Permeable Surfacing Guidance
Why Clay Pavers are Ideal for SUDS
Sustainable drainage systems (SUDS) are drainage solutions that provide an alternative to the direct channelling of surface water through networks of pipes and sewers to nearby watercourses. Permeable paving plays a key role in creating sustainable drainage systems and helps to get rainwater back into the ground where it should be.
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Construction Standards
There are British construction standards and codes of practices that relate to laying precast concrete paving blocks and clay pavers for flexible pavements. For SUDS compliance, pavers should be laid in accordance with BS 7533 Part 13, which allows surface water to percolate through the joints of the paving, dissipating through the system into the subgrade. Part 13 of the standard deals specifically with permeable pavements.
Contributory factors causing drainage systems to overload.
We’re experiencing an increasing rate of flooding throughout the UK. Land in its natural undeveloped state, directs only approximately 5% surface water run into natural watercourses, with 95% naturally infiltrating into the subgrade.
In farmland, surface water run is affected by the agricultural process and compaction of the surface by livestock increasing water run off to 30% with 70% infiltrating into the subgrade.
In suburban areas where the majority of domestic driveways are impermeable, the surface water run off into natural watercourses significantly increases to 70%, with only 30% infiltrating into the subgrade.
In towns and cities, apart from specific green areas such as municipal parks/green open spaces etc, a massive 95% of surface water enters drainage systems and watercourses with only 5% infiltrating into the subgrade.
As housing, urban development and agriculture continues to increase, so does the need for controlling this water at source to ease the burden on drainage systems and water courses.
Basic Design of Permeable Paving
If we start looking at the basic design of permeable pavements, there is some similarity with traditional flexible construction method, but using different components.
It is important to recognize there are two factors which determine the depth of sub base required.
One is the Structural requirement, which determines the pavement strength required to accommodate the anticipated traffic loadings and takes into consideration the type and strength of the subgrade.
Secondly, the Hydraulic requirements which provide sufficient capacity to allow the water permeating through the surface into the sub base and the need for this to be temporarily stored until it either permeates naturally into the subgrade, or is held until dispersion is complete, or alternatively stored as part of an attenuation system for grey water use etc.
Principles of Permeable Paving and its Constituents
If we consider this diagram as the basic principle providing Total Infiltration, or System A as it is known. Surface water permeates through the wider joints, passes through the laying course which and flows through the permeable sub base eventually entering the subgrade.
These different layers are separated by a geotextile, generally woven or non-woven The job of the geotextile is to allow water to flow through and prevent migration of particulates between the construction layers whilst providing a trap for any sediment. The geotextile reduces the rate of infiltration and is required to perform both structurally and hydraulically.
All CGA within the system, which we’ll address later, is clean, no fines, and angular, allowing the controlled free flow of water through the system. The depth of the sub base is designed subject to both the loading and hydraulic calculation requirements, which we’ll cover shortly, and it is often the case where the more substantial depth of the sub base is a prerequisite of the hydraulic requirement.
If the total depth of excavation required provides a problem due to levels, there is an alternative option to use a sub base replacement system which can provide a shallower excavation. This is a geo-cellular box system incorporated into permeable pavements consisting of plastic cellular units connected together to form a raft structure. Their main advantage is that they have a high void ratio which, when compared with a granular sub base, can reduce the overall depth of sub base by 30-40%. This is useful in reducing the disposal of excavated material to landfill making these systems particularly suited to the economics for brownfield and contaminated sites.
Comparison
If we compare these diagrammatically side by side, in the cases of Systems B & C, you will observe that careful consideration must be given to controlling the outflow from the drainage pipe, particularly in System C, as this is the only way water leaves the system.
The volume or rate of water entering a permeable pavement is often greater than the rate percolating into the subgrade, or being slowly discharged to a secondary system via the drain pipes.
Consequently, this requires the volume of water required to be stored within the sub base to be calculated and it is necessary in Types B & C to adopt a flow control device limiting the discharge from the drainage pipes into existing systems as no restriction would result in overloading the receiving system, particularly in the case of prolonged heavy rainfall periods.
This restriction of flow through the overflow pipes requires calculation as it may be required to be ,for example, 5 litres/sec/hectare to prevent the overloading of the drainage system it enters. Water In, minus Water Out = Storage Volume Required.
Water In, minus Water Out = Storage Volume Required.
Assessing Subgrade Suitability
- The permeability of the subgrade must be determined to identify which filtration system is required, i.e. Types A, B or C (shown later).
- Determined by the CBR value, or by measurement of the plasticity index.
- For domestic applications, a recommended minimum subbase thickness of 250mm coarse graded aggregate (CGA) be used for Systems A & B. Increased minimum thickness of 350mm for driveways exposed to larger private vehicles and footways with the occasional vehicular overrun.
- System C - Subbase thickness for a patio/private driveway would be 250mm CGA + 150mm capping layer.
- Driveways exposed to larger private vehicles; Subbase thickness will require increasing to 350mm + 150mm capping layer.
- Low CBR subgrades - adjustment to the pavement design will be required. Subgrades with a CBR of <5% won’t provide sufficient permeability.
- This type of subgrade requires System C - seek expert guidance as additional capping layers to subbase design are required.
Suitability of Subgrade
Remove topsoil to expose subgrade
Excavate hole 400 x 400 x 400mm
Fill with water and allow to drain naturally
Repeat 3 times
If not drained away, the permeability is deemed to be insufficient
Test hole every 20m2 | Driveway – minimum of 2 holes
This process sounds technical and complicated but there is further guidance available in the Standard. If there are any concerns, particularly relating to construction on subgrades of questionable performance, it would be beneficial to involve an engineer.
Subbase Specification
- Granular sub-base material shall comprise crushed rock or concrete possessing well defined edges.
- The upper sub-base layer to consist of 5-20mm clean crushed stone.
- Sieve sizes % passing
40mm 100%
20mm 90-100%
10mm 25-75% 4mm 0-15%
2mm 0-5% - Minimum depth of 350mm should be used for subgrade achieving a CBR >15%
- For increased depths of subbase, a lower sub-base layer can be introduced to consist of 10-63mm clean crushed stone.
Laying Course/Jointing Material
- Laying course 2-6mm clean crushed stone compacted thickness of 50mm BS EN 13242:2002 Material supplied shall be referred to as 2-6mm clean crushed stone typically limestone or granite. Material to conform to sieve analysis and aggregate testing.
- Jointing material to consist of the same material as the laying course. Surface dressing 2-4mm of Quartzite gritstone may be used, the material supplied shall be referred to as 2-4mm clean Quartzite and conform to sieve analysis and aggregate testing.
- This defines the 2-4mm surface dressing to be applied to the surface of the clay paving and subsequently brushed/vibrated between pavers.
Crushed Leicestershire granite recommended by the supplier
Bedding layer currently used on site
Bedding & Joint Filling Material
The aboves images show the correct specification of material, being angular crushed rock meeting the required BS sieve analysis test, in this particular case being a crushed Leicestershire granite, and a naturally occurring crushed gravel which meets the sieve analysis requirements but the rounded shape of the gravel produces no interlock and as a consequence is not a suitable material.
So what does a permeable clay paved surface look like?
The image below a standard clay pavers laid on edge with spacing of 6mm. The image below that shows the same area where the joints were partly filled with 2-6mm grey CGA and the top 25-30mm filled with a 2-4mm red/brown crushed Scottish granite to complement the paver colour.
Clay pavers laid on edge with spacing of 6mm
Joints were partly filled with 2-6mm grey CGA and the top 25-30mm filled with a 2-4mm red/brown crushed Scottish granite
Comparisons with Conventional Paving
The top image shows a typical flexible application laid to falls where surface water is directed towards the drainage channel and should there be any low spots, water will accumulate and puddle. In comparison, permeable paving can be laid flat, will not puddle and dries quickly.
Conventional Paving
- Laid to falls
- Channels & Gullies
- Risk of ponding Pedestrian / vehicular loads
Permeable Paving
- Ideal for flat areas
- No 'fittings'
- Dries quickly Pedestrian / vehicular loads
Comparison
Can permeable paving be used on a sloping drive is a question we are often asked. The answer is yes, but with the following considerations:
In System B, where there is a reduced rate of permeability into the subgrade and System C, where there is no permeability at all, water will naturally accumulate at the lower end which will reduce the storage capacity of the overall system. You will also see water coming up through the joints as the system starts to fill up.
In the case of both B & C Systems, It is recommended to have a minimum gradient of approximately 0.5% towards the drainage pipe outlets. This is equivalent to a 50mm fall over a 10m length, or 1 in 200.
For a sloping site where the gradient exceeds 5%, which is 500mm over a 10m length or 1 in 20, terraced or compartmental sections are created by incorporating check dams which restrict the flow of water from higher to lower sections by utilising the storage capacity available in the higher sections.
The concrete check dam can also be used as a lateral restraint to prevent the movement or creep experienced with flexible paving when subject to heavy channelized braking, typically experienced at the bottom of drives in any form of flexible paving.
The top face of the check dam can be either exposed at the surface, or a double row of pavers fixed directly to the top of the concrete to make it visually more attractive.
Abutments to Buildings
An impermeable membrane is required for System C only where effectively a tank is formed using typically HDPE, polypropylene or EPDM membranes, however, it is recommended to incorporate an impermeable membrane up against the building wall for Types A & B also as above.
These membranes should be durable, robust and capable of loads applied during the construction process and throughout the full design life of the pavement. They should also be resistant to puncture and stresses and strains associated with movement and environmental stress cracking and also unaffected by potential pollutants present in groundwater. The strength, suitability and performance also applies to the geotextiles separating the different layers.
Particular attention has to be paid to this detail, especially in older properties, where there may not be a DPM, or over the years where it may have been compromised.
There is also a likelihood that the new depth of excavation to accommodate a permeable pavement may exceed the original depth of any existing drive, so it would be wise to adopt the detail above to prevent any ingress of moisture back into the building and turning an existing cellar into a plunge pool overnight.
Permeable Pavement Design And Maintenance
Can permeable pavements flood – well yes, but all drainage systems are designed around the possibility of not being able to cope with a 1 in 30 or 1 in 100 year storm occurrence for economic reasons.
Designed correctly, permeable pavements are calculated to block by as much as 90% over the design life of 20 years and, as is stated above, will still perform if only 10% of the system remains working successfully.
We would like to draw your attention to avoid dumping a delivery of sand or topsoil tipped directly on to any permeable driveway, as the fine particles will block the joints and possibly drop into the sub base.
We have seen where permeable paving maintenance has been described as requiring a ‘reactive’ maintenance regime, which I think is a fair statement, in so much that the system will only need attention if it no longer works.
We would recommend removing moss, weeds with an application of a ‘safe’ weed killer and the removal of general detritus by general sweeping of the surface to maintain the cosmetic appearance.
If ponding on the surface is noticed then remove the aggregate within the blocked joints with jet wash/stiff brush or suction sweeper and replace the aggregate.
Designed correctly offers 20 year
design life Design infiltration 10% of initial rate
Infiltration capability significantly higher than rainfall intensity
Avoid sand/topsoil deliveries on permeable surfaces
Jet wash and suction sweeper
Permeable Paving FAQs
Q: Can it be used on clay soils?
A:
Use on clay soils – yes system C as previously discussed
Q: Is Permeable Paving damaged by frost/freezing conditions?
A:
Damage by frost/freezing- no, it is highly unlikely the water within the sub base will freeze due to the depth of the sub base beneath the surface and the whole construction is flexible and will accommodate any ground movement successfully.
Q: Does it clog and stop working?
A:
As long as it is well maintained, there is no reason why it should clog up.
Q: Is it expensive compared with conventional flexible construction?
A:
It is difficult to draw a direct comparison with traditional flexible paving as drainage channels, gullies, inclusion of manhole construction, soakaway construction, connection to drain facilities etc. need to be factored in as these are not required by a typical System A permeable construction.
Q: Is there a lack of aesthetic options?
A:
Though the concrete manufacturers have led the field in permeable pavement development for over 20 years now in small element block paving, clay is now a viable option, The use of either clay, concrete and natural stone as a permeable pavement is governed by exactly the same British Standard 7533 Part 13, so we think the onset of clay now provides you with a visually more appealing durable product…and it doesn’t fade!!!
Please note the above information offers guidance only and outlines the Design process in order to attain a successful permeable pavement Installation. We recommended an engineer is involved at the design stage. There are quite a few factors which have to be deliberated when considering a permeable pavement and the british standard 7533 part 13 takes you through the whole process of design with useful tables to consider and lead you through the design process. Chelmer valley brick don’t offer a design service, but there are companies out there who do, but we are here to offer any advice we can.