Difference between revisions of "Underdrains"
m (→‎Spacing drainage pipes to reduce groundwater mounding)
m (→‎Underdrains for exfiltrating practices)
In a [[bioretention]] facility, after the rooting depth of the plants has been accommodated, the reservoir gravel layer can be increased for storage. Reservoir aggregate has a void ratio of 0.4, whilst most bioretention [[filter media]] may have a void ratio of 0.3 or lower.
In some cases where the underdrain layer has sufficient depth to accommodate it, a larger bore perforated pipe (e.g. ≥ 300 mm) may be used to add further storage capacity. Ultimately this idea may result in the use of [[infiltration chambers]] to create significant reservoir storage beneath a planted area. Be sure to check with manufacturers about the compatibility of their systems with [[trees]].
Underdrains comprise a length of perforated pipe embedded into a layer of reservoir aggregate. They are an optional component of bioretention systems, stormwater planters, swales and soil cells used to support urban trees. Their design varies according to the drainage requirements of the installation, and the available maintenance access.
1 Underdrains for exfiltrating practices
2 Underdrains for non-exfiltrating practices
2.1 Below ground
2.2 Above ground
3 Maintenance and inspection
4 Material specifications
4.1 Pipes
4.2 Reservoir gravel
4.3 Choker course
4.4 Geotextiles
4.5 Performance research
5 Alternative Technology
Underdrains for exfiltrating practices
The pipe within the drain should be elevated from the base to promote infiltration of the water stored beneath. The depth of this internal water storage should be sized according to the desired drainage timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). and the infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. of the native soils below. An alternative design configuration permits the head of water to be stored by using an upturned outflow pipe.
At least one pair of vertical cleanout pipes/wells should be included in the design, for inspection and periodic flushing of accumulated sedimentSoil, 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.. As most hydro-jetting apparatus used for this has some trouble accommodating narrow 90 deg bends, it is important that both ends of a perforated pipe be connected with a pair of 45 deg elbows/Y connectors instead.
Clean out spacing[1]
Pipe internal diameter (mm)
Maximum distance between cleanouts (m)
200 or greater 30
In some cases where the underdrainA perforated pipe used to assist the draining of soils. layer has sufficient depth to accommodate it, a larger bore perforated pipe (e.g. ≥ 300 mm) may be used to add further storage capacity. Ultimately this idea may result in the use of infiltration chambers to create significant reservoir storage beneath a planted area. Be sure to check with manufacturers about the compatibility of their systems with trees.
During a 25 mm storm event, a bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell receives concentrated flow from a catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. 20 times larger than its own footprint (25 x 20 = 500 mm = 0.5 m). The bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell comprises 0.6 m filter mediaThe 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. (K = 2.4 m/day), laid over 0.6 m clear, coarse reservoir gravel. The pipes are laid within the reservoir, 0.9 m below the surface. The system is designed to fill entirely during the rainstorm event. i.e. Depth to water tableThe 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. = 0 m.\[Drain\ spacing=\sqrt{\frac{4\times 2.4\ m/day\left(0.9\ m-0\ m\right)^{2}}{0.5\ m/day}}=6\ m\]
Underdrains for non-exfiltrating practices
Below ground
Where a stormwater planterA vegetated practice that collects and treats stormwater through sedimentation and filtration. Contributions to water cycle/water balance are through evapotranspiration only; no infiltration. or biofiltration cell is contained within a concrete box or completely lined to prevent infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface., the perforated pipe should be bedded on a thin layer of fine aggregateA 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.. This thin layer is to hold the pipe in place during construction, and to permit free ingress of accumulated water through holes on the underside of the pipe. As storage in a non-infiltrating practice is predominantly through soil/water tension, the depth of reservoir should be minimised to just accommodate the pipe. A pair of vertical clean out pipes/wells should be included in the design, for inspection and periodic flushing of accumulated sedimentSoil, 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.. As most hydro-jetting apparatus used for this has some trouble accommodating narrow 90 deg bends, it is important that both ends of a perforated pipe be connected with a pair of 45 deg elbows/Y connectors instead.
Where possible the underdrainA perforated pipe used to assist the draining of soils. pipe should be designed without any bends in order to facilitate easy maintenance. Otherwise see advice above regarding connectors.
To promote infiltration the base of the gravel reservoir and the underdrainA perforated pipe used to assist the draining of soils. pipe should be horizontal to optimize distribution of the water within.
Where drainage or conveyance to a downstream facility is a greater priority, the base of the reservoir and the underdrainA perforated pipe used to assist the draining of soils. pipe may have a gradient of up to 1-2%.
Schematic of pipes and connectors
Diagram of hydrojetting cleaning apparatus
To permit access by cameras or cleaning apparatus, 90 degree connectors must not be used in subterranean underdrains. Instead 2 x 45 degree connectors, or preferably 3 x 30 degree connectors should be used instead.
For the same reason, dual walled perforated pipes with smooth internal walls are highly recommended to reduce the potential snagging of maintenance equipment.
The recommended distances between clean outs is based on advice for filter beds in the Ontario Building Code[2]. However, the access capabilities of difference apparatus and contractors varies and designers are advised to take advice from maintenance operators in this matter.
Pipes are available with perforations on just one side, these should be situated on the lower half of the pipe. Pipes with 360° perforations should have a strip of geotextileFilter 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. or membrane placed over the pipe to reduce the migration of finesSoil particles with a diameter less than 0.050 mm. from overlying media.
Perforated pipes are a common component of underdrains used in bioretention, permeable pavements, infiltration trenches and exfiltration systems.
Pipes should be manufactured in conformity with the latest standards by the Canadian Standards Association (CSA) or ASTM International.
Perforated pipes should be continuously perforated, smooth interior HDPE or PVC.
Wherever possible pipes should be ≥200 mm internal diameter to reduce potential of freezing and to facilitate push camera inspections and cleaning with jet nozzle equipment.
Smooth interior facilitates inspection and maintenance activities; internal corrugations can cause cameras or hydrojetting apparatus to become snagged.
A perforated pipe with many rectangular slots has better drainage characteristics than a pipe with similar open area provided by fewer circular holes [3].
Non-perforated pipes should be used for conveyance of stormwater to and from the facility, including overflow. It is good practice to extend the solid pipe approximately 300 mm within the reservoir or practice to reduce the potential for native soilThe natural ground material characteristic of or existing by virtue of geographic origin. migration into the pipe.
Pipe with slotted perforations
Perforated pipes awaiting installation, note the 45 degree couplings used to facilitate push camera inspection and jet nozzle cleaning.
See also: Flow through perforated pipe
Reservoir gravel
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 aggregateA 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. to be used to store water for infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface.. This is usually called 'clear stone' at aggregateA 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. yards.
To see an analysis of Ontario Standard Specifications for granularGravel, 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. materials, see OPSS aggregates.
For advice on decorative surface aggregatesA 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. see Stone
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 porosityThe 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) of 0.4[4].
The clean wash to prevent rapid accumulation of finesSoil particles with a diameter less than 0.050 mm. from the aggregateA 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. 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 porosityThe 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)). PorosityThe 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) and permeability are directly influenced by the size, gradation and angularity of the particles [5]. 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 aggregatesA 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. are maintained by ASTM D2940
The highest porosityThe 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) is found in uniformly graded aggregateA 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., as there are no smaller particles to occupy the inter-particle pores. [5]
Higher permeability is found in larger, angular, uniformly graded aggregateA 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.. This is due to larger pore sizes and lower tortuosity. [5]
Choker course
medium sized granularGravel, 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., free from finesSoil particles with a diameter less than 0.050 mm.
In bioretention systems a choker layer of ≥ 100 mm depth is the recommended method to prevent migration of finer filter media into the underlying storage reservoir aggregate. These same mid-sized granularGravel, 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. materials are recommended for use in Stormwater planter underdrains and may be useful in the fine grading of foundations courses for permeable pavements.
Suitable materials include:
High performance bedding (HPB)
Clean, angular aggregateA 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. screened to between 6 and 10 mm. Widely available and designed specifically for drainage applications. Free from finesSoil particles with a diameter less than 0.050 mm. by definition.
HL 6
Is a clean, angular aggregateA 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. screened between 10 and 20 mm. Free from finesSoil particles with a diameter less than 0.050 mm. by definition.
Pea Gravel
Rounded natural aggregateA 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., screened between 5 and 15 mm, and washed free from finesSoil particles with a diameter less than 0.050 mm..
In most scenarios, a geotextile layer is unnecessary and has been associated with rapid decline and clogging in some circumstances.
The properties of geotextiles vary widely.
See Clogging for notes on their application in LIDLow 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. structures.
Geotextiles can be used to prevent downward migration of smaller particles in to larger aggregatesA 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., and slump of heavier particles into finer underlying courses. Geotextiles are commonly used on low strength soils (CBR<4). The formation of biofilm on geotextiles has also been shown to improve water quality:
By degrading petroleum hydrocarbons[6]
By reducing organic pollutant and nutrient concentrations [7]
When installing geotextiles an overlap of 150 - 300 mm should be used.
Material specifications should conform to OPSS 1860 for Class II geotextileFilter 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. fabrics [8]. Note when expansive clays are present, a non-infiltrating design may be necessary. If used, geotextileFilter 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. socks around perforated pipes should conform to ASTM D6707 with minimum water flow rate conforming to ASTM D4491 (12,263 L/min/m2 at 5 cm head).
Fabrics should be woven monofilament or non-woven needle punched.
Woven slit film and non-woven heat bonded fabrics should not be used, as they are prone to clogging.
In choosing a product, consider:
The maximum forces that will be exerted on the fabric (i.e., what tensile, tear and puncture strength ratings are required?),
The load bearing ratio of the underlying native soilThe natural ground material characteristic of or existing by virtue of geographic origin. (i.e. is the geotextileFilter 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. needed to prevent downward migration of aggregateA 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. into the native soilThe natural ground material characteristic of or existing by virtue of geographic origin.?),
The texture (i.e., grain size distribution) of the overlying and underlying materials, and
The suitable apparent opening size (AOS) for non-woven fabrics, or percent open area (POA) for woven fabrics, to maintain water flow even with sedimentSoil, 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. and microbial film build-up.
Recommended criteria for selection of geotextileFilter 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. fabric
Percent soil/filter mediaThe 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. passing 0.075 mm (#200 sieve)
Non-woven fabric apparent opening size (AOS, mm)
Woven fabric percent open area (POA, %)
Permittivity (sec-1)
>85 ≤ 0.3 - 0.1
50 - 85 ≤ 0.3 ≥ 4 0.1
5 - 15 ≤ 0.6 ≥ 4 0.5
≤ 5 ≤ 0.6 ≥ 10 0.5
Performance research
http://www.mdpi.com/2073-4441/7/4/1595/htm
Alternative Technology
Smart drain is a polymer ribbon-like material with capillary drains on the underside; it's use has recently been demonstrated in bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.[9]. It's low profile may make it particularly well suited to non-infiltrating practices, such as Stormwater planters.
Ribbon-like drainage material
↑ Province of Ontario. (2018). O. Reg. 332/12: BUILDING CODE. Retrieved February 23, 2018, from https://www.ontario.ca/laws/regulation/120332
↑ Hazenberg, G., and U. S. Panu (1991), Theoretical analysis of flow rate into perforated drain tubes, Water Resour. Res., 27(7), 1411–1418, doi:10.1029/91WR00779.
↑ Porosity of Structural Backfill, Tech Sheet #1, Stormtech, Nov 2012, http://www.stormtech.com/download_files/pdf/techsheet1.pdf accessed 16 October 2017
↑ 5.0 5.1 5.2 Judge, Aaron, "Measurement of the Hydraulic Conductivity of Gravels Using a Laboratory Permeameter and Silty Sands Using Field Testing with Observation Wells" (2013). Dissertations. 746. http://scholarworks.umass.edu/open_access_dissertations/746
↑ Newman AP, Coupe SJ, Spicer GE, Lynch D, Robinson K. MAINTENANCE OF OIL-DEGRADING PERMEABLE PAVEMENTS: MICROBES, NUTRIENTS AND LONG-TERM WATER QUALITY PROVISION. https://www.icpi.org/sites/default/files/techpapers/1309.pdf. Accessed July 17, 2017.
↑ Paul P, Tota-Maharaj K. Laboratory Studies on Granular Filters and Their Relationship to Geotextiles for Stormwater Pollutant Reduction. Water. 2015;7(4):1595-1609. doi:10.3390/w7041595.
↑ ONTARIO PROVINCIAL STANDARD SPECIFICATION METRIC OPSS 1860 MATERIAL SPECIFICATION FOR GEOTEXTILES. 2012. http://www.raqsb.mto.gov.on.ca/techpubs/OPS.nsf/0/2ccb9847eb6c56738525808200628de1/$FILE/OPSS%201860%20Apr12.pdf. Accessed July 17, 2017
↑ Redahegn Sileshi; Robert Pitt, P.E., M.ASCE; and Shirley Clark, P.E., M.ASCE Performance Evaluation of an Alternative Underdrain Material for Stormwater Biofiltration Systems, Journal of Sustainable Water in the Built Environment, 4(2), May 2018 https://doi.org/10.1061/JSWBAY.0000845
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