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Permeable block paving
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An environment-friendly solution for integrated water management policy
Permeable block pavings play an important part in an integrated water management policy, which aims at minimizing flooding risks by keeping water in place for as long a time as possible. Using a concept of full-depth permeability from the surface to the subgrade, rainwater is buffered (preferably) in the subbase layer, and allowed to infiltrate into the subgrade soil if possible. Permeable block pavings are used as buffering and infiltration systems for parking areas, squares, low-volume roads, cycling tracks, etc.
The main causes of flooding are insufficient collecting capacity of sewer systems and waterways, and increasing urban development and application of impermeable pavements. An important step in preventing flooding and overloading of sewer systems and avoiding the activation of storm overflows is to separate rainwater from waste water collection, by buffering the rainwater and allowing it to infiltrate as much as possible. If local buffering and infiltration systems can be applied when constructing new pavements, the environmental nuisance and flooding caused by heavy rain showers will be strongly reduced.
In addition to preventing surface water runoff from flowing directly into sewers or waterways, permeable pavements help to raise the groundwater table. When infiltration into the soil is impossible, the water is temporarily stored in the structure and slowly carried off to a nearby infiltration basin or ditch.
The full structure (i.e. paving blocks, paving bed, base layer and subbase layer, if any) should have a permeability of at least 5.4 x10-5 m/s. This value corresponds with a 10-min rain shower of 16 mm or 270 l/s/ha - taking into account a safety factor of 2 -, which has a statistical return period of thirty years. Ideally the value should increase with depth in the structure, to minimize the risk of obstruction or loss of permeability.
The permeability of the paving blocks is obtained by the porous structure of the blocks themselves or by the presence of draining holes or widened joints between the blocks. Concrete grid paving slabs (so-called grasscrete slabs) may also be used as permeable pavings, provided the voids are filled with aggregates and grass. Permeability requirements for concrete paving blocks are given in Belgian technical specifications PTV 122, and property requirements for the full structure in Belgian technical specifications PTV 827.
Permeable paving structures are designed using a decision tree which clearly illustrates the working principle: the choice of drainage system at the bottom of the structure is determined by the permeability of the subgrade and, hence, by the type of soil in place; the thickness of the base layer ensuring the bearing capacity of the structure depends on the category of traffic; and the subbase layer is to offer sufficient buffering capacity and to keep the subgrade soil free from frost if required.
Besides correct design and skilful construction (especially adequate compaction), an appropriate choice of materials and adequate checks are essential factors for obtaining high-quality permeable pavings. Various test methods have been developed for those checks. The so-called “column” test helps to determine permeability before construction with a view to choosing appropriate materials, whereas the open-end and double-ring tests are used to determine the permeability of the subgrade soil and of the hardened surface on site, respectively. Test methods for determining the permeability of the base layer are underway.
Permeable concrete block pavings are built on the following principle:
- precipitation water infiltrates through the permeable blocks, the joints and the paving bed into the base layer. The blocks allow the water to pass, thus avoiding surface water runoff;
- the infiltrated precipitation water is temporarily stored (“buffered”) in the structure, preferably in the subbase layer. The base layer provides additional safety and sufficient capacity to withstand traffic loads;
- the precipitation water infiltrates into the subgrade, depending on the permeability of the soil;
- any precipitation water not able to enter the subgrade soil is carried off slowly through a pinch pipe to nearby ditches or infiltration systems or to the sewer system.
Standard structures have been defined with traffic category and subgrade soil permeability as input parameters.