Infiltrometer Tests
by: Sebastien Fortin, E.I.T., M.Sc.
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.
Forward to Other Methods.
Return to Infiltrometer Methods.
|
|
/ Surface Water
