4.4.6 Construction

Prior to construction, meet with contractor, subcontractors, construction management, and inspection staff to review critical design elements and confirm specification requirements, proper construction procedures, construction sequencing, and inspection timing. Runoff from construction activity should not be allowed into the bioretention areas unless there is no other option for conveying construction stormwater, there is adequate protection of the subgrade soil and BSM, and introduction of stormwater is approved by the project engineer.

Excavation
Soil compaction can lead to facility failure; accordingly, minimizing compaction of the base and sidewalls of the bioretention area is critical. Excavation should never be allowed during wet or saturated conditions (compaction can reach depths of 2 to 3 feet during wet conditions and mitigation is likely not possible). Excavation should be performed by machinery operating adjacent to the bioretention facility and no heavy equipment with narrow tracks, narrow tires, or large lugged, high pressure tires should be allowed on the bottom of the bioretention facility. If machinery must operate in the bioretention cell for excavation, use light weight, low ground-contact pressure equipment and rip the base at completion to re-fracture soil to a minimum of 12 inches (Prince George’s County, 2002). If machinery operates in the facility, subgrade infiltration rates must be field tested and compared to design rates and verified by the project engineer. Failure to meet or exceed the design infiltration rate for the subgrade will require revised engineering designs to verify achievement of treatment and flow control benefits that were estimated in the Stormwater Site Plan.

Prior to placement of the BSM the finished subgrade should:
  • Be scarified to a minimum depth of 3 inches.
  • Have any sediment deposited from construction runoff removed (to remove all introduced sediment, subgrade soil should be removed to a depth of 3 to 6 inches and replaced with BSM).
  • Be inspected by the project engineer to verify required subgrade condition.

Sidewalls of the facility beneath the surface of the BSM can be vertical if soil stability is adequate. Exposed sidewalls of the completed bioretention area with BSM in place should be no steeper than 3H:1V (see bottom area and side slopes in section 4.4.2.2: Bioretention components). The bottom of the facility should be flat.

Vegetation protection areas with intact native soil and vegetation should not be cleared and excavated for bioretention facilities.

Bioretention soil media installation

Placement
On-site soil mixing or placement should not be performed if BSM or subgrade soil is saturated. The bioretention soil mixture should be placed and graded by machinery operating adjacent to the bioretention facility. If machinery must operate in the bioretention cell for soil placement, use light weight equipment with low ground-contact pressure. If machinery operates in the facility, the BSM infiltration rates must be field tested and compared to design rates and verified by the project engineer. Failure to meet or exceed the design infiltration rate for the BSM will require revised engineering designs to verify achievement of treatment and flow control requirements. The soil mixture should be placed in horizontal layers not to exceed 12 inches per lift for the entire area of the bioretention facility.

Compact the BSM to a relative compaction of 85 percent of modified maximum dry density (ASTM D1557-12). Compaction can be achieved by boot packing (simply walking over all areas of each lift) and then apply 0.2-inch of water per 1-inch of BSM depth. Water for settling should be applied by spraying or sprinkling.

Verification
If using the guidelines in Section 4.4.2.2: Bioretention components, pre-placement laboratory analysis for saturated hydraulic conductivity of the BSM is not required. Verification of the mineral aggregate gradation, compost guidelines, and mix ratio in Section 4.4.2.2: Bioretention components, must be provided to verify performance guidelines in that section.

If the BSM uses a different mineral aggregate gradation, compost guidelines, and mix ratio than Section 4.4.2.2: Bioretention components, then verification of the BSM composition (2-5 percent passing the #200 sieve, 4-8 percent OM content, CEC > 5 MEQ/100 grams dry soil, pH in the range of 5.5 – 7) and hydraulic conductivity (initial rate less than 12 inches per hour and a long-term rate more than 1-inch per hour) must be provided before placement through laboratory testing of the material that will be used in the installation.

BSM infiltration rates are determined per ASTM Designation D2434-68 (Standard Test Method for Permeability of Granular Soils) at 85 percent compactionper ASTM Designation D1557-12 (Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort). Determine the organic matter content before and after permeability test using ASTM D2974-07a (Standard Test Method for Moisture, Ash, and Organic Matter of Peat and Other Organic Soils).

Testing should be performed by a Seal of Testing Assurance, AASHTO, ASTM or other standards organization accredited laboratory with current and maintained certification. Samples for testing must be supplied from the BSM that will be placed in the bioretention areas.

Filter fabrics

Filter fabrics between the subgrade and the BSM are typically not needed. The gradation between existing soils and BSM is generally not great enough to allow significant migration of fines into the BSM. Additionally, filter fabrics may clog with downward migration of fines from the BSM.

Temporary Erosion and Sediment Control (TESC)
Controlling erosion and sediment are most difficult during clearing, grading, and construction; accordingly, minimizing site disturbance to the greatest extent practicable is the most effective sediment management.
During construction:
  • Bioretention areas should not be used as sediment control facilities and all drainage should be directed away from bioretention areas after initial rough grading. Flow can be directed away from the facility with temporary diversion swales or other approved protection (Prince George’s County, 2002).
  • Construction on bioretention facilities should not begin until all contributing drainage areas are stabilized according to erosion and sediment control BMPs and to the satisfaction of the project engineer.
  • If the design includes curb and gutter, the curb cuts and inlets should be blocked until BSM and mulch have been placed and planting completed (when possible), and dispersion pads are in place (see Figure 4.4.20).

Every effort during design, construction sequencing, and construction should be made to prevent sediment from entering bioretention areas. However, bioretention areas are often distributed throughout the project area and can present unique challenges during construction. Minimizing sedimentation, removing sediment from bioretention areas, and replacing any soil removed with new BSM when project is complete are necessary for a proper functioning system. Deep compaction in bioretention areas is very difficult, if not possible, to mitigate and must be prevented.

Erosion and sediment control practices should be inspected and maintained on a regular basis.


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Fig4-4-20_6.1.28 bioretention sediment control
Fig 4.4.20
Proper erosion and sediment control for bioretention installation. Note the permeable pavement sidewalk is protected with filter fabric and the curb inlets to the bioretention area are blocked, until site is stabilized. Source: Photo by Curtis Hinman