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Instructors stress stricter soil‑testing rules, sampling density and Ksat verification after audit findings
Summary
Lisa Schaefer reviewed March 2024 updates to Chapter 12: minimum exploration depths, pit/boring counts, HSG determination steps, required hydraulic conductivity tests, post‑construction verification and a factor‑of‑safety rule for design Ksat.
Lisa Schaefer, presenting the soil‑testing module, told attendees Chapter 12 was updated in March 2024 and that auditors continue to find incomplete or incorrect soil testing in submitted stormwater reports.
She reviewed methods to determine hydrologic soil group (HSG): consult the NRCS Web Soil Survey first; where the survey has no rating or soils are disturbed, use the chapter‑12 default method or field HSG testing procedures. Lisa emphasized that dual HSG ratings (A/D, B/D, C/D) indicate drained vs undrained conditions and that reviewers must verify whether a site is drained to select the appropriate HSG.
On sampling density and depth, Lisa said the chapter requires minimum exploration depths and a spreadsheet/workbook to compute required profile pits and borings by soil mapping unit. For HSG determination the earlier 6‑ft depth guidance changed: chapter 12 requires soil explorations to extend at least 40 inches and to include any restrictive layers and the seasonal high water table (SHWT) identification.
For infiltration BMPs she summarized the rules: at least two profile pits inside the infiltration area (plus additional pits per 10,000 ft²), at least one Ksat laboratory or field test per profile pit, and careful placement of pits equidistant in each mapping unit. If soil replacement is proposed the testing depth is measured from the bottom of the replaced layer.
Testing methods allowed in chapter 12 include double‑ring infiltrometers, well permeameters, single ring infiltration tests, basin flooding tests and modified basin flooding tests; the former case borehole infiltration test has been removed. Lisa stressed that laboratory tests must use undisturbed cores, continuous sampling and compliance with ASTM practices for borings and sampling.
Design guidance and safety factors: a safety factor of two must be applied to the slowest tested saturated hydraulic conductivity to compute the design Ksat (and capped at 10 in/hr where tested rates exceed 20 in/hr). Post‑construction permeability tests must be performed before placing the sand or soil bed, use non‑invasive methods (single ring, double ring or modified basin flooding), be certified by a licensed professional engineer and be submitted to the reviewing agency.
Why it matters: Lisa said correct soil testing affects whether an infiltration BMP can be used, the design’s ability to meet TSS removal credit, and whether groundwater mounding will create adverse impacts to nearby structures. She urged reviewers to require complete, dated soil logs, a Web Soil Survey extract, test locations on plan sheets and certified post‑construction test results.
