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INFILTRATION TESTING FOR STORMWATER

"THE METHODOLOGY MUST CONSIDER SITE SPECIFIC CONDITIONS"

Sue Ahern, 10/22/2008

While soil infiltration testing for stormwater management facilities may be relatively simple to undertake in the field, generation of an appropriate infiltration rate for use in the stormwater management design is not as simplified. There are numerous field testing methodologies that are acceptable along with several models and equations for determining an appropriate infiltration design rate. However, simply utilizing a field rate of drop as the infiltration rate, even with consideration of a safety factor, is not scientifically sound.

The purpose of conducting field infiltration testing should be to determine a Saturated Hydraulic Conductivity (Ksat) value for use in the stormwater management design. At most project sites, Evans Mill Environmental, LLC (EME) typically performs infiltration testing as a falling head test in a sealed borehole, and utilizes a Hvorslev equation to generate a Ksat value for design use considerations. But it is important to understand that the field testing methodology along with the appropriate Ksat calculations are dependent upon the site conditions encountered, particularly the soil characteristics identified during soil test pit investigations.

A summary of the test methodology typically used by EME was presented to our regional DEP office, and EME has obtained concurrence from DEP that our standard protocol for conducting infiltration testing and generating a Ksat value for the engineer’s use in stormwater management design efforts is an acceptable protocol. It should be noted that this testing protocol is not a new concept or test methodology. EME’s professional staff, as well as other soil professionals, has utilized the procedure and/or other similar procedures for evaluating site suitability and design considerations for large volume, land-based wastewater disposal. After years of utilizing such testing for land-based wastewater applications, such level of testing is now being applied to stormwater management designs.

It is noteworthy to recognize that the testing protocol summary provided to DEP is in fact only a summary of the standard protocol. There are several site specific factors to consider when performing the falling head test and applying the Hvorslev equation. Even slight deviations in certain aspects of the field testing methodology would necessitate variations in the corresponding equation to calculate the hydraulic conductivity. As examples... did you know?

• That the shape factor in the Hvorslev equation is not a constant and is based upon the lithology of the soil as well as the borehole geometry?

• That the soil conditions above and below the proposed infiltration test depth influence the field testing protocol and the hydraulic conductivity formula?

• That the depth of the testing in relation to an impervious soil boundary can affect the hydraulic conductivity formula to be used?

• That the thickness of the soil horizon being tested affects the depth of field testing and/or the hydraulic conductivity formula used?

Conducting a falling head test in a sealed borehole is only one of many acceptable methodologies to evaluate the infiltration capability of the soil and to generate a hydraulic conductivity value. EME believes this methodology makes best use of time and monies, and if appropriately performed as part of a thorough soil test pit investigation, will provide the user with a realistic measurement of the hydraulic conductivity of the soil. However, caution is warranted when conducting such testing without full knowledge and understanding of the supporting science.

IN SUMMARY

Users of the EME testing methodology, as well as other similar test methodologies, need to understand the science behind the testing so that the appropriate site specific conditions can be properly addressed such that the design engineer can be provided with a reliable soil hydraulic conductivity value for further design use.

If you have nay questions on existing or upcoming projects, please do not hesitate to contact our professional staff. The success of our mutual projects relies on our cooperative efforts!

 

   
 

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   Updated:

18 Nov 2013

 

 

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