Sub-surface components

The construction guidance in this section applies to all sub-surface and ground-level LID practices: bioretention gardens, bioswales, rain gardens, exfiltration trenches, enhanced swales, permeable pavements, infiltration trenches, infiltration chambers, and soakaways. In general, this section describes construction and inspection processes for below ground installed materials and infrastructure that are part of the LID practices noted above.

Overview

This section gives guidance for project managers, engineers, and contractors on the installation of sub-surface components of LID practices.

Geotextiles

Geotextiles are filter fabrics that can be installed to separate dissimilar soils and prevent the migration of materials. Geotextiles are recommended for installation at the bottom and side limits of the LID feature and should be implemented at the discretion of the engineer.

Construction Steps:

1. Roll out fabric on the flattened sub-grade surface.
2. Provide a minimum overlap of 300 mm between adjacent lengths of geotextile. If the manufacturer specifies a greater overlap, follow the manufacturer’s specification.
3. Secure fabric in place with stakes along the edge of the sub-grade surface and where lengths of fabric overlap.
4. Provide 500 mm of excess at the ends of each length of fabric. Fold excess materiel on top of the infiltration medium/choker course once installed.
5. Provide cuts for trees and shrubs where appropriate.
6. Assess on-site conditions to ensure that the tensile, tear, and puncture strength ratings of the geotextile are suitable for expected structural loads (e.g. traffic). If not properly considered, geotextiles can be adversely impacted by loading from adjacent infrastructure uses.

Key Inspection Points:

• Geotextile is clean and free of damage.
• Geotextile delivered to the site matches the design specifications and is approved by the engineer prior to installation.
• Sufficient overlap is provided between lengths of fabric.
• Geotextiles have been applied as per the manufacturer’s guidelines.
• Structural considerations for geotextiles have been made when applied adjacent to infrastructure and the geotextile can handle expected structural loads.
• Class II geotextiles conform to OPSS 1860.

Mistakes to Avoid:

• Wrinkles in the fabric: Follow the manufacturer’s procedure for installation to ensure geotextiles lie smooth on sub-grade.
• Unnecessary application of geotextile: Mixing of media is negligible where structural loads are not expected, so geotextile does not need to be applied. Also, geotextiles should not be used where root growth is encouraged to penetrate different layers of media.

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  The shape of the LID feature required the geotextile to be rolled out prior to being laid on the subgrade surface. Photo taken during construction of IMAX Corporation Headquarters. (Photo Source: CVC, 2012)

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  Excess fabric is draped over the edges of the LID practice, and backfilling with granular is taking place. Photo taken during construction of IMAX Corporation Headquarters. (Photo Source: CVC, 2012)

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  Footprint with geotextile, granular, and underdrain. Photo taken during construction of IMAX Corporation Headquarters. (Photo Source: CVC, 2012)

Underdrains

Underdrain installed to subgrade. (Photo Source: CVC, 2010)

Underdrain installed with maintenance risers. Note the 'T' joints, which are no longer recommended. (Photo Source: CVC, 2010)

Underdrains are perforated pipes that collect subsurface water and convey water out of an LID facility once it reaches capacity. Their design is determined by the drainage requirements of the feature.

Construction Steps:

1. Install the underdrain to grade with a consistent slope of 0.5% to 1.0%.
2. Connect overflow risers with single-elbowed ‘y’ connections or 45° elbows.
3. Situate maintenance risers where siltation is expected in drain lines (such as at junctions or where grade and direction changes occur), in accordance with the design drawings.
4. Mark the locations of risers.
5. Use video inspections to confirm that the interior of the underdrain remains free of debris after construction.

Key Inspection Points:

• Size, type, and material of the underdrain conforms to the design specifications.
• Underdrain is perforated prior to arrival on-site.
• Drain material is resistant to UV radiation and any chemicals in the soil and groundwater.
• Clean-outs are positioned correctly and spaced appropriately for the size of the underdrain.
• Any manual perforations are approved by the design or field engineer.
• Pipe grade matches design specifications.
• Tie-in locations to municipal sewers meet municipal specifications.

Mistakes to Avoid:

• Incorrect overflow risers – ensure single-elbowed ‘y’ connections or 45° elbows are used for overflow risers instead of ‘T’ connections.
• Incorrect drain size – confirm underdrain has an internal diameter greater than or equal to 200 mm to reduce freezing and allow cleaning and camera inspections.
• Potential for clogging – Remove sock from underdrain prior to installation or geofabric wrap. A choker course is recommended, instead, as field change.

For more information on the design of underdrains, such as materials and connections, see the page on underdrains linked above.

Impermeable liners

Impermeable liners can be applied when LID features are not intended to infiltrate runoff into the underlying soil. They protect building foundations or other subsurface infrastructure. They also prevent infiltration through contaminated soils. Where building foundations next to the LID feature are not waterproofed, liners can be used to protect adjacent infrastructure.

Construction Steps:

1. Compact 30 – 50 mm of sand over the soil onto which the membrane will be installed to protect against punctures. Alternatively, a geotextile can be used in the place of sand.
2. Place lengths of liner on the bed of the facility as per the manufacturer’s specifications.
3. Provide 150 mm of overlap between adjacent lengths of liner.
4. Secure liner in-place and bond together the overlapping portions of liner as specified by the manufacturer.
5. Seal a flange to the pipe and liner when pipes provide drainage from the practice.
6. Compact sand or apply cushion fabric on top of the liner to protect against punctures.

Key Inspection Points:

• Liner matches design specifications.
• Liner installation procedures conform to manufacturer’s specifications.
• Minimum overlap between lengths of liner is achieved.
• Need for protection of building footprints and adjacent infrastructure confirmed through field verification of setbacks.
• Sand and/or geotextile are applied on both sides of the liner to protect against punctures.
• Lengths of liner are bonded together to ensure protection against groundwater penetration and contamination.

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  An impermeable liner was placed along one side of the LID facility to prevent infiltrated water from contacting the nearby watermain in the road right-of-way, which runs parallel to the feature. (Photo Source: CVC, 2020)

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  Impermeable Bentofix® liner being rolled out in LID practice area. (Photo Source: CVC, 2012)

Overflows

Overflows are features of inline facilities and convey larger storm events out of the LID feature.

Construction Steps:

1. Connect the overflow drain to the underdrain.
2. Fit a metal, domed grate to the overflow.
3. Install overflow pit to grade at the location specified in the contract documents.

Key Inspection Points:

• Overflow drain matches design specifications.
• Overflow pit is positioned at the maximum water surface elevation of the practice, as per the contract documents.
• Sufficient freeboard is provided between the overflow and inlet such that the inlet is not inundated by design storm flows.
• Overflow grate matches design specifications.
• Overflow grates located in high-traffic areas are screwed on or equipped with locks.

Mistakes to Avoid:

• Backflows: Ensure that backflows are avoided by confirming that inlet and overflow elevations match design specifications with surveys.
• Incorrect sequencing: Install overflow drain prior to backfilling with granular material and engineered soil.
• Incorrect grate material: Avoid using plastic overflow grates due to breakages and UV degradation.

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  Overflow pipe installed to an underdrain T-connection. (Photo Source: CVC, 2010)

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  Overflow installed with a plastic cap. Note that the use of plastic grates is discouraged. (Photo Source: CVC, 2012)

Monitoring wells

Monitoring well stack installed next to underdrain within the LID practice. (Photo Source: CVC, 2010)

Monitoring wells allow for measurement of water levels in the sub-surface components of LID practices. They are important for sampling procedures and ensuring that the LID feature are performing as intended.

Construction Steps:

1. Install a cap stand at the required elevation to anchor the monitoring well.
2. Wrap the perforated pipe with a geotextile sock (to prevent migration of soil into the well).
3. Anchor the perforated pipe to the bottom of the LID feature.
4. Install a lockable cap to the top of the standpipe to protect against vandalism.
5. Install monitoring equipment (e.g., a level logger) if required.

Key Inspection Points:

• Pipe is perforated, rigid, and matches design specifications.
• Well is installed according to contract documents.
• Geotextile sock is wrapped around the perforated portion of the well.

Mistakes to Avoid:

• Incorrect sequencing: Ensure that standpipe is installed prior to backfilling with granular material.

Storage reservoir

The storage reservoir layer holds and directs the stormwater into the underlying/native soils. The LID facilities, if applicable, should be filled with uniformly graded, washed stone (20 mm – 50 mm) that provides 30 to 40% void space.

Construction steps:

1. Backfill material from outside of the LID facility to avoid compaction and sediment entering the facilities. Use a slinger truck if possible.
2. Place the material to the elevation and thickness as per the design specifications.

Key Inspection Points:

• Material arrival to the site:
  • Check chain of custody
  • Verify it meets specifications as per design
  • Verify no debris or fines within the aggregate (it’s a washed stone)
• Granular material should be 19 - 50 mm clear stone or as per design. See Aggregates for further details.
• Installed aggregate is at the correct elevation as per design.

Mistakes to Avoid:

• Accepting material that does not meet design requirements and specifications.
• Installing material with heavy equipment from the inside of the LID facility
• Installing frozen aggregate. Do not install frozen aggregate
• Leaving heavy equipment (excavator) for long periods of time within the LID facility.

For more guidance on materials specifications, refer to Aggregates, OPSS aggregates and (Ontario Provincial Standards, 2013)

Permeable pavement sub-base and base course

Permeable pavement sub-base and base course

Stone choker layer

In LID facilities, a choker layer of ≥ 100 mm depth is recommended to prevent migration of finer filter media into the underlying storage reservoir aggregate. Similar to the storage reservoir material, this aggregate layer should be a washed 5 – 10 mm stone free from fines and debris. Installation of aggregate choker layer should not be done when frozen.

Follow the construction guidance shown above in the section Storage reservoir

For more guidance on materials specifications, refer to Aggregates, OPSS aggregates and (Ontario Provincial Standards, 2013)

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

Credit Valley Conservation

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.

A perforated pipe used to assist the draining of soils.

A perforated pipe used to assist the draining of soils.

The water below the surface, and typically below the groundwater table.

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.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Refers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet.

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.

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.

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.

Soil particles with a diameter less than 0.050 mm.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

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.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

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.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.