Paving units of concrete, clay, brick, timber, stone or tile are popular and attractive options, though it’s essential they are installed correctly and designed to manage run-off and permeability requirements
The following is the first of a three-part guide to how paving surfaces can be made permeable and work with concrete and aggregate. It also provides information on how hardfill and bedding material interact with pavers, and a guide to the range of materials from which pavers are made.
Sustainability
Typically, a high percentage of urban and suburban land is covered by impervious surfaces. Rooftops and paved surfaces such as streets, pavements and parking lots form huge catchments for stormwater collection.
Strategies for sustainable design and construction include:
Minimise the area of impervious surfaces – only use where absolutely necessary.
Use locally produced paving materials.
Use smaller unit sizes to provide a larger number of joints through which surface water can drain.
Use a paving design that incorporates permeable surfaces between areas of impervious paving. In addition to sustainability, selection of paving materials is based on a variety of factors, including:
Cost to purchase and labour to install.
Appropriateness to the site.
Relationship to other landscape and built elements.
Function of the space.
Style (formality or informal).
Shape of the area to be paved (to minimise onsite cutting or otherwise machining).
Permeable paving surfaces
Permeable paving surfaces are designed to minimise run-off and to maximise absorption on site. Permeable paving is best suited to relatively flat areas of less than 1:20 gradient, where soils are highly pervious (for example, sandy soils or well drained gravels).
In areas of instability, expansive clays, where areas of more than 250m2 are involved or where permeable surfaces are required by a local TA, the paving should be designed by a qualified engineering consultant. Some areas are unsuitable for permeable paving, for example, where there has been cut-and-fill benching with highly compacted material or where the cut bench is essentially rock.
Permeable paving products are available in unit paver form in a variety of thicknesses, sizes and colours or can be placed as an in-situ product. Depending on the design of the sub-base, the product can be used for light pedestrian walkways to driveways and even streets to accommodate heavy vehicular traffic. The cost of the product may mean that it’s selectively used, for instance, as a strip drain between adjoining areas of impervious paving to add a decorative feature to its functional qualities. It is also often seen as a tree surround at the base of street trees and can be easily integrated into adjoining paved surfaces.
Depending on location, permeable paving may need maintenance such as hosing to remove fine silts and debris.
Permeable paving generally consists of the following elements:
Paving layer – of one of the types described below.
Jointing sand – clean crushed stone jointing sand (2-5mm chip).
Bedding sand – usually 20-25mm sand bedding.
Geotextile membranes – to minimise movement of bedding sand into the base course (and possibly also from prepared base course to the underlying substrate).
Base course – selected permeable (high-void) crushed rock, thickness determined by the average maximum hourly rainfall and traffic load. This layer is sometimes replaced with 19mm aggregate no fines porous concrete.
Underdrain or strip drain (where required) – to discharge water into an approved stormwater system.
Surface drainage – falls must be provided to direct excess surface water to stormwater systems.
Edge restraint – a sturdy concrete kerb or perimeter edging to hold pavers neatly in place, particularly when subjected to vehicle loads.
Concrete or brick unit paving
These are described in further detail below, but in terms of permeability, smaller unit paving sizes result in more cracks through which water can percolate to the prepared base and underlying substrate. Field studies by the Auckland University Department of Civil and Environmental Engineering have shown that permeable paving can decrease run-off by 40% compared to an adjoining equivalent area of asphalt. Some unit paving products are specifically manufactured to provide a nominal gap between pavers as they are laid. Spacers can also be used. Graded gap sand of 2-5mm should be used. In any case, unit paved surfaces are better in principle than conventional monolithic concrete or asphalt surfaces for areas such as driveways and parking lots.
Open-cell concrete paver units
Open-cell concrete paver units (often referred to as Gobi blocks or grass pavers) provide a practical permeable paving surface for light vehicle access or parking areas. The concrete units are egg crate-like in appearance, usually about 400 x 400 x 80mm in size, with the voids filled with a free draining (high sand content) topsoil. They spread the load of light vehicles while minimising compaction of the soil beneath and still allowing grass to grow through the voids.
A similar open-cell concrete product that is cast in situ can accommodate heavier traffic loads. In this case, concrete is simply poured on site on to polypropylene or HDPE egg crate-type permanent formwork to create the voids. These types of surfaces provide a high absorption rate of surface water and are particularly useful in low-frequency traffic areas such as overflow parking lots or amenity verges. An additional benefit of these types of surface treatment is that they are cooler than conventional paved surfaces. They can also be used for slope retention. Regular watering and the occasional application of fertiliser may be required to encourage good grass growth.
Plastic grid pavers
Similar in principle to open-cell concrete paver units, lightweight plastic grid mats (usually about 0.25m2 in area) are placed on a prepared base and the open-cell voids filled with topsoil to allow grass or other vegetation to grow through. The product is better suited to lighter vehicle loads such as overflow parking or intermittently used parking areas. Several manufacturers offer similar open-cell plastic products designed to be used in conjunction with planting for slope retention.
Porous or pervious concrete and asphalt
The permeability of conventional paving materials can be improved by using relatively coarse aggregates (15–19mm aggregate) with no fines in the mixes.
Loose aggregates, stone mulches and rocks
River rocks and stone are frequently used as an attractive form of ground cover in areas such as rain gardens and drainage swales, especially when used in combination with appropriate native plants.
The best-known examples of the use of loose aggregates are in the raked gravel courtyards in Japanese gardens. The suggestion of a dry riverbed can easily be created by using boulders, rocks and cobbles.
Crushed limestone, limestone chip, crushed stone pea metals and crushed glass all provide alternatives to hard-paved surfaces. Coarse grades of bank-run rounded river gravels (which are obtained from natural deposits) are appropriate for use as mulching.
The interlocking angular facets of finer grades of crushed stone provide a better walking surface for paths. Placing a geotextile fabric under loose aggregates will prevent the material from being compacted into the soil.
Low-level timber decking
Open slat decking allows the passage of water, while providing a practical walking surface that protects the soil layer below.
Other permeable paving
A recent development in permeable paving is a product manufactured from clean New Zealand stone aggregate (of 3-10mm no fines) bound in epoxy resin in a structure that allows water to drain rapidly through the surface and a prepared sub-base to the substrate beneath.
This article was first published in BRANZ’s Landscape Construction book, Second Edition and is reproduced with permission.
The book covers the design, building and planning rules as they apply to landscape construction and provides guidance on design and construction to ensure good practices are followed. It can be purchased at www.branz.co.nz.