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). This method is not allowed within areas of fill.

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

1. As the Soil Grain Size Analysis method is only approved for soils with d₁₀ between 0.1 and 2.5 millimeters, the Hazen formula (below) may be used to calculate the soil permeability (K).

1. 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).

The rate at which stormwater percolates into the subsoil measured in inches per hour.