Infiltrometer Methods

Infiltrometer Tests

by: Sebastien Fortin, E.I.T., M.Sc.

Introduction

Measurement of field saturated hydraulic conductivities is often done by borehole permeameters, which measures Kfs at depth (e.g. Elrick and Reynolds, 1992). In many cases, however, measurement of the soil surface Kfs is essential, especially in infiltration-related applications (e.g. irrigation management or tailings discharge). Ring infiltrometers are often used for measuring the water intake rate at the soil surface. The following sections present and discuss some of the infiltration test methods that are commonly used for field permeability testing in the vadose zone.

Infiltrometer test methods measure the rate of infiltration at the soil surface, which is influenced both by the field-saturated hydraulic conductivity as well as capillarity effects of the soil. Capillary effect refers to the ability of a dry soil to pull or wick water away from a zone of saturation faster than would occur if soil were uniformly saturated. The magnitude of the capillary effect is determined by the initial moisture content at the time of testing, the pore size, soil physical characteristics (texture, structure), and a number of other factors. Capillary effects are minimized by waiting until steady-state infiltration is reached.

This paper presents a detailed discussion of the following infiltrometer testing methods in the vadose zone:

• Tension Infiltrometer;
• Single-Ring Infiltrometer;
• Ponded Infiltrometer; and
• Double-Ring Infiltrometer

For each one of these methods, the operating principles, field procedures, and the analysis of field data are systematically presented, followed by a list of relevant references.

Tension Infiltrometer

Tension (disk) infiltrometers (TI) have been widely used for in situ determination of the field-saturated hydraulic conductivity (Kfs) of soils under near-saturated conditions in the vadose zone (Meiers, 2002). Near-saturated conditions refer to measurements made over the negative pressure range, -20cm to 0.0cm, where water contents are nearly as high as those for saturation (Reynolds and Elrick, 1991). Tension infiltrometers determine the steady-state infiltration rate into the soil through a porous plate on which a constant negative water pressure (tension) is applied.

As for the Guelph permeameter, the tension infiltrometer is also capable of giving estimates of several useful parameters for characterizing soil structure. Soil Moisture Equipment Corp. manufactures the Guelph Tension Infiltrometer Adapter (Model 2825KI), which is designed to attach directly to the GP reservoir (Figure 6).

Tension infiltrometers are especially useful for quantifying the effects of macropores and preferential flow paths on infiltration in the field. A new method has recently been established for determining the Kfs from the TI method (Reynolds and Elrick, 1991, Ankeny et al. 1991). In this method, sequences of steady infiltration measurements are taken at several tensions on a single infiltration surface are used for calculating the Kfs.

Principles

White et al. (1992) reviewed the use of tension infiltrometers and presented alternative methods of measurement and analysis of the resulting data. Figure 7 presents a schematic description of the tension infiltrometer and its components. All Mariotte-type instruments operate on the same physical principles. The major components of a TI are (i) the bubbling tower, which contains the air-entry tubes that control tension at the soil surface, (ii) the water reservoir, in which the water level falls as water flows into the soil, (iii) the porous base plate, which establishes hydraulic continuity with soil, and (iv) data logger and pressure transducers (optional). Tension (negative pressure) in the air pocket at the top of the water reservoir is linearly related to the height of water in the column (Ankeny, 1992), e.g. a 1mm drop in water level corresponds to a 1mm decrease in tension in the air pocket. Thus, cumulative infiltration can be monitored by recording tension changes over time.

Measurements conducted with the TI require that the negative water pressure in the soil be transferred to the TI through the porous membrane, which requires a good hydraulic contact. A flat surface, free of debris or coarse particles, is favorable for making "good" contact with the TI membrane. In order to achieve these conditions, the soil surface is prepared by removing any occasional fragments (>2 mm) from the site or by locating a 20 cm diameter area free of coarse fragments (Meiers, 2002).

A support ring slightly larger than the TI foot assembly is placed on the ground. The bottom edge of the support ring is inserted about 0.5cm into the ground; this acts to support the contact material so a uniform thickness can be established (Reynolds, 1993). Once the foot assembly is positioned in the testing area and in good contact with the soil, the sides of the foot assembly must be sealed to the ground using a saturated soil paste made onsite with saturated material. When determining Kfs from a tension infiltrometer, the radius of the support ring is used for calculating the area of the infiltrating surface. The applied tension must be corrected when using a contact material. The following correction factor is given by Meiers (2002):

The use of a contact material is required in situations where the surface is irregular or not level. Contact materials should only be used where necessary since they can cause a difference between the pressure head set on the tension infiltrometer membrane and the pressure head at the soil surface (Azevedo et al., 1998). Contact materials shall have an air-entry value greater than the greatest suction being applied and a hydraulic conductivity greater than the material being tested (i.e. contact material is hydraulically "transparent"). Contact material, usually a silicate sand, should have a grain size distribution in the range of 53mm to 105mm (Meiers, 2002).

Poor contact results in poor data. The sand should have a conductivity greater than that of the soil being measured to avoid impeding flow. If too fine a sand is used, conductivities may be underestimated because of the impedance of the contact layer. Coarse sand, however, may not wet fully, which could also lead to underestimation of infiltration rates. If contact material conductivity is greater than soil conductivity, the maximum error in water potential at the contact-soil interface is the thickness of the contact layer itself. Therefore, the thickness of the contact layer should be kept to a practical minimum (Arkeny, 1992).

Summary of Field Procedures

The typical approach to infiltration measurements using the tension infiltrometer is briefly summarized here (adapted from Ankeny, 1992):

1. Soil surface preparation: The surface crust or top 10 or 20m is carefully removed unless the crust itself is being tested.
2. Ring insertion: A sharpened ring is pushed a short distance (~1cm) into the soil to define the area of the infiltration surface and prevent lateral surface flow of ponded water, and cheesecloth is placed in the ring.
3. Filling with contact material: If necessary, contact sand is added to fill the inside of the inserted ring and leveled.
4. TI installation: A Tension infiltrometer is centered over the sand-filled ring, and the legs of the device are pushed into the soil until contact is made with the sand. This step can be adapted when using the TI-kit to adapt to the Guelph permeameter.
5. Measurements: Measurements are typically made from low to high tension.

Note: The air contained in the porous base plate must be removed by submerging the apparatus in water for a period of 24 hours.

A complete step-wise description of the field set up and operating procedures is provided by Ankeny (1992).

The time required to reach steady-state in unconfined infiltration measurements depends on initial soil water content and on hydraulic properties of a given soil. In general, drier soil and lower hydraulic conductivity result in the need for a longer infiltration period in order to reach steady-state infiltration. The change in rate over time should therefore be monitored to confirm that steady rates are reached. Not reaching steady-state results in an overestimate of hydraulic conductivity (Kfs). Typically, Arkeny et al. (1990) suggest collecting data for 1000 seconds at each tension measuring from low to high tension under most condition. This methodology is normally adequate except for very dry and/or high bulk density porous media. In very porous or sandy soil, steady-state rates are reached much faster and times can be shorter. As a practical field guide, if a third of the 25,4 mm-diameter water reservoir has emptied, most likely more than enough water has been added to the soil wetting bulb for the infiltration rate to approach steady-state.

Analysis of Field Data

Wooding's equation for steady-state unconfined (three dimensional) infiltration rates is used in calculation hydraulic conductivities. The Kfs can be calculated by the following equation (Meiers, 2002):

Note: When using equation 12 (i.e. steady-state flow from two applied negative heads), the recommended procedure indicates that the lowest negative head (or lowest tension, e.g. -5cm) is applied first, followed by the greater negative head (e.g. -10cm) (Arkeny, 1992).


Consult the Reference list on Tension Infiltrometers.

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Single-Ring Infiltrometer

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).

Principles

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.

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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Ponded Infiltrometer

The Ponded Infiltrometer is a variant of a single ring infiltrometer. Figure 8 presents a schematic diagram of the ponded infiltrometer. Bouwer (1986) stated that cylinder infiltrometers are typically 0.30m in diameter but that infiltrometers of 1m in diameter or greater should be used to obtain meaningful results. However, driving large cylinders into most soils may disrupt soil macropores and other structural features affecting infiltration. Soil variability necessitates infiltration measurements at many locations to characterize infiltration accurately on a field scale. Because of size and set-up time required for existing cylinder infiltrometers, infiltration measurements at multiple sites are difficult to obtain in a reasonable length of time.

Photo 6 presents a ponded infiltrometer used for testing the permeability of an alluvial cover installed on mine tailings. The ponded infiltrometer is a self-regulating, single-ring infiltrometer which is simple in design and operation and can be used with a variety of water containment rings. The device allows (i) accurate control of ponded water height, (ii) precise measurement of water flow, (iii) direct delivery of water into the containment ring, and (iv) rapid setup and transport.

Obtaining data from these permeameters is a lot easier than with other single-ring devices or with the double ring infiltrometer, although it is a lot more complicated to analyze due to the flow being in three dimensions. When analyzing this data, absorption and capillary forces, which act in all directions, and the geometry of the water source have to be considered (White et al, 1992). When using this device, a good intimate contact between the disc and the soil surface needs to be established, e.g. fine sand. A drawback of using such a material is that it will interfere with the measurements especially in the early stages of infiltration giving inaccurate sorptivity values. Another disadvantage when using the ponded infiltrometer is that if there is a large macropore in the site the water tower may not be able to supply water quick enough, also causing inaccurate results.

Photo 6.Ponded Infiltrometer used for testing the permeability of an alluvial cover installed on mine tailings (courtesy of Robertson GeoConsultants Inc., 2002) .

Principles

The major components of the ponded infiltrometer are a Mariotte reservoir, a valved base, a containment ring and a tripod (see (Figure 8) adapted after Prieksat et al., 1992). Optionally, a datalogger connected to two pressure transducers at the top and base of the water column can be used for automating the water flow measurements. Prieksat et al., (1992) describe the design for an automated, self-regulating ponded (single-ring) infiltrometer. Commonly, the water reservoir and the base are constructed of plastic polycarbonate. A rubber stopper is used to seal the top of the reservoir after filling. Pressure, created by pushing the stopper into the reservoir, starts water flow out of the base when the base valve is opened.

The base consists of a bubble chamber, and bubbling tube, a high-flow air-impermeable nylon membrane, two ports and a two-port valve. The bubble tube regulates the height of water ponded on the soil to +/-1mm. The bubble tube is adjusted up or down within the bubble chamber to raise or lower, respectively, the height of the ponded water in the ring from 0.5 to 1.0cm. This means that the water level in the containment ring can be adjusted without having to raise or lower the entire Mariotte reservoir as is required by previous designs. Because water flow from the device is partly determined by the ponded water height, water heights of < 1.0cm will minimize the size of the water reservoir required to make infiltration measurements.

Two ports connect the Mariotte reservoir to the bubble chamber. The two-port valve is opened during measurement and is closed for movement between sites. The bubble chamber is design to funnel air bubbles up through only one of the two ports. Thus, only water flows through the other port and air bubbles do not limit water flow. Having a second port with unrestricted water flow reduces water-height fluctuations in the containment ring and thus increases measurement precision.

A low-impedence nylon filter covers the bottom of the base, which helps to disperse water flowing from the Mariotte reservoir and limit disturbance of the soil surface. The nylon filter also prevents air from entering the device, except through the bubble tube. The membrane provides a direct link between water ponded in the containment ring and water contained in the Mariotte reservoir without allowing air to enter the system.

Much discussion has occurred about the size of the containment ring that is required to obtain accurate infiltration data. This question remains unanswered, but scaling the dimensions of the device to fit the desired conditions of specific studies will allow the device to be used with a variety of containment ring sizes.

A unit change in water height in the Mariotte reservoir causes a unit change in air pressure above the water (Constantz and Murphy, 1987). Thus, water flow from the reservoir can be calculated from the change in air pressure in the reservoir with time.

Summary of Field Procedures

The typical approach to infiltration measurements using the ponded infiltrometer is briefly summarized here (after Ankeny, 1992):

• Soil surface preparation: The soil surface crust or top 10 or 20mm is carefully removed ub a 150mm diameter area, unless the crust itself is being tested.
• Ring insertion: A sharpened ring is pushed a short distance (~1cm) into the soil to define the area of the infiltration surface and prevent lateral surface flow of ponded water, and prevent disruption of the soil structure. Cheesecloth (or geotextile) is normally placed in the ring to act as a separation medium.
• Measurements: The ponded infiltrometer is set over the ring and ponded measurements are made.

Infiltration can be measured with or without removal of any soil crust. A pointing trowel works well to prepare the surface. If the soil is too wet to avoid smearing, the measurement should wait. The skirted ring is gently pressed into the prepared surface up to the stop ring (the larger diameter outer ring). Next, layers of cheesecloth are placed on the soil surface in the ring to reduce soil slaking into macropores. Initially, the infiltrometer is centered and leveled above the containment ring by adjusting the angle of each tripod legs with the leveling screws. The pointed tripod legs can also be pushed into the ground to stabilize the device. After leveling and centering, the water reservoir and the base are lowered until the base makes contact with the containment ring.

The water reservoir tube and base are then locked in place with the collar lock so that the weight of the infiltrometer is supported by the tripod and not by the containment ring. Using this procedure allows the base and the bubbling tube to be placed at the same relative height above the soil surface each time the device is set up.The water valve can then be open, and water level adjusted prior to starting infiltration test proper.

Analysis of Field Data

During infiltration events, the water enters the soil in response to potential gradients of water potential and gravitational potential. The water potential term is governed by the dryness of the soil and the pore structure of the soil. These two factors combine to form a sorptivity factor which is made up of the combined influences of capillary action and adhesive forces to soil solid surfaces. The sorptivity of the soil is often expressed as "S". The gravity term is a constant for different soils and is due to the impact the pore size, continuity and distribution on the rate of water flow through soil under the influence of gravity. This term is known as "A".

Infiltrometer tests are useful for measuring the rate of infiltration but do not provide a direct measure of field-saturated hydraulic conductivity. Since entrapped air exists within the wetting front, true saturated conditions do not form during infiltration tests. Experience indicates that field saturated Kfs is approximately 50-75% less than Ks (Reynolds and Elrick, 1986).

The initial water infiltration rate is largely governed by the sorptive forces of the dry soil, this is then replaced once the soil wets up by the gravitational term. Equations describing infiltration include the Green et al. empirical model (described by Bouwer, 1986):

Consult the Reference list on Ponded Infiltrometers.

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Double-Ring Infiltrometer

This test method describes a procedure for field measurement of the infiltration rate of water into soils. A detailed description of the double-ring infiltrometer test method is provided in ASTM standard D3385-94. This test method is particularly applicable to relatively uniform fine-grained soils, with an absence of very plastic (fat) clays and gravel-size particles and with moderate to low resistance to ring penetration. This test method may be conducted at the ground surface or at given depths in pits, and on bare soil or with vegetation in place, depending on the conditions for which infiltration rates are desired. However, this test method cannot be conducted where the test surface is below the groundwater table or perched water table.

The double ring infiltrometer is a way of measuring saturated hydraulic conductivity of the surface layer, and consists of an inner and outer ring inserted into the ground. Each ring is supplied with a constant head of water from a Mariotte bottle. Hydraulic conductivity can be estimated for the topsoil when the water flow rate in the inner ring is constant.

Having the two rings eliminates the problem of overestimating the hydraulic conductivity in the field due to 3-D flow. The outer ring supplies water, which contributes to lateral flow so that the inner ring is contributing only to downward flow.

Water moves from the Mariotte bottles into the rings via a tap at the base of the vessells until the height equals that of the base of the bubble tube. When water moves into the soil, reducing the height of ponded water to below that of the bubble tube, more water is fed into the ring.

Some draw-backs of the double ring are that it is very time consuming, requiring trial and error when adjusting the bubble tubes to get the water levels in each ring equal. The practicality of the instrument is reduced by the fact that the rings are extremely heavy to move. It also requires a flat undisturbed surface, which sometimes is not available. During the experiment it is sometimes necessary to refill the Mariotte bottles. To do this, the tap has to be turned off and this disrupts the experiment.

This test method is difficult to use or the resultant data may be unreliable in pervious or impervious soils (i.e. soils with hydraulic conductivity > 10-2cm/s or < 10-6cm/s) or in dry or stiff soils that most likely will fracture when the rings are installed.

Principles

The double-ring infiltrometer method consists of driving two open cylinders, one inside the other, into the ground, partially filling the rings with water, and maintaining the liquid at constant level. The volume of water added to the inner ring, to maintain the water level constant is the measure of the volume of water that infiltrates the soil. The volume infiltrated during timed intervals is converted to an incremental infiltration velocity, usually cm/hour and plotted versus elapsed time. The maximum steady-state or average incremental infiltration velocity, depending on the purpose/application of the test is equivalent to the infiltration rate.

The underlying principles and method of operation of the double ring infiltrometer are similar to the single ring infiltrometer, with the exception that an outer ring is included to ensure that one-dimensional downward flow exists within the tested horizon of the inner ring. Water that infiltrates through the outer ring acts as barrier to lateral movement of water from the inner ring. Double-ring infiltrometers may be either open to the atmosphere, or most commonly, the inner ring may be covered to prevent evaporation. For open double ring infiltrometers the flow rate is measured directly from the rate of decline of the water level within the inner ring for falling head tests, or from the rate of water input necessary to maintain a stable head within the inner ring for the constant head case. For sealed double ring infiltrometers (see below), the flow rate is measured by weighing a sealed flexible bag that is used as the supple reservoir for the inner ring.

ASTM method D 5093 - 90 describes an alternative double-ring infiltrometer method using a sealed inner ring for field measurement of infiltration rate through soils. Briefly, the infiltrometer consists of an open outer and a sealed inner ring. The rings are embedded and sealed in trenches excavated in the soil. Both rings are filled with water such that the inner ring is submerged.

The rate of flow is measured by connecting a flexible bag filled with a known weight of water to a port on the inner ring. As water infiltrates into the ground from the inner ring, an equal amount of water flows into the inner ring from the flexible bag. After a known interval of time, the flexible bag is removed and weighed. The weight loss, converted to volume, is equal to the amount of water that has infiltrated into the ground. An infiltration rate is then determined from this volume of water, the area of the inner ring, and the interval of time. This process is repeated and a plot of infiltration rate versus time is constructed. The test is continued until the infiltration rate becomes steady or until it becomes equal to or less than a specified value.

The following discussion focuses on standard double-ring infiltrometer method (i.e. sealed-inner ring is not covered).

Summary of Field Procedures

After a test site has been selected and the soil surface has been prepared, the outer ring is driven into the soil using a driving cap on top of which a wood block can be used to absorb the blow from a sledge hammer. The outer ring is inserted by moving the wood block around the edge of the driving cap. The ring is inserted to a depth that will (a) prevent the test water from leaking to the ground surface surrounding the ring, and (b) be deeper than the depth to which the inner ring will be driven. A depth of about 15cm is usually adequate.

Once the outer ring is in place, the inner ring can be centered inside the large ring and driven to a depth that will prevent leakage of water to the ground surface surrounding the ring. A depth of about 5-10 cm is usually adequate. Both the outer and the inner rings should be level. The soil surrounding the wall of the ring (s) should be exempt of excessive disturbance. In case extensive cracking or heave are observed, the ring (s) should be reset to a different location using a technique that will minimize such disturbance.

There are three ways to maintain a constant head (water level) within the inner ring and annular space between the two rings: manually controlling the flow of liquid, the use of constant-level float valves, or the use of a Mariotte tube. The latter option is the preferred one since it auto-regulates water flow to the ring. A pair or water bottles is used to fill both rings with water to the same desired depth in each ring. The water flow from the Mariotte tube can then be initiated. As soon as the fluid level becomes constant, the water level in the inner ring and in the annular space is measured (and recorded) to the nearest 2 mm using a ruler or a tape measure. The water level is maintained at a selected head (level) in both the inner ring and annular space between rings throughout the test to prevent flow of water from one ring to the other.

The volume of water that is added to maintain a constant head in the inner ring and annular space during each timing interval is determined by measuring the change in elevation of the water level in the appropriate graduated Mariotte tube. For average soils, the volume of water used to maintain the head is recorded at every 15 min intervals for the first hour, 30 min for the second hour, and 60 min during the remainder of a period of at least 6 hours, or until a relatively constant infiltration rate is achieved. The appropriate reading frequency may be determined only through experience and may be more frequent for high-K materials.

Analysis of Field Data

As with the single ring infiltrometers the wetting front is allowed to advance below the bottom of the ring, but it is assumed that infiltration through the outer ring functions as an effective barrier to lateral flow beneath the ring. However, the accuracy of this assumption may be limited.

The volume of water used during each measured time interval is converted into an incremental infiltration velocity for both the inner ring and annular space using the following equations:

For the inner ring, calculate as follows:

The infiltration rate calculated with the inner ring should be the value used for results if the rates for the inner ring and annular space differ. The difference in rates is due to divergent flow.


Consult the Reference list on Double-Ring Infiltrometers.

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