Determining Head Height Repeat-Ability and Other Related Parameters for Aardvark Permeameter.

Hydraulic conductivity is a critical parameter in almost all applications and studies related to soil and water relationship. Conventional constant-head permeameters use Marriott Bubble Tower method for establishing a constant water head. However, Aardvark Permeameter uses a float valve to do so.  Considering significant differences between Aardvark methodology and permeameters based on Marriott Bubbler, there is a need for determining important technical specifications (e.g. measurement range, head height repeatability, etc) for Aardvark.

Over head pressure (vertical distance between reservoir and Aardvark unit in borehole) has a significant impact on Aardvark water supply. Aardvark water supply rate was measured under minimum practical overhead pressure (7 kPa or 1 psi) and maximum nominal applicable pressure (34 kPa or 5 psi). Results were used for determining Aardvark operational rage.

Float valve leaking rate can affect accuracy of Aardvark Permeameter measurements. Since it is not possible to detect and measure leakage rate in soil, Aardvark system was installed in laboratory and in a clear pipe with 10.2 cm (4 inch) diameter (recommended borehole diameter for Aardvark). The pipe was completely impermeable to water. Therefore it was possible to detect very small amounts of water loss (due to leakage or evaporation) from Aardvark system. Evaporation from borehole was measured using another identical cylinder and Aardvark measurements were corrected for evaporation.

Overhead pressure above Aardvark Permeameter Unit was adjusted on 41 kPa (6 psi). It was 7 kPa (1 psi) more than maximum nominal pressure (34 kPa or 5 psi) for Aardvark to get reliable results. Aardvark software application was used to perform automated readings (every minute) for more than 20 hr. Reading procedure was repeated 15 times. Reading data was used for simulating leakage rate and evaporation rate at any time increment.

A mathematical model was created to calculate Saturated Hydraulic Conductivity (K) based on four methods of calculation and due to different scenarios. Calculation methods include Glover solution, Reynolds and Elrick solution, Radcliff and West method and Earth Manual method. For each level (order of magnitude) of K, maximum and minimum of Flow Rate were calculated. Measurement Error was calculated using Aardvark Scale accuracy and resolution provided by the manufacturer and also system water loss due to leakage and evaporation.

A mathematical model was created for simulating Aardvark leakage rate at any time increment (1 minute) after opening Aardvark Reservoir valve. Since leakage Error reduces over time, Reading Start Time was defined as time duration between opening Aardvark Reservoir valve and starting “reliable” (less than 5% Measurement Error) readings (due to small Leakage Errors). Using Flow Rate, Leakage Rate and Measurement Error (less than 5%), Reading Intervals and Reading Start Time were optimized for each level of K.

Aardvark practical operational range was determined to be between 10^-4 and 10^-9 m/s. A Reading Interval of 0.3 to 1 min is enough for soils with K of 10^-4 m/s. Reading Interval in the range of 10^-5 m/s is 1 to 10 min. Reading Interval in the range of 10^-6 m/s is 10 to 30 min. Reading Interval in the range of 10^-7 to 10^-8 m/s is 30 min (longer Start Time and less Reservoir Water has to be used for smaller K values). Reading Intervals for K values in range of 10^-9 m/s are very long (10 hr to 48 hr). However, using Aardvark automated reading feature, it is still possible to measure these soils.

A mathematical model was created to simulate water head height based on total water consumed in 20 hours, borehole (clear pipe) diameter and the volume of Aardvark Permeameter Module.

Water head average had an average of 7.41 cm (2.92 inch) with standard deviation of 0.7 cm (0.28 inch). Head height CV was calculated as 9.5%. Water head height confidence interval was calculated as 0.36 cm (0.14 inch) for 5% and 0.47 cm (0.18 inch) for 1%. Total variation range was determined as 2.23 cm (0.88 inch) equivalent to 30.02% of the whole range.

In order to have a better understanding about the effect of head height repeatability on ultimate results, head height confidence intervals were converted to K values. The results suggest that head height changes in range of 1% confidence interval (including 99% of head height variation) changes K value about 2.5% or less.

Aardvark was capable of performing reliable and repeatable measurements automatically and for long periods of time which makes it a suitable instrument for measuring saturated hydraulic conductivity in laboratory and field condition.