Specific Yield--Column drainage and centrifuge moisture content

The specific yield of a rock or soil, with respect to water, is the ratio of (1) the volume of water which, after being saturated, it will yield by gravity to (2) its own volume. Specific retention represents the water retained against gravity drainage. The specific yield and retention when added together are equal to the total interconnected porosity of the rock or soil. Because specific retention is more easily determined than specific yield, most methods for obtaining yield first require the determination of specific retention. Recognizing the great need for developing improved methods of determining the specific yield of water-bearing materials, the U.S. Geological Survey and the California Department of Water Resources initiated a cooperative investigation of this subject. The major objectives of this research are (1) to review pertinent literature on specific yield and related subjects, (2) to increase basic knowledge of specific yield and rate of drainage and to determine the most practical methods of obtaining them, (3) to compare and to attempt to correlate the principal laboratory and field methods now commonly used to obtain specific yield, and (4) to obtain improved estimates of specific yield of water-bearing deposits in California. An open-file report, "Specific yield of porous media, an annotated bibliography," by A. I. Johnson, D. A. Morris, and R. C. Prill, was released in 1960 in partial fulfillment of the first objective. This report describes the second phase of the specific-yield study by the U.S. Geoligical Survey Hydrologic Laboratory at Denver, Colo. Laboratory research on column drainage and centrifuge moisture equivalent, two methods for estimating specific retention of porous media, is summarized. In the column-drainage study, a wide variety of materials was packed into plastic columns of 1to 8-inch diameter, wetted with Denver tap water, and drained under controlled conditions of temperature and humidity. The effects of cleaning the porous media; of different column diameters; of dye and time on drainage; and of different methods of drainage, wetting, and packir? were all determined. To insure repeatability of porosity in dupliacte columns, a mechanical technique of packing was developed. In the centrifuge moisture-content study, the centrifuge moisture-equivalent (the moisture content retained by a soil that has been first saturated and then subjected to a force equal to 1,000 times the force of gravity for 1 hour) test was first reviewed and evaluated. It was determined that for reproducible moisture-retention results the temperature and humidity should be controlled by use of a controlled-temperature centrifuge. In addition to refining this Al A2 HYDROLOGIC PROPERTIES OF EARTH MATERIALS standard test, the study determined the effect of length of period of centrifuging and of applied tension on the drainage results. The plans for future work require the continuation of the laboratory standardization study with emphasis on investigation of soil-moisture tension and unsaturated-permeability techniques. A detailed study in the field then will be followed by correlation and evaluation of laboratory and field methods. INTRODUCTION The specific yield of a rock, or soil, with respect to water, is the ratio of (1) the volume of water which, after being saturated, it will yield by gravity to (2) its own volume (Meinzer, 1923, p. 28). Specific retention represents the water retained against gravity drainage. The specific yield and retention when added together are equal to the total interconnected porosity of a rock or soil. That is, any isolated openings that do not take part in the draining and refilling of a body of rock or soil are not counted as a part of the total porosity as used here. Gravity ground water is available free water, or the water that will be. withdrawn from a given body of rock or soil in the zone of saturation by the direct action of gravity if the water table and capillary fringe move downward to a new position that is below the top of the given body. The distinction between gravity water and retained water is not entirely definite because the amount of water that will drain out depends on the length of time it is allowed to drain, on the temperature and mineral composition of the water (which affect its surface tension, viscosity, and density), and on various physical relations of the body of rock or soil. Because specific retention is more easily determined than is specific yield, most methods for obtaining specific yield first require the determination of specific retention. As indicated above, specific yield may then be found by subtracting the specific retention from the total porosity. OBJECTIVES AND PROPOSED SCOPE OF BESEABCH PBOGEAM Recognizing the great need for developing improved methods of determining or estimating specific yield of water-bearing materials, the U.S. Geological Survey and the California Department of Water Resources began a cooperative investigation of this subject in July 1957. The major objectives of this research program are as follows: 1. To review pertinent literature and to prepare an annotated bibliography on specific yield and related subjects. (See Johnson and others, 1960.) '2. To increase basic knowledge of specific yield and rate of drainage and to determine the most practical methods of obtaining them for general application.