Texture

Many of the physical and chemical properties of soil are affected by texture. The filter media component of bioretention, swales and green roofs must meet very specific design specifications related to texture in order for the BMP to achieve drainage and water treatment performance targets. If the media texture is too fine (i.e., contains more silt- and clay-sized particles than specified) it may have low permeability and drain too slowly or retain too much water for excessively long periods of time. If the media texture is too coarse (i.e., contains more sand and gravel-sized particles than specified) it will have high permeability and may drain too quickly to provide adequate treatment of run-off, and may not retain enough water between storm events to sustain healthy vegetation cover.

Testing

Soil texture is most accurately characterized by submitting a representative sample to a soil laboratory for a particle-size distribution (PSD) test. Other commonly used terms for the PSD test by soil laboratories are “Particle-Size Analysis”, “Grain-Size Distribution” and “% Sand, % Silt, % Clay”. For bioretention filter media and green roof growing media, “Sand Fraction Analysis” should also be requested.

Recommended methods for determining PSD of a soil sample are provided in ASTM D6913 and ASTM D7928 . These methods are recommended for use in inspection and testing of LID BMPs because they include assessment of the pebble-sized particles of the soil (i.e., particles that are greater than 2 mm in diameter).

Most soil laboratories will summarize PSD test results according to the proportions of the sample made up of pebble/gravel-, sand-, silt- and clay-sized particles. When Sand Fraction Analysis is requested, a more detailed breakdown of gradations of sand-sized particles is provided, which is important for evaluating the acceptability of filter media for bioretention and dry swales. Figure 8.8 describes the Wentworth soil particle-size classification system (Wentworth, 1922) that should be used to classify pebble, sand, silt and clay fractions of a soil sample.

Evaluation

A critical part of Construction, Assumption and Verification inspections involves sampling and testing the soil component of BMPs to ensure it meets design specifications related to texture or is still within acceptable ranges for important gradations (e.g., percent silt- and clay-sized particles).

Construction inspections

If laboratory testing indicates any media texture-related parameter is not within the design or product specification ranges, notify the media or topsoil supplier, issue a “do not install” order to the construction site supervisor and contact the design professionals and property owner or project manager to determine corrective actions.

Assumption and Verification inspections

If laboratory testing indicates any media texture-related parameter is not within the design or product specification ranges, or the acceptance criteria ranges, schedule investigative work to do further sampling and testing to determine the affected area and depth and decide on corrective actions. Corrective actions for bioretention and bioswale filter media where the proportion of silt- and clay-sized particles is too high may involve removal of mulch, stone cover and plantings and tilling the top 20 - 30 cm, or removal and replacement of part or all of the filter media with material that is within acceptable tolerance ranges of design specifications.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.