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Structural performance
There are three elements to be considered in the structural design of vehicular
trafficked pervious surfaces:
- The surface course (porous asphalt, concrete slab, concrete blocks, reinforced
gravel or grass). The function of the surface material is to provide good
quality ride to be combined with appropriate skid resistance and resistance
to crack propagation in the bituminous and concrete surfaces. Texture and
durability under trafficking are key considerations.
- The underlying construction layers which spread the applied loads (binder
course, base, concrete slab and sub-base ). These layers are required to
provide a stable construction platform and spread the applied construction
and traffic loading so that the underlying foundation layers are not over
stressed.
- The foundation soils (capping layer and subgrade). This is the platform
upon which the structural layers are placed.
Each layer must be considered in turn and the key to successful structural
design and performance is to recognise the difference between pervious and
conventional pavements. Some factors to consider in the design and specification of each type of surface include:
- Pervious pavements use materials with high permeability and void space.
All the current structural pavement design methods commonly used in the
UK are based on the use of conventional materials (which are dense and relatively
impermeable). The stiffness of the materials to be used must, therefore,
be assessed. This can be done based on equivalence.
- Water is present within the construction and can soften and weaken materials
and this must be allowed for, for example by measuring CBR values on saturated
samples. The effects of freezing must be allowed for in the design of water
storage layers. Specification
of aggregates must allow for this.
- The pavement designers should satisfy themselves that the materials to
be used do not invalidate the assumptions made in the structural design
methods.
- The design methods assume full friction between layers. Any
geotextiles or geomembranes must be carefully specified to minimise
loss of friction between layers. Geotextiles
and geogrids can be used to enhance the strength of pervious surfaces.
- Porous asphalt loses adhesion (binder stripping) and becomes brittle as
air passes through the voids. Its durability is therefore lower than conventional
materials.
- The single sized grading of the materials used means care must be taken
to ensure that loss of finer particles between unbound layers does not occur.
Geotextiles are important in this respect.
- Block paving is one of the most common pervious surfaces constructed in
the UK and may be designed using BS 7533.
There is no current structural design method in the UK specifically for pervious
pavements. They have, however, been in service in car parks in the USA for
over 20 years and are used widely in Germany for applications such as bus
and lorry parks, where heavy axle loads occur. Adverse structural effects
have not been reported.
A common misconception with pervious pavements is that the presence of water
in the unbound layers will reduce their strength and stiffness. Although this
is true for materials such as Type 1 sub-base it does not apply to the single
sized materials used in pervious pavements. Type 1 sub-base has a relatively
high fines content and is therefore affected by changes in moisture content.
The single sized nature of the sub-base required in a pervious pavement requires
a low fines content. Although it will have a lower stiffness than Type 1,
it will not to be significantly reduced further by the presence of water within
it, provided there is sufficient friction between particles when saturated.
Pervious pavements require a single size grading to give open voids. The choice
of materials for use in capping and sub-base layers below pervious pavements
is therefore a compromise between stiffness, permeability and storage capacity.
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