Infiltration material specifications
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Component Infiltration basin Bioinfiltration basin Infiltration trench Underground infiltration Dry well Dry swale with check dams Permeable pavement Tree trench/box/planter
Observation well
  • Vertical 4- to 6-inch Schedule 40 PVC perforated pipe according to MNDOT Manual Section 3248 with a lockable cap and anchor plate within gravel portion of bed
  • Install one per 50 ft. of length of infiltration practice
  • Cap should be flush with or above the surface of the infiltration practice
  • Vertical 4- to 6-inch Schedule 40 PVC perforated pipe according to MNDOT Manual Section 3248 with a lockable cap and anchor plate within gravel portion of bed
  • Install one per 50 ft. of length of infiltration practice
  • Install at downstream end of all permeable pavement systems >10,000 ft2 contributing drainage area
  • Vertical 4- to 6-inch Schedule 40 PVC perforated pipe according to MNDOT Manual Section 3248 with a lockable cap and anchor plate within gravel portion of bed
  • Install one per 50 ft. of length of infiltration practice
  • Cap should be flush with or above the surface of the infiltration practice
Emergency overflow structure
  • All infiltration beds should be designed with overflow spillway for extreme storm events
  • Properly designed vegetated filters are required from the overflow of all infiltration systems
  • See detail
NA A surcharge pipe should be constructed as an emergency overflow device for Dry Wells which receive runoff directly from a roof leader. [1] NA See Design criteria for permeable pavement Tree box filters should be designed with an emergency overflow pipe
High flow bypass structure
  • A high flow bypass or diversion structure should be included to safely convey high flows from large storm events
  • Control in beds is usually provided as an outlet control structure
  • Modified inlet box with internal concrete weir (or weir plate) and low-flow orifice is common
  • High flows must be bypassed within 48 hours and allow the infiltration practice to meet the 48 hour drawdown requirement
  • A flow bypass structure should be located upgradient of the infiltration structure to convey high flows around the structure
  • High flows must be bypassed within 48 hours and allow the infiltration practice to meet the 48 hour drawdown requirement
NA NA NA NA
Buffer vegetation
  • Keep adjacent vegetation from forming an overhead canopy above infiltration practices. This keeps leaf litter, fruits, and other vegetative material from clogging stone.
  • Trees may be planted along side slopes but must be 15 ft away from pipes and 25 ft away from risers
  • Grass strip or other vegetated buffer at least 20 feet wide should be maintained around trenches accepting surface runoff as sheet flow
  • Slope of filter strip should be approximately 1% along entire length and 0% across width
Keep adjacent vegetation from forming an overhead canopy above infiltration practices. This keeps leaf litter, fruits, and other vegetative material from clogging stone.
  • Planted with turf grass, tall meadow grasses, decorative herbaceous cover, or trees
  • See MNDOT Standards and Specs Section 3876 for Seed Requirements for Dry Swales
  • If receiving runoff from an impervious area, construct an adjacent grass filter strip, gravel diaphragm, or gravel flow spreader to provide pretreatment
NA
  • Although deep rooted, native species are preferred, noninvasive cultivars may be used or combined with native species to achieve desired landscape aesthetic qualities
  • See MNDOT Standards and Specifications for Native Species
  • Energy dissipator Stones placed over the mulch layer may be used as a pretreatment to reduce velocity of inflow
Surface cover For stone cover: apply a two inch layer of pea gravel or river stone
  • Establish dense vegetation on basin side slopes and floor:
    • Dry- and mesic-prairie species for side slopes
    • Wet meadow species for lower portions
  • Mulch (2-3 inch layer) must be free of weeds and consist of aged, double-shredded hardwood bark mulch or shredded leaf mulch
  • Sand, if used, must comply with MNDOT Sand Cover in Section 3149 K
  • Deep-rooted, native species for vegetative cover
  • If sod is used, grow over permeable soils
3-inch layer of river stone or pea gravel with filter fabric and additional aggregate on top
  • Permeable topsoil and planted with grass.
  • Deep-rooted plants, such as prairie grass, should be used to increase infiltation capacity
  • Bottom and side slopes of basin should be planted with dense water-tolerant grass (such as fescues)
  • Soil cover (12 - 18 inches) should be maintained above infiltration bed to allow for a healthy vegetative cover
  • Overly infiltration bed with native grasses, meadow mix, or other low-growing, dense vegetation
  • In place of turf, use basin liner of 6 to 12 inches of fill material (coarse sand)
Covered by a minimum of 12 inches of topsoil
  • 3" layer of mulch on surface to enhance plant survival
  • See MNDOT Standards and Specs Section 3882 for Mulch Requirements
  • Erosion control matting can be used in lieu of mulch where flow velocities may wash mulch away
  • Porous Asphalt Course: 3 to 7 inch thick layer with 15% to 20% void content
  • Pervious Concrete Course: 5 to 8 inch thick layer with 15% to 25% void content1
  • Plastic Grid Pavers: depends on fill material
  • Concrete Grid Pavers: 3.5 inch thick layer with 20% to 50% void content
  • Concrete Interlocking Pavers: 3 inch thick layer with 5% to 15% void content1
  • Min Thickness (NAPA, 2008)
    • Parking: 2.5 in
    • Residential Trucks: 4.0 in
    • Heavy Truck: 6.0 in
Intermediate layer NA NA NA NA NA NA Choker Layer: A 2 to 8 inch bedding coarse of Medium Filter Aggregate (MNDOT Section 3149 J.1) is usually placed over the top of the base material to help stabilize the irregular surface NA
Filter bed (stone) Bioinfiltration engineered soil mix 20-30 inch layer of permeable manufactured soil mixture meeting the bioinfiltration soil mix
  • Reservoir Layer: 1 to 4-foot deep bed of double-washed, uniform (uniformity coefficient of 2 or smaller with a void ratio of 0.40) aggregate between 1.5 and 3.0 inch diameter
  • MNDOT Coarse Aggregate Bedding (Section 3149 G.2) ranging 1-2 inch in gradation
3' of bioinfiltration Soil Media
Filter fabric
  • Top of geotextile should be 6-12 inches below upper surface of drainage media with 12" overlap of fabric seams
  • Fabric should be placed between side walls and between stone and gravel layers but not on the basin bottom
  • Geotextile must meet MNDOT Type I Non-woven geotextile
  • Examples: Mirafi 140N, Amoco 4547, and Geotex 451.
  • Geotextile or a pea gravel filter separating stone and soil media
  • Geotextile must meet MNDOT Type I non-woven geotextile
  • Examples: Mirafi 140N, Amoco 4547, and Geotex 451
  • Surround fill on sides with filter fabric with 12" overlap of fabric seams. A separate piece should be used for the top layer to act as a failure plane. Fabric should be placed between side walls and between stone and gravel layers but not on the trench bottom.
  • Geotextile must meet MNDOT Type I non-woven geotextile
  • Examples: Mirafi 140N, Amoco 4547, and Geotex 451
  • Surround fill on sides with filter fabric with 12" overlap of fabric seams
  • Geotextile must meet MNDOT Type I non-woven geotextile
  • Examples: Mirafi 140N, Amoco 4547, and Geotex 451
  • A pea gravel choking layer (3 to 4 inches thick) should be placed below the filter media and above the bottom aggregate
  • If a geotextile is used in lieu of the pea gravel layer, it must meet MNDOT Type I non-woven geotextile
  • Examples: Mirafi 140N, Amoco 4547, and Geotex 451
  • Filter Blanket could include any of the following:
  • Examples: Mirafi 140N, Amoco 4547, and Geotex 451 (K)
  • Geotextile should be placed on filter bed sides between filter media and in-situ soils
Bottom
  • Any of the following can be used on the bottom:
    • Uniformly graded, crushed, clean-washed stone. (less than 0.5% wash loss, by mass, per AASHTO T-11 wash loss test
    • Grass turf that can be inundated for +72 hrs
    • MNDOT Coarse Aggregate Bedding (Section 3149 G.2)
    • Coarse organic material (composted mulch or erosion control mix) tilled into soil, soaked and allowed to dry
Bottom Layer: 4" min depth of .75" crushed stone MNDOT Coarse Aggregate Bedding (Section 3149 G.2) 2' layer of clean, washed angular gravel 0.75 to 1.5 inch diameter
in-situ soils NRCS Type A and B soils are the most efficient soils for proper infiltration. Type C soils are acceptable of the infiltration practice meets the required 48 hour drawdown period
Miscellaneous All material must be placed such that compaction is avoided. See the Construction Specifications webpage for more details To increase the runoff capture storage volume of trenches, plastic, aluminum or concrete gallery frames can be inserted All material must be placed such that compaction is avoided. See the Construction Specifications webpage for more details
  • Non-erosive material such as wood, gabions, riprap, or concrete
  • Underlay with geotextile and include 1/2 inch weep holes
  • Construct check dams 6 to 12 inches high
  • 9" min diameter rock on outside edges, 12-18" diameter larger rocks at core. Gaps filled with 4-6" diameter smaller rock
  • Designed so pond behind dam will drawdown within 12 hours

1Thickness will vary depending on traffic conditions.Typically, thicker configurations are needed for heavier traffic loads.

This page was last edited on 28 June 2017, at 16:54.

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