Permeable pavements
To prevent localised flooding in a developed environment, the traditional management of rainwater has been to capture it into the stormwater drainage system, or where this is not present or insufficient to cope with the flow, into the sewerage system as quickly as possible. Rainwater is then transported either to treatment works or watercourses. In developed areas this means that there is less natural infiltration of the rainwater back into the ground, while the sewerage systems also have to cope with large quantities of water. During heavy rainfall, storm overflows into watercourses are required which can in turn lead to serious pollution. Sustainable Drainage Systems (SuDS) help alleviate these problems by allowing the management of rainwater at a level as local as reasonably possible. Precast concrete elements are key components of the SuDS solution to the problem of rainwater management.
There are a number of concrete block paving solutions designed specifically to create permeable pavements. These designs incorporate enlarged joints created by engineered spacers, (which maintain interlock) and are larger than conventional spacer nibs on the sides of each paving block, or voids generated by geometric block shapes (which maintain interlock). Joints or voids are filled with a nominal 6mm single sized permeable joint filling material that allows rainwater to freely drain through it into the ground below.
There are three forms of precast concrete block paving construction offering total, partial or no infiltration to meet specific requirements:
- Total infiltration (System A) Suitable for existing subgrade with good permeability, System A allows all the water falling onto the pavement to infiltrate down through the constructed layers and eventually into the subgrade. Some attenuation will occur in the sub base layer before it eventually passes through. No water is discharged into conventional drainage systems, completely eliminating the need for pipes.
- Partial infiltration (System B) Where the existing subgrade may not be capable of absorbing all the water, outlet pipes drain the structure of excess water into other elements, such as swales, ponds, watercourses or sewers. This is one way of achieving the requirement to reduce the volume and rate of runoff, and will most likely remove the need for any long-term storage. An orifice plate flow control on each outlet pipe can increase water retention time and infiltration, and protect downstream devices.
- No infiltration (System C) This system is used where the subgrade is poor or contaminated (to avoid pollution being washed down to groundwater), where infiltration would present a down-slope risk or where treated water is to be re-used as an asset. All the water is captured using an impermeable, flexible membrane placed on top of the subgrade and up the sides of the permeable sub-base to effectively form a storage tank. Outlet pipes through the membrane transfer the collected water and orifice plate flow controls ensure that designed storage/discharge rates are achieved.
Soakaways
As part of a Sustainable Drainage ystem (SuDS), Building Regulations require that rainwater from a drainage system should discharge to a soakaway as a first priority and only to a sewer when this is not reasonably practicable. Precast concrete soakaways offer an excellent solution, providing an infiltration drainage solution. Individual soakaways can be linked together with pipes to increase capacity and provide a greater infiltration area. Their strength enables them to take loads from vehicular traffic.
Concrete block permeable paving (CBPP)
Concrete block permeable paving provides a coloured hard surface which is aesthetically pleasing, comfortable to walk on, suitable for traffic, extremely durable and easy to maintain. The paving blocks are fully engineered products and accurately fashioned in local factory conditions. Rainwater passes between the concrete blocks through a graded stone fill and is directed into a storage volume beneath the blocks. This volume may be provided by the provision of plastic ‘crates’ or the use of a larger graded stone fill with approximately 30 per cent air voids. Laid with an edge restraint over a granular bedding course, individual blocks interlock with the graded granular fill to act compositely and thus safely distribute the design loading into underlying materials. CBPP can be put into use immediately after installing and requires only minimal maintenance. Mechanical installation techniques allow large areas to be laid with a minimum of manpower, saving both time and energy.
Benefits
Performance
Concrete block permeable paving (CBPP) offers outstanding strength and durability and is resistant to most chemicals. Blocks are routinely tested for parameters such as splitting strength and weathering, abrasion and slip/skid resistance, to ensure conformity with the requirements of BS EN 1338: 2003, Concrete Paving Blocks - Requirements and Test Methods. This offers specifiers and suppliers maximum confidence in the wide range of products available on the market.
Applications
CBPP is suitable for a range of applications from heavy-duty areas, such as Container Ports, heavy industrial units, to lightly-trafficked residential areas, car parks, driveways and hard landscaping projects. CBPP can also be used adjacent to buildings, or grassed verges for additional amenity to give variety and opportunity for easy access to services in the future
Site considerations
Unlike impermeable paving, the surface of CBPP can be completely flat, avoiding ponding. This means that CBPP surfaces are independent of cross-falls, channels, gulleys and other impediments to accessibility. Therefore, designers have complete freedom to introduce level changes for other reasons unrelated to drainage, for example to suit site topography. A level surface is recommended for supermarket car parks to eliminate the risk of wandering trolleys. The level surface does not restrict the hydraulic performance of the CBPP in any way.
Retrofitting
The provision of CBPP to replace impermeable paved areas (e.g. roads, car parks and driveways) can reduce flood risk to lower parts of a surface water catchment. This approach is likely to be considered favourably by authorities seeking to reduce flood risk within heavily urbanised areas. CBPP can also be fitted in some circumstances as an overlay to existing road surfaces, maximising re-use of the existing roadbase and its embodied carbon.
Adoption
CBPP have been adopted by forward-thinking local authorities in exchange for a commuted sum paid by the developer for the future maintenance of the CBPP. Legislation is also changing to facilitate an increased uptake of SUDS solutions to combat climate change.
Reinstatement and recycling
CBPP allows sectional removal and reinstatement of paved areas using the original materials and blocks, without leaving behind any evidence of repair. The ability to fully recycle concrete block permeable paving and foundations at the end of their design life contributes further to its impressive environmental credentials.
Aesthetics
CBPP is available in a variety of shapes, sizes, colours and finishes and can be installed in numerous bonds and laying patterns, enabling delineation of space or focus on specific features.
Environment
The adequate control of pollution within surface waters released into the environment is an important consideration to regulators.
CBPP have been proven by CIRIA to greatly reduce the levels of contaminants such as suspended solids and biodegradables including troublesome hydrocarbons released from vehicles and a wide variety of contaminants
A comprehensive review of all aspects of concrete block permeable paving can be found in the latest edition 8 of Understanding Permaeble Paving published by Interpave, MPA Precast.
The Interpave website also includes a collection of exemplar case studies and other useful guidance related to block paving.