# Grain size analysis

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 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 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

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 rateThe rate at which stormwater percolates into the subsoil measured in inches per hour.$K=C\cdot {\left (d_{10}\right )^{2}}$

Where:

• d10 = the soil particle diameter for which 10% of all soil particles are finer (smaller) by weight (mm),
• C = is a shape factor (see below), and
• K = permeability (mm/hr)
Shape factors (C) to yield K in mm/hr
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 correction before beginning design 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.