Grain size analysis

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This method of determining design infiltration rate is only approved for sandy native soils with d10 between 0.1 - 2.5 mm (i.e. soils to which the Hazen formula is applicable)[1]. This method must not be applied within areas of fill.

Procedure[edit]

  1. 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 paving, 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 maximum depth of water in the facility, but not less than 3 m.  
  2. Submit the soil samples for laboratory testing of particle size analysis according to ASTM D422.

Data Analysis[edit]

  1. As the Soil Grain Size Analysis method is only approved for soils with d10 between 0.1 and 2.5 millimeters, the Hazen formula [2] may be used to calculate the soil permeability. The calculated soil permeability shall be considered to be the measured infiltration rate:

{{plainlist|Where: d10 = the soil particle diameter for which 10% of all soil particles are finer (smaller) by weight (mm), C = is a shape factor (see below), and K = permeability (mm/hr)

Shape factors to yield K in mm/hr
Very fine to fine sand 144000
Medium to coarse sand 288000

Safety Correction Factor[edit]

Adjust it!


  1. 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
  2. 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.