New Expression Relating Kfs to Percolation Time

Perc Test

The outdated percolation test does not control for variables that can substantially affect the test results.

The Percolation Test (PT) rate of fall or “Perc Time” is still used in many jurisdictions to determine suitability for onsite sewage disposal or for sizing of soil absorption systems (drainfields) and stormwater infiltration BMPs. It has been understood for some time, however, that PT is less than ideal because it is not just a function of soil permeability, but also a function of test conditions.

The field saturated hydraulic conductivity, Kfs, determined using the Constant Head Well Permeameter (CHWP) technique, is a much more scientifically and technically sound indicator of soil permeability than the outdated PT. Kfs testing controls for variables that can substantially affect the PT such as pit/borehole dimensions, depth of water ponding, soil capillary properties, and background soil moisture content at the time of the test.

Various correlations between Perc Time (PT) and field-saturated hydraulic conductivity (Kfs) have been proposed. Reynolds et al (2015) analysed six PT versus Kfs correlations from Virginia, Georgia, Connecticut, and Ontario. None of the correlations were found to be generally applicable, accurate or scientifically defensible, in part because they did not completely describe the factors affecting PT and Kfs.

CHWP diagram

Constant Head Well Permeameter (CHWP) method.

An accurate and physically-based analytical expression relating PT to Kfs for cylindrical test holes has now been proposed and is described in detail by Reynolds et al (2015). Using this analytical expression, it is now possible to show the relationship between PT and Kfs. A procedure has been described which shows how to determine PT from Kfs using the extended single-ponded height CHWP method (which is directly comparable to the original single-head Glover analysis). This procedure was also described in a paper presented by Kelly Galloway, P.Eng. at the 2015 Annual Conference of the National Onsite Wastewater Recycling Association (NOWRA), West Virginia Beach.

A summary of a simplified procedure to determine PT from Kfs which is applicable to the current ETC Pask Permeameter kit is outlined below. Refer to Reynolds et al (2015) for a detailed discussion of the factors and applicability of the newly developed Kfs-to-PT relationship.

Step 1: Determine Kfs and α* using the Single-Head method outlined in the ETC Pask Permeameter User Guide.

Step 2: Determine the appropriate Kfs to PT conversion factor, m, from Table 1.0 below.

Step 3: Determine the “equivalent” PT (in units of minutes/inch or minutes/cm) that corresponds to the H, d (a), α* and Kfs values, using the relationship, PT = m/Kfs, where Kfs is in meters/sec.

The calculated PT value is referred to as “equivalent” because borehole water level is held constant (at H) by the CHWP, thereby preventing direct measurement of PT = ∆t/∆H.

 

Table 1.0: Conversion factor, m, relating Perc Time (PT) to Kfs for constant head, H=15.0cm and approx. well hole diameter, d=8.3cm.

PT=m/Kfs, where Kfs is in meters/sec.

Capillarity Category Representative
α* (cm-1)
m (for PT in min/cm) m (for PT in min/inch)
Negligible 1.0 7.74E-06 1.97E-05
Weak 0.36 7.00E-06 1.78E-05
Moderate 0.12 5.39E-06 1.37E-05
Strong 0.04 3.18E-06 8.07E-06
Very Strong 0.01 1.05E-06 2.68E-06


It must be emphasized that the conversion factors given in Table 1.0 apply only to the well characteristics indicated above.  Alternative conversion tables applicable to other auger/well hole sizes and constant heads may be ordered from Dynamic Monitors, or they may be derived using the procedure outlined in Reynolds et al (2015)

 

REFERENCES

Reynolds,W.D. and Galloway, K.A. and Radcliffe, D.E. 2015. The relationship between perc time and field-saturated hydraulic conductivity for cylindrical test holes. Proceedings of the Annual Conference of the National Onsite Wastewater Recycling Association, West Virginia Beach, West Virginia. November 2015.

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