Borehole Permeameter Tests
by: Sebastien Fortin, E.I.T., M.Sc.
Guelph Permeameter
The Guelph Permeameter (GP) is perhaps the most well known/used borehole permeameter technique and is described in detail below. It is sometimes referred to as the Guelph Pressure Infiltrometer (PI) to describe the same instrument.
The GP is an easy to use instrument for quickly and accurately measuring in situ hydraulic conductivity. Accurate evaluation of soil hydraulic conductivity, soil sorptivity, and matrix flux potential can be made in all types of soils. The equipment can be transported, assembled and operated easily by one person. Measurements can normally be made in ½ to 2 hours, depending on soil type, and require only about 2.5L of water. The Guelph permeameter is manufactured by Soil Moisture Equipment Corp., based in California, USA. Reynolds et al. (1985) provide a complete description of the GP apparatus, which is essentially an in-hole Mariotte bottle constructed of concentric, transparent plastic tubes.
The GP comes as a complete kit and extension tubes can be used for determining hydraulic conductivity to a maximum operating depth of approximately 3m. Ring attachments allow ring infiltrometer measurements from 10-cm and 20-cm diameter rings. A tension adaptor can also convert the GP to a tension infiltrometer (see section on tension infiltrometer), which allows measurements to be made under tensional and very low head conditions
Principles
Guelph permeameter measurements are carried out in the vadose zone above the water table, where the soil is unsaturated (Figure 3, Photo 1). Steady flow produces a small inner saturated zone adjacent to the well, encased within a larger outer wetted, but unsaturated volume. As a consequence, combined saturated-unsaturated flow occurs as shown in Figure 4.
The GP method measures the steady-state rate Q (m3/s) necessary to maintain a constant depth of water H (m) in an uncased cylindrical well of radius a (m), above the water table. Then the field saturated hydraulic conductivity Kfs (m/s), and the matric flux fm (m2/s) are calculated from Q, H, and using the following approximate analytical solution (Reynolds et al., 1985).
where C is a dimensionless shape factor primarily dependent on the H/a ratio and soil type (see Soil Moisture operating manual, Fig. 45 on p. 25).
Photo 1. Guelph Permeameter testing in mine tailings, note dessication cracks at the surface
(courtesy of Robertson GeoConsultants Inc., 2003)
The field saturated hydraulic conductivity (Kfs) is a measure of the capacity of porous medium to conduct a wetting liquid, and the matric flux potential (fm) is a measure of the capacity of porous medium to absorb a wetting liquid. Hence the first term of equation (8) represents the pushing action, and the second term represents the capillary pulling action.
Although the GP is normally used under constant head conditions, the test tube can also be used for falling head testing in special applications such as investigating the change in permeability associated with tailings settlement (Photo 2).
The GP can be used anywhere a hole can be augered in the soil. Soils typically possess a three dimensional heterogeneity, while the GP method essentially provides a "point" measurement. Therefore, the number of measurements to adequately represent field variability will depend on factors such as soil type, type of application, project objectives, etc...A description of the soil profile (by sampling or from soil survey reports) will greatly complement the value and understanding of data obtained with the Guelph Permeameter.
A complete description of the operating mode of the GP is provided with the Operating Instructions manual from Soil Moisture Equipment Corp., which can be found on the manufacturer website . For the sake of brevity, the standard procedures are only summarized below.
Photo 2. Set up for falling head testing in using the GP test tube in freshly deposited mine tailings
(courtesy of Robertson GeoConsultants Inc., 2003)
Summary of Field Procedures
i. Excavate a cylindrical well to the desired depth in the material to be tested;
ii. Fill the permeameter with liquid and inserting it in the well;
iii. Start the permeameter by raising the air-inlet tube out of the outlet port;
iv. Set the desired H level by adjusting the height of the air-inlet tube;
v. Monitoring the rate of fall, r, of the liquid surface in the reservoir until a steady rate, r, is attained;
vi. Calculate Q using r and the cross-sectional area of the reservoir;
vii. Calculate Kfs, S and a using the solution equations (see Appendix A).
A convenient field data sheet for GP testing is provided in Appendix A of this document, which permits calculations to be performed in the field (adapted from Reynolds et al., 1985).
Baragello (1997), Bagarello and Provenzano (1995), Reynolds and Elrick (1985) and Meiers (2002) provide useful recommendations with respect to the augering of a borehole and comment on the importance of avoiding smearing (which should be removed by brushing and puncturing the side walls of a hole) in fine-grained materials and cleaning the base of the hole free of debris. Negligence at following these recommendations could result in a substantial underestimation of Kfs.
Subsidence of the GP tip into the base of the hole during a measurement can produce an underestimate of Kfs because the actual ponded water in the well becomes lower then assumed and the outflow from the GP is reduced by a smaller effective infiltration area (Bagarello, 1997). Meiers (2002) recommends additional care may be needed to support the weight of the GP when filled with water so that the water outlet tip does not sink into the base of the well during measurements. In soft and/or unstable materials (sands), it is advisable to use a screen insert to minimize collapse of the well during measurements, since this may block flow from the tip (Meiers, 2002). Alternatively, a platform (e.g. made of wood) can be used for supporting the weight of the GP water reservoir and prevent subsidence in soft materials (Photo 3).
Photo 3. Set up for GP Testing in soft tailings using a wooden support platform
(courtesy of Robertson GeoConsultants Inc., 2003)
Analysis of Field Data
The values of Kfs that can be measured with the GP range from 10-4 to 10-8 m/sec. Beyond these limits there is a reduction in accuracy and precision. In soils with Kfs <10-8 m/s the rate of infiltration is too low to me monitored accurately.
Currently, two methods are widely accepted for solving equation (9) and hence determining the field saturated hydraulic conductivity; one consist of a single (or one ponded) height analysis (Elrick et al., 1989) while the other consists of using a dual (or two ponded) height analysis (Simulataneous Equation, or Richards analysis) (Reynolds et al., 1985). The Richards analysis accounts for all three forces that contribute to the 3D flow of water into soils. The gravitational pull of liquid out through the bottom of the well and the capillary pull of water out of the well into the surrounding soils). If used in combination in a complementary fashion the results will be more representative of the field conditions (Meiers, 2002).
Equation 9 below summarizes the calculation involved in determining Kfs when using the GP unit from Soil Moisture Equipment Corp. with the dual height analysis method.
Where R1' and R2' are the steady-state drop in water level during the first and second test, respectively.
Note that the value of the Kfs is dependent not only on the calculation procedure but it is also strongly influenced by the accurate detection of the steady-state flow rate (Q). Bagarello et al. (1999) provide a discussion and comparison of the various procedures to determine when steady-flow rate is achieved. The Three Quasi-Equal Readings and the Three Equal Readings (TR) are the most widely used criteria for evaluating steadiness in the flow rate.
The heterogeneous nature of soils can potentially lead to inaccuracies in flow measurements. Bagarello and Provenzano (1996) provide recommendations as to how the data should be treated to minimize the effects of heterogeneities on the calculation of Kfs. Reynolds et al. (2002) conducted field experiment comparing the tension infiltrometer (TI), the Guelph pressure infiltrometer (PI) and the intact soil core (SC) methods for measuring Ksat for various soil types and land management practices and concluded the PI method gave the most consistent and most reliable results of the three methods studied.
Consult the reference list on Guelph Permeameter testing.
Forward to Air-Entry Permeameter.
Return to Borehole Permeameter Tests.
|
|
/ Surface Water
