Permeable pavements: Life Cycle Costs

Placement of permeable pavers during construction.

Overview

Permeable Pavers are an alternative to traditional impervious pavements that allow stormwater to drain through them and into a storage reservoir below. Depending on the native soil properties and site constraints, the system may be designed for full infiltration, partial infiltration, or as a non-infiltrating detention and filtration only practice. They can be used for low traffic roads, parking, driveways, and walk ways, and are ideal where space for other surface BMPs is limited. Permeable pavement types include:

• permeable interlocking pavers (concrete or composite materials)
• grid systems (concrete or composite materials)
• pervious concrete (poured-in-place or pre-cast)
• porous asphalt
• permeable articulating block/mat systems

For the sake of this page and associated costs/figures below the information found here relate to Permeable Pavers, for costs and information associated with Porous Asphalt click here. STEP has prepared life cycle costs estimates for each design configuration, based on a 2,000 m² asphalt drainage area, runoff control target of 25 mm depth and 72 hour drainage period, for comparison which can be viewed below. To generate your own life cycle cost estimates customized to the development context, design criteria, and constraints applicable to your site, access the updated LID Life Cycle Costing Tool (LCCT) here.

Design Assumptions

Permeable pavers are ideal for sites with limited space and projects such as low traffic roads, parking lots, driveways and walkways. Components include: interlocking pavers, precast pervious slabs, cast in place surface, bedding course, and underground storage layer. Additional components include an underdrain to remove excess water and soil additives to enhance pollutant removal.

STEP's LCCT Assumptions:

Design and operation and maintenance program assumptions used to generate cost estimates are based on tool default values and the following STEP recommendations:

• Native soil infiltration rates for Full, Partial and No Infiltration Design scenarios were assumed to be 20 mm/h, 10 mm/h and 2 mm/h, respectively, and a safety factor of 2.5 was applied to calculate the design infiltration rate.
• Operation and maintenance (O&M) cost estimates assume annual inspections, removal of trash and debris twice a year, removal of sediment from pretreatment structures annually, and removal of weeds twice a year (where applicable). Verification inspections are included every 5 years to confirm adequate maintenance, and every 15 years to confirm adequate drainage performance through in-situ surface infiltration rate testing (where applicable).
• Maximum impervious drainage area to permeable surface area (I:P area) ratio of 1:1 (or 4:1 for roofs that contribute clean runoff to the practice)
• Default Bedding depth of 50 millimeters.
• Default Base depth of 100 millimeters.
• Default Sub-base depth (50 mm dia clear stone) of 200 millimeters.
• An underdrain (minimum 150 mm perforated pipe) is included in Partial Infiltration and No Infiltration design configurations only.
• Bedding layer and joint filler should consist of clear stone and gravel rather than sand to prevent clogging.
• Granular materials should not be applied as anti-skid agents during winter because they can quickly clog the system.
• Winter maintenance practices should be limited to plowing, with de-icing salts applied sparingly.
• The slope of the permeable pavement surface should be at least 1% and no greater than 5%.
• The impervious land surrounding and draining onto the pavement should not exceed the area of the permeable pavement (1:1 / I:P ratio).
• Pervious surfaces should not drain onto the pavement.
• The storage layer must be sized to accommodate runoff from the pavement and any impermeable areas draining to it.
  

Notes

• Operation and maintenance cost estimates assume replacement of severely degraded/failing pavers after the first 8 years of operation.
• The tool calculates costs for new (greenfield) development contexts and includes costs for contractor overhead and profit, material, delivery, labour, equipment (rental, operating and operator costs), hauling and disposal.
  • Land value and equipment mobilization and demobilization costs are not included, assuming BMP construction is part of overall development site construction.
  • Design and Engineering cost estimates are not calculated by the tool and must be supplied by the user.
  • The tool adds 10% contingency and additional overhead as default.
• All cost estimates are in Canadian dollars and represent the net present value (NPV) as the tool takes into account average annual interest and discount rates over the 25 and 50 year operating life cycle periods.
• Unit costs are based on 2018 RSMeans standard union pricing.
• Additional costs associated with retrofit or redevelopment contents is assumed to be 16% higher than the cost for new (greenfield) development contexts.
  • Retrofit construction cost estimates are included in the 'Costs Summary' section for comparison.
    

Construction Costs

Construction Costs Per Unit Drainage Area (CAD$/m²) - No Infiltration Design, 25 mm Retention

Construction Costs Per Unit Drainage Area (CAD$/m²) - Full Infiltration Design, 25 mm Treatment

Construction Costs Per Unit Drainage Area (CAD$/m²) - Partial Infiltration Design, 25 mm Retention

Note: Please click on each image to enlarge to view associated construction cost results.

Above you can find a cost breakdown of a 1000m² in three different configurations:

1. Non-infiltrating/filtration only Permeable Pavement,
2. Partial infiltration Permeable Pavement
3. Full infiltration Permeable Pavement

As can be seen, regardless of configuration type, Material & Installation expenses represent the largest portion of the total construction costs (82 to 85%). These include costly components such as the, impermeable membrane/liner, the underdrain, the sub-base of 50mm clear stone, base of 19mm clear stone and the bedding pavers themselves.

Life Cycle Costs

Below are both the capital and life cycle costs of the three permeable pavement configurations over a 25- and 50-year time periods. horizon based on a detailed assessment of local input costs, maintenance requirements, rehabilitation costs and design scenarios relevant to Canadian climates. The costs of maintenance and rehabilitation (Life cycle costs) are set at "Present Value" of these activities in 2022. The estimates of maintenance and rehabilitation (life cycle) costs represent net present values (NPV). Operation and maintenance costs are predicted to represent between 18 to 20% of total life cycle costs over the 25-year evaluation period, and increase to between 36 to 29% of total life cycle costs over the 50-year period, due to costs associated with increased surface vacuuming, replacement of joint material after vacuuming, restriping, replacement of pavers and cleaning out underground pipes (underdrain and overflow) to prevent clogging assumed to be continually required after 25 years of operation.

25-Year life cycle cost break down

Permeable Pavement: Full infiltration

Permeable Pavement: Non-infiltrating

Permeable Pavement: Partial infiltration

Note: Please click on each image to enlarge to view associated life cycle cost results.

50-Year life cycle cost break down

Permeable Pavement: Full infiltration

Permeable Pavement: Non-infiltrating

Permeable Pavement: Partial infiltration

Note: Please click on each image to enlarge to view associated life cycle cost results.

Total Cost & Design Summary

As previously discussed the three Permeable pavement configurations total cost summary vary greatly dependent on whether you want your feature to possess full infiltration, no infiltration, or partial infiltration. In short the most expensive of these options is the Permeable Pavement: No infiltration option ($187,356.33 vs. $168,085.10 - partial infiltration and $162,491.78 - full infiltration). The same can be said for construction + associated retrofit costs with each configuration design ($217,333.34 vs. $194,978.72 - partial infiltration and $188,490.47 - full infiltration). This configuration costs more than the other two predominantly due to the addition of an over 1,100m² Impermeable membrane (0.762 mm High Density Polyethylene-HDPE), which costs $24,181.47. This difference in price accounts for all three configurations possessing the exact same surface area footprint of 1,000m². As a reminder, it is important to understand your site's surrounding native soil infiltration rate to ensure you are selecting the appropriate design and if your site is located within a WHPA or Pollution hotspot, thus requiring a non-infiltration practice such as the aforementioned configuration of a permeable pavement feature.

A final note regarding the accuracy of the LCCT. A follow up sensitivity analysis study was conducted by CVC & STEP back in 2019 to test the tool's accuracy. The analysis took designs from 6 completed projects (4 bioretention, 1 permeable pavement, and 1 infiltration trench), and ran them through the tool comparing construction costing results from the LCCT to actual construction costs for the projects. The accuracy target set for the tool was plus-or-minus 30% of actual construction costs.

The analysis found that the tool was on average (±14%) to actual construction costs

Full Infiltration

Permeable Interlocking Concrete Pavement (PICP) on a narrow road between buildings in Chicago (Source: ICPI)

Partial Infiltration

Non-Infiltrating/filtration only

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

The natural ground material characteristic of or existing by virtue of geographic origin.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

The technique of removing pollutants from runoff as it infiltrates through the soil.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

Sustainable Technologies Evaluation Program

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A perforated pipe used to assist the draining of soils.

The natural ground material characteristic of or existing by virtue of geographic origin.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Credit Valley Conservation

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.