G.3.6 Grain Size Analysis

The design infiltration rate for grain size analysis shall be determined by a correlation to grain size distribution from soil samples. This method uses the ASTM soil size distribution test procedure (ASTM D422), which considers the full range of soil particle sizes, to develop soil size distribution curves. The following grain size analysis may be used to determine initial K_sat if the site has soils unconsolidated by glacial advance and is allowed for rural (outside the UA and UGA) residential project sites only.

The Grain Size Analysis report shall be prepared by a licensed professional. The test method may be modified due to site conditions if recommended by the licensed professional and the reasoning is documented in the report. Refer to report submittal requirements in Volume II, Chapter 1.

G.3.6.1 Procedure

1. For each defined layer below an infiltration pond to a depth below the pond bottom of 2.5 times the maximum depth of water in the pond, but not less than 10 feet, estimate the initial K_sat in cm/sec using Equation G.1. For large infiltration BMPs serving drainage areas of 10 acres or more, soil grain size analyses should be performed on layers up to 50 feet deep (or no more than 10 feet below the water table).

   Equation G.1. Estimate Initial Saturated Hydraulic Conductivity

   log₁₀(K_sat) = -1.57 + 1.90D₁₀ + 0.015D₆₀ - 0.013D₉₀ - 2.08f_fines

   where:

   K_sat = saturated hydraulic conductivity (cm/s) (1 cm/sec = 1417 in/hr)

   D₁₀ = grain size in mm for which 10 percent of the sample is more fine

   D₆₀ = grain size in mm for which 60 percent of the sample is more fine

   D₉₀ = grain size in mm for which 90 percent of the sample is more fine

   f_fines = fraction of the soil (by weight) that passes the number-200 sieve

   For BMP T7.30: Bioretention, analyze each defined layer below the top of the final bioretention area subgrade to a depth of at least 3 times the maximum ponding depth, but not less than 3 feet (1 meter).

   For BMP T5.15: Permeable Pavements, analyze for each defined layer below the top of the final subgrade to a depth of at least 3 times the maximum ponding depth within the base course, but not less than 3 feet (1 meter).

   If the licensed professional conducting the investigation determines that deeper layers will influence the rate of infiltration for the facility, soil layers at greater depths must be considered when assessing the site’s initial K_sat. Massman (2003) indicates that where the water table is deep, soil or rock strata up to 100 feet below an infiltration BMP can influence the rate of infiltration. Note that only the layers near and above the water table or low permeability zone (e.g., a clay, dense glacial till, or rock layer) need to be considered, as the layers below the ground water table or low permeability zone do not significantly influence the rate of infiltration. Also note that this equation for estimating K_sat assumes minimal compaction consistent with the use of tracked (i.e., low to moderate ground pressure) excavation equipment.

   If the soil layer being characterized has been exposed to heavy compaction (e.g., due to heavy equipment with narrow tracks, narrow tires, or large lugged, high pressure tires) the K_sat for the layer could be approximately an order of magnitude less than what would be estimated based on grain size characteristics alone. In such cases, compaction effects must be taken into account when estimating K_sat.

   For clean, uniformly graded sands and gravels, the reduction in K_sat due to compaction will be much less than an order of magnitude. For well graded sands and gravels with moderate to high silt content, the reduction in K_sat will be close to an order of magnitude. For soils that contain clay, the reduction in K_sat could be greater than an order of magnitude.

   If greater certainty is desired, the in-situ saturated hydraulic conductivity of a specific layer can be obtained through the use of a PIT, as described above.

2. Once the K_sat for each layer has been identified, determine the effective average K_sat of the native soils. K_sat estimates from different layers can be combined using the harmonic mean (Equation G.2).

   Equation G.2. Saturated Hydraulic Conductivity Using the Harmonic Mean

   

   where:

   d = total depth of the soil column,

   d_i = thickness of layer “i” in the soil column

   K_i = saturated hydraulic conductivity of layer “i” in the soil column

   The depth of the soil column, d, typically would include all layers between the infiltration pond bottom and the water table. However, for sites with very deep water tables (>100 feet) where ground water mounding to the base of the infiltration pond is not likely to occur, it is recommended that the total depth of the soil column in Equation G.2 be limited to approximately 20 times the depth of infiltration pond, but not more than 50 feet. This is to ensure that the most important and relevant layers are included in the hydraulic conductivity calculations. Deep layers that are not likely to affect the infiltration rate near the infiltration pond bottom should not be included in Equation G.2.

   Equation G.2 may over-estimate the effective K_sat value at sites with low conductivity layers immediately beneath the infiltration BMP. For sites where the lowest conductivity layer is within five feet of the base of the BMP, it is suggested that this lowest K_sat value be used as the equivalent hydraulic conductivity rather than the value from Equation G.2. Using the layer with the lowest K_sat is advised for designing bioretention (see Vol II–5.4.6 Bioretention Cells, Swales, and Planter Boxes) and permeable pavement (see Vol II–5.4.8 Permeable Pavement). The harmonic mean given by Equation G.2 is the appropriate effective hydraulic conductivity for flow that is perpendicular to stratigraphic layers, and will produce conservative results when flow has a significant horizontal component such as could occur due to ground water mounding.

If using the soil Grain Size Analysis Method for estimating infiltration rates: Complete laboratory testing as necessary to establish the soil gradation characteristics and other properties, to complete the infiltration facility design. At a minimum, conduct one-grain size analysis per soil stratum in each test hole within 2.5 times the maximum design water depth, but not less than 10 feet. When assessing the hydraulic conductivity characteristics of the site, soil layers at greater depths must be considered if the licensed professional conducting the investigation determines that deeper layers will influence the rate of infiltration for the BMP, requiring soil gradation/classification testing for layers deeper than indicated above.