The Permeable Paving Guide – Part 2: Components of Permeable Pavements
Permeable pavements aren’t just the surfacing element visible to the public, but a completely functional below ground system. Effective systems require all components to work together and permeable paving is no different.
The figure below shows a typical cross section with a breakup of materials.
The aim of Part 2: Components is to explain in greater detail how these materials are used, and how particular characteristics of these materials can alter the behaviour of the permeable pavement.
The surface of the permeable pavement is the largest visible component and the only one that comes into direct contact with pedestrian and vehicular traffic – therefore it needs to be durable, slip resistant, wear resistant, and allow constant infiltration of water. The most common surface materials are no fines concrete, porous asphalt or interlocking permeable paving units, with the latter being the most widely used around the world. Interlocking permeable paving units allow for a more aesthetically appealing surface with a range of colours, textures, laying patterns, and shapes readily available for selection by designers.
The various surface types will have different initial infiltration rates so the designer should select one that is adequate for the rainfall intensity and distribution. It is important to keep in mind that the infiltration rate of the surface is greatest upon pavement completion, however after 5 years of service and depending on maintenance practices the infiltration rate may have reduced by up to 50%. A responsible approach is to use a 6-10 year average infiltration rate as the design value.
The substructure in any pavement is arguably the most critical component. It can be comprised of different materials in multiple layers, which the designer can vary in thickness to successfully distribute the loads from the surface to the subgrade. In the case of permeable paving the substructure serves a dual purpose. Firstly it is the main structural element of the pavement system, and secondly it store’s water within its voids for harvesting, re-use or discharging.
There are many materials available to designers to use within the substructure and careful consideration should be given to the materials suitability based on the load distribution and rainfall intensity in the pavements geographical location. The two most common substructure materials are variously graded unbound granular materials, and cement bound permeable materials which includes no fines concrete. A single sized material has a higher void ratio and will be able to store more water than a well graded material however, it may not have the same bearing capacity. Generally speaking as the void ratio of a material increases its bearing capacity decreases.
The figure below illustrates different graded materials.
In addition to the above, the permeable pavement will be required to withstand the design loads whilst the material in the substructure has water within the voids and is in a saturated condition. Saturated material in the substructure can be more than 50% weaker than the same material at normal saturation conditions. The designer should select a material and thickness that is adequate for the load distribution and the water storage requirement.
The subgrade is the natural material on which pavements are constructed. The strength of the subgrade is expressed as a Californian Bearing Ratio (CBR) or modulus value and is an important parameter required by designers for pavement structural design. In addition to this, the subgrade will have a corresponding permeability value which is required for hydraulic design.
The permeability of the subgrade may require the designer to select a full, partial or no infiltration system
The figure below illustrates the different types of permeable paving systems.
As the CBR of the subgrade decreases, so does the permeability. For some clay subgrades the permeability is so low the designer may assume no water infiltrates the subgrade and opt for a no infiltration system.
Various forms of geotextiles are used within permeable pavements. The 3 main geotextiles are filter fabrics, impermeable liners, and high tensile subgrade stabilisation grids.
Filter fabrics help to stop the infiltration of fine particles into the substructure from the surface. Over time, the introduction of fine particles into the substructure can clog the voids and reduce the overall storage capacity. The filter fabric is usually placed between the bedding layer and substructure where it can be easily replaced in the future.
Impermeable liners are used to stop the infiltration of water from the substructure into the subgrade to create a no filtration system. Impermeable liners are typically manufactured from high density polyethylene or polypropylene materials with a thickness of 1mm or more. Any joints or laps should be welded or bonded to ensure they are water tight. Introducing a cushion layer above and below the impermeable liner can help in reducing the chance of larger angular particles in the subgrade or substructure material puncturing the liner during installation and compaction.
The structural performance of the permeable pavement can be raised without the need to increase the thickness of the substructure material by introducing a high tensile grid at the subgrade, or within the substructure to mechanically stabilise the layer. This leads to a more cost effective design by reducing the excavation depth, material quantities, and labour costs associated with installation.
By using a combination of geotextiles within the permeable pavement, designers are able to optimise the performance of the pavement creating a longer lasting, economically efficient system.
Drainage is still a very important part of a permeable paving system. A partial or no infiltration system will require an internal drainage network to move the water from the substructure to a point where it can be pumped, treated, stored and re-used, or discharged to an existing storm water network. Falls are designed into the subgrade to move water to the lowest point.
In addition to including a fall in the structure, surface drainage can also be incorporated into the design in case of an intense storm where the surface infiltration rate cannot withstand the intensity of the rainfall. In the event of complete loss of surface infiltration, surface drainage can also be used as a backup mechanism to adequately remove water from the pavements surface.
Understanding the role of various components allows designers to select the most appropriate materials to create an effective, functional, long lasting, durable permeable pavement system.
This Permeable Paving Guide will explore the common aspects of permeable pavements by covering the following topics:
Part 2: Components
Part 3: Material Specifications
Part 4: Applications
Part 5: Design
Part 6: Maintenance
Stay tuned for Part 3: Material Specifications to be published. This will contain the detailed technical specifications of the above mentioned components to ensure long term performance of the permeable pavement throughout its intended design life.