Grain size analysis
This method shall not be applied within areas of fill or in regions where hydraulic conductivity is controlled by vertical fractures in the soil matrix.
- Collect soil samples for each defined soil layer below the bottom of the proposed infiltration facility. Layers must be evaluated to the following depths, depending upon facility type:
- For bioretention facilities, analyze each defined layer below the proposed facility bottom to a depth of at least 3 times the maximum ponding depth, but not less than 1 m.
- For permeable pavements, analyze each defined layer below the proposed facility bottom to a depth of at least 3 times the maximum depth of water within the base course, but not less than 1 m.
- For other types of infiltration facilities serving drainage areas up to 4 Ha, analyze each defined layer below the proposed facility bottom to a depth of at least 2.5 times the water storage reservoir depth, but not less than 3 m.
- Submit the soil samples to a certified soil testing laboratory for grain-size, or particle-size distribution analysis according to ASTM D422 Standard Test Method for Particle-size Analysis of Soils.
NB: The Hazen formula method of estimating soil permeability based on grain size distribution analysis is only suitable for coarse-textured soils with d10 between 0.1 and 2.5 millimeters . The soil permeability value estimated using the Hazen method can be considered to be the measured infiltration rate of the soil, f, in mm/h
- d10 = the soil particle diameter for which 10% of all soil particles are finer (smaller) by weight (mm), median value of all samples tested.
- C = is a shape factor (see below)
|Shape factors (C) to yield f in mm/h|
|Very fine to fine sand||144000|
|Medium to coarse sand||288000|
Don't forget to apply a safety correction factor before beginning your sizing calculations
- San Francisco Public Utilities Commission. (2017). Determination of Design Infiltration Rates for the Sizing of Infiltration‐based Green Infrastructure Facilities. Retrieved from http://sfwater.org/modules/showdocument.aspx?documentid=9681
- Hazen, A. (1893). Some physical properties of sand and gravel with special reference to the use in filtration. 4th Annual Report, State Board of Health, Boston.