Difference between revisions of "Grain size analysis"
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| − | + | ==Don't forget to apply [[Design infiltration rate#Safety correction| correction]] before beginning design calculations== | |
Revision as of 16:24, 18 March 2018
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]
- 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. The calculated soil permeability shall be considered to be the measured infiltration rate:
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 sand | 144000 |
| Medium to coarse sand | 288000 |
Don't forget to apply correction before beginning design calculations[edit]
- ↑ 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.