Difference between revisions of "Infiltration trenches"

Revision as of 19:16, 18 June 2019

This article is about underground systems which distribute concentrated flow along a level, linear facility to promote infiltration to native soils.
For a similar structure, which differs in being designed to receive excess flow and convey it, whilst promoting infiltration to native soils, see exfiltration trenches.

The infiltration trench at Kortright is topped with filter fabric and decorative stone, both of which provide some pretreatment and can easily be removed and replaced as an occasional maintenance task. For more details click here

Overview

As their name suggests infiltration trenches work primarily to infiltrate and convey stormwaterSurface runoff from at-grade surfaces, resulting from rain or snowmelt events.. They are an underground facility and are excellently suited to connecting other components in the treatment trainStormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.A combination of lot level, conveyance, and end-of-pipe stormwater management practices..

The fundamental components of an infiltration trench are:

• Layers of coarse aggregate to bed the pipe, store and redistribute water.
• Perforated pipe
• Geotextile

Planning considerations

As shown in the illustration above a surface inlet to an infiltration trench may simply be a channel of decorative stone supported by a geotextile. So that at grade it may be indistinguishable from a gravel diaphragm. In function though, the decorative surface course of the infiltration trench needs to remain free-draining down into the trench, whereas the gravel diaphragmA level spreading device placed at a runoff discharge location, perpendicular to flow, to maintain sheet flow and distribute runoff as evenly as possible across a pervious area or stormwater infiltration practice. A gravel diaphragm acts as a pretreatment device, settling out suspended sediments before they reach the practice. is designed to spill over onto adjacent land, leaving 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. behind in the gravel or stone channel.

Design

Sizing

Infiltration: Sizing and modeling

• Virginia up to 10' (3 m) deep. ^[1]
• Minnesota up to 12' (3.6 m) deep. ^[2]
• "...not normally be deeper than 3 to 4 m in order to maximise the length of the flow path to the 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. through the unsaturated zone." ^[3]
• There is a cost implication with digging deeper practices, trench boxes may be required to retain the side walls during construction.

Materials

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.

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]

Perforated Pipe

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 ^[6].
• 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

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  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

• Geotextiles

Construction

• Construction

Gallery

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  Trench laid in advance of the surrounding catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. being finished. This system would benefit from some 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. on the surface to prevent 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. from accumulating before the asphaltA mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces. is laid.

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  Good use 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., being limited to preventing finesSoil particles with a diameter less than 0.050 mm. from entering the trench from above.

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  View of infiltration trench in U.S. Photo credit: Moreau1