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− | This method of determining [[design infiltration rate]] is only approved for sandy native soils with d<sub>10</sub> between 0.1 - 2.5 mm (i.e. soils to which the Hazen formula is applicable). This method is not allowed within areas of fill. | + | This method of determining [[design infiltration rate]] is only suitable for coarse-textured native soils with d<sub>10</sub> between 0.1 - 2.5 mm (i.e. soils to which the Hazen formula is applicable)<ref>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</ref>. |
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| + | '''This method shall not be applied within areas of fill or in regions where hydraulic conductivity is controlled by vertical fractures in the soil matrix.''' |
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| ==Procedure== | | ==Procedure== |
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| + | # 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: |
− | # 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 [[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 [[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 maximum depth of water in the facility, but not less than 3 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 reservoir depth, but not less than 3 m. |
− | # Submit the soil samples for laboratory testing of particle size analysis according to [https://www.astm.org/Standards/D422.htm ASTM D422]. | + | # Submit the soil samples to a certified soil testing laboratory for grain-size, or particle-size distribution analysis according to [https://www.astm.org/Standards/D422.htm ASTM D422] Standard Test Method for Particle-size Analysis of Soils. |
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| + | ===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 d<sub>10</sub> between 0.1 and 2.5 millimeters <ref>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.</ref>. The soil permeability value estimated using the Hazen method can be considered to be the measured infiltration rate of the soil, f, in mm/h |
| + | :<math>f=C\cdot {\left (d_{10}\right )^{2}}</math> |
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− | ==Data Analysis== | + | {{Plainlist|1=Where: |
− | # As the Soil Grain Size Analysis method is only approved for soils with d<sub>10</sub> between 0.1 and 2.5 millimeters, the Hazen formula (below) may be used to calculate the soil permeability (K).
| + | *''d<sub>10</sub>'' = 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)}} |
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− | # Soil permeability shall be considered to be the measured infiltration rate:
| + | {| class="wikitable" |
| + | ! colspan =2| Shape factors (C) to yield f in mm/h |
| + | |- |
| + | |Very fine to fine sand |
| + | |144000 |
| + | |- |
| + | |Medium to coarse sand |
| + | |288000 |
| + | |} |
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− | K = C (D10)
| + | ==Don't forget to apply [[Design infiltration rate#Safety correction| a safety correction factor]] before beginning your sizing calculations== |
− | 2
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− | Where K is in cm/s, C = 1, and D10 is the soil particle diameter in mm for which 10% of all soil
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− | particles are finer (smaller) by weight.
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− | 2. Adjust the measured infiltration rate using the correction factor described in Section 4 to
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− | establish the Design Infiltration Rate in inches per hour (in/hr).
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