DESCRIPTION. Bioretention facilities are small landscaped basins intended to provide water quality management by filtering stormwater runoff before release into stormdrain systems.  This work shall consist of installing bioretention facilities as specified in the Contract Documents, including all materials, equipment, labor and services required to perform the work.

Bioretention Soil Mixture.  The Bioretention Soil Mixture (BSM) is a mixture of planting soil, mulch, and sand consisting of the following:

The USDA textural classification of the Planting Soil for the BSM shall be LOAMY SAND OR SANDY LOAM.  The Planting Soil shall be salvaged or furnished.  Additionally, the Planting Soil shall be tested and meet the following criteria:

The textural analysis for the Planting Soil shall be as follows:

At least 45 days prior to the start of construction of bioretention facilities, the Contractor shall submit the source of the Planting Soil for the BSM to the Engineer for approval.  No time extensions will be granted should the proposed Planting Soil fail to meet the minimum requirements stated above.  Once a stockpile of the Planting Soil has been sampled, no material shall be added to the stockpile. (See Comment 4)

The Bioretention Soil Mixture (BSM) shall be a uniform mix, free of stones, stumps, roots or other similar objects larger than two inches excluding mulch.  No other materials or substances shall be mixed or dumped within the bioretention area that may be harmful to plant growth, or prove a hindrance to the planting or maintenance operations.

The Bioretention Soil Mixture shall be tested and meet the following criteria:

* Use authorized soil test procedures.

Should the pH fall outside of the acceptable range, it may be modified with lime (to raise) or iron sulfate plus sulfur (to lower).  The lime or iron sulfate must be mixed uniformly into the BSM prior to use in bioretention facilities.

Should the BSM not meet the minimum requirement for magnesium, it may be modified with magnesium sulfate.  Likewise, should the BSM not meet the minimum requirement for potassium, it may be modified with potash.  Magnesium sulfate and potash must be mixed uniformly into the BSM prior to use in bioretention facilities.

Planting soil and/or BSM that fails to meet the minimum requirements shall be replaced at no additional cost to the Administration.  Mixing of the corrective additives to the BSM is incidental and shall be at no additional cost to the Administration.

Mixing of the BSM to a homogeneous consistency shall be done to the satisfaction of the Engineer.

PLANT MATERIALS.  All plants, unless otherwise specifically permitted, shall conform to the standards of the current edition of American Standard for Nursery Stock as approved by the American Standards Institute, Inc.  All plant grades shall be those established in the current edition of American Standards for Nursery Stock. Only one size per grade will be listed rather than a size range. The one size shall mean the minimum size for that grade and shall include plants from that size up to but not including the next larger grade size. 

All plant materials shall have normal, well developed branches and a vigorous root system. They shall be healthy plants free from physical defects, plant diseases, and insect pests.  Shade and flowering trees shall be symmetrically balanced. Major branches shall not have V shaped crotches capable of causing structural weakness. Trunks shall be free of unhealed branch removal wounds greater than a 1 in. diameter.

Shade trees shall have a single main trunk. Trunks shall be free of branches below the following heights:

Plant materials shall be able to tolerate saturated soil conditions for the length of time anticipated in the design storm event, as well as snow melt chemicals and other anticipated runoff constituents.

Water.  Water used in the planting, establishing, or caring for vegetation shall be free from any substance that is injurious to plant life.

Limestone.  Limestone shall contain not less than 85 percent calcium and magnesium carbonates. Dolomitic (magnesium) limestone shall contain at least 10 percent magnesium as magnesium oxide and 85 percent calcium and magnesium carbonates.

Limestone shall conform to the following gradation:

Iron Sulfate.  Iron sulfate shall be a constituent of an approved horticultural product produced as a fertilizer for supplying iron and as a soil acidifier.

Magnesium Sulfate.  Magnesium sulfate shall be a constituent of an approved horticultural product produced as a fertilizer.

Potash.  Potash (potassium oxide) shall be a constituent of an approved horticultural product produced as a fertilizer.

Excavation.  If the soil is classified as a clay soil (USDA textural classification), the roto-tilling shall not be done where the soil supports the aggregate bed underneath the underdrain.  (See underdrain specifications below.)  Clay soils beyond the edge of the aggregate bed shall be first overlain with a BSM layer 3 inches thick, and  then roto-tilled to a depth 6 inches below the surface of the BSM layer.

Underdrain.  Underdrains shall be outletted into drainage structures whenever possible. Outlets that empty into a drainage structure shall be positioned a minimum of 6 in. above the normal flow level in the structure and shall be constructed of normal underdrain pipe. A minimum of 18 in. of cover over the pipe shall be maintained.  When outletted to a slope or ditch, the outlet pipe shall slope a minimum of 0.5% percent unless otherwise directed by the Engineer. (See Comment 5)

CONSTRUCTION.  Bioretention facilities shall not be constructed until all contributing drainage areas are stabilized as shown on the Contract Plans and to the satisfaction of the Engineer.  Bioretention facilities shall not be used as sediment control facilities. No heavy equipment shall operate within the perimeter of a bioretention facility during excavation, underdrain placement, backfilling, planting, or mulching of the facility.

Excavation. The bioretention facility shall be excavated to the dimensions, side slopes, and elevations shown on the Contract Plans.  The method of excavation shall minimize the compaction of the bottom of the bioretention facility.  Excavators and backhoes, operating on the ground adjacent to the bioretention facility, shall be used to excavate the facility if possible.  Low ground-contact pressure equipment may also be used for excavation.  No heavy equipment shall be allowed on the bottom of the bioretention facility. (See Comment 6)

Excavated materials shall be removed from the bioretention facility site.  Excavated materials shall be used or disposed of in conformance with Section 201.

Prior to placing the underdrain and the BSM, the bottom of the excavation shall be roto-tilled to a minimum depth of 6 inches to alleviate any compaction of the facility bottom. Any substitute method for roto-tilling must be approved by the Engineer prior to use.  Any ponded water shall be removed from the bottom of the facility and the soil shall be friable before roto-tilling.

Underdrain. The underdrain system shall be placed on a 3-ft wide bed of No. 57 aggregate.  The minimum thickness of the ASTM No. 57 aggregate shall be 3 inches. Underdrain shall be covered with 6 inches of No. 57 aggregate and topped with an additional layer of No. 7 aggregate (layer thickness shall be 2 inches, minimum).  All aggregate shall be placed according to dimensions shown on the Contract Plans.  (See Comment 7)

Observation wells/cleanouts of 6-inch non-perforated pipe shall be placed vertically in the bioretention facility as shown on the Contract Plans. The wells/cleanouts shall be connected to the perforated underdrain with the appropriate manufactured connections as shown on the Contract Plans.  The wells/cleanouts shall extend 6 inches above the top elevation of the bioretention facility mulch, and shall be capped with a screw cap.  The ends of underdrain pipes not terminating in an observation well/cleanout shall be capped. (See Comment 8)

Placement and Compaction of the Bioretention Soil Mixture.  The Bioretention Soil Mixture (BSM) shall be placed and graded using low ground-contact pressure equipment or by excavators and/or backhoes operating on the ground adjacent to the bioretention facility.  (See Comment 9)  No heavy equipment shall be used within the perimeter of the bioretention facility before, during, or after the placement of the BSM.  The BSM shall be placed in horizontal layers not to exceed 12 inches for the entire area of the bioretention facility.  The BSM shall be compacted by saturating the entire area of the bioretention facility after each lift of BSM is placed until water flows from the underdrain.  Water for saturation shall be applied by spraying or sprinkling.  Saturation of each lift shall be performed in the presence of the Engineer.   An appropriate sediment control device shall be used to treat any sediment-laden water discharged from the underdrain.  If the BSM becomes contaminated during the construction of the facility, the contaminated material shall be removed and replaced with uncontaminated material at no additional cost to the Administration.  Final grading of the BSM shall be performed after a 24-hour settling period.  Final elevations shall be within 2 inches of elevations shown on the Contract Plans.

Plant Installation. Trees, shrubs, and other plant materials specified for Bioretention Facilities shall be planted as specified in the Contract Plans and applicable landscaping standards with the exception that pesticides, herbicides, and fertilizer shall not be applied during planting under any circumstances.  Furthermore, pesticides, fertilizer, and any other soil amendments shall not be applied to the bioretention facility during landscape construction, plant establishment, or maintenance.

Mulching. Once the plants are in place, the entire bioretention facility shall be mulched to a uniform thickness of 3 inches.  Well aged (minimum age of 6 months) shredded hardwood bark mulch is the only acceptable mulch. (See Comment10)

MEASUREMENT AND PAYMENT. Bioretention Facilities will not be measured but will be paid for at the Contract Bid Lump Sum Price. The payment will be full compensation for all applicable excavation, sheeting, shoring, de-watering, hauling, storing, re-handling of material, removal and disposal of excess and unsuitable material, underdrain, aggregate, underdrain outlet, furnishing, mixing, pH correction, potassium correction, magnesium correction, and placement of the BSM backfill, compaction of the BSM backfill by saturation, grading and slope adjustments, and for all material, labor, equipment, tools, and incidentals necessary satisfactorily complete the work.

Mulch. Shredded hardwood bark shall consist of the bark from hardwood trees which has been milled and screened to a maximum 4 in. particle size and provide a uniform texture free from sawdust, foreign materials, and any artificially introduced chemical compounds that would be detrimental to plant or animal life.

Plantings. Plant materials will be measured and paid for at the Contract unit price per each item specified in the Contract Documents. The payment will be full compensation for all supports, watering during the construction period and for all material, labor, equipment, tools, and incidentals necessary to complete the work. If at any time during the Contract period any plants become unacceptable, they shall be replaced at no additional cost to the contracting highway agency.

Additional Watering of Plants.  Additional Watering of Plants within bioretention facilities will be measured and paid for at the Contract unit price per 1000 gallons of water applied to the planting beds.  Water will be measured by means of satisfactorily installed meters, or by measurements of tank capacities, or by acceptable documentation of tanks of predetermined capacity. The Contractor shall furnish the Engineer's representative with measurement and capacities to provide a complete and accurate record of the quantity of water furnished and applied. The payment will be full compensation for all water, material, labor, equipment, tools, and incidentals necessary to complete the work. No compensation will be allowed for water wasted or excess watering as determined by the Engineer. (See Comment 11)

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