Specific yield - laboratory experiments showing the effect of time on column drainage

The increasing use of ground water from many major aquifers in the United States has required a more thorough understanding of gravity drainage, or specific yield. This report describes one phase of specific yield research by the U.S. Geological Survey's Hydrologic Laboratory in cooperation with the California Department of Water Resources. An earlier phase of the research concentrated on the final distribution of moisture retained after drainage of saturated columns of porous media. This report presents the phase that concentrated on the distribution of moisture retained in similar columns after drainage for various periods of time. Five columns, about 4 cm in diameter by 170 cm long, were packed with homogenous sand of very fine, medium, and coarse sizes, and one column was packed with alternating layers of coarse and medium sand. The very fine materials were more uniform in size range than were the medium materials. As the saturated columns drained, tensiometers installed throughout the length recorded changes in moisture tension. The relation of tension to moisture content, determined for each of the materials, was then used to convert the tension readings to moisture content. Data were then available on the distribution of retained moisture for different periods of drainage from 1 to 148 hours. Data also are presented on the final distribution of moisture content by weight and volume and on the degree of saturation. The final zone of capillary saturation was approximately 12 cm for coarse sand, 13 cm for medium sand, and 52 cm for very fine sand. The data showed these zones were 92 to 100 percent saturated. Most of the outflow from the columns occurred in the earlier hours of drainage 90 percent in 1 hour for the coarse materials, 50 percent for the medium, and 60 percent for the very fine. Although the largest percentage of the specific yield was reached during the early hours of drainage, this study amply demonstrates that a very long time would be required to reach drainage equilibrium. In the layered columns the middle (medium sand) layer functioned as a hanging water column accelerating the drainage of the overlying coarse-sand layer. After the middle layer started to drain, the moisture distribution as retained in all three layers showed trends similar to that obtained when the same materials were tested in homogenous columns. Bl B2 HYDROLOGIC PROPERTIES OF EARTH MATERIALS INTRODUCTION The increasing use of ground water from many major aquifers in the United States has required a more thorough understanding of gravity drainage, or specific yield. Recognizing this need, the California Department of Water Resources and the U.S. Geological Survey began cooperative studies to identify and determine accurately the parameters that are related to and affect specific yield and to perfect more accurate and reliable techniques for laboratory and field determinations of specific yield. The specific yield of a rock or soil has been defined by Meinzer (1923, p. 28) as the ratio of (1) the volume of water which, after being saturated, the rock or soil will yield by gravity to (2) its own volume. Specific retention represents the water retained against gravity drainage and when added to the specific yield will equal the total interconnected porosity of the rock and soil. Both specific yield and specific retention are usually expressed as a percentage. SCOPE OF RESEARCH As the first phase of this study, an annotated bibliography on specific yield and related properties was prepared and released as an open-file report (Johnson, Morris, and Prill, 1961). To the second phase of the research project namely, to investigate laboratory methods for determining specific yield a detailed study was made of the centrifuge-moisture-equivalent and column-drainage methods (Johnson, Prill, and Morris, 1963). Among other results this -study showed that the effect of temperature on the centrifuge moisture equivalent was of sufficient magnitude to warrant establishment of a standard temperature for the test (Prill and Johnson, 1959). Also a comparison was made between the centrifuge and the columndrainage techniques (Prill, 1961), and a standard method was developed for packing columns or porous media (Morris and Kulp, 1961). The column-drainage study concentrated on the final distribution of water retained after drainage of saturated columns of porous media. Another step in the laboratory phases of the research has been a detailed study of moisture tension and centrifuge scale modeling, and reports on these investigations are in preparation. A third and final phase of the research project involves the detailed study of gravity drainage (specific yield) at a number of field sites in California and the evaluation of, and eventual correlation between, laboratory and field methods. The field phases also include detailed studies of the nuclear moisture meter as applied to the determination of changes in moisture content as drainage progresses. SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B3 PURPOSE AND SCOPE OF THIS REPORT This report, which is the second report of the laboratory phase of the research, presents the results of a study of laboratory drainage of columns continued beyond that summarized in the earlier report on column drainage (Johnson, Prill, and Morris, 1963). The scope of this report concentrates on the distribution of water retained in columns of three different sand-size materials after drainage for various periods of time up to approximately 6 days. Both layered and nonlayered conditions are included in this study. All laboratory work was done in the Hydrologic Laboratory, U.S. Geological Survey, Denver, Colo. The report was prepared under the general supervision of H. D. Wilson and Fred Kunkel, successive district engineer and district geologist of the Ground Water Branch, U.S. Geological Survey, Sacramento, Calif. , and under the direct supervision of A. I. Johnson, chief of the Hydrologic Laboratory. The following personnel of the Hydrologic Laboratory assisted with the various laboratory experiments: R. P. Moston, A. H. Ludwig, W. K. Kulp,andN.N. Yabe. ACKNOWLEDGMENTS Valuable technical advice was provided by J. F. Poland, research geologist, U.S. Geological Survey in Sacramento, Calif. , and V. C. Fishel, district engineer, Ground Water Branch, Lawrence, Kans. , who provided useful ideas on the construction of drainage columns. Many individuals at university and private research laboratories contributed to this report by giving advice and furnishing information relative to the problem. Especially valuable assistance was provided by Mr. R. T. Bean, supervising engineering geologist, and others on the staff of the California Department of Water Resources, Sacramento, Calif. ; also Prof. A. T. Corey, Colorado State University, Fort Collins, Colo. REVIEW OF PREVIOUS WORK Most methods for obtaining specific yield first require the determination of specific retention. Methods using the drainage of saturated columns of porous media almost universally rely upon the determination of retained moisture. As indicated earlier, the specific yield then may be found by subtracting the specific retention from the total porosity. During the past 70 years the drainage of columns of porous media has been the subject of many research studies, but very few of these studies have determined the moisture retained in the media after different periods of drainage. A few of these studies are summarized below. B4 HYDROLOGIC PROPERTIES OF EARTH MATERIALS Hazen (1892) presented significant results of tests of the waterretaining and water-yielding capacities of eight different sands used for filtering sewage. These tests related specific yield and specific retention to the effective size and uniformity coefficient as obtained from particle-size analyses. King (1899) packed five sorted sands of different particle sizes in columns 8 feet in length and 5 inches in diameter. The materials were slowly saturated from below and then drained for 2i/£ years. Outflow readings were taken during the period of drainage and the moisture content was then determined at the end of the drainage period. The apparatus was designed to prevent moisture loss by evaporation. The results of King's tests showed some discrepancy between the porosity and the total water content and gave lower values for specific retention than Hazen's tests, especially for the coarser samples. Lebedeff (1927) drained tubes, 4 to 5 cm in diameter and 1 to 3 m in length, filled with sandy soils. A head of water of 2 cm was used to saturate the soil. After drainage, the water retained in 10-cm segments of the column was determined gravimetrically and showed that a uniform moisture content prevailed in the upper part of the columns. The author also saturated 6 pairs of sand tubes 10, 20, 30, 40, 50 and 100 cm high. One tube of each pair was immediately sampled for moisture content. The other tube was allowed to drain until drainage ceased and then sampled to determine the moisture-content distribution throughout the tube. In the longest column, after drainage for 3i£ days, the moisture content became constant at a height of about 40 cm. Changing the height of column did not change this distribution; it gave only a longer section of column with constant moisture content. Saturated columns with layers of sand and loess also were drained. More water was retained in the fine-textured material when underlain by coarse-textured material than when underlain by material of the same type. Stearns, Robinson, and Taylor (1930) collected undisturbed columns of soil in metal cylinders of 18-inch diameter and 36-inch length in the Mokelumne area of California. A bottom was soldered on each cylinder after it was collected, and small-diameter observation wells were installed in each soil column. A water table was established at a high stage in the column. Measured volumes of water were then withdrawn and adde