Borehole Permeameter Tests
by: Sebastien Fortin, E.I.T., M.Sc.
Air-Entry Permeameter
The air-entry permeameter (AEP) is similar to a single-ring infiltrometer in design and operation in that the volumetric flux of water into the soil with a single permeameter ring is used to calculate the vertical field-saturated hydraulic conductivity (Kfs) of geologic materials in the unsaturated zone. The primary differences between the two test methods are that the AEP typically penetrates deeper into the soil profile and measures the air-entry pressure of the soil. Air-entry pressure is used as an approximation of the wetting front pressure head for determination of the hydraulic gradient, and consequently field-saturated hydraulic conductivity.
Bouwer (1966) first proposed the AEP, and more recently Stephens et al. (1988) and Havlena and Stephens (1991) have shown that the AEP produces good results in low permeability clays and engineered clay liners. A short discussion of the applications of AEP's is included in ASTM standard D 5126. Useful information regarding the AEP is summarized in Table 2 below.
Principles
The AEP consists of a single permeameter ring, typically 30 cm in diameter and 25 cm-deep, sealed at the top by an air-tight cover, that is driven into the soil approximately 15 to 25 cm. Water is introduced into the permeameter through a standpipe, to the top of which is attached a graduated water supply reservoir and a pressure gauge (Figure 5). Daniel B. Stephens & Associates, Inc. has recently developed an automated AEP, which includes a pressure transducer and datalogger that automatically controls the water valve and records the water level data.
Water is allowed to infiltrate into the soil within the permeameter ring, and the flow rate is measured by observing the decline of the water level within the reservoir. After a predetermined amount of water has infiltrated (based upon the estimated available storage of the soil interval contained within the ring), and the flow rate is relatively stable, infiltration is terminated and the wetted profile is allowed to drain. The air-entry value is the minimum pressure measured over the standing water inside of the permeameter ring attained during drainage. Once the mimimum pressure is achieved, the permeameter is removed, and the depth to the wetting front is determined.
Figure 5. Automated air-entry permeameter (after Pegram et al., Daniel B. Stephens & Associates)
Summary of Field Procedures
Measurements of Kfs using the AEP in low-permeability soils can be performed in 1-8 hours. The testing is conducted in two stages. The first stage is the infiltration stage, in which water is allowed to enter the ring and infiltrate into the soil. The rate of decline in the water level within the water supply reservoir is measured during this stage.
The second stage is the drainage stage, during which the water supply is shut off and the water within the soil is allowed to redistribute. As the water redistributes, the tension within the water inside the ring increases until the point where the air-entry pressure (or bubbling pressure) is reached and bubbles migrate upward through the soil into the ring. The pressure during the drainage stage must be recorded at regular intervals. The minimum pressure (tension) achieved during this portion of the test is used to calculate the air-entry pressure of the soil, which is used in the determination of saturated hydraulic conductivity.
Analysis of Field Data
As soon as minimum pressure is reached, air begins to bubble up through the wetting front. Field-saturated Kfs can be calculated from the critical "air-entry" value or minimum pressure. Field-saturated Kfs is approximately equal to ½ of Ks in most soils or ¼ of Ks in fine-texture (clayey) soils (ASTM 5126).
Determination of vertical field-saturated hydraulic conductivity (Kfs) is based on measurement of the infiltration rate during Stage 1 and the air-entry pressure from Stage 2, along with the various values for the permeameter geometry , using the following Darcy-type equation, first proposed by Bouwer (1966).
This equation for vertical saturated hydraulic conductivity using an AEP assumes that the air-entry pressure of the material within the permeameter ring can be used to approximate the critical pressure along the wetting front, thus allowing the AEP to measure the actual hydraulic gradient applied to the soil. Such direct measurement offers a distinct advantage over other ring type permeameter tests, for which the wetting-front pressure head and resulting hydraulic gradient must be estimated. This solution also assumes that the critical pressure during drainage is approximately twice that for imbibition, and therefore, ½ the value for air-entry is used in the solution equation.
Several assumptions are included within the use of the AEP, which are the same for all ring-type permeameters and infiltrometers presented in the section on infiltrometer tests presented below.
Consult the reference list on Air-Entry Permeameter testing.
Forward to Infiltrometer Tests.
Return to Borehole Permeameter Tests.
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