Permeable Paving Using Chelmer Valley Clay Paving
Since 1st October, 2008 planning permission has been necessary if a new or replacementdriveway exceeding 5m2 is to be constructed using an impermeable surface which discharges surface water run-off onto the public highway.
Planning permission is not required if a permeable (or porous) surfacing, such as gravel, permeable block paving or porous asphalt, or if the rainwater is directed to a lawn or border to drain naturally, or be assisted by a soakaway.
Three key principles of permeable paving, as part of a Sustainable Urban Drainage System, (often known as SuDS) comprises of components and techniques deemed to be more sustainable dealing with storm water at source following flash floods and are as follows: –
- Reduce water quantity – requires a reduction in the rate and volume of runoff so preventing overloading of natural watercourses and drains
- Improve water quality – by replicating the natural drainage from an area before development and to treat runoff by removing pollutants. This also reduces the
impact on receiving natural drainage such as watercourses
- Biodiversity/amenity – providing environmental benefits such as wildlife habitat and
For permeable clay pavements to comply with SuDS, pavers should be laid in accordance with BS 7533 Part 13:2009 – “Guide for the design of permeable pavements”.
This standard, which applies to clay, concrete and natural stone, deals specifically with permeable pavements which allow surface water to infiltrate through the joints of the paving dissipating through the system into the subgrade. It applies to all pavements including both domestic and commercial applications with the usual road spectrum of vehicle axle loadings up to a maximum of 8,000kg., but excludes heavy duty pavements.
When designing a permeable pavement there are basically 3 types of construction methods where consideration must be given to the following factors before system selection :-
- Position of ground water table –is the water table a minimum 1m from bottom of permeable sub base?
- Ground permeability and suitability?
- Presence of pollutants?
- Influence of topographical features – does surface water run-off from surrounding area enter the system or discharged water enter into natural water courses?
Total infiltration – System A
- suitable for an existing subgrade with good permeability eg. Sandy/gravel subgrade
- allows all the water to infiltrate down into the subgrade through the constructed layers
- initial partial storage before it passes through
- no water discharged into conventional drainage system
- eliminates need for pipes and gullies
Partial infiltration – System B
- used where existing subgrade is not able to absorb all the water eg. sandy/silty clay subgrade
- partial infiltration into the subgrade through the constructed layers
- outlet pipes required to swales, ponds, watercourses, sewers
- reduces volume and rate of run-off
No infiltration- System C
- used where existing subgrade permeability is poor or contains pollutants eg. either clay subgrade or previous brownfield site possibly containing contaminants. Contaminants are to be prevented from entering natural watercourses such as water supply wells
- allows complete capture of water
- impermeable membrane on top of subgrade (and sides of sub-base) to contain water
- where polluted run-off may be present use additional SuDS techniques (eg. Swales and wetlands) to treat run-off before discharge
- storage for recycling of ‘grey water’ as a water attenuation system
How does a conventional flexible construction, using a sand bed with narrow sanded joints, differ to permeable pavements?
Laid to falls
Requires gullies and channels
Risk of ponding
ideal for flat areas
no fittings required
surface quickly dries
Commonly held misconceptions relating to permeable pavements: –
Cannot be used on clay soils – clay subgrades require a “Type C- No infiltration” method of
Damaged by frost/freezing in winter – frost heave not experienced
Clogs easy and stops working – system capacities are designed so that if the system ‘clogs’ to 90%, the remaining 10% capacity will still perform adequately offering a design life of up to 20 years. The introduction of fine sands/soil into the system must be prevented as this will lead to premature clogging of the system and will obviously reduce the design life of the pavement.
Expensive in comparison to ‘traditional’ construction – permeable paving offers an alternative to the traditional pipes, gullies and culvert approach.
Can’t be used in heavy loaded areas – in accordance with BS 7533:2009 Part 13, vehicle axle loadings up to a maximum of 8,000kg including highway and light industrial pavements are permitted but excludes heavy duty pavements such as airport pavements, ports etc.
Lack of aesthetic options – an extensive choice of colours, textures and bond patterns available
Constructing Permeable Pavements
- Preparation and checking suitability of subgrade
- Geotextile layer separating the subgrade and sub base preventing aggregate punching through into subgrade and reduces rate of infiltration
- Sub-base layer(s) – depth determined by traffic loadings and/or hydraulic requirements
- Geotextile between sub base and bedding course. This provides trap for sediment and residence for bacterial digestion to take place.
- Bedding course
- Wearing course (paving units.) Lay standard clay paving products providing a nominal 6-8mm space between adjacent units, the joints aligned using a string line every 6-8 courses with joints between the string lines corrected. Images below show alignment of joints followed by infilling of joint spacings.
- Coarse Graded Aggregate (CGA) generally 4-20mm angular, clean no fines
- CGA 2-6mm angular, clean no fines
- CGA 2-6mm angular, clean no fines
- NO SAND
Sub base Replacement Systems
When insufficient construction depth causes problems, sub base replacement systems using high voided modular geocellular structures. These are designed to be used in place of, or in conjunction with a traditional aggregate sub base, provide a unique high strength consistent structural raft.
Sub base replacement systems can be incorporated into the full range of traffic conditions from domestic driveways to HGV applications and are suitable for permeable block paved areas. The system allows shallow excavation, even under heavy loads, the modular cells providing 95% void ratio (compared with 30% void ration offered by traditional aggregates). This high void ratio makes them ideal for water attenuation systems reducing the depth of excavation required.
Sloping site solutions
As with any flexibly laid block paved area, a steep incline may require a rigidly constructed lateral restraint to prevent creeping of the paved elements. Within a permeable pavement construction water must be allowed to pass through these restraints from higher to lower sections and by providing flow control to restrict the passage of water between adjacent compartments this optimizes the use of all available storage space.
Combination of impermeable/permeable paved areas
It is also feasible, particularly if steep inclined areas also require a tight turning movement, to combine both traditional impermeable flexible construction (which uses sand bedding with sand jointing) alongside permeable construction (using CGA with wider joints) using the same paved surfacing. The surface water run off from the impermeable paved can be directed onto the permeable paved area which provides an effective ‘soakaway’ which delivers a decorative, functional extension to the paved area.