Infiltration chambers

Overview

As their name suggests infiltration chambers work exclusively to infiltrate stormwater. Infiltration chambers include a range of proprietary manufactured, modular structures embedded in clean, crushed angular stone that are installed underground, typically under parking or landscaped areas, that create large void spaces for temporary storage of stormwater, allowing it to infiltrate into the underlying native soil. They may be designed to provide sufficient load bearing capacity to allow construction of structures on top of them. They can be installed individually or in series, in trench or bed configurations. Due to the large volume of underground void space they create, and the modular nature of their design, they are well suited to sites where available space for other types of BMPs is limited, or where it is desirable for the facility to have little or no surface footprint (e.g., high density development contexts).

Infiltration chambers are an ideal technology for:

• Installing below any type of surface or landscape
• Receiving and infiltrating large volumes of water

The fundamental components of an infiltration chamber system are:

• Pretreatment devices to retain trash, debris, sediment and floatables and prevent clogging of inlets, underlying native soil and outlets;
• Storage reservoir consisting of structurally reinforced chambers, vaults, crates, perforated pipes or other void-forming structures embedded in coarse aggregate;
• Pipes and possibly flow control structures to convey water into and out of the practice; and
• Geotextiles to maintain separation between the storage reservoir and surrounding native soil.

System type	Perforated pipes	Vaults	Arched chambers	Crates
Materials	Plastic or Metal	Concrete	Plastic	Plastic
Footprint	Medium	Small	Medium	Small
Stackable	Yes	No	Yes	Yes
Effective Porosity (n')	0.60 - 0.65	0.75 - 0.85	0.50 - 0.65	0.95
Maintenance Access	Moderate	Excellent	Moderate	Difficult
Standard Strength	H-20	H-25	H-20	H-20 - H-25

Planning Considerations

All types of modular infiltration systems require a bedding of angular clear stone to permit infiltration, and provide a foundation for the installation:

• Plastic chambers usually have a parabolic shape to support the load above. The spaces between rows of plastic pipes or chambers are filled with clear stone to support overlying structures.
• Concrete vaults and plastic crates are often rectangular-shaped. Concrete vault systems can, in some circumstances, be employed without any additional cover. However, a minimum of 20 cm cover is recommended for most applications. Where this cover is planting soil this can support turf grass. Greater planting soil depths are required to support more deeply rooting plants like perennials and shrubs (45 to 60 cm) and trees (85 to 100 cm).

Native Soil

Infiltration trenches, chambers and soakaways can be constructed over any soil type, but hydrologic soil group (HSG) A or B soils are best for achieving water balance and erosion control objectives. Facilities should be located on portions of the site with the highest infiltration rates. Native soil infiltration rate at the proposed facility location and depth should be confirmed through in-situ measurements of hydraulic conductivity under field saturated conditions.

Wellhead Protection

Facilities receiving road or parking lot runoff should not be located within two (2) year time-of-travel wellhead protection areas (see drinking water source protection plan).

Available Space

Recommended under parking, walkway or landscape areas and require little surface space, solely for inlets, outlets and access structures.

Site Topography

Facilities cannot be located on natural slopes greater than 15%.

Water Table

Maintaining a separation of one (1) metre between the elevations of the base of the practice and the seasonally high water table, or top of bedrock is recommended. Lesser or greater values may be considered based on groundwater mounding analysis. See STEP LID Planning and Design Guide wiki page, Groundwater for further guidance and spreadsheet tool.

Pollution Hot Spot Runoff

To protect groundwater from possible contamination, runoff from pollution hot spots should not be treated by infiltration trenches, chambers or soakaways.

Proximity to Underground Utilities

Designers should consult local utility design guidance for the horizontal and vertical clearances required between storm drains.

Karst

Infiltration trenches, chambers and soakaways are not suitable in areas of known or implied karst topography.

Setback from Buildings

Facilities should be setback a minimum of four (4) metres from building foundations.

Design

Geometry and Site Layout

Infiltration chambers and soakaways can be designed in a variety of shapes, although facilities should have level or nearly level bottoms to spread flow evenly throughout. Typically designed with an impervious drainage area to pervious facility footprint area ratio (i.e. I:P ratio) between 5:1 on low permeability soils (HSG C & D) to 20:1 on high permeability soils (HSG A & B). Not typically deeper than 4 m.

Inlets

Recommended to distribute inflow between multiple inlets to reduce risk of failure. May include delivery of runoff to the facility as concentrated overland or concentrated underground (i.e. pipe) flow. For concentrated overland flow:

• Catch basins or other inlet structures should be located at all sag points in the gutter grade and immediately upgrade of median breaks, crosswalks and street intersections,
• inlet types include curb openings, side inlet catch basins, trench drains, and other pre-fabricated inlet structures,
• spillways aid in turning flow 30, 45 or 90 degrees into the practice,
• if the inlet structure itself does not provide sedimentation or filtration pre-treatment, incorporate a geotextile and stone filter inlet at curb opening or spillway locations to isolate sediment, trash and debris for ease of removal, and
• provide a 50 to 100 mm drop in elevation between inlet invert and top elevation of the stone cover.

For concentrated underground flow, inlets may include plastic or concrete pipes from roof drains or catch basins, storm sewers, OGSs or outlet leader from an upstream BMP. Solid pipe transitions to perforated pipe within the storage reservoir 0.3 m from the sides. Trench plugs should be installed around solid pipes where they enter and exit the storage reservoir.

Sizing

Infiltration: Sizing and modeling

Materials

Chambers

Plastic chambers

Chambers should be compliant with:

• CSA B184-17 "Polymeric subsurface stormwater management structures".
• ASTM F2418-16a “Standard Specification for Polypropylene (PP) Corrugated Wall Stormwater Collection Chambers”.

Allowable loads for the chambers must be determined in accordance with ASTM F2787-13 “Standard Practice for Structural Design of Thermoplastic Corrugated Wall Stormwater Collection Chambers”.

Concrete vaults

Concrete vault-type systems should be compliant with:

• CSA A23.3-14 "Design of concrete structures",
• CSA A23.1-09/A23.2-09 (R2014) "Concrete materials and methods of concrete construction/Test methods and standard practices for concrete", and
• ASTM C858 - 10e1 "Standard Specification for Underground Precast Concrete Utility Structures".

Aggregates

Note the uniform size and angularity of this clear stone sample. Note also that the fragments all appear to have a film of fine particles adhering; this material would be improved by being washed prior to use.

This article gives recommendations for aggregate to be used to store water for infiltration. This is usually called 'clear stone' at aggregate yards.

To see an analysis of Ontario Standard Specifications for granular materials, see OPSS aggregates.

For advice on decorative surface aggregates see Stone

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Gravel used for underdrains in bioretention, infiltration trenches and chambers, and exfiltration trenches should be 20 or 50 mm, uniformly-graded, clean (maximum wash loss of 0.5%), crushed angular stone that has a porosity of 0.4.

The clean wash to prevent rapid accumulation of fines from the aggregate particles in the base of the reservoir. The uniform grading and the angularity are important to maintain pore throats and clear voids between particles. (i.e. achieve the porosity). Porosity and permeability are directly influenced by the size, gradation and angularity of the particles . See jar test for on-site verification testing protocols.

Gravel with structural requirements should also meet the following criteria:

• Minimum durability index of 35
• Maximum abrasion of 10% for 100 revolutions and maximum of 50% for 500 revolutions

Standard specifications for the gradation of aggregates are maintained by ASTM D2940

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  The highest porosity is found in uniformly graded aggregate, as there are no smaller particles to occupy the inter-particle pores.

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  Higher permeability is found in larger, angular, uniformly graded aggregate. This is due to larger pore sizes and lower tortuosity.

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Other

• Geotextiles
• Pipes

Construction

Construction

Gallery

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  Infiltration chambers being installed.

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  Chamber systems can be very large, both in terms of depth and footprint...

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  ...or very small as this residential soakaway/dry well.

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  Parking lot stormwater detention system, partially installed. Photo credit: Arbitrarily0

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  Contractors construct an underground soakaway on the runway extension of Taxiway Alpha as shown here Oct. 5, 2012, at RAF Mildenhall, England. Photo credit: Karen Abeyasekere

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  Infiltration chambers being installed.

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  Example of "crate style" Infiltration chambers being installed in East Gwillimbury. Photo credit: Make-Way Environmental Technologies Inc.

External links

In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.

Plastic chambers

• ADS chambers design tool
• Armtec
• Echelon
• Cultec
• Nilex
• Triton

Concrete chambers

• Stormtrap

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Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

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

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.

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.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

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

Penetration of water through the ground surface.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

Includes the protection of soil from dislocation by water, wind or other agents.

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.

A parameter that describes the capability of a medium to transmit water.

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.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

Sustainable Technologies Evaluation Program

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.

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

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.

Open space floodway channels, road reserves, pavement expanses and other flow paths that convey flows typically in excess of the capacity of the Minor Drainage System.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

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

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.

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.

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.

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.

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.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

Soil particles with a diameter less than 0.050 mm.

An underground water storage reservoir into which stormwater is directed and allowed to percolate into the underlying native subsoil.

An excavated area lined with geotextile filter cloth and filled with clean granular stone or other void forming material, that receives runoff and allow it to infiltrate into the native soil; can also be referred to as infiltration galleries, French drains, dry wells or soakaway pits.

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