Soil Permeability Testing¶

Soil permeability controls whether an infiltration BMP can function as designed and what design permeability may be credited in recharge, drain-time, and quantity calculations. In the live source set, the governing technical framework comes from BMP Manual Chapter 12 together with the recharge and hydraulic-impact provisions in N.J.A.C. 7:8.

Structured citation evidence for this page lives in the sibling claim manifest.

Concept Explanation¶

Permeability testing is the bridge between site soils and BMP design. The source corpus uses field-tested soil hydraulic conductivity to support decisions about:

whether infiltration is feasible at all
what design permeability can be used after applying the required factor of safety
how far investigations must extend below the BMP bottom
what post-construction confirmation testing is allowed

County or NRCS soil mapping can help with planning, but final infiltration design depends on site-specific soil exploration and hydraulic conductivity testing at the proposed BMP location.

Chapter 12 Requirements¶

Accepted test methods¶

Chapter 12 does not restrict the engineer to a single ASTM method. The live source set supports a broader method family that includes:

percolation testing
tube permeameter testing
single-ring infiltration testing
basin flooding testing for the applicable cases
ASTM D3385 double-ring infiltrometer testing
other recognized in-situ methods accompanied by a published reference

That matters because legacy summaries that treated ASTM D5126 as the controlling infiltration method were wrong.

Design permeability¶

The credited design permeability is the tested field permeability divided by a factor of safety of 2.0. In other words, the source-backed design rule is:

K_design = K_field / 2.0

That safety factor carries through into drain-time and recharge calculations. A page or spreadsheet that uses the raw tested permeability as the design value is overstating infiltration performance.

Investigation depth and testing density¶

For infiltration BMP design, Chapter 12 requires soil profile pits and borings to extend to at least eight feet below the lowest basin-bottom elevation or to twice the maximum potential water depth generated by the largest design storm, whichever is greater.

For green infrastructure BMPs, Chapter 12 also ties the number of required pits and borings to the BMP layout and soil mapping units. In the standard GI case, one soil profile pit is required per soil mapping unit and two soil borings are required per BMP, with additional testing for linear systems.

For small distributed infiltration work, the chapter is even more specific. Where multiple infiltration BMPs each have infiltration areas of 500 square feet or less, Chapter 12 requires at least one soil profile pit per soil mapping unit and one soil boring per BMP. It also requires a minimum of one soil hydraulic conductivity test at each soil profile pit and soil boring location.

Post-construction verification¶

Post-construction hydraulic conductivity confirmation is narrower than pre-construction exploration. The live source set limits post-construction testing to:

single-ring infiltration testing
modified basin flooding testing
double-ring infiltration testing
similar methods that do not require a test hole in the soil layer

Chapter 12 also fixes the location and record standard for the as-built test. The post-construction hydraulic conductivity test must be run at the bottom of the excavation before the sand layer or soil bed is placed, and the results must be certified by a licensed professional engineer, submitted to the review agency, and recorded in the as-built documents.

Engineering Implications¶

The permeability result affects several downstream decisions:

Design question	Why permeability matters
Can the BMP infiltrate?	The tested soil conditions determine whether infiltration is technically supportable
What permeability enters the calculations?	The factor of safety reduces the tested value to a design value
How deep must the investigation go?	Chapter 12 sets a minimum exploration depth tied to BMP bottom elevation and storm depth
What happens after construction?	The as-built BMP must be checked with an allowed post-construction method

Because permeability feeds directly into recharge, drain-time, and hydraulic-impact analysis, a weak soil evidence chain usually invalidates more than one part of the design.

N.J.A.C. 7:8¶

N.J.A.C. 7:8-5.4 for groundwater recharge standards
N.J.A.C. 7:8-5.2(h) where hydraulic impact on the groundwater table must be assessed

BMP Manual Chapters¶

Chapter 12 for soil testing criteria
Chapter 6 for groundwater recharge calculations
Chapter 13 for groundwater mounding analysis