Permeable pavements
To prevent localised flooding in a developed environment, the traditional management of rainwater has been to capture it 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 filtration 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 urban 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 larger than conventional spacer nibs (which maintain interlock) on the sides of each paving block, or voids generated by geometric block shapes (which maintain interlock). Joints or voids are subsequently filled with a nominal 6mm single sized permeable joint filling material that allows rainwater to 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 concrete block paving allows water falling onto the pavement to infiltrate down through the joints or voids between the concrete blocks, passing through the constructed layers below and eventually into the subgrade.
- Partial infiltration concrete bock paving builds on the total infiltration model, this design includes a series of perforated pipes or fin drains at the formation level to allow the remaining water to be drained to other systems such as sewers, swales or watercourses.
- No infiltration concrete block paving allows for the complete capture or attenuation of the water using an impermeable flexible membrane placed on top of the formation level. Pipes or fin drains transmit the water away as in partial infiltration systems. The no infiltration solution is particularly suitable for contaminated sites.
Soakaways
As part of a sustainable urban drainage system (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 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 laying 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 industrial units, to lightly-trafficked residential areas, car parks, driveways and hard landscaping projects. CBPP can also be used in conjunction with either impermeable surfaces which may be preferred adjacent to buildings, or grassed verges for additional amenity to give variety and opportunity for easy access to services in the future
Site considerations
CBPP is suitable for construction on both level or sloping sites. If surface gradients exceed 1 in 20 then a terraced approach is recommended to maximise the use of the storage volume provided within the pavement. 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.
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.
For more information on paving, visit the Interpave website.