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 is not allowed within areas of fill.
Procedure[edit]
- 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.
- Submit the soil samples for laboratory testing of particle size analysis according to ASTM D422.
Data Analysis[edit]
- 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.
- Soil permeability shall be considered to be the measured infiltration rate:
K = C (D10) 2 Where K is in cm/s, C = 1, and D10 is the soil particle diameter in mm for which 10% of all soil particles are finer (smaller) by weight. 2. Adjust the measured infiltration rate using the correction factor described in Section 4 to establish the Design Infiltration Rate in inches per hour (in/hr).
- ↑ 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.