will slow down the flow of surface water through the aggregate and if used in a Type A system will compartmentalise the pavement and infiltrate back to ground. If using a Type C system, the flow will have to be managed through each of the dams to allow the surface water to convey through to the outlet.

Laying Course and Joints:

Once the sub-base has been constructed then the laying course can be installed at a depth of 50mm. Rather than using a sharp or grit sand, a permeable chipping must be used to allow the rainwater through the surface of the pavement into the sub-base below. 

The specification and grading for the laying course and jointing material is detailed as 2/6.3 Gc 80/20  according to BS EN 13242:2007 within its grading being classed in Table A2 of BS 7533-13: 2009.

Suitable Laying Patterns and Block Styles. 

Dependant on the application of the pavement, consideration needs to be given to the laying pattern; a stretcher course laid at either 90 degree or 45 degree to the direction of travel is acceptable for a domestic driveway as is a 90 degree or 45 degree herringbone pattern. 

Top Tip: Basket Weave laying pattern is only suitable for pedestrian applications and should not be used where light or heavy vehicular traffic is present, including domestic driveways. 

The permeable block paving requires to be installed in compliance with BS 7533-3: 2005 + A1 2009. Due to the larger joint required around the around the block the accuracy of cuts is critical and ensuring that nibs adjoin the kerb surrounding the pavement and cut edges face nibs of the adjacent blocks. 

Top Tip: Don’t place any cut edges against the perimeter kerbs.  Cuts should be no less than a quarter of a block.

Joint topping & Structural Nib.

Some Concrete Permeable Paving Blocks are produced with a male and female interlocking nib. The nibs provide added benefit for maintaining a 6mm joint for hydraulic performance by allowing the surface water to pass through the aggregate filled joint. They also provide increased  structural integrity to the pavement surface by ‘locking’ the blocks together. 

The male and female interlock ensure that the blocks are held in place both through trafficking of the pavement and if joints maintenance is not kept up to ensures the blocks will not move if there is a loss of jointing aggregate. 

Once the pavement has been completed, it is good practice to check the integrity of the jointing material after a 3 to 6 month period, as upon initial installation the 2-6.3mm laying course and jointing material can become wedged part way up the side of the joint between adjacent blocks. 

As concrete block permeable pavements are a flexible pavement, once the surface is trafficked the wedged particles of jointing material can dislodge and drop down the joint. When this happens the rest of the jointing material above the wedged particle will also drop and consolidate further down the joint leaving it below the surface of the pavement. This results in reduced structural integrity and an increased risk of clogging of the joint with detritus and silts

Top Tip: Check the pavement after 3 months of trafficking and top up any hungry joints.  This will improve the structural integrity and reduce the chances of joint clogging.

By following the guidance outlined above, you will be constructing a Concrete Block Permeable pavement in accordance to the details set out in the British Standard BS 7533-13:2009 and MPA Precast guidance. 

Discover more information on permeable pavements and the Brett Landscaping range of permeable paving.

Written by Jamie Gledhill, Brett Landscaping Engineering Technical Manager.