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The planting soil is critical to the success of any bioretention facility.  From a practical standpoint, it must drain appropriately while having the necessary organic elements to sustain the plant community and biological processes. 

Soil for bioretention facilities should have a sandy loam, loamy sand, or loam texture per USDA textural triangle. Maximum clay content is <5%; soil mixture shall be 50-60% sand; 20-30% leaf compost; and 20-30% topsoil. Leaf compost is essentially composed of aged leaf mulch and provides added organic matter to improve the health of the soil and ensure adequate soil structure.  The soil must be a uniform mix, free of stones, stumps, roots, or other similar objects larger than two inches. No other materials or substances should be mixed or dumped within the bioretention that may be harmful to plant growth, or prove a hindrance to the planting or maintenance operations. The planting soil must be free of plant or seed material of non-native, invasive species, or noxious weeds.

Planting soil for bioretention areas must be tested prior to installation for PH and organic matter. The soil should meet the following criteria (Landscape Contractors Association, 1986).

• PH Range: 5.5 – 6.5

• Organic Matter: 1.5 – 3.0%

Sieve analysis, PH, and organic matter tests must be performed per each bioretention area.

Soil preparation can be performed onsite or offsite and transported to the facility location when ready for installation. Prior to transport, the soil mix should be certified as meeting the criteria established for the soil medium and approved by the site inspector.

Soil preparation can be accomplished by thoroughly mixing soil components, amendments and additives, as needed utilizing a backhoe or front-end loader.

In-situ Preparation

In-situ (or in-place) soil used for bioretention must also be prepared. Scarification of soil surfaces by manually raking to aerate and reduce soil compaction is recommended. When in-situ soils are being used without underdrain systems, soils investigation/geotechnical reports should be required.

Placement of the planting soil in the bioretention area should be after scarifying the invert area of the proposed facility, in lifts of 12 to 18 inches and lightly compacted.  Minimal compaction effort can be applied to the soil by tamping with a bucket from a dozer or backhoe. Lifts are not to be compacted but are performed in order to reduce the possibility of excessive settlement.  Installation of soils must be done in a manner that will ensure adequate filtration.

Avoid over compaction by allowing time for natural compaction and settlement. No additional manual compaction of soil is necessary. Rake soil material as needed to level out. Overfill above the proposed surface invert to accommodate natural settlement to proper grade. Depending upon the soil material, up to 20% natural compaction may occur.

For facilities designed with a liner, no scarification of the invert area is required. 

It is very important to minimize compaction of both the base of the bioretention area and the required backfill. When possible, use excavation hoes to remove original soil. If bioretention areas are excavated using a loader, the contractor should use wide track or marsh track equipment, or light equipment with turf-type tires. Use of equipment with narrow tracks or narrow tires, rubber tires with large lugs, or high pressure tires will cause excessive compaction resulting in reducing infiltration rates and storage volumes and is not acceptable. Compaction will significantly contribute to design failure.  Compaction can be alleviated at the base of the bioretention facility by using a primary tilling operation such as a Chisel Plow, Ripper, or Subsoiler. These tilling operations are to refracture the soil profile through the 12 inch compaction zone. Substitute methods must be approved by the engineer.  Rototillers typically do not till deep enough to reduce the effects of compaction from heavy equipment.  Rototill 2 to 3 inches of sand into the base of the bioretention facility before back filling the facility and placement of underdrain. Pump any ponded water before preparing (rototilling) base. 

When back filling the bioretention facility, do not use heavy equipment within the bioretention basin. Heavy equipment can be used around the perimeter of the basin to supply soils and sand. Grade bioretention materials with light equipment such as a compact loader or a dozer/loader with marsh tracks. 

In order to speed up the natural compaction process, presoaking the placed soil may be performed.  Significant settlement can occur after the first presoak, and additional settlement may occur subsequent to the initial wetting. If time and construction scheduling permits, it is preferable to allow natural settlement to occur with the help of rain events to presoak the soil medium.

Soil Investigation and Testing Criteria for Bioretention WITHOUT UNDERDRAINS

Whenever bioretention without underdrains is to be utilized, close attention to the soil conditions and limitations are of obvious importance. For bioretention areas that do not incorporate underdrain systems, at least one soil test per facility should be performed that includes the following as a minimum:

Identification of soil horizons and the corresponding USDA soil classification

Grain size distribution (sieve analysis) indicating the % clay, sand and silt.

Depth to the groundwater table or impervious layer (>2 feet below the bioretention invert), if present. This requirement may vary per specific state or local guidelines.

Depth of test should be at least 3 feet below the proposed invert, or as specified by local guidance..

Infiltration rates must be determined using standard acceptable practices such as a percolation test.

The mulch layer plays a vitally important role in the overall bioretention design. This layer serves to prevent erosion and to protect the soil from excessive drying. Soil biota existing within the organic and soil layer are important in the filtering of nutrients and pollutants and assisting in maintaining soil fertility. Bioretention areas can be designed either with or without a mulch layer. If a dense herbaceous layer or groundcover (70 to 80% coverage) is established, a mulch layer is no longer necessary. Areas should be mulched once trees and shrubs have been planted. Any ground cover specified as plugs may be installed once mulch has been applied.

The mulch layer recommended for bioretention may consist of either a standard landscape fine shredded hardwood mulch or hardwood chips.   Both types of mulch are commercially available and provide excellent protection from erosion for very low velocity flows, although shredded is less likely to float.

The mulch may be either aged or fresh to maximize nitrogen and metal uptake by the facility. Mulch should be free of weed seeds, soil, roots, or any other substance not consisting of either bole or branch wood and bark. The mulch should be uniformly applied approximately 2 to 3 inches in depth. Mulch applied any deeper than three inches reduces proper oxygen and carbon dioxide cycling between the soil and the atmosphere, and keeps plant roots from making good contact with the soil.

Grass clippings are unsuitable for mulch, primarily due to the excessive quantities of nitrogen built up in the material. Adding large sources of nitrogen could limit the capability of bioretention areas to filter the nitrogen associated with runoff and possibly create a net gain of nitrogen.

While mulching provides an important function in the bioretention process, the establishment of a herbaceous layer or groundcover may be preferred over mulching for several reasons. First, the mulch material has the ability to float up-and-out during heavy rain events. Second, the herbaceous layer provides more opportunities to capture and hold water though interception and evapotranspiration. Finally, providing thick, lush, groundcover increases the aesthetic appeal and adds to the landscape character. A combination of groundcover and mulch is an equally preferable option.

Sand should be clean and free of deleterious materials, meeting AASHTO M-6 or ASTM C-33 with grain size of 0.02”- 0.04”.

Geotextile fabric should meet ASTM D-751 (puncture strength - 125 LB), ASTM D-1117 (Mullen burst strength – 400 PSI), and ASTM D-1682 (Tensile strength – 300 LB)

Fabric should have 0.08” thick E.O.S. of #80 sieve, and maintain 125 GPM per SQ. FT. flow rate.

The above information was derived from the Prince George's County Bioretention Manual.

CONTINUE TO UNDERDRAINS

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The information on this website was compiled by T.E. Scott & Associates, Inc. from various sources.

We make no guarantees or warranties to the applicability of this material to specific applications. 
Last modified: 04/27/08