Inspection and Maintenance: Permeable Pavement
Permeable Pavement contain many small openings (i.e., joints or pores) that allow rainfall and snowmelt (i.e., stormwater) to drain through them instead of running off the surface as it does on impervious pavements like conventional asphalt and concrete. An overflow outlet is needed to safely convey flows during flood events. Depending on the permeability of the underlying soil and other constraints, the pavement may be designed with no sub-drain for full infiltration, with a sub-drain for partial infiltration, or with an impermeable liner and sub-drain for a no infiltration practice. The sub-drain pipe may feature a flow restrictor (e.g., orifice cap or valve) for gradually releasing detained water and optimizing the amount drained by infiltration into the underlying soil.
Key components of Permeable pavements to pay close attention to are the:
- Permeable Interlocking Pavers (i.e., Block Pavers) – Precast modular units made of concrete, pervious concrete or rubber/plastic composite designed to create open joints between pavers that are filled with fine, washed aggregate and installed on an open graded aggregate (i.e., clear stone) base and sub-base.
- Permeable Interlocking Grid Systems (i.e., grid pavers) – Precast concrete or manufactured plastic grids with open cells that can be filled with aggregate or a mixture of sand, gravel and topsoil and planted with grass or low-growing ground covers and are installed on an open-graded aggregate base.
- Pervious Concrete – a rigid pavement installed on an open-graded aggregate base that uses a cementitious binder to adhere aggregate together, similar to conventional concrete, except that the fine aggregate component is minimized or eliminated which results in the formation of connected pores throughout.
- Porous Asphalt – a flexible pavement installed on an open-graded aggregate base that uses a bituminous binder to adhere aggregate together, similar to conventional asphalt, except that the fine aggregate component is minimized or eliminated which results in the formation of connected pores throughout.
Inspection and Testing Framework
Routine Operation Inspection
|Contributing Drainage Area|
|Underdrain & Monitoring Well|
|Monitoring well condition||x||x||x||x|
|Sub-drain/Perforated pipe obstruction||x||x|
|Outlets||Overflow outlet obstruction||x||x||x||x|
|Pavement Surface||Pavement surface condition||x||x|
|Pavement surface sediment accumulation||x||x||x||x|
|Control structure condition||x||x||x||x|
|Control structure sediment accumulation||x||x||x||x|
Routine Operation Inspection
|Surface infiltration rate testing||x||(x)|
|Natural or simulated storm event testing||x||(x)|
|Note: (x) denotes indicators to be used for Performance Verification inspections only (i.e., not for Maintenance Verification inspections)|
Construction Inspection Tasks
Construction inspections take place during several points in the construction sequence, specific to the type of LID BMP, but at a minimum should be done weekly and include the following:
- During site preparation, prior to BMP excavation and grading to ensure the CDA is stabilized and/or flow diversion devices are in place and confirm that construction materials meet design specifications
- At completion of excavation and grading, prior to backfilling and installation of pipes to ensure depths, slopes and elevations are acceptable
- At completion of installation of pipes, prior to completion of backfilling to ensure slopes and elevations are acceptable
- After final grading, prior to surface course installation to ensure depths, slopes and elevations are acceptable
- Prior to hand-off points in the construction sequence when the contractor responsible for the work changes (i.e., hand-offs between the storm sewer servicing, paving, building and landscaping contractors)
- After every large storm event (e.g., 15 mm rainfall depth or greater) to ensure flow diversion devices are functioning and adequately maintained. View the table below, which describes critical points during the construction sequence when inspections should be performed prior to proceeding further. You can also download and print the table here
Construction Sequence Step & Timing
|Site Preparation - after site clearing and grading, prior to BMP excavation and grading||Natural heritage system and tree protection areas remain fenced off|
|ESCs protecting BMP layout area are installed properly|
|CDA is stabilized or runoff is diverted around BMP layout area|
|BMP layout area has been cleared and is staked/delineated|
|Benchmark elevation(s) are established nearby|
|Construction materials have been confirmed to meet design specifications|
|BMP Excavation and Grading - prior to backfilling and installation of pipes/catchbasins||Excavated soil is stockpiled outside the CDA|
|Excavation location, footprint, depth and slope are acceptable|
|BMP Installation – after installation of pipes/catchbasins, prior to completion of backfilling||Structural components (e.g., pavement base, curbs) installation is acceptable|
|Impermeable liner installed correctly, if applicable|
|Installations of sub-drain pipes (e.g., locations, elevations, slopes), standpipes/monitoring wells are acceptable|
|Sub-drain trench dams installed correctly (location, elevation)|
|Surface coarse installation (elevation, slope, monitoring wells) is acceptable|
Routine Maintenance - Key Components and I&M Tasks
Regular inspections (twice annually, at a minimum) done as part of routine maintenance tasks over the operating phase of the BMP life cycle to determine if maintenance task frequencies are adequate and determine when rehabilitation or further investigations into BMP function are warranted.
Table below describes routine maintenance tasks for permeable pavements, organized by BMP component, along with recommended minimum frequencies. It also suggests higher frequencies for certain tasks that may be warranted for BMPs located in highly visible locations or those receiving flow from high traffic (vehicle or pedestrian) areas or those designed with higher than recommended impermeable drainage area to permeable BMP footprint area ratios (I:P ratios). Tasks involving removal of trash, debris and sediment and weeding/trimming of vegetation for BMPs in such contexts may need to be done more frequently (i.e., higher standards may be warranted).
Individuals conducting vegetation maintenance and in particular, weeding (i.e., removal of undesirable vegetation), should be familiar with the species of plants specified in the planting plan and experienced in plant identification and methods of removing/controlling noxious weeds. Key resources on these topics are provided below at the links provided:
- Agriculture and Agri-food Canada’s Weed Info database
- Ontario Ministry of Agriculture, Food and Rural Affairs’ Ontario Weed Gallery
- Ontario Ministry of Agriculture, Food and Rural Affairs’ Noxious Weeds In Ontario list
- Ontario Invasive Plant Council’s Quick Reference Guide to Invasive Plant Species
- Oregon State University Stormwater Solutions, 2013, Field Guide: Maintaining Rain Gardens, Swales and Stormwater Planters, Corvallis, OR.
- Plants of Southern Ontario (book), 2014, by Richard Dickinson and France Royer, Lone Pine Publishing, 528 pgs.
- Weeds of North America (book), 2014, by Richard Dickinson and France Royer, University of Chicago Press, 656 pgs.
|Comnponent||Description||Inspection & Maintenance Tasks||(Pass) Photo Example||(Fail) Photo Example|
|Contributing Drainage Area (CDA)||
Area(s) from which runoff directed to the BMP originates; includes the pavement itself and any adjacent impermeable pavement or roofs that drain to it.
Devices or features that retain trash, debris and sediment; help to extend the operating life cycle; examples are eavestrough screens, catch basin inserts and sumps, oil and grit separators, geotextile-lined inlets, gravel trenches, grass filter strips and forebays.
Structures that deliver water to the BMP (e.g., impermeable pavement edges, pipes from roof downspouts or catchbasins).
The surface of the pavement, including pavers/pavement, joints and edge restraints (e.g., curbs, edging); should not allow water to pond on the surface so any observation or evidence of surface ponding (e.g., sediment caking on the pavement) indicates a drainage problem.
Applies to grid paver systems only; a mixture of deep rooting perennial grasses or low growing ground covers, tolerant to both wet and dry conditions and salt (if receiving impermeable pavement runoff); roots uptake water and return it to the atmosphere, provide habitat for soil organisms that break down pollutants trapped in the soil and help maintain soil structure and permeability.
Structures (e.g., catchbasin, curb-cut, swale) that convey flow that exceeds the storage capacity of the BMP to another drainage system (e.g., municipal storm sewer or other BMP).
Optional component; perforated pipe(s) surrounded by gravel and may be wrapped in geotextile filter fabric; installed in the base or sub-base gravel layer to collect and convey treated water to an adjacent drainage system or other BMP; may also include a flow restrictor.
Perforated standpipe that extends from the bottom of the excavation to just below the pavement surface and contains perforations or slots to allow measurement of subsurface water level; used to track drainage performance over the operating life cycle of the BMP.
Manhole or catchbasin to which the subdrain outlets that provides access to the underdrain and flow restrictor. Inspect for accessibility, damage and sediment.
Tips to Preserve Basic BMP Function
- Never use sealants on porous asphalt nor pervious concrete.
- Prohibit access by construction vehicles to prevent tracking of sediment on to the surface.
- Prohibit storage of soil, compost, sand, salt or unwashed gravel on permeable pavements to prevent clogging of joints or pores, or protect the pavement surface with tarps or geotextile during temporary storage of such materials.
- Landscaped areas adjacent to permeable pavements should be covered with vegetation and not drain to the pavement where possible to prevent eroding soil from reaching the surface.
- Use a mulching mower to mow permeable interlocking grid systems with grass cover.
- Permeable pavements can be plowed for snow removal like conventional pavements. To reduce the risk of dislodging pavers or grids and minimize displacement of joint/cell fill material, the plow blade should be slightly raised off the pavement surface (e.g., 0.6 cm or 1/4”) with a shoe attachment.
- Plowed snow piles should not be stored on permeable pavements to reduce the risk of clogging from sediment accumulation upon melting.
- Do not spread sand on permeable pavements as part of winter maintenance as it will quickly clog the joints or pores and impair drainage function. On permeable interlocking pavers and grid systems filled with gravel, if application of an anti-skid material is desirable, spread the same fine washed gravel material used to fill the paver joints or grid cells; and
- De-icers should be used sparingly, as needed during winter. Due to their freely draining design, ice will not form on permeable pavements as readily as it does on conventional impermeable pavements during winter thaw-freeze cycles and thereby require fewer salt applications during winter. In a study of a porous asphalt parking lot in New Hampshire, it was found that application of de-icing salt could be reduced by between 64 to 77% while still maintaining equivalent or better surface conditions compared to rates needed for impermeable asphalt. In a comparative study of subsurface temperature during freezing winter conditions, Danz et al. (2021) found that PICP surpassed PC and PA with fewer days below freezing, higher temperatures on melt days, slower freeze and faster thaw times, and less penetration of freezing temperatures at depth.
Rehabilitation & Repair
Table below provides guidance on rehabilitation and repair work specific to permeable pavements organized according to BMP component.
|Pavement Surface||Major cracks, spalling or raveling of the porous asphalt or pervious concrete surface||
|Paver or grid unit is missing, damaged or displaced||
|Surface infiltration rate is < 250 mm/h ||
Poor grass cover on interlocking permeable grid system
Sub-drain perforated pipe is obstructed by sediment.
Inspection Time Commitments and Costs
Estimates are based on a typical partial infiltration permeable pavement design (i.e., includes a sub-drain); estimates for other designs (i.e., full infiltration and no-infiltration) are described in
the Low Impact Development (LID) Stormwater Management Practice Inspection and Maintenance Guide
Estimates of the life cycle costs of inspection and maintenance have been produced using the latest version of the LID Life Cycle Costing Tool for three design variations (full infiltration, partial infiltration and no infiltration) to assist stormwater infrastructure planners, designers and asset managers with planning and preparing budgets for potential LID features.
Assumptions for the above costs and the following table below are based on the following:
- Design variations for permeable pavements can be broken down into three main categories:
- Full Infiltration design, where the pavement drains through infiltration into the underlying subsoil alone (i.e., no sub-drain);
- Partial Infiltration design, where drainage is through the combination of a subdrain and infiltration into the underlying subsoil (i.e., with a sub-drain); or,
- No Infiltration (i.e., filtration only design) that includes an impermeable liner between the base of the BMP and the underlying native sub-soil, where drainage is through a sub-drain alone (i.e., with a sub-drain and impermeable liner).
- For permeable pavements it is assumed that rehabilitation of the pavement surface will be needed once the BMP reaches 30 years of age in order to maintain surface drainage performance at an acceptable level. Included in the rehabilitation costs are (de)mobilization costs, as equipment would not have been present on site.
- Design costs were not included in the rehabilitation as it was assumed that the original LID practice design would be used to inform this work. The annual average maintenance cost does not include rehabilitation costs and therefore represents an average of routine maintenance tasks, as outlined in Table Permeable Pavements: Key Components, Descriptions and Routine I&M Requirements, above. All cost value estimates represent the NPV as the calculation takes into account average annual interest (2%) and discount (3%) rates over the evaluation time periods.
- The costing presented in this section is specific to permeable interlocking concrete pavers (PICP), as defined in the Tool. This product has been selected for costing due to its popularity and well understood maintenance needs.
- For all permeable pavement design variations, the CDA has been defined as 2,000 m2 of which 1,000 m2 is impermeable pavement draining to the pavers, and 1,000 m2 is permeable pavement. The impervious area to pervious area ratio (I:P ratio) used to size the BMP footprint is 1:1, which is in accordance with recommendations in the LID SWM Planning and Design Guide (CVC & TRCA, 2010).
- The Full Infiltration design does not include a sub-drain and assumes a native sub-soil infiltration rate of 20 mm/h. The base granular reservoir is 350 mm deep and is capable of storing runoff from a 61 mm rain event over the CDA. A monitoring well is included for inspection purposes.
- The Partial Infiltration design includes a sub-drain and assumes a native sub-soil infiltration rate of 10 mm/h. The base granular reservoir is 350 mm deep and is capable of storing runoff from a 9 mm rain event before the stored volume reaches the perforated underdrain pipe located 50 mm above the native sub-soil. Although a flow restrictor is recommended to maximize infiltration, the cost of this feature is not included due to its relatively low cost.
- The No Infiltration design includes a sub-drain pipe installed on the bottom of the sub-surface water storage reservoir and an impermeable liner. All other features are the same as the Partial Infiltration design variation.
- Estimates of the life cycle costs of PICP permeable pavements in Canadian dollars per unit CDA ($/m2) are presented in the table below. LID Life Cycle Costing Tool allows users to select what BMP type and design variation applies, and to use the default assumptions to generate planning level cost estimates.
- Users can also input their own values relating to a site or area, design, unit costs, and inspection and maintenance task frequencies to generate customized cost estimates, specific to a certain project, context or stormwater infrastructure program.
- For all BMP design variations and maintenance scenarios, it is assumed that rehabilitation of the pavement surface will be necessary when the BMP reaches 30 years of age to maintain acceptable surface drainage performance. Rehabilitation of PICP pavements is assumed to typically involve the following tasks and associated costs:
- Remove pavers, bedding and joint fill and top 5 cm (2”) of base aggregate and replace with new material that meets design specifications
- Construction and Assumption inspection and testing associated with rehabilitation work to confirm that materials meet design specifications and installation is acceptable, including compaction and surface infiltration rate testing.
- Estimated life cycle costs represent NPV of associated costs in Canadian dollars per square metre of CDA ($/m2).
- Average annual maintenance cost estimates represent NPV of all costs incurred over the time period and do not include rehabilitation costs.
- Rehabilitation cost estimates represent NPV of all costs related to rehabilitative maintenance work assumed to be needed after 30 years in service, including those associated with inspection.
- Full Infiltration design life cycle costs are lower than Partial and No Infiltration designs due to the absence of a sub-drain to construct, inspect and routinely flush.
- Rehabilitation costs for Full Infiltration designs are estimated to be 54.8% to 55.6% of the original construction costs for High and Minimum Recommended Frequency maintenance program scenarios, respectively.
- Rehabilitation costs for Partial Infiltration designs are estimated to be 53.5% to 54.3% of the original construction costs for High and Minimum Recommended Frequency maintenance program scenarios, respectively.
- Rehabilitation costs for No Infiltration designs are estimated to be 47.4% to 48.1% of the original construction costs for High and Minimum Recommended Frequency maintenance program scenarios, respectively.
- Maintenance and rehabilitation costs over a 25 year time period for the Minimum Recommended maintenance scenario are estimated to be 22.3%, of the original construction costs for Full Infiltration design, 22.6% for Partial Infiltration design, and 20.0% for No Infiltration design.
- Maintenance and rehabilitation costs over a 25 year time period for the High Frequency maintenance scenario are estimated to be: 39.0% of the original construction costs for Full, 38.9% for Partial Infiltration designs, and 34.5% for No Infiltration designs.
- Maintenance and rehabilitation costs over a 50 year time period for the Minimum Recommended Frequency maintenance scenario are estimated to be approximately: 1.07 times the original construction cost for Full, 1.06 times the original construction costs for Partial Infiltration designs, and 93.9.% the original construction cost for No Infiltration designs.
- Maintenance and rehabilitation costs over a 50 year time period for the High Frequency maintenance scenario are estimated to be approximately: 1.43 times the original construction cost for Full, 1.41 times the original construction costs for Partial Infiltration designs, and 1.25 times the original construction cost for No Infiltration designs.
Inspection Field Data Sheet
Feel free to download (downward facing arrow on the top righthand side) and print (Pinter emoticon on top right hand side) the following Permeable pavements Inspection Field Data Form developed by TRCA, STEP and its partners for the Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide.
The 4 page document prompts users to fill out details previously mentioned above on this page in other sections about various zones associated with [[[Permeable pavements]] features (i.e. CDA, pavement surface, associated control structures, outlets, etc.) and describe why each area is a pass or fail, and if remediate action is required and under what timeframe it would be completed by. Furthermore, the forms prompt the reviewer to determine what type of inspection is being conducted for the feature in question: Construction (C), Routine Operation (RO), Maintenance Verification (MV), or Performance Verification (PV).
- TRCA. 2016. Fact Sheet - Inspection and Maintenance of Stormwater Best Management Practices: Permeable Pavement. https://sustainabletechnologies.ca/app/uploads/2018/02/Permeable-Pavement-Fact-Sheet.pdf
- STEP. 2018. Fact Sheet - Inspection and Maintenance of Stormwater Best Management Practices: Permeable Pavements. https://sustainabletechnologies.ca/app/uploads/2018/02/Permeable-Pavement-Fact-Sheet.pdf
- Guertin, M. 2010. Simple Screen Gutter Guards Better Than Pro-Installed Systems (and way cheaper). Fine Homebuilding - The Daily Fix. Accessed July 17 2022. https://www.finehomebuilding.com/2010/08/17/simple-screen-gutter-guards-better-than-pro-installed-systems-and-way-cheaper
- My Gutter Pro. 2021. Clogged Downspout : Causes and Solutions. 10 April 2021. Accessed July 19 2022. https://mygutterpro.com/downspout-clog/
- Fairfax County. 2014. Protecting our Environment, one Stormwater Practice at a Time - Permeable Pavements. April 2014. Accessed: July 20 2022. https://www.fairfaxcounty.gov/publicworks/sites/publicworks/files/assets/documents/pdf/factsheets/permeable-pavement.pdf
- Weiss, P.T., Kayhanian, M., Gulliver, J.S., Khanzanovich, L. 2019. Permeable pavement in northern North America urban areas: research review and knowledge gaps. International Journal of Pavement Engineering. Vol.20, No.2, 143-162. https://www.tandfonline.com/doi/full/10.1080/10298436.2017.1279482
- Al-Rubaei, A.M., Stenglein, A.L., Viklander, M., Blecken, G.T. 2013. Long-Term Hydraulic Performance of Porous Asphalt Pavements in Northern Sweden. Journal of Irrigation and Drainage Engineering. Vol.139 No.6: 499-505. https://ascelibrary.org/doi/10.1061/%28ASCE%29IR.1943-4774.0000569
- Permeable Pavements Task Committee. 2015. Permeable Pavements. Edited by Eisenberg, B., Collins Lindow, K. and Smith, D.R., Amercian Society of Civil Engineers (ASCE), Reston VA. https://ascelibrary.org/doi/book/10.1061/9780784413784
- Huang, J., Valeo, C., He, J., Chu, A. 2016. Three Types of Permeable Pavements in Cold Climates: Hydraulic and Environmental Performance. Journal of Environmental Engineering. 2016:04016025. https://ascelibrary.org/doi/abs/10.1061/%28ASCE%29EE.1943-7870.0001085
- American Society of Civil Engineers (ASCE). 2018. ASCE/T&DI/ICPI Standard 68-18. Permeable Interlocking Concrete Pavements. Reston VA.https://sp360.asce.org/PersonifyEbusiness/Merchandise/Product-Details/productId/244074874
- Roseen, R.A., Ballestro, T.P., Houle, K.M., Heath, D., Houle, J.J. 2014. Assessment of Winter Maintenance of Porous Asphalt and Its Function for Chloride Source Control. Journal of Transportation Engineering. 2014.140. https://ascelibrary.org/doi/abs/10.1061/%28ASCE%29TE.1943-5436.0000618
- Danz, M.E., Buer, N.H., Selbig, W.R. 2021. Water. 2021, 13, 3513. https://www.mdpi.com/2073-4441/13/24/3513
- Western Interlock Inc. 2020.How to Remove Pavers and Replace Them. Repair & Maintenance. Authored June 2020. Accessed: 21 July 2022. https://westerninterlock.com/how-to-remove-and-replace-a-paver/
- American Society of Civil Engineers (ASCE). 2018. ASCE/T&DI/ICPI Standard 68-18. Permeable Interlocking Concrete Pavements. Reston VA. https://sp360.asce.org/PersonifyEbusiness/Merchandise/Product-Details/productId/244074874
- Winston, R.J., Al-Rubaei, A.M., Blecken, G.T., Viklander, M., Hunt, W.F. 2016. Maintenance measures for preservation and recovery of permeable pavement surface infiltration rate - The effects of street sweeping, vacuum cleaning, high pressure washing and milling. Journal of Environmental Management. 169 (2016) 132-144. https://www.sciencedirect.com/science/article/pii/S0301479715304412?via%3Dihub
- TRCA. 2018. Inspection and Maintenance of Stormwater Best Management Practices. Permeable Pavements - Fact Sheet. https://sustainabletechnologies.ca/app/uploads/2018/02/Permeable-Pavement-Fact-Sheet.pdf
- TRCA. 2018. Inspection and Maintenance of Stormwater Best Management Practices. Permeable Pavements - Fact Sheet. https://sustainabletechnologies.ca/app/uploads/2018/02/Permeable-Pavement-Fact-Sheet.pdf
- TRCA. 2018. Inspection and Maintenance of Stormwater Best Management Practices. Permeable Pavements- Fact Sheet. https://sustainabletechnologies.ca/app/uploads/2018/02/Permeable-Pavement-Fact-Sheet.pdf
- TRCA. 2016. Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide. Prepared by the Sustainable Technologies Evaluation Program. Vaughan, Ontario. https://sustainabletechnologies.ca/app/uploads/2016/08/LID-IM-Guide-7.4-Permeable-Pavements.pdf
- STEP. 2016. Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide. https://sustainabletechnologies.ca/app/uploads/2016/08/LID-IM-Guide-2016-1.pdf