Infiltration trenches

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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.

Schematic diagram of an underground infiltration trench


As their name suggests infiltration trenches work primarily to infiltrate and convey stormwater. They are an underground facility and are excellently suited to connecting other components in the treatment train. Template:Bextbox The fundamental components of an infiltration trench are:

Planning considerations[edit]



Infiltration: Sizing and modeling



This is a collection of three articles with the common theme of being aggregate products for various applications in LID.

Underground construction aggregates[edit]

For reservoirs[edit]

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

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[1].

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 [2]. 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

For choking/choker layers[edit]

medium sized granular, free from fines

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 granular 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 aggregate screened to between 6 and 10 mm. Widely available and designed specifically for drainage applications. Free from fines by definition.
HL 6
Is a clean, angular aggregate screened between 10 and 20 mm. Free from fines by definition.
Pea Gravel
Rounded natural aggregate, screened between 5 and 15 mm, and washed free from fines.

In most scenarios, a geotextile layer is unnecessary and has been associated with rapid decline and clogging in some circumstances.

OPS Aggregates[edit]

Of the standard granular materials in the standard OPSS.MUNI 1010 only Granular O is recommended as a substitute for clear stone in LID construction.

Where Granular O is substituted for clear stone in underground reservoir structures, the porosity used in design calculations shall be 0.3 unless laboratory testing proves otherwise.

Examples of BMPs with underground reservoirs include Underdrains, infiltration trenches, permeable pavements, infiltration chambers, exfiltration trenches.

All other mixes must be avoided for free drainage or storage as they are permitted to contain a higher enough proportion of fines to reduce permeability below 50 mm/hr.

For more information see OPS aggregates

Landscaping aggregates[edit]

This rain garden in a school yard uses stone as both decorative edging and for erosion control.
This bioswale in a parking lot uses stone at the inlets and along the bottom of the swale to prevent erosion, as the sides are sloped.

For advice on aggregates used in underdrains, see Reservoir aggregate.

Stone or gravel can serve as a low maintenance decorative feature, but it may also serve many practical functions on the surface of an LID practice.

Stone for erosion control[edit]

Aggregates used to line swales or otherwise dissipate energy (e.g. in forebays) should have high angularity to increase the permissible shear stress applied by the flow of water. [3] However, in some surface landscaped applications there may be a desire to use a rounded aggregate such as 'river rock' for aesthetic reasons. Rounded stones should be of sufficient size to resist being moved by the flow of water. Typical stone for this purpose ranges between 50 mm and 250 mm in diameter. The larger the stone, the more energy dissipation.

  • Stone beds should be twice as thick as the largest stone's diameter.
  • If the stone bed is underlain by a drainage geotextile, annual inspection and possible replacement should be performed as there is a potential for clogging of this layer to occur.

Stone mulch[edit]

Finer inorganic mulch materials can be of value applied in areas with extended ponding times i.e. in the the centre of recessed, bowl shaped bioretention, stormwater planters, trenches or swale practices. Inorganic mulches resist movement from flowing water and do not float. Applying a thin layer of inorganic mulch over the top of wood based mulch has been shown to reduce migration of the underlying layer by around 25% [4]. Inorganic mulches which may be available locally, include:

  • Pea gravel
  • River rock/beach stone
  • Recycled glass
  • Crushed mussel shells

On-site verification[edit]

Steps in conducting a jar test to detect fines in construction materials

Specifying that aggregates for the construction of LID practices must be free from fines is important. But checking that the delivered materials meet specification is essential to reduce problems with construction and longer term performance.

When possible, Construction Managers should observe the offloading of materials to watch for dust clouds. Aggregates or sand for LID construction should not give rise to clouds of dust when dumped.

A simple jar test can be used to gauge the proportion of fines in an aggregate product before acceptance.


  • A large wide-mouthed jar - glass or clear plastic are both fine,
  • Tap water, and
  • The aggregate to be tested.


  1. Collect approximately 5 cm of material in the jar (or at least two complete layers of 50 mm clear stone),
  2. Add water to around 3/4 full,
  3. Secure cap and shake,
  4. Leave for at least 30 minutes and until the water is clear - plan to run the test overnight when possible,
  5. Examine the layer of sediment - if > 3 mm has been washed from 5 cm of product, the material should be rejected,

Note that the sediment may collect on top of, or at the bottom of the construction material.

External references[edit]

Perforated Pipe[edit]

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 geotextile or membrane placed over the pipe to reduce the migration of fines 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 [5].
  • 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 soil migration into the pipe.

See also: Flow through perforated pipe


The following presents a summary of considerations when planning the construction of a low impact development project. More details can be found in the following reference:[6]

  • The site of the infiltration facility must remain outside the limit of disturbance and blocked from site traffic until construction of the facility begins, to prevent soil compaction by heavy equipment.
  • This area must not be used as the site of sediment basins during construction, as the concentration of fines will reduce post-construction infiltration.
  • This area must not be use as a staging area, for storing materials.
  • To prevent sediment from clogging the surface, stormwater must be diverted away from the facility until the drainage area is fully stabilized.
  • As many infiltration facilities are installed in the road right-of-way or tight urban spaces, considerations of traffic control and utility conflicts must be part of the plans and inspections.


The following is a typical construction sequence to properly install an infiltration practice:

  • The area should be fully protected by silt fence or construction fencing to prevent compaction by construction traffic and equipment.
  • Installation may only begin after entire contributing drainage area has been either stabilized or flows have been safely routed around the area. The designer should check the boundaries of the contributing drainage area to ensure it conforms to original design.
  • The pretreatment part of the design should be excavated first and sealed until full construction is completed.
  • Excavators or backhoes working adjacent to the proposed infiltration area should excavate to the appropriate design depth.
  • The soil in the bottom of the excavation should be ripped to promote greater infiltration.
  • Any accidental sediment accumulation from construction should be removed at this time.

  1. Excavate subsurface water storage reservoir to base elevation,
  2. Check base elevation and slope,
  3. Fracture/rip bottom and roughen side of the excavation to remove smeared surfaces,
  4. Install optional geotextiles (or liner for biofilter); overlapping according to design drawings,
  5. Install coarse reservoir gravel, and any void forming structures (e.g. underdrains, infiltration chambers, or wells),
  6. Check elevation and slope at top of reservoir,
  7. Install choking layer and optional geotextile (typically only over the perforated pipe),
  8. Check elevation and slope at top of choking layer,
  9. Install filter media with additional 30 cm over finish grade of the filter bed,
  10. Thoroughly saturate and allow to settle for at least one week. After this time, tamp manually to check settling is complete. Alternatively, installations made in the fall can be left to settle over the whole winter season at this point,
  11. Install temporary erosion and sediment control practices,
  12. Conduct all other site construction activities (buildings/servicing etc.)
  13. Check condition of bioretention after settling period, remediate any deficiencies,
  14. Install curbs and pavements and concrete pretreatment devices,
  15. Check elevations of curb cuts and other inlets
  16. Install erosion control to all inlets!!
  17. Remove excess filter media along with any accumulated construction sediment,
  18. Install any surface applied additives,
  19. Conduct fine grading to surface of filter bed, checking elevations/slopes/compaction,
  20. Apply stone or mulch cover for decorative systems, or turf reinforcement for grassed systems,
  21. Install erosion control blankets or matting
  22. Plant or lay sod,
  23. Saturated system thoroughly to settle filer media particles around the roots of new plants,
  24. Irrigated the system as required to establish healthy vegetation cover,
  25. Inspect and remediate deficiencies after any significant rainfall within the next 3 months or remainder of the first growing season.

Facilities containing media[edit]


Sequencing depends on the design:

  • Full infiltration:Pack 50 mm diameter clear stone to storage design depth, top with 100 mm of the choker course,
  • Partial infiltration:Place design depth of 50 mm diameter clear stone for the infiltration volume on bed and then lay the perforated underdrain pipe over it. Pack more clear stone to 75 mm above the top of the underdrain, top with 100 mm of choker layer.

Stormwater planters[edit]

  • Place an impermeable liner on the bed with 150 mm overlap on sides. Lay the perforated underdrain pipe, Pack 50 mm diameter clear stone to 75 mm above top of underdrain, top with 100 mm of choker layer;

Rain gardens[edit]

No storage or drainage is required, filter media or amended topsoil is laid onto native soils

Media installation[edit]

Media installed over the choker course in 0.3 m lifts until desired top elevation is achieved. Each lift must be thoroughly wetted and drained before adding the next. Wait three weeks to check for settling, and add additional media and regrade as needed.

  • Prepare planting holes for any trees and shrubs, install vegetation, and water accordingly.
  • Install any temporary irrigation.
  • Plant landscaping materials as shown in the landscaping plan, and water them weekly in the first two months.
  • Lay down surface cover in accordance with the design (mulch, riverstone, or turf).
  • Conduct final construction inspection, checking inlet, pretreatment, bioretention cell and outlet elevations.
  • Remove erosion and sediment controls, only when the entire drainage area is stabilized.



Incentives and Credits[edit]

In Ontario[edit]

LEED BD + C v. 4[edit]

Sustainable sites: Rainwater management (up to 3 points)

  • Two points (or 1 point for Healthcare) will be awarded if the project manages "the runoff from the developed site for the 95th percentile of regional or local rainfall events."
  • Three points (or 2 points for Healthcare) will be awarded if the project manages "the runoff from the developed site for the 98th percentile of regional or local rainfall events."


  • For zero-lot-line projects only, 3 points (or 2 points for Healthcare) will be awarded if the project manages "the runoff from the developed site for the 85th percentile of regional or local rainfall events."

SITES v.2[edit]


  1. Porosity of Structural Backfill, Tech Sheet #1, Stormtech, Nov 2012, accessed 16 October 2017
  2. 2.0 2.1 2.2 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.
  3. Roger T. Kilgore and George K. Cotton, (2005) Design of Roadside Channels with Flexible Linings Hydraulic Engineering Circular Number 15, Third Edition
  4. Simcock, R and Dando, J. 2013. Mulch specification for stormwater bioretention devices. Prepared by Landcare Research New Zealand Ltd for Auckland Council. Auckland Council technical report, TR2013/056
  5. 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.
  6. [ Construction Guide for Low Impact Development, CVC (2013)