Grain size analysis

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This method of determining design infiltration rate is only suitable for coarse-textured native soils with d10 between 0.1 - 2.5 mm (i.e. soils to which the Hazen formula is applicable)[1].

This method shall not be applied within areas of fill or in regions where hydraulic conductivityA parameter that describes the capability of a medium to transmit water. is controlled by vertical fractures in the soil matrix.

Procedure

  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 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 reservoirAn underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe. depth, but not less than 3 m.  
  2. 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.

Data Analysis

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 [2]. The soil permeability value estimated using the Hazen method can be considered to be the measured infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. of the soil, f, in mm/h \[f=C\cdot {\left (d_{10}\right )^{2}}\]

Where:

  • 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 sandMineral 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. 144000
Medium to coarse sandMineral 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. 288000

Don't forget to apply a safety correction factor before beginning your sizing calculations

  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.