Soil Permeability Testing
Permeability of Soil, continued
Soil permeability can be estimated using empirical methods like soil survey mapping, soil texture, or particle size distribution. However, a variety of different laboratory and field test methods make it just as easy to measure these properties directly. The soil type and purpose of the test, accuracy required, and specimen type influence the selected test method.
Materials with a high permeability coefficient will allow fluids to move rapidly through them while those with a low permeability will not. The measuring of a material's permeability is a key design characteristic for all earthen structures, such as structural foundations; embankments; earthen dams; flood management, and effluent infiltration.
While testing permeability, it is important to make sure all specified conditions are met. The discharged water must be under laminar flow conditions and move under a unit hydraulic gradient at standard temperature conditions (20°C). Additionally, consistency of pressure is important to maintain. Certain control products are useful in regulating the testing environment.
Factors that Affect Soil Permeability
The structure of the voids between grains of soil create an easy path for the movement of water, however, other factors such as hydraulic gradient, soil type, texture, and particle size distribution also affect permeability. Darcy’s law, which dictates all results from soil permeability tests, is an equation describing the movement of fluids through a porous medium. This equation defines the coefficient of permeability or hydraulic conductivity of soils, as a ratio of fluid velocity through the soil matrix to the hydraulic gradient.
Coefficient of Permeability
The coefficient of permeability (k) is the velocity of water in meters or centimeters per second as it passes through soils. Fine-grained soils such as clays might have values of around 10-8 meters/sec or lower, where as a sand and gravel formation could be 10-4 meters/sec or higher.
A permeability coefficient is most commonly determined through the use of two main laboratory tests: the constant head permeability test and the falling head permeability test. For highly granular soils such as sands and gravels, the constant head method is best and can yield accurate results even if the sample has been disturbed or reconstituted. The falling head method is typically used for fine-grained soils and is more accurate when testing undisturbed samples. These two methods work in different ways to determine a sample’s permeability coefficient.
The constant head permeability test:
The constant head permeability test utilizes a test apparatus with a reservoir on the top to hold de-aired water and one at the bottom to hold water that has permeated through the soil sample. Because the samples are large-grained, their hydraulic conductivity is rather fast. The coefficient of permeability can be calculated using the pressure measurements and volume of the permeated water during the set time interval, as well as the height and cross-sectional area of the soil sample. To ensure accurate results, repeat the test three or more times and find an average coefficient.
The falling head permeability test:
The falling head permeability test is performed using a standpipe and a relatively small sample size. A small sample size is due to the fine-grained nature of the test samples, which results in slow hydraulic conductivity. The soil sample is first placed in a container and saturated with water. From there, the sample is attached to standpipes filled with a specific level of de-aired water. The permeability coefficient can be calculated when the water in the standpipe has reached a previously determined level. The calculation takes into account the size of the sample, the cross-sectional area of the standpipe, as well as the time it took to change the water level.
Soil Permeability Testing Equipment:
Permeability Cells (Flexible-Wall):
Humboldt's HM-4188B Permeability Cells and our HM-4199B Triaxial Cells can be used to measure hydraulic conductivity (permeability) of soils using several methods as described in ASTM D5084. These methods outline several variations on the constant and falling-head permeability methods, including tests for the constant rate of flow and tests measuring constant volume with controlled pressures. Undisturbed material from Shelby tubes as well as loose material compacted into a mold at a specified density can be used in these tests. Once the sample has been obtained it is encased in a latex membrane and placed inside a fluid-filled, pressurized test cell (either permeability or triaxial cell). These cells use a system of valves and burettes mounted on a distribution panel, like our FlexPanels, which permit three-dimensional control of confining pressures on the sample, as well as the permanent (typically water) used. The monitoring of sample deformation and volume change takes place throughout the procedure. Although this test is standard and widely specified, it requires considerable sample preparation and can take several days to complete.
Automated Pressure Controllers
The HM-5810 is an automated three-channel pressure controller designed to handle saturation, consolidation and permeation of a triaxial test sample. In this application it can be used solely for permeability testing as well, eliminating the need for distribution panels, etc. The HM-5810 is all you need to successfully do permeability testing for soil.
Cell, head and tail pressure can be set in increments of 0.1 PSI while volume change is measured to 0.0001cc. The flow rate for permeation can be set from 1 cc/sec (60 cc/min) down to less than 0.000002 cc/sec (0.00012 cc/min). There are three data input channels – one for each pressure transducer, or if you are only doing permeability, you will only need one of the channels/pressure transducers.
The HM-5810 provides an accurate and simplified permeability setup, which eliminates the need for separate distribution panels, and simplifying tubing and control cable setup. By using the integral staging platform, the HM-5810 provides an extremely compact and organized setup.
FlexPanels - Distribution Panels for Permeability Testing
Humboldt FlexPanels provide a simple and highly efficient distribution system for providing air, water and de-aired water for use in permeability and triaxial testing applications. The FlexPanel’s simple, straight-forward configuration, with its integral burettes provides a condensed/compact design that takes up less counter space than competing systems with air/water bladders.
The long, narrow burette design of Humboldt’s FlexPanels provide faster test processing times when compared to larger, shorter burette systems, while providing the same volume. This is due to the reduced amount of meniscus formation in the narrower burettes, which allows the water level to drop faster, resulting in faster readings. In addition, the use of longer/narrower burettes and a scale graduation of 0.02ml, also provides an easier-to-read and more accurate scale.
FlexPanels also feature a bias regulator and bridge. The bias regulator maintains the differential pressure when confining and back pressures are increased. The bridge delivers simultaneous control of base and top pressures through the use of just one regulator. This feature minimizes operator time and reduces the margin of error in opening and adjusting regulators during a test. The Humboldt FlexPanel system is comprised of 5 separate panel configurations, which can be grouped together to accommodate from 1 to 6 cell setups.
Constant/Falling Head Permeameters & Test Sets
Constant/Falling Head Permeameters allow the option of testing granular soils under either constant- or falling-head conditions but do not meet a current ASTM test standard. However, they do comply with the historic ASTM standard D2434. For use with granular soils in determining the coefficient of permeability via the falling-head method for laminar flow of water. Compact and portable unit includes a spring incorporated into top cap to apply 5-10 lbs. force against screen to prevent soil density changes. End caps and clamping ring are anodized aluminum. Each cell comes complete with valves and perforated screens.
Compaction Permeameters are used for determining permeability of clay, sand and gravel soils. They are available in 4" or 6" (102 or 152mm) Proctor soil moisture/density compaction molds supplied with upper and lower plates fitted with valves and ports to allow the mold to function as a permeameter. Either constant-head or falling-head permeability testing can be performed directly on the Proctor or California Bearing Ratio (CBR) compacted samples molded using standard compaction methods. Porous stones at either end of the samples provide drainage. These permeameters and test methods are covered in ASTM D5856. In use, double-tube manometers are connected to the inlet and outflow ports to measure the flow of the de-aired water through the sample.
Shelby Tube Permeameters
The Shelby tube permeameter allows you to perform permeability tests within a Shelby tube without removing the sample. This test method does not comply with ASTM or AASHTO standards, but it provides constant or falling head testing on minimally disturbed specimens. Ideal for cohesion-less materials and sands. It is suggested that the specimen be ejected at test conclusion and examined for voids or large aggregate, which possibly could affect the test results. Two sets of end caps fit over a Shelby tube liner up to 6" long. End caps each contain valve to control flow of permeants through the specimen, along with a porous stone to prevent material from flowing into and clogging the valve. End caps are anodized aluminum. Includes o-rings, connecting rods, clamping knobs, and tubing.
Shelby Tube Permeameters are kits of components to build a permeameter around a section of an undisturbed sample retained in a 3" (76mm) Shelby tube. The specimen does not need to be extruded, ensuring minimal disturbance to silty or sandy materials or bedding structures of sensitive soils. ASTM or AASHTO documents do not cover the test method, but it allows constant or falling head testing on minimally disturbed specimens. A section of the tube is cut to a maximum length of 6in (152mm) using a large pipe cutter, or band saw. End caps have ports for water inflow and egress positioned at each end of the sample for connection to double-tube manometers. Threaded rods secure the permeameter assembly.