Hydraulic conductivity laboratory tests

Atterberg-limit tests determine the water content influence in defining liquid, plastic, semisolid and solid states of fine-grained soils. Permeability tests may be carried out in the laboratory or in the field. Such tests are used to determine the hydraulic conductivity coefficient k. ... 

This section discusses soil liners and their use in hazardous waste landfills. The section focuses primarily on hydraulic conductivity testing, both in the laboratory and in the field. It also covers materials used to construct soil liners, mechanisms of contaminant transport through soil liners, and the effects of chemicals and waste leachates on compacted soil liners. 

FIGURE 26.7 Hydraulic conductivity as a function of PI for soils in Austin Laboratory Tests. (Adapted from U.S. F.PA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)... 

The hydraulic conductivity20 of a soil liner is the key design parameter. The important variables in hydraulic conductivity testing in the laboratory are5... 

For a laboratory test on this soil, the test specimen would need to measure about 3 in. in diameter and 3 in. in height. Finding a 3-in. diameter sample representative of this large mass of soil presents a challenge, since small samples from larger quantities of material inevitably vary in hydraulic conductivity. 

The tests were replicated under controlled conditions using soil collected from the liner in thin-walled 3-in. diameter sample tubes. The laboratory measures of hydraulic conductivity were consistently 1 x 10 9 cm/s, five orders of magnitude lower than the field value of 1 x 10-4 cm/s. The laboratory tests yielded a hydraulic conductivity 100,000 times different from that from the field test. Apparently, the flow through the 3-in. specimens did not mimic the flow on a larger scale... 

The hydraulic conductivity obtained in a laboratory test can also be affected by the amount of gas present in the soil. Dry soils are less permeable than wet soils. A dry soil is primarily filled with air. Because invading water does not flow through air-filled voids, but flows only through water-filled voids, the dryness of a soil tends to lower the permeability. 

Some engineers believe that hydraulic conductivity tests on compacted clay soil should be performed on fully saturated soils in an attempt to measure the highest possible hydraulic conductivity. Most, if not all, of the gas can be eliminated from laboratory hydraulic conductivity tests by backpressure saturation of the soil. This technique pressurizes the water inside the soil, compressing the gas and dissolving it in the water. Increasing the backpressure will increase the degree of water saturation and reduce the amount of air, thereby increasing hydraulic conductivity.5,21... 

One implication of these experiments for laboratory hydraulic conductivity testing is that conductivity values can vary remarkably depending on the confining stress. It is essential that the confining stress used in a laboratory test be of the same magnitude as the stress in the field. 

When conducting laboratory hydraulic conductivity tests, two criteria should be met before testing is terminated. First, the rate of inflow should be within 10% of the rate of outflow. Measuring both the rate of inflow and the rate of outflow is necessary to detect problems such as a leak in the system or evaporation from one of the reservoirs. Second, a plot of hydraulic conductivity versus time or pore volume of flow should essentially level off, indicating that hydraulic conductivity is steady. 

There is a radical variation in the reliability of field tests versus laboratory tests. In the Houston test pad discussed earlier, the real value for hydraulic conductivity in the field was 1 x 10 4 cm/s, while the lab value was 1 x 10 9 cm/s, a 100,000-fold difference in the values. 

Laboratory tests have achieved hydraulic conductivities of less than 4 x 10 ° cm/sec in soils. 

Table I. Hydraulic conductivity of the 14-40 SMZ measured in the laboratory and after installation in the pilot-test tank.

Compaction of SMZ under the loading conditions of a permeable barrier is a potential problem. Since the hydraulic conductivity of SMZ can be tailored by varying the particle size, SMZ with a laboratory conductivity significantly greater than that of the aquifer material should be used in permeable barriers. Based upon the pilot-test data collected, it appears that contaminant retention by SMZ in a permeable barrier can be well-predicted from laboratory sorption measurements. 

The permeability coefficient (k) has the units of velocity, that is, distance/time. It is determined either in laboratory experiments or derived from pumping tests. Both methods are semiquantitative, but are still highly informative, as the values observed for common rocks span more than seven orders of magnitude. A variety of units are in use—m/day being a common one. The following are a few of the average permeability or hydraulic conductivity values floating around in the literature, expressed in m/day ... 

To determine groundwater flow at a site, hydraulic conductivity must be measured, either in a permeameter test in a laboratory or in situ. For the permeameter, or column, test (Fig. 3-6), aquifer material is placed in a labo-... 

The sample used in the laboratory permeability test had the same characteristics as the phosphogypsum in the piles. It was formed by settling a phosphogypsum slurry, followed by compaction, which is the same way the stockpiles are created. The permeability figure that resulted from the tests was the saturated hydraulic conductivity since the sample was completely immersed in a column of water during the test. However, phosphogypsum in the piles is not saturated with water but contains an average of about 15% moisture. Permeability decreases drastically with decreases in moisture content because of the loss of water head. The unsaturated hydraulic conductivity would be 10% or less of the saturated value I/O], or approximately 0.024 cm/h (2.0 m/year). 

The network model is developed for estimating the hydraulic conductivity and soil-water retention taking into account the spatial differences of the pore size. The model is checked with published laboratory tests. It is clear that the model demonstrates the hysteresis between the wetting and drying, and shows good similarities of the relative hydraulic conductivity with the laboratory values. 

Hydraulic tests were conducted at 356-m depth on a single fracture intersecting a 500-m deep borehole in crystalline rock at the Rock Mechanics Laboratory of Lulea University of Technology in Sweden. Three types of hydraulic tests were included (i) pulse test. 

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