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Soil Testing and Stormwater Infiltration

"A General Soil Discussion for Our Fellow Engineers"

Sue Ahern, 03/26/2007


As most of you know, the PA Stormwater Best Management Practices Manual was finalized by PADEP in December of 2006, and many of you have attended the PADEP training seminar which was held in the area last month at Valley Forge and at Villanova.  While the focus of the stormwater management BMP's over the last few ears has emphasized the need for infiltration in order to protect our surface waters, it has become increasingly evident that there could be many development hurdles to overcome if infiltration requirements can not be achieved on a given site.  This fact alone makes it critical for the design engineer and soils consultant to work together closely in the early planning stages of a project in order to address this issue for our clients.


Some general soil information to know:


The Use of Percolation Testing

For those who attended the training session, you may recall that it was repeatedly stated by the presenters that the use of the standard percolation test procedure (used for on-lot sewage disposal) is not a recommended testing method for infiltration testing for stormwater.  While the BMP manual does provide for the use of such percolation testing, there is a required reduction formula that needs to be applied to the field rate.  In addition, a minimum 2 to 3-fold design safety factor must be applied to the already reduced rate depending upon the texture of the soil.


As an example to consider,  a field measured percolation test rate of 2.5 inches per hour is reduced to 0.9 inches per hour after dividing the measured rate by the required 2.5 reduction factor.  Assuming a very common soil texture of a silt loam, the reduced rate would need a 3-fold safety factor for design purposes, therefore reducing the rate to 0.24 inches per hour.


Preferred Testing Methods for Infiltration Testing

A variety of field tests exist for determining the infiltrative capacity of the soil.  As presented at the PADEP training seminar, the preferred methodology for infiltration testing is a permeability test procedure that yields a saturated hydraulic conductivity rate, commonly referred to as the Ksat.  As clarified at the Valley Forge seminar, no additional safety factors are needed for design purposes if the Ksat value is calculated.


What Type of Infiltration Testing does EME Conduct?

EME typically conducts vertical infiltration testing in a cased, sealed borehole.  The saturated hydraulic conductivity rate is then calculated based upon the field parameters of the testing (i.e.: thickness of soil horizon, diameter of pipe, water height, bottom conditions, etc.). EME has been utilizing this general testing protocol over the past few years for infiltration testing for stormwater.


When referring to testing in engineering reports, please refrain from referring to EME test results as "percolation tests".  This is inaccurate and can be confusing to the regulators when considering the application of "safety factors" for design purposes.  You can refer to our testing as an infiltration test or hydraulic conductivity test.  And as you know, we calculate and provide you with the the saturated hydraulic conductivity (Ksat) rate based upon the test methodology.


Any Questions for EME?

If you have any questions on existing projects or have any new or upcoming projects that need alternatives evaluated for stormwater infiltration, please do not hesitate to contact our professional staff.  The success of projects relies on our cooperative efforts!


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18 Nov 2013



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