by: Sebastien Fortin, E.I.T., M.Sc.
This category includes ponded infiltrometer, drum (cylinder) test and other types of single-ring apparatus.
The single ring apparatus typically consist of a cylindrical ring 30cm or larger in diameter that is driven about 5cm into the soil (Bouwer, 1986). Water is ponded within the ring above the soil surface (Photo 4). The upper surface of the ring is often covered to prevent evaporation. The volumetric rate of water added to the ring sufficient to maintain a constant head within the ring is measured. Alternatively, if the head of water within the ring is relatively large, a falling head type test may be used wherein the flow rate, as measured by the rate of decline of the water level within the ring, and the head for the later portion of the test are used in the calculations. Infiltration is terminated after the flow rate has approximately stabilized. The infiltrometer is removed immediately after termination of infiltration, and the depth to the wetting front is determined either visually, with a penetrometer-type probe, or by moisture content determination for soil samples.
A special type of single-ring infiltrometer called the pounded infiltration basin is presented in ASTM method D5126 but is not discussed here because it is seldom used.
It is good practice to establish the soil strata to be tested from the soil profile determined by the description of soil samples from an adjacent auger hole. The test site should be nearly level, or a level surface should be prepared. The test may be set up in a pit if infiltration rates are desired at depth rather than at the surface. In low permeability materials where test duration are expected to be considerable, provisions should be made to protect the test apparatus from direct sunlight, which could promote water evaporation from the rings and/or water level fluctuation in the Mariotte reservoir.
Photo 4. Single-ring infiltrometers installed in "freshly deposited mine tailings (courtesy of Robertson GeoConsultants Inc., 2003).
Ring infiltrometers are often used for measuring the water intake rate at the soil surface. Water flow from a single-ring infiltrometer into soil is a 3-D problem (Reynolds and Elrick, 1990). The total flow rate into the soil from a single-ring infiltrometer is a combination of both vertical and horizontal flow.
Most infiltrometers generally employ the use of a metal cylinder placed at shallow depths into the soil (Photo 5a, b), and include the single ring infiltrometer, the double-ring infiltrometer and the infiltration gradient method. Various adaptations to the design and implementation of these methods have been employed to determine the field-saturated hydraulic conductivity of material within the unsaturated zone. The principles of operation of these methods are similar in that the steady volumetric flux of water infiltration into the soils within the infiltrometer ring is measured. Saturated hydraulic conductivity is derived directly from solution of Darcy's Equation for saturated flow. Primary assumptions are that the volume of soil being tested is field-saturated and that the saturated hydraulic conductivity is a function of the flow rate and the applied hydraulic gradient across the soil volume. Additional assumptions common to infiltrometer tests are as follows:
The movement of water into the soil profile is 1-D downward;
Equipment compliance effects are minimal and may be disregarded or easily accounted for;
The pressure of soil gas does not offer any impedance to the downward movement of the wetting front;
The wetting front is distinct and easily determined;
Dispersion of clays in the surface layer of finer soils is insignificant;
The soil is non-swelling, or the effects of swelling can easily be accounted for.
Photo 5a,b. Set up for single-ring infiltrometer test in mine tailings with "clean hole" base (a) and gravel-geofabric base (b) to simulate underdrain bottom condition (courtesy of Robsertson GeoConsultants Inc., 2003).
Analysis of Field Data
A method to calculate the Ks from data obtained from a pressure or ring-infiltrometer for both early-time and steady-state infiltration was developed by Reynolds and Elrick (1990), Elrick and Reynolds (192) and Elrick et al. (1995). Their steady-state method uses a shape factor based on Garder's (1958) relationship between hydraulic conductivity and matric pressure head.
Wu et al. (1999) developed new single-ring infiltrometer methods that use a generalized solution to measure the field saturated hydraulic conductivity (Kfs). The Kfs values can either be calculated from the whole cumulative infiltration curve (Method 1) or from the steady-state portion of the cumulative infiltration curve by using a correction factor (Method 2).
The generalized equation (Wu and Pan, 1997) is:
In equations 15 through 20, a and b are dimensionless constants (a=0.9084, b=0.1682) from the generalized equation, H is the ponded depth in the ring, d is the ring insertion depth, r is the radius of the ring infiltrometer, Ks and Ki are the hydraulic conductivity at saturated water content (q0) and at initial water content (qi), h and hi are matric and initial matric pressure heads, and K'(h) is the modified van Genuchten hydraulic conductivity pressure head function (Wu and Pan, 1997).
There are two ways to calculate Ks by applying the generalized infiltration equation to the measured infiltration curves from a single-ring infiltrometer. Method 1 is based on the cumulative infiltration equation. By integrating equation 15 from t=0 to t=t, we have:
By applying scaling theory, Wu and Pan (1997) developed a generalized solution for single-ring infiltrometers, and showed that infiltration rate from a single ring infiltrometer is approximately f times greater than the 1D infiltration rate for the same soil, where f is a correction factor that depends on soil initial and boundary conditions and ring geometry. For a relatively small ponded head, the 1D infiltration rate of a soil is approximately equal to the field saturated hydraulic conductivity (Kfs).
For a fine soil, constant and falling head methods produce very similar infiltration rates (Wu and Pan, 1997) for a time period practical for field measurements (e.g., a few hours) because the head drop in the ring is small. However, for a coarse textured soil, the head drop is fast. Thus, the falling head method measures substantially lower infiltration rates if the measurement is taken when the ponded head is small. Measurements taken immediately after refilling the infiltrometer will be close to the infiltration rate by the constant head method.
The effect of layering on infiltration measurement is time and position dependent. For a limited period of measurement, the layering effect is more profound when the underlying soil is closer to the surface. The time required for the wetting front to reach the interface of texture discontinuity can be estimated from the correction factor, f, and the cumulative infiltration.
Consult the reference list on Single-Ring Infiltrometers.
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