Porous or permeable pavers are designed with large voids that temporarily store stormwater, allowing runoff to infiltrate the soil below. Permeable pavement, when properly maintained, usually has a significantly longer lifespan than conventional asphalt because it is less prone to frost heave. Permeating hard surfaces on your property can be accomplished using a variety of methods including the following:
Block pavers are concrete blocks designed with gaps between them filled with small stone or sand. The gaps compose approximately 10% of the surface area, and allow stormwater to infiltrate into the reservoir below.
Permeable pavers involve interlocking concrete paving blocks connected by sand or gravel. The concrete blocks prevent the sand or gravel from washing away, which is an advantage over plain crushed rock.
Grass pavers are reinforced, vegetated surfaces supported by a durable plastic or concrete grid structure. The grid provides root protection, support for vehicle and pedestrian traffic, and prevents compaction of the vegetated area. This method is best suited for low-traffic or pedestrian areas with a variety of designs available.
Paving strips are strips composed of concrete or cement, embedded into a green area where vehicles can drive. This technique can be accomplished by digging shallow trenches and filling with cement or concrete, or by aligning a series of paving blocks.
Pervious Concrete and Porous Asphalt
Pervious concrete and porous asphalt involve pavement mixes that contain aggregate with the majority of the fine material removed, providing a typical void space of over 30%. These voids allow stormwater to percolate into a bed of crushed gravel below, and then seep slowly into surrounding soil. The pouring and setting procedures differ from impervious materials; however, they are typically used in the same situations.
The slope of the permeably paved area should be 1-5%.
The slope of the surrounding impermeable area should be no more than 20%.
The permeably paved area should not drain onto pavement.
The base of the permeably paved area should be at least 1 meter above the water table.
If soil below the permeably paved area has low permeability (an infiltration rate of less than 15 mm/hr) a perforated pipe underdrain must be included.
The permeably paved area receiving the stormwater should treat an impervious area approximately 1.2 times its size.
The permeably paved area should be located downslope from building foundations, or set back at least 4 metres from the foundation.
It is recommended to hire a professional with permeable paving experience for larger jobs such as driveways.
Call Ahead: Before beginning any major projects or alterations, make sure you are aware of the locations of any underground pipes, gas lines, or electrical wires and be sure to obtain any permits that may be necessary.
Test Soil: Do a percolation test that will indicate how fast the soil will absorb water. If percolation rate is less than 1.3 cm per hour, a drainage pipe may be required.
Start Digging: Remove existing pavement and dig to a depth of at least 40 cm. If percolation rate is less than 1.3 cm per hour, a deeper base will be required.
Prepare Base: Cover excavated area with 15 cm with a layer of 3/4- to 2-inch washed crushed stone. Compact twice with a plate compactor in both the width and length directions. Cover this layer with a 10 cm layer of 1/4- to 3/2-inch washed crushed stone and compact twice.
Screed the Bedding: Install concrete or plastic edging to prevent the pavers from shifting. Starting from one corner, lay two 2-inch-diameter steel pipes about 6 feet apart and parallel on the compacted base. Cover them with 1/4-inch stone, then pull a 2×4 screed board over the pipes to create a flat bed for setting the pavers. Lift out the pipes, fill the gaps, and repeat across the remaining area.
Place the Layers: Starting at the lowest corner, set the pavers on the bedding layer, tight to the edging and one another. Check every 6 to 10 feet to make sure the pavers are square to the first row. Place the full-size pieces, then go back and cut pieces to fit in any gaps along the edges.
Fill the Joints: Once the pavers are in, sprinkle 1/4-inch stone, the same used in Step 3, on the surface, and sweep it into the gaps with a wide push broom. Push the broom diagonally across the grid so that you don’t dislodge stones already in the joints.
Tamp: Sweep the surface clean, then run a plate compactor diagonally over the entire driveway. The machine’s vibrations pack the pavers firmly into the bedding layer and lock them in place. Refill joints that have settled deeply, and compact again.
Maintenance guidelines must be strictly followed to prevent clogging of the system. Mitigation techniques include the following:
Sweep the surface annually or biannually with a commercial vacuuming sweeping unit to remove accumulated sediment from the voids.
Sand should never be used for winter maintenance, as it will clog the system. De-icers should be used sparingly since dissolved constituents are not removed by the permeable pavement system.
Do not pressure wash the surface as it drives the sediment deeper into the voids.
Periodically clean out drainage systems within, or that drain to, the bedding.
Do not allow heavy vehicles to track or spill dirt on the permeable surfaces. Heavy vehicles tend to grind dirt into the voids of the surface, clogging the system.
Impervious areas draining into the permeable pavement should be swept clear of garbage and debris. Drainage from landscaped areas should be diverted away from the permeable area unless the vegetation has stabilized the soil.
Grass paver systems should be mowed, irrigated and fertilized regularly. Grass clippings should be removed to avoid clogging.
Seal coats should never be applied to permeable surfaces.