Specific YieldLaboratory Experiments 

Showing the Effect of 

Time on Column Drainage

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1662-B

Prepared in cooperation with the 

California Department of ff^ater 

Resources
Specific YieldLaboratory Experiments 

Showing the Effect of 

Time on Column Drainage

By R. C. PRILL, A. I. JOHNSON, and D. A. MORRIS

HYDROLOGIC PROPERTIES OF EARTH MATERIALS

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1662-B

Prepared in cooperation with the 

California Department of Heater 

Resources

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

First printing 1965 

Second printing 1967

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D .C. 20402 - Price 25 cents
CONTENTS

Page

Abstract._____________________________________________________ Bl

Introduction._ ____________________________________________________ 2

Scope of research._____________________________________________ 2

Purpose and scope of this report______________________________ 3

Acknowledgments. ____________________________________________ 3

Review of previous work- ______________________________________ 3

Theory........______________________________________________ 8

Techniques._______________________________________________________ 12

Materials____________________________________________________ 12

Apparatus and controls_.._______-_____________________________- 12

Procedure.____________________________________________________ 15

Discussion of test results___--_-_______-___-___-_______-_-_-_______- 18

Drainage of 0.120-millimeter glass beads________________________ 19

Drainage of Fresno medium sand_._..____________________________ 26

Drainage of 20-mesh Del Monte sand.......________-_________.___ 34

Drainage of layered 20-mesh Del Monte sand and Fresno medium

sand _______________________________________________________ 42

Summary______________________________________________________ 51

References_ _____________________________________________________ 54

ILLUSTRATIONS

Page 

FIGURE 1. Graph showing relationship of tension to height above water

table for a hypothetical sand____________________________ Bll

2. Particle-size distribution curves..____-____-__--_-_-__--_-_- 13

3. Drawing of assembly for segmented column with ring-type

tensiometers--..--__.._________________-_--________--- 14

4. Photograph of tensiometer and manometer system for drainage

columns ____--_-___________________________-_--_------ 16

5-31. Graphs showing 

5. Tension distribution during drainage of 0.120-millimeter glass beads--.-....--_-----_----------__- 20

6. Distribution of dry unit weight and porosity after

draining a column of 0.120-millimeter glass beads. . 22

7 Moisture retained after drainage of 0.120-millimeter

glass beads.__________________--_-_----__----- 23

8. Relation of moisture content to tension for 0.120-millimeter glass beads___...-.-.--_-_-_---_-_------- 24

9. Moisture distribution during drainage of 0.120- 

millimeter glass beads________________----- 25

10. Quantity of water drained from a column of 0.120- 

millimeter glass beads_____._-__-_____--------- 26

11. Tension distribution during drainage of Fresno

medium sand (column l)__________-____-------- 28

m
IV CONTENTS

FIGURE 5-31. Graphs showing Continued Page

12. Tension distribution during drainage of Fresno

medium sand (column 2)_______________________ B29

13. Distribution of dry unit weight and porosity after

draining columns of Fresno medium sand...-__-__ 30

14. Moisture retained after drainage of Fresno medium

sand-_.-----_-__-____---__-__--------___--____ 31

15. Relation of moisture content to tension for Fresno

medium sand____-____--____-_--_--__--_-_____ 33

16. Moisture distribution during drainage of Fresno

medium sand (column !)_______________________ 34

17. Moisture distribution during drainage of Fresno

medium sand (column 2)_______________________ 35

18. Quantity of water drained from columns of Fresno

medium sand_______-________-_-______________ 36

19. Tension distribution during drainage of 20-mesh Del

Monte sand (column 1).______________________ 37

20. Tension distribution during drainage of 20-mesh Del

Monte sand (column 2)___._____________________ 38

21. Distribution of dry unit weight and porosity after

draining columns of 20-mesh Del Monte sand_____ 39

22. Moisture retained after drainage of 20-mesh Del

Monte sand...---.__________________________ 40

23. Relation of moisture content to tension for 20-mesh

Del Monte sand_-.___-_______-____-__-________ 41

24. Moisture distribution during drainage of 20-mesh

Del Monte sand (column l)__--_-_-_--__-_______ 43

25. Moisture distribution during drainage of 20-mesh

Del Monte sand (column 2)___-.-_-._-_______-__ 44

26. Quantity of water drained from columns of 20-mesh

Del Monte sand as related to time of drainage.--- 44

27. Tension distribution during drainage of a layered 

column of 20-mesh Del Monte sand and Fresno 

medium sand_________________-_-__-.-____-_-__ 45

28. Distribution of dry unit weight and porosity after 

draining a layered column of 20-mesh Del Monte 

sand and Fresno medium sand-_________________ 48

29. Moisture retained after drainage of a layered column 

of 20-mesh Del Monte sand and Fresno medium 

sand__--....-_______-______--_________--_-_--- 49

30. Moisture distribution during drainage of a layered 

column of 20-mesh Del Monte sand and Fresno 

medium sand_________________________________ 50

31. Quantity of water drained from a layered column of

20-mesh Del Monte sand and Fresno medium sand__ 51

TABLES

Page; 

TABLE 1. Distribution of test materials...__-_--_______.________ B13

2. Chemical analysis of water from Denver, Colo___._____._-__ 17
HYDROLOGIC PROPERTIES OF EARTH MATERIALS

SPECIFIC YIELD LABORATORY EXPERIMENTS SHOWING 

THE EFFECT OF TIME ON COLUMN DRAINAGE

By K. C. PRILL, A. I. JOHNSON, and D. A. MORRIS

ABSTRACT

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 added alternately and water levels were observed. The 

time intervals for the water levels to reach equilibrium were very 

short only one was over 50 minutes. Greater specific yields were 

obtained for a rising water table than for a falling water table.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B5

White (1932) collected undisturbed soils in steel cylinders 12 or 18 

inches in diameter and 18, 36, or 54 inches long. With the bottom of 

the cylinders sealed by means of a steel plate, small wells were sunk 

into the material enclosed within the cylinders. In these wells the 

effects on the water level produced by adding or withdrawing measured quantities of water were observed. Equilibrium was not established until 24 hours after the addition of water and about 48 hours 

after its removal. Data indicated the average specific yield for clay 

and clay loam ranged from about 1 to T percent.

Eckis and Gross (1934) packed three 4- by 40-inch cylinders with 

carefully sorted sand and saturated the columns with a measured 

quantity of water. Water was then drained from the columns by 

means of a well attached through a U-tube to the outside bottom of 

the cylinder. The water table was lowered by slowly removing a 

measured quantity of water for 1 month, and a period of 30 to 60 

days was allowed for the new water table to reach equilibrium. The 

difference between the water levels was then used to calculate the specific yield. The columns were allowed to drain for an additional 18 

months, the cylinders were split, and the moisture contents were determined for 1-inch lengths of column. Evaporation effects were found 

at depths as great as 27 inches below the top of the columns. Less 

than 10 percent of the water retained in the column at the end of the 

30- to 60-day period drained out in 18 months.

Columns 18 inches in diameter and at least 42 inches long were used 

by Piper and others (1939) to study specific yield in the Mokelumne 

area in California. The specific yield was determined by measuring 

the volume of material saturated or unwatered when measured quantities of water were added or withdrawn from the columns of undisturbed materials. Water levels were affected by both temperature and 

barometric pressure. No less than 21 days were allowed for a new 

water level to attain equilibrium and some tests continued for as long 

as 220 days. The longer drainage periods increased the specific yield 

by only 1 to 3 percent over that obtained from the short-term drainage. 

For fine materials, specific yield by saturation was about 3 times as 

great as that obtained by unwatering, apparently because the columns 

were too short and were primarily occupied by the capillary fringe. 

Average values of specific yield obtained were: gravel and coarse sand, 

34.8 percent; medium and fine sand, 24.2 percent; very fine sand, silt, 

and clay, 4.2 percent.

Leverett (1941) used pairs of tubes 10 feet long and of 94-inch 

diameter, filled with sand. Sand in one tube of each pair was saturated with water and then allowed to drain for at least 2 weeks whereas 

the initially dry sand in the other tube was fed water as fast as the
B6 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

sand would imbibe it. His depth-moisture content curves are of the 

same general character as those of King (1899), but he found that the 

curves were not the same for the drainage conditions as they were for 

the imbibition conditions a hysteresis zone was formed.

Drainage tests of unconsolidated Wilcox sand packed in pip<; 

columns 8 feet long and 2 or 4 inches in diameter were made by Stahl, 

Martin, and Huntington (1943). The columns were equipped with 

water jackets to maintain temperature control. Holes were drilled 

and plugged at 6-inch intervals vertically so cores of the sand could 

be obtained at any time during drainage. The sand-packed columns 

were saturated with fluid from the bottom, drained for several hours, 

and saturated again, prior to each drainage run. Data relating retained moisture to time of drainage were obtained by removing core 

samples after drainage for given intervals of time. The cavities 

caused by taking the cores were filled with the fresh sand before the 

columns were resaturated and drained for a new interval of time. The 

rate of drainage was found to be proportional to the temperature.

Coleman (1946) packed air-dry clay-loam soil in a brass tube 7 feet 

long and 6 inches in diameter. Holes were drilled in the tube at 6- 

inch vertical intervals to provide access for the installation of tensiometers and to obtain samples for moisture content. The column 

was saturated and drained under >a variety of moisture tensions from 

0 to 160 cm of water. The proper drainage tension for the soil seemed 

to be about 125 cm of water, which was equivalent to 19 percent moisture content, the field capacity of the soil.

Gate wood and others (1950) obtained three undisturbed columns of 

alluvium, each 42 inches in length and 14 inches in diameter. The 

columns were saturated from the bottom for 15 days, then allowed 

to drain, and the decline in water level was observed for about 25 

days. Water level in the 3 columns reached drainage equilibrium at 

the end of 9, 10, and 15 days. The volume of water withdrawn from 

each column was divided by the volume of material unwatered and 

multiplied by 100 to obtain the coefficient of drainage, in percent, for 

the period of drainage.

Lambe (1951) drained 2%-inch saturated soil columns with 

piezometers spaced vertically on the side. They measured and recorded the quantity of outflow and the pressure heads at selected 

intervals in the column. Pressure heads below the visual line of 

saturation were measured by piezometers and it was pointed out that 

the visual line of saturation was not the true line of saturation because 

a considerable amount of drainage occurred after the visual line of 

saturation appeared to have reached its equilibrium position.

Terwillinger and others (1951) packed a clean silica sand, by means 

of mechanical vibration, into a vertical plastic tube 13 feet long and
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B7

2 inches in diameter. To eliminate the possibility of boundary flow 

along the wall of the tubing, the plastic was heated and was forced 

under pressure to conform to the outside surface of the sand. The 

column was saturated with a 0.25 normal sodium chloride solution 

and then allowed to drain. When drainage equilibrium was reached, 

the saturation distribution was obtained by electrical conductivity 

readings for probes spaced at 5-cm intervals down the length of the 

column.

By the use of tensiometers, Luthin and Miller (1953) measured the 

volume of outflow and recorded the pressure distribution in soil 

columns, about 8 cm in diameter and 120 cm in length, during drainage. Null-type tensiometers were installed at intervals in the column 

to determine moisture distribution during drainage of the column. 

As drainage proceeded, the upper part of the soil column became unsaturated, the water draining until the capillary forces resisting the 

downward movement of water were sufficient to neutralize the downward forces. The hydraulic gradient approached zero in the lower 

part of the soil column and the hydraulic conductivity remained high. 

Water continued to move out of the upper zone under the driving 

action of a high hydraulic gradient, but movement was slow because 

of a reduced hydraulic conductivity. Static equilibrium was reached 

when the hydraulic gradient was zero throughout the column. At 

this time the soil columns were separated into sections, the moisture 

content was determined, and the percentage of pore saturation was 

calculated.

Day and Luthin (1956) did their experimental drainage work with 

a column of Oso Flaco fine sand, about 87 cm high and 7 cm in diameter. 

Tensiometers were installed at heights of about 5, 20, 36, 51, 65 and 

83 cm from the bottom of the column. The column was saturated, 

and then drained while the tension and outflow volume measurements 

were made at selected times. The data were then used to test a numerical solution of a differential equation for vertical flow.

Marx (1956) used a continuous sandstone cylinder, 154 cm long and 

5 cm in diameter, with its lateral surfaces sealed in plastic tubing. 

The volume of water draining from the saturated core was obtained 

by weighing. The data obtained from the column drainage were 

compared with drainage data obtained from centrifuging small 

samples of the same sandstone. The correlation between the two 

methods seemed to be neither more nor less precise than the sampling 

technique itself.

Johnson, Prill, and Morris (1963), made a preliminary study of 

column drainage in which factors such as cleaning of media, method 

of drainage, column diameter, method of wetting, methods of packing,
B8 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

and length of drainage period were evaluated. They found that 

cleaning with acid only slightly affected the drainage characteristics 

for glass beads; that the diameter of the column made little difference 

in the moisture distribution after drainage of mechanically packed 

1-, 4-, and 8-inch-diameter columns of 0.120-mm (millimeter) glass 

beads; that different procedures for wetting the porous media gave 

similar results for the distribution of water after drainage and for the 

rate of discharge; and that a mechanical method of packing produced 

repeatability of moisture contents.

A theoretical study by Smith (1961) showed that as the meniscus 

of maximum capillary rise falls through the sand the meniscus enclosing the water bodies retained in draining sand columns is detached 

from it. The initial drainage under a maximum capillary-rise meniscus accounts for the bulk of the drained water and it is also shown that 

the data of Hazen (1892) and King (1899) are in good agreement with 

the developed theory.

The preceding review of the literature shows that in the 60 years or 

more since the classic experiments of Hazen (1892) and King (1899), 

relatively little experimental work has been done by hydrologists to 

determine the specific yield of water-bearing sediments by columndrainage techniques. Many of the controlled experiments that have 

been completed, both in the laboratory and in the field, have been made 

by scientists of the agriculture or petroleum-geology disciplines and 

are not completely applicable to problems of hydrology. The review 

also indicates that most of the experimental work has been completed 

on the ultimate drainage, or specific yield, and only a very limited 

amount of the research has been on drainage obtained for different 

periods of time. This lack of information and the importance of 

determining the effect of time on drainage thus provided the impetus 

for the present study.

THEORY

If a column of porous material is saturated with water and is then 

allowed to drain to equilibrium conditions, it will be found that the 

soil column will have retained some water. The amount of water 

retained in the column of porous media will vary with the height above 

the water table and with the texture of the media. For media with 

equal porosities, the smaller the pore spaces the greater will be the 

amount of retained water.

Three types of soil-moisture distribution called by Versluys (1917, 

1931), the pendular, funicular, and capillary stages will be found 

in a column of porous media after it has drained. In the pendular 

zone, in the upper part of the column of media, the water is retained 

in rings around the contact points. Lower down in the funicular zone
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B9

of the column, the rings of water are combined into more complicated 

forms and are characterized by having two or more contacts embedded 

in a single mass of water. Near the bottom of the column there is a 

zone of essentially complete saturation.

Molecular forces tend to arrange the retained water in the pendular 

zone in the form of small rings around the contact points of the solid 

particles. These air- water interfaces are convex toward the contact 

points and must have a curvature that will result in a vapor pressure in 

equilibrium with the surrounding vapor pressure. Presumably, there 

will be a thin film of water covering the whole solid particle, but it 

will not appreciably affect the mass distribution of the water except 

at a very low water content. A thin film or connecting link of water 

must exist between the rings of water if the porous media have capillary and electrical conductivity. The experiments by Buckingham 

(1907) indicated that the soils tested had electrical conductivity for 

water contents as low as 4 percent by weight.

When a solid water-air system is in stable thermodynamic equilibrium, the vapor pressures have definite values throughout the column 

of porous media. In other words, the vapor pressures at any two 

points of equal distance above the water table are equal. According to 

Kelvin's theorem (Smith, 1933), if two similar masses of liquid are 

at equal distances above a free liquid, and if one is in communication 

with, and the other isolated from the free liquid, then not only the 

vapor pressures over each but the respective curvatures of corresponding parts of each surface are equal. If the curvatures of liquid masses 

at the same height above a water table are not equal there will be 

evaporation from one and condensation onto the other until the curvatures become equal. A single isolated liquid mass can therefore be 

considered as if it were in communication with the water table, so 

far as transfer of moisture is concerned.

A curved interface between two liquids or between a liquid and a 

gas indicates that a difference in pressure exists between the two. The 

difference in pressure, pc , between a liquid and air is given by the 

equation

where <r equals surface tension of the liquid and n and r2 are the major 

and minor radii of curvature of the two curves formed by the intersection of two planes at right angles to each other and passing perpendicular through the water surface at the point where it is desired that the 

curvature be known. If both radii have their centers of rotation on 

the same side of the interface in question, both radii have the same 

sign. If the centers of rotation are on opposite sides of the interface,
BIO HYDROLOGIC PROPERTIES OF EARTH MATERIALS

one radius is positive and the other is negative. If we adopt the convention that r2 is negative, the mean curvature of the air-water interface will be given by C1-1)

\ri r2 /

The difference in pressure at the air- water interface in a capillary 

tube is equal to ygh where y is the density of the water, g is the acceleration due to gravity, and h is the height above the water table. According to Kelvin (Smith, 1933) , the difference in pressure on the two 

sides of the air- water interface in the isolated masses of water is also 

equal to ygh, and thus,

We can thus conclude that the curvature of an air-water interface 

is proportional to its height above the water table. Because the 

amount of water at each contact point decreases as the curvature increases, the amount of water retained in a column of porous media 

under equilibrium conditions will decrease with height above the 

water table.

Smith (1933) analyzed the relation of a free-liquid surface, or water 

table, to the distribution of soil moisture in an ideal soil under equilibrium conditions. He concluded that the position of the free liquid 

determines the vapor pressures and curvatures existing in the porous 

media and also determines the types of distribution zones present. 

For example, the free-liquid surface may be so far below the bottom 

of the column of porous media that only the pendular distribution of 

single rings exists. On the other hand, if the water table is sufficiently 

close to the bottom of the column, all three distribution zones pendular, funicular, and saturation will be present. Thus, the length of 

the column of porous media is important : it may be so short that it 

contains only a funicular or saturation zone depending on the relative position of the water table free-liquid surface.

A brief discussion of tension measurements and relationships is 

appropriate at this point. If a column of porous material is in equilibrium with a water table and there is no tendency for the water in the 

media to move either upward or downward, then for each foot of 

increase in elevation above the water table the tension in the water 

should increase by an amount equal to 1 foot of water. This relation 

between the tension and the elevation in the column can be represented 

(fig. 1) by a straight-line extension upward from the water table 

with unit slope that is, 45° if both ordinates are to the same scale. 

Where the plotted tension readings (negative values) for the tensiometers lie on such a line, it is an indication that hydrostatic equilibrium 

has been reached or that the hydraulic gradient in the column of
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE Bll

100

80

60

40

20

40

30 c/5

20

10 I

20 40 60

TENSION, IN CENTIMETERS OF WATER

80 100

FIGURE 1. Graph showing relationship of tension to height above water table for a 

hypothetical sand in general, the greater the tension the lower the moisture content.

material being drained is equal to the total mechanical potential 

the difference in tension between two points, plus the difference in 

height above the water table for the two points divided by the length 

between these points. Line B (fig. 1), having a 1-to-l slope, thus 

represents a hydraulic gradient of zero or a condition of hydrostatic 

equilibrium. There is a rapid reduction in unsaturated permeability 

for sand-size media for a small increase in tension. At the higher 

tensions, flow may become very slow and may be primarily a function 

of vapor transfer. In such instances a large increase in tension may 

correspond to only a very slight decrease in water content.

Although equilibrium drainage is reached when the hydraulic 

gradient is zero, a hydraulic gradient of close to zero may exist from 

the water table to several feet above the water table after a relatively 

short period of drainage of sand-size materials. However, at some 

distance above the water table, as shown in line A on figure 1, the
B12 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

hydraulic gradient may be only slightly less than one and yet all but 

a fraction of a percent of the total potential yield may have been 

discharged from the material. If the zero tension intercept of the 

line B is not zero on the depth scale, the intercept value then indicates 

the elevation of the zero pressure level, or water table, in the soil 

water.

When the hydraulic gradient is positive, the soil water tends to 

move downward, whereas when the hydraulic gradient is zero, the 

soil water is in equilibrium with the water table.

The general principles for operation of a tensiometer are simple. 

If the water content of the soil decreases, water moves into the soil 

through the semipermeable tip of the tensiometer. Water then is 

drawn down in the manometer of the tensiometer system until the 

pressure difference across the semipermeable tip is balanced by the 

pressure difference due to the column of water in the manometer tube. 

In general, the greater the tension in a media, the lower the moisture 

content.

TECHNIQUES

MATERIALS

One of the prerequisites in the selection of a material to be used 

in the investigation of moisture distribution with time of drainage 

was that the material have a capillary rise less than the 170-cm length 

of columns used in the study. This restricted the materials used in 

this study to those of the sand- or gravel-size range: glass beads 

3M "Superbrite," Minnesota Mining Co., St. Paul, Minn. of 

0.120-mm diameter (very fine sand size) ; Del Monte sand DM-20, 

Del Monte Properties Co., Del Monte, Calif. of a 20~mesh size 

(coarse-sand size) ; and Fresno medium sand natural sand of 

medium-sand size collected near Fresno, Calif. figure 2 shows the 

particle-size distribution of these materials.

APPARATUS AND CONTROLS

The drainage columns were formed by packing the appropriate 

media into transparent plastic tubing having an inside diameter of 

2.8 cm and a wall thickness of 0.6 cm. The plastic tubing was cut 

into 2- and i-cm segments which were calibrated for volume. The 

segments, with accurately milled end surfaces, were placed end to 

end and built into leak-proof columns approximately 170 cm in length, 

by the use of a pressure frame (fig. 3). The bottom segment, containing the outflow tap, was filled with a 80-mesh copper screen for 

the 20-mesh Del Monte sand and a 230-mesh copper screen for the 

0.120-mm glass beads and the Fresno medium sand.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B13

100

90

PARTICLE-SIZE DIAMETER, IN MILLIMETERS 

FIGURE 2. Particle-size distribution curves of test materials. (See table 1 for values.)

TABLE 1. Distribution of test materials 

[Curves for these values are shown on fig. 2]

Test material

Glass beads. .___

Fresno sand.

Del Monte sand .

Silt- and 

clay-size, 

< 0.062

4.3

Distribution, in percent, for indicated particle size, in 

millimeters

Very fine 

0.062-0.125

52.2 

5. 5 

. 1

Fine 

0.125-0.25

47.8 

17.3

5

Medium 

0.25-0.5

49.3 

9.6

Coarse 

0.5-1

22.4 

85.0

Very coarse 1-2

1.2

4.8
B14 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

Plastic cylinder 

section

Detail of 

tensiometer ring

Brass 

outlet

Scale: lcm = 4cm

FIGURE 3. Drawing of assembly for segmented column with ring-type tensiometers.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B15

The water level during drainage was maintained at the base of 

the column by adjusting a constant-head overflow tank which was 

connected to the discharge tap.

Tensiometer rings segments of tubing containing a tensiometer 

were placed at appropriate intervals in the column for measuring the 

tension at selected times of drainage. These ring-type tensiometers 

were made by gluing strips of a semipermeable plastic sheet ("Porvic" 

Filter Sheet Grade "M," Prichett and Gold, London), into 1-cm 

grooves machined on the inside of 2-cm long plastic segments (fig. 3). 

The main grooves were machined just deep enough to permit the installation of the "Porvic" flush with the inside wall, but had a deeper 

groove in the center portion to permit ready movement of water to 

the outlet tap. A hole was drilled from the outside of the plastic 

segment to the groove and a brass tap of 0.1 cm inside diameter was 

threaded into the hole from the outside. Flexible plastic tubing (0.15 

cm inside diameter) extended from the tap to the manometers. The 

"Porvic" plastic, while serving as a barrier to the entry of air into 

the manometer system, allowed water to move through the system 

rapidly. Only a short lag thereby occurred between the true tension 

in the media and the tension recorded by the manometer.

The manometer system is shown in part in figure 4. The manometer 

tubes in 4- or 8-foot lengths were made of capillary glass of a uniform 

inside diameter of 1 mm. A water-level change of 128 cm in a manometer would result in a volume change of 1 ml (milliliter). All the 

tensiometers from one column were connected to the same manometer 

board. Millimeter graph paper was placed under the manometers so 

the head in the manometers could be read directly.

The columns were drained in a room where the humidity was maintained at 20 to 45 percent and the temperature was controlled at 20 °C. 

To reduce evaporation, the columns were sealed at the top with cellophane tape through which was a 1-mm hole. In addition, most columns were enclosed within a flexible polyethylene jacket, held together 

by use of a good nylon-tape fastener, extended from a pan of water 

immediately below the column to the top of the column and thereby 

provided an atmosphere having a humidity approximately 10 to 15 

percent higher than normal room humidity.

PROCEDURE 

CONSTRUCTION AND SATURATION OF THE COLUMN

Prior to constructing a column, the tensiometers were purged of 

air by first placing them in water that had been boiled and then 

applying enough vacuum to draw water through the tensiometers and 

into the flexible tubing. The column was constructed with 2- and 4-cm
B16 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

FIGURE 4. Photograph of tensiometer and manometer system for drainage columns.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B17

segments, placing the 2-cm segments containing the tensiometers at 

appropriate intervals. The column assembly in the pressure frame 

was mounted on a vibratory-type packer (Johnson, Prill, and Morris, 

1963) and media added by means of a long "tremie" tube.

Unless otherwise noted, all columns packed with a single medium 

were filled and then vibrated for 10 seconds at a rheostat setting that 

provided an amplitude of 0.09 cm. For columns containing layers of 

different media, the column was vibrated after each layer was added.

Deaired Denver tapwater was used as the wetting fluid. (See 

table 2.) Air was removed from the water by means of a vacuum system especially designed for this purpose. The column was saturated 

from below by slowly raising the head tank. As the water level in the 

column reached the level of each tensiometer, the flexible tubing connected with that tensiometer was attached to the appropriate manometer on the manometer board. Water was allowed to move up each 

manometer to a preselected level based on an estimate of the tension 

that would exist for that manometer after one-half hour of drainage. 

The flexible tubing was then pinched with a hose clamp to prevent 

any further upward movement. The total wetting period for a 

column was approximately 65 hours.

TABLE 2. Chemical analysis of water from Denver, Colo. 

[Analysis by Quality of Water Laboratory, U.S. Geol. Survey, Denver, Colo.]

Constituent

Calcium (Ca)

Magnesium (Mg)

Sulfate (S04) ------------------------------------------------

Chloride (Cl)_.__. .._....._.__..._____. ____ .__ _.-____._ 

Fluoride (F)._______ ______________ _______ ___.________---___.__

Nitrate (NO-0 __________ __ ________ _ __________ -----------

Dissolved solids _

Hardness (as CaC03) ________________ __ __ __ ___ ___ ___ _ _

pH

Concentration 

(ppm)

20

4

3.4

47

31

1.5

.2

.6

99

66

161

7.3

DRAINAGE AND SAMPLING OF THE COLUMN

Drainage was started by allowing water to discharge through the 

lower head tank as the water level was maintained at the base of the 

column. The zero reference point of the graph paper on the manometer board was placed at the same level as the base of the column and 

the height above zero measurement was scaled off in centimeters. At 

the start, of drainage, the hydraulic head at any location in the media
B18 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

would be equal to the distance from that location to the top of the 

column. During drainage, manometer and accumulated discharge 

readings were recorded frequently. Manometer readings were converted to tension readings by subtracting the manometer reading 

minus a correction for capillary rise in the manometer tube from 

the height of the tensiometer above the water table.

During the early minutes of drainage there was a noticeable lag 

in manometer readings caused by rapid change from positive to negative head values. After 1 hour of drainage, the rate of change in 

manometer readings reduced considerably and there was no noticeable 

lag. Because of the lag in early manometer readings, tension values 

were not reported usually until after 1 hour of drainage.

After drainage the pattern of moisture distribution and the dry 

unit weight throughout the column were determined. The pressure 

on the segments of the drainage column was released and the porous 

medium was removed and weighed as the segments were separated 

one by one from top to bottom of the column.

DISCUSSION OF TEST RESULTS

The procedures used in the drainage studies were generally the same 

for all drainage tests. One drainage test was made using 0.120-mm 

glass beads and 2 drainage tests each were made using 20-mesh Del 

Monte sand and Fresno medium sand. In addition, one drainage test 

was made of a column of layered media composed of layers of 20- 

mesh Del Monte sand and Fresno medium sand.

A wide variety of data was obtained on the moisture and physical 

properties of the porous media during or following drainage of the 

columns. Dry unit weight was determined by the following equation:

W, ^vT 

where

y<z=dry unit weight, in grams per cubic centimeter; 

Ws ~ weight of over-dry sample in segment, in grams; 

V,= volume of sample in segment, in cubic centimeters.

Specific gravity of solids (absolute), 6?, is the ratio of the unit 

weight of the solid particles (grain density) to the unit weight of 

distilled water at 4°C.

The volumetric-flask method was used for determining specific gravity. The sample was disaggregated into individual particles and a 

weighed oven-dry portion was placed in water in a calibrated volumetric flask. The volume of the particles was equivalent to the volume of displaced water. The unit weight of the solid particles was 

obtained by dividing the dry weight of the sample by the volume of 

the solid particles. As the density of water at 4°C is unity in the
SPECIFIC YIELD EiFFECT OF TIMEJDN COLUMN DRAINAGE B19

metric system, the specific gravity is numerically equivalent to this 

unit weight. 

Porosity was determined by the following equation :

(100) /« 

where

»=porosity, in percent ;

y»=average unit weight of solid particles (numerically equal to specific 

gravity of solids in metric units), in grams per cubic centimeter.

The moisture content by dry weight is defined as the ratio of (1) 

the weight of water contained in a sample to (2) the oven-dry weight 

of solid particles, expressed as a percentage. Moisture content by 

volume was determined by multiplying the moisture content by weight 

by the dry unit weight (7*), expressed as a percentage. The degree 

of saturation, expressed as a percentage, was determined by dividing 

the moisture content (percent of volume) by the porosity (in percent).

Measurements of outflow during drainage were plotted as accumulated outflow in milliliters against time of drainage in hours. Flow 

into or out of the media from the manometer during drainage was 

determined by subtracting the manometer readings, after selected 

intervals of drainage, from the readings at the start of the experiment. 

The total change in manometer readings in centimeters was then 

divided by 128 to give the computed flow in milliliters (128 cm length 

on manometer equals 1 ml volume).

Tension after selected periods of drainage was plotted against height 

above water table. Time periods were selected to represent nearly 

equivalent changes in the period. Tension readings taken just prior 

to segmenting the column were compared to the moisture content after 

drainage of the appropriate tensiometer segment and these values 

were used for developing a curve relating moisture content to tension. 

Tension values measured during the drainage tests were then converted to moisture contents and plotted against height above the water 

table. A pictorial estimate of the distribution of water above the 

water table after selected periods of drainage thus could be obtained.

DRAINAGE OF 0.120-MILLIMETEB GLASS BEADS

A single drainage test was run using a column of 0.120-mm glass 

beads. The vertical thickness of the media was 171 cm and the period 

of drainage was 99 hours.

TENSION IN RELATION TO TIME OF DRAINAGE

The data shown in figure 5 indicate the pattern of tension distribution over the period of 99 hours of drainage. Tension increased 
B20 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

160

ELAPSED TIME, IN HOURS 

12 5 10 15 31 70 99

20 40 60 80 100 

TENSION, IN CENTIMETERS OF WATER

120

PIGUBB 5. Graph showing tension distribution during drainage of 0.120-mlllimeter

glass beads.

moisture content decreased very rapidly in the early hours of drainage and after 5 hours reached in the lower 60 cm of the column a value 

approximately equal to the height above the base of the column. The 

hydraulic gradient computed from the tension data in this Tower 

part of the column was approximately three-tenths by the end of 1 

hour, one-tenth after 2 hours, and close to zero after 5 hours of 

drainage.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B21

The vertical distance from the base of the column to the point where 

the hydraulic gradient was close to zero increased with time and 

reached about 90 cm at the end of the 99-hour drainage period.

In the upper part of the column after 1 hour of drainage the hydraulic gradient was close to one. The gradient decreased with time of 

drainage, being approximately seven-tenths at the end of drainage. 

Tension in this upper section of the media decreased at a gradually 

diminishing rate during the period of drainage. However, at the time 

the beads were sampled for moisture content, the rate of change of 

tension with time of drainage had reached a very low value.

The progressive decrease in rate of change in tension with increased 

time of drainage suggests that a period of drainage much longer than 

4 days would be necessary before a hydraulic gradient of zero (or 

hydrostatic equilibrium) would exist throughout the vertical section 

of the beads. However, the percentage of the total discharge that 

would occur in the upper section of the columns after 4 days of drainage would be very small. This decrease in rate of change in tension 

during the later period of drainage indicates that 1 or more years of 

time would be required before a hydraulic gradient of zero would exist 

throughout the column. The principal change in tension probably 

would be caused by vapor transfer, although it is reported to be a very 

slow process (Eichards and Moore, 1952) in comparison to film 

transfer.

PHYSICAL, AND HYDKOLOGIC PROPERTIES AFTER DRAINAGE

Figure 6 shows the vertical distribution within a column of 0.120- 

mm glass beads of the dry unit weight and porosity after drainage. 

Figure 7 shows the moisture retained after drainage. As shown in 

figure 6, dry unit weight and porosity varied over a narrow range 

throughout the column of beads. The average dry unit weight was 

1.47 g per cc and the range was from 1.44 to 1.50. The average porosity 

was 41.2 percent and the range was from 40.0 to 42.4.

The plot of vertical distribution of moisture content and the degree 

of saturation (fig. 7) show that the zone of capillary saturation, or 

near saturation, for the glass beads extended to a height of 53 cm. 

This zone has a moisture content of approximately 38 percent of volume 

and was approximately 92 percent saturated. The degree of saturation decreased rapidly above 53 cm to a value of 6 percent in the upper 

part of the column. Except for a sharp decrease in moisture content 

at the top segment, probably caused by evaporation, essentially no 

change occurs at heights above 100 cm that have a moisture content of 

approximately 3 percent of volume.
B22 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

1 /U

160

140

120

CO

a:

UJ 

UJ

z 100

UJ 

O

Z

z

=)

8 80

LJ_

o

UJ 

CO

CO

UJ

> 60 

O 

CO

hC5

UJ

I

40

20 

0

<

i

- .

r .

0

~ .

~ 0

9

~'f

~ .

 \

i

1.4 1.6 1.8

DRY UNIT WEIGHT, IN

GRAMS PER CUBIC

CENTIMETER

4

<

 -

i

»

/

,

»

»

 65

 60

 50

CO

UJ

 40 z

Z

z"

8

LJ_

-30 °

UJ 

CO

CO

ABOVE T 

-20 S2

UJ

I

 10 

. n

JO 40

POROSITY, IN

PERCENT

FIGURE 6. Graph showing distribution of dry unit weight and porosity 

after draining a column of 0.120-millimeter glass beads.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B23

180

160

140

120

100

40 60 

PERCENT

80 100

FIGUBB 7. Graph showing moisture retained after drainage of 0.120-millimeter

glass beads.
B24 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

The curve shown on figure 8 relates moisture content to tension. 

The points for this curve were obtained by recording the tension 

values at the end of the 99-hour drainage period and then determining 

moisture content of the beads in the segments. In addition to using 

the moisture and tension values of the beads in the segments containing tensiometers, the moisture and estimated tension values of the 

beads in all segments below 85 cm from the base of the column were 

used. The estimated tension values in this lower section of the column 

were considered to be equal to the height above the base of the column 

(see fig. 5). The equilibrium moisture content in the tension zone 

above 100 cm of water is 2.5 percent of volume.

MOISTURE DISTRIBUTION AND DISCHARGE IN REI^ATION TO TIME

OF DRAINAGE

Using the moisture-tension curve (fig. 8), the tension data (fig. 5) 

were converted to moisture content. The data indicating the pat160

140

120

I

o 100

1 80

~ 60

O 

V)

LU

K 40

20

60

50

40

30

20

10

10 15 20 25 30 

MOISTURE CONTENT, IN PERCENT OF VOLUME

40

FIGURE 8. Graph showing relation of moisture content to tension for 0.120-millimeter

glass beads.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B25

tern of water distribution over a period of 99 hours of drainage are 

shown on figure 9. The discharge of water in relation to time of 

drainage is shown on figure 10.

The accumulated discharge includes an estimated volume of 5.5 ml 

contributed by the manometer system. The outflow data provide a 

direct measurement of the accumulated discharge from the column 

and the moisture-distribution curves provide an indirect measurement 

of discharge and indicate the point of discharge in the column. After 

1 hour of drainage, approximately 60 percent of the total discharge 

had occurred; after 5 hours, approximately 90 percent; and after 15 

hours, approximately 97 percent. Although no measurable outflow 

occurred between YO and 99 hours, the moisture-distribution curves 

indicate a very slight decrease in moisture content in the upper 

half of the column over this same period of time. Figure 5 shows a 

change in tension values of from 4 to Y cm of water between the 

period of YO and 99 hours of drainage. At the higher tensions there

ELAPSED TIME, IN HOURS

70 15 5 

99 31 10 2 1

160

0 5 10 15 20 25 30 35 

MOISTURE CONTENT, IN PERCENT OF VOLUME

FIGURE 9. Graph showing moisture distribution during drainage of 0.120-millimeter glass beads.
B26 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

280

80

60 O

40

20

30 40 50 60 70 

ELAPSED TIME, IN HOURS

80 90 100

FIGURE 10. Graph showing quantity of water drained from a column of 0.120-millimeter 

glass beads as related to time of drainage.

is a very small change in moisture content with a large change in tension (fig. 8). This suggests that although hydrostatic equilibrium 

was not obtained in this test, the outflow between this time and that 

when the drainage was terminated would be a small fraction of a 

percent of the total drainage. Results obtained and observations 

made during the tests indicated that very precise controls would 

need to be employed in order to obtain reliable data for hydrostatic 

equilibrium. These would include humidity controls that would 

insure that there was no evaporation from the column, the outflow 

system, or from the manometers, and temperature controls of a very 

precise nature to insure against temperature gradients being developed 

in the media.

DRAINAGE OF FRESNO MEDIUM SAND

Two drainage tests were run using Fresno medium sand. In the 

first test the column was filled with sand and mechanically packed 

as described earlier. After settling by vibration, the level of the 

sand was 148 cm above base of column. Sand was added in increments of 5 cm and manually packed, by tapping each addition with a 

rubber-padded stick, to a level of 163 cm. The porosity in the handpacked section was found to be less than that in the section of column 

that was packed by vibration.

In the second test the column was filled with sand and mechanically 

packed only. The vertical thickness of the sand after vibration was
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B27

150.5 cm. The drainage period for the first test was 148 hours and 

for the second was 94 hours.

During drainage of column 1 the temperature-control unit became 

inoperative and the temperature increased to 25 °C.

TENSION IN RELATION TO TIME OF DRAINAGE

The data shown on figures 11 and 12 indicate the pattern of tension distribution in the sand during drainage of columns 1 and 2, 

respectively. Although the two columns of sand were not comparable 

in vertical thickness, the general pattern of tension distribution with 

time of drainage is quite similar.

Tension increased very rapidly during the early hours of drainage, 

approaching a hydraulic gradient close to zero after 4 hours in the 

lower 25 cm of the columns of sand. The vertical thickness of media 

having a hydraulic gradient near zero increased with increased time 

of drainage, becoming about 45 cm after nearly 48 hours of drainage 

and 60 cm in column 1 after about 148 hours of drainage. In the 

upper part of the columns the change in tension decreased with time 

and had reached a very low rate at the time the sand was sampled 

for moisture content and porosity. The hydraulic gradient decreased 

from slightly less than 1 after 1 hour of drainage to seven-tenths after 

96 hours of drainage.

PHYSICAL AND HYDROL.OGIC PROPERTIES AFTER DRAINAGE

Figures 13 an'l 14 show, for both columns 1 and 2 of Fresno sand, 

the distribution 01 Jry unit weight, porosity, moisture content, and 

degree of saturation at various heights above the base of the column. 

Dry unit weight and porosity were not determined in the segments 

below 25 cm where a number of sand grains remained on the inside 

wall of each segment after the bulk of the sample had been removed. 

.Because the moisture contents would have been affected by evaporation, these sand grains were not removed.

As shown on figure 13, the vertical distribution of dry unit weight 

and porosity for the two drainage columns had similar trends. The 

values are similar throughout the sections of the columns between 75 

and 150 cm, but between 25 and 75 cm there is a trend towards an increase in dry unit weight and decrease in porosity with height above 

the base of the column. The average dry unit weight and porosity 

were 1.71 g per cc and 36.7 percent, respectively, for column 1, and 

1.70 g per cc and 37.0 percent for column 2.

The vertical distribution of moisture content and the degree of 

saturation for columns 1 and 2 are plotted in figure 14. The data show 

that the moisture content and the degree of saturation have similar 

trends for the two columns. Because no porosities were available
B28 HTDROLOGIC PROPERTIES OF EARTH MATERIALS

140

120

100

ELAPSED TIME, IN HOURS

16 47 75 95 148

20 40 60 80 

TENSION, IN CENTIMETERS OF WATER

100

FIGURE 11. Graph showing tension distribution during drainage of Fresno medium sand

(column 1).
HEIGHT ABOVE BASE OF COLUM

S Z o

HEIGHT ABOVE BASE OF 
draining columns ofFresno mediumsand. Solid circles referto column1 ;

open circles referto column2.

1.468 304

DRY UNITWEIGHT, INPOROSITY, IN GRAMS PERCUBIC PERCENT CENTIMETER

hO 4* CTtOO

3OOOOO

1 1 1 1 1 1 1 1 

0 6

* ° 0 ° ^ 0 

m ^ W o w m, _ 

*^ Q 

t °\ °. o *°* /° ** *~ 

1 1 1 1 1 1 1 1 

3 1 1 1 

i-1 Nl U) 

O O O 

HEIGHT ABOVE BASE OF COL
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B31

40 60 

PERCENT

80 100

 0

FIGURE 14. Graph showing moisture retained after drainage of Fresno medium sand. 

Solid circles refer to column 1; open circles refer to column 2.
B32 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

for the lower sections of the columns, the degree of saturation in those 

sections was obtained by using average porosities.

The zone of capillary saturation extends from the base of the column 

to a height of approximately 12 cm. The average degree of saturation 

in this zone was calculated to be approximately 96 percent for column

I and 92 percent for column 2. Because these values were calculated 

using average porosity values, the resultant degree of saturation will 

represent only approximate values. The average moisture content 

for both columns in this zone is approximately 22 percent by weight 

and 37 percent by volume.

The degree of saturation decreased with increasing height above 12 

cm, with the rate of decrease being less pronounced than for the 

0.120-mm glass beads. In the section of the column above 110 cm, the 

degree of saturation was essentially constant, at 22 percent.

The curve relating moisture content to tension for Fresno medium 

sand is shown in figure 15. The hydraulic gradient at the end of drainage was nearly zero in the vertical section of the column below 60 cm 

for test 1 and 45 cm for test 2. Because the tension values are virtually 

equal to the heights of the tensiometers above the base of the column, 

the moisture values in these sections were used in forming the calibration curve.

The points from which the moisture-tension curve was extrapolated 

are more scattered than the points for 0.120-mm glass beads. This 

wide scatter is probably due to the more heterogeneous nature of the 

Fresno sand (fig. 2) and the difficulty of constructing a column of 

Fresno sand that would have a uniform distribution of particle size.

MOISTURE DISTRIBUTION AND DISCHARGE IN RELATION TO TIME

OF DRAINAGE

Using the moisture-tension curve (fig. 15) the tension data (fig.

II and 12) were converted to moisture content in percent of volume. 

The moisture content data are shown in figure 16 for test 1 and figure 

17 for test 2.

The change in moisture content for tests 1 and 2 from 75 hours of 

drainage to the time the columns were segmented was very small, yet, 

there was an appreciable change in tension over this period of time.

As experienced in the drainage of the 0.120-mm glass beads, hydrostatic equilibrium was not obtained in either of the two drainage tests 

of Fresno medium sand. However, here also, all but a small part of 

the total potential drainage appeared to have occurred during 4 days 

of drainage.

The accumulated discharge compared with time of drainage for 

tests 1 and 2, shown in graph form in figure 18, was 50 percent of the 

total discharge after one hour of drainage, 75 percent after 4 hours
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B33

100,-

15 20 25 30 35 

MOISTURE CONTENT, IN PERCENT OF VOLUME

40

FIGURE 15. Graph showing relation of moisture content to tension for Fresno medium 

sand. Solid circles refer to column 1; open circles refer to column 2.
B34 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

of drainage, 90 percent after 24 hours, and over 99 percent after 75 

hours of drainage. The reasons for the differences in total discharge 

of the two columns are (1) the different vertical thicknesses of column, 

and (2) a difference in outflow from the manometer systems.

DRAINAGE OF 20-MESH DEL MONTE SAND

Two drainage tests were made using 20-mesh Del Monte sand. The 

vertical thicknesses of the columns were 152 cm and 148 cm respectively. The period of drainage for each test was 28 hours.

TENSION IN RELATION TO TIME OP DRAINAGE

Figures 19 and 20 show the tension distribution during drainage 

for columns 1 and 2, respectively. In the two tests, essentially no 

change in tension at distance of less than 20 cm above the base of 

the column was observed over the drainage period of 1 to 28 hours.

ELAPSED TIME, IN HOURS

148

140

5 10 15 20 25 30 35 

MOISTURE CONTENT, IN PERCENT OF VOLUME

FIGURE 16. Graph showing moisture distribution during drainage of Fresno medium

sand (column 1).
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B35

ELAPSED TIME, IN HOURS

5 10 15 20 25 30 35 

MOISTURE CONTENT, IN PERCENT OF VOLUME

FIGURE 17. Graph showing moisture distribution during drainage of Fresno medium

sand (column 2).

Even at heights of over 30 cm above the base of the column, the change 

in tension was, in general, slightly less than 10 cm the maximum 

change in tension at any tensiometer location over this period being 

11.1 cm. Because of the small change in tension in the upper part of 

the columns, all tension readings above 30 cm for each selected time 

period were used to extrapolate the time-tension trend lines (figs. 19 

and 20). Thus, these trend lines smooth out the effects of contact 

with media, differences in manometer diameter, changes in porosity 

of media, and other factors that cause variations in the readings of 

the tensiometers.

At the end of 1 hour of drainage, the hydraulic gradient in the 

two tests approached zero in the lower 22 cm of the columns and was 

close to one above 22 cm. After 28 hours of drainage the hydraulic 

gradient was nearly zero in the lower 28- to 29-cm section of the 

column and was close to one above this height. During the last 16
B36 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

hours of drainage the change in tension above a vertical height of 28 

cm was about 2 cm of water.

PHYSICAL, AND HYDROLOGIC PROPERTIES AFTlSK DRAINAGE

Data in figures 21 and 22 give the vertical distribution of dry unit 

weight, porosity, moisture content, and degree of saturation with 

heights above the base of the column for the two columns of 20-mesh 

Del Monte sand. As shown in figure 21, dry unit weight and porosity 

changed very little throughout the column. The average dry unit 

weight was 1.71 g per cc for test 1 and 1.73 g per cc for test 2, and 

the average porosity was 35.5 percent for test 1 and 34.7 percent for 

test 2.

The vertical distribution of moisture content and the degree of 

saturation (fig. 22) show that the zone of capillary saturation extends 

from the base of the column to a height of about 10 to 12 cm. Only 

one porosity value was obtained in this zone because tensiometers were 

concentrated in the lower section of the columns. Using this porosity, 

the degree of saturation was calculated to be 96.5 percent. All other 

calculations for degree of saturation in this section of the column 

were made using the average porosity in the column. The degree of 

saturation was found to range from 95 to slightly greater than 100 

percent, the values greater than 100 percent probably resulting from 

use of the average porosity in the calculations of saturation.

Above the zone of capillary saturation the degree of saturation in 

the Del Monte sand decreased sharply to 12 percent, this decrease being

200

20 40 60 80 100 

ELAPSED TIME, IN HOURS

120 140 160

FIGURE 18. (Graph showing quantity of water drained from columns of Fresno medium 

sand as related to time of drainage. Solid circles refer to column 1 (163.2 cm long) ; 

open circles refer to column 2 (150.5 cm long).
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B37

140

120

80

60

40

20

ELAPSED TIME, IN HOURS 

1 4 12 28

10 20 30 

TENSION, IN CENTIMETERS OF WATER

-50

-40

-20

 10

40

FIGURE 19. Graph showing tension distribution during drainage of 20-mesh Del 

Monte sand (column 1).
HEIGHT ABOVE BASE OF COLUMN, IN C

a

a $

N> W

o o 

HEIGHT ABOVE BASE OF COLUMN
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B39

160

140

120

CO

cc

Ul 

Ul

p 100

Z

Ul

O

Z

z"

d 80

u_

0

Ul 

CO

m 

£ 60

|

1 

I

S2

Ul I

40

20

0 

1

-

-

_

4

i

<

<

i

<

c

*

|0

o

o 

o

0

D

 

0

 

o

3 

 o

 0

o

o

 0

 o

o

m

o

0

o

c

(

o

o

a

d

c

o

01

1

0 

0

c

c

o

) 

at

0 

0

o

o

#

y»

o

1

1

I

»

ID

>o

0*

-

-

-

_

-

 60

 50

CO

UJ

-40 g 

Z

Z

z" 

2

8

 30 O

Ul 

CO

m

Ul

8m

t

 20 S3

Ul I

 10

n

4 1.6 1.8 30 40

DRY UNIT WEIGHT, IN POROSITY, IN

GRAMS PER CUBIC PERCENT

CENTIMETER

FIGURE 21. .Graph showing distribution of dry unit weight and porosity after 

draining columns of 20-mesh Del Monte sand. Solid circles refer to column 

1; open circles refer to column 2.
B40 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

140

120

100

80

60

40

cc»

 O 

3

O 

O O

Average porosity, 

35.1 percent

50

40

sCO

20 <

ho

10

40 60 80 100

PERCENT

FIGURE 22. Graph showing moisture retained after drainage of 20-mesh Del Monte 

sand. Solid circles refer to column 1; open circles refer to column 2.

more pronounced than for the Fresno medium sand. At heights 

greater than 30 cm, moisture content remained at 4 percent of volume. 

The curve relating moisture content to tension is shown in figure 23. 

The hydraulic gradient at the completion of drainage was nearly zero 

in the section of the media below 30 cm for tests 1 and 2. Because the 

tension values in that section were very close to the heights of the 

tensiometers above the base of column, all the moisture values in the 

section were used in developing the curve. Above 30 cm in height the
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B41

35

30

25

20

O

Z

Z 15

10

10

10 20 30 

MOISTURE CONTENT, IN PERCENT OF VOLUME

40

FIGURE 23. .Graph showing relation of moisture content to tension for 20-mesh 

Del Monte sand. Solid circles refer to column 1; open circles refer to 

column 2.
B42 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

moisture and tension values of the sand in the segments containing 

tensiometers also were used in developing the curve (fig. 23).

As observed with the Fresno medium sand, the points from which 

the 20-mesh Del Monte sand moisture-tension curve was extrapolated 

are more scattered than the points representing the moisture-tension 

values for the 0.120-mm glass beads. The scatter is probably due 

to the greater heterogeneity, and consequently greater variation in 

porosity, of the Del Monte sand.

MOISTURE DISTRIBUTION AND DISCHARGE IN RELATION TO TIME

OF DRAINAGE

Using the moisture-tension curve (fig. 23), the tension data (figs. 19 

and 20) were converted to moisture content and are shown in figure 24 

for test 1 and in figure 25 for test 2. The accumulated discharge compared with time of drainage for the two runs is shown in figure 26. 

After 1 hour of drainage, approximately 90 percent of the total discharge from the 2 columns had occurred. After 6 hours, over 97 percent, and after 12 hours, over 99 percent of the total discharge had 

occurred. The differences in discharge of the two columns are due 

to the different vertical thickness and a difference in outflow from the 

manometer system.

Although there was a noticeable change in tension between 12 and 

24 hours, there was only a very small change in outflow or moisture 

content in the media as indicated by the outflow data and the moisturedistribution curves. (See figs. 24, 25, and 26.) Thus, although the 

tensions in the upper part of the column had not reached theoretical 

equilibrium conditions, it appears that all but a small amount of the 

potential drainage had occurred in 28 hours.

DRAINAGE OF LAYERED 20-MESH DEL MONTE SAND AND FRESNO

MEDIUM SAND

The layered column consisted of a layer of 20-mesh Del Monte sand 

in the lower 49.5 cm of the column, overlain by a layer of Fresno 

medium sand extending to a height of 131.2 cm, and a layer of 20-mesh 

Del Monte sand extending from 131.2 to 170 cm.

TENSION IN RELATION TO TIME OF DRAINAGE

The vertical distribution of tension in the layered column during 

a 99-hour period of drainage is shown in figure 27.

Visual observation of the zone of apparent saturation during drainage indicated that the 20-mesh Del Monte sand in the lower section 

of the column was saturated until about half an hour after drainage 

started. The outflow data substantiated this observation by showing 

a sharp increase in outflow after about half an hour, of drainage, indicating that the lower layer of Del Monte sand had then started to
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B43

drain. After half an hour of drainage there was some evidence of a 

slight lag between tensiometer readings and actual tensions for several 

of the tensiometers. Although the half-hour tension curve thus represents only an approximate tension distribution, it at least should reflect the general trend of tension distribution during the early period 

of drainage.

The tension pattern during the early half-hour period indicates a 

very low hydraulic gradient in the bottom layer of Del Monte sand, 

a hydraulic gradient in excess of 1 in the middle layer of Fresno sand, 

and a hydraulic gradient close to 1 in the top layer of Del Monte sand. 

The saturated permeability of Fresno medium sand is 0.5 cc per sec per 

sq cm (cubic centimeter per second per square centimeter), and the 

saturated permeability of the 20-mesh Del Monte sand is 8 cc per sec 

per sq cm. Because the permeability of the Del Monte sand is about 

16 times that of the Fresno sand, only a low hydraulic gradient would 

be necessary to cause water movement through the Del Monte sand 

from the overlying Fresno sand during the first half hour of drainage.

ELAPSED TIME, IN HOURS

12

28 4 1

0(120 

tr

100

80

60

O 40 

DO

10 15 20 25 30 

MOISTURE CONTENT, IN PERCENT OF VOLUME

35

50

40

30

20

40

FIGURE 24. Graph showing moisture distribution during drainage of 20-mesh Del Monte

sand (column 1).
B44 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

140

120

ELAPSED TIME, IN HOURS

12

28 4 1

100

80

60

40

20

10 15 20 25 30 

MOISTURE CONTENT, IN PERCENT OF VOLUME

50

40

30

20

10

40

FIGURE 25. Graph showing moisture distribution during drainage of 20-mesh Del Monte

sand (column 2).

320

^ 240

_T 160

12 16 20 

ELAPSED TIME, IN HOURS

28

FIGURE 2,6. Graph showing quantity of water drained from columns of 20-mesh Del Monte 

sand as related to time of drainage. Solid circles refer to column 1 (151.7 cm long) ; 

open circles refer to column 2 (147.5 cm long).
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B45

ELAPSED TIME, IN HOURS

180

160

40 60 80 

TENSION, IN CENTIMETERS OF WATER

100

 70

120

FIGURE 27. Graph showing tension distribution during drainage of a layered column 

of 20-mesh Del Monte sand and Fresno medium sand.
B46 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

Thus, in effect, the Del Monte sand was acting as a hanging water 

column during this period of drainage.

Two lines having a 1-to-l slope are drawn in figure 27. The line 

starting at zero distance above the base of the column is the theoretical 

tension curve representing equilibrium drainage conditions for the Del 

Monte sand. The other line, in the layer of Fresno medium sand, is 

extrapolated from that part of the 99-hour tension curve that has a 

hydraulic gradient close to zero. This line crosses the left vertical 

axis at 19 cm. The tension at the top of the bottom layer of Del Monte 

sand or at the bottom of the layer of Fresno sand (49.5 cm above the 

base of the column) at the end of drainage was about 30.5 cm (the 

tension recorded at a height of 44 cm above the base of the column after 

99 hours of drainage). In drainage of homogeneous columns of 

Fresno sand, the tension at 49.5 cm after drainage was approximately 

49.5 cm. Thus, there was at this height an estimated difference of tension of 19 cm of water between the layered column and a homogeneous 

column of Fresno medium sand. It is assumed then that the pattern 

of tension distribution after about 100 hours of drainage within the 

Fresno sand in the layered column would be similar to that in a column 

of Fresno sand having a vertical thickness of about 112 cm. However, 

some adjustment would be necessary because of the changing tension 

in the underlying strata and because of water entering the media from 

the overlying strata. The lower boundary of the Fresno medium sand 

in the layered column then should be comparable in position to a height 

of 30.5 cm in a homogeneous column. A comparison of the tension 

curves for the Fresno medium sand in the layered column (fig. 27) 

with the curves for homogeneous columns of this same sand (figs. 11 

and 12) supports this assumption.

After 2 hours of drainage the hydraulic gradient in the bottom 

layer of Del Monte sand was nearly zero in the section of the column 

between 0 and 25 cm and about 1 between 25 and 49.5 cm. After 100 

hours, it was approximately zero from 0 and 35 cm and almost 1 from 

35 to 49.5 cm. A similar pattern of hydraulic gradients at these heights 

was observed in the two drainage tests for the homogeneous columns of 

20-mesh Del Monte sand (figs. 19 and 20).

The hydraulic gradient in the top layer of Del Monte sand (between 

131.2 and 170 cm) was about one throughout the test. The hydraulic 

gradient and the tension values are similar to the values at comparable 

heights in the two drainage tests for homogeneous columns of 20-mesh 

Del Monte sand.

PHYSICAL AND HYDROLOGIC PROPERTIES AFTER DRAINAGE

Figures 28 and 29 show the vertical distribution of dry unit weight, 

porosity, moisture content and degree of saturation after drainage. As
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B47

shown in figure 28, dry unit weights and porosities for the Del Monte 

sand do not vary appreciably whether in the top or bottom layer of the 

column. The average porosity of 35.1 percent and average dry unit 

weight of 1.72 g per cc for this sand in the layered column are similar 

to those values obtained for columns filled only with Del Monte sand 

(fig. 21).

Porosity and dry unit weight were consistent throughout the layer of 

Fresno medium sand. The average porosity was 37.0 percent and the 

average dry unit weight was 1.70 g per cc. These average values for 

the Fresno medium sand in the layered column are similar to those 

obtained for columns filled only with Fresno sand (fig. 13).

In figure 29 the moisture content and degree of saturation of the Del 

Monte sand in the layered column are shown to be similar to those 

values obtained in the drainage of columns filled only with 20-mesh 

Del Monte sand (fig. 22).

Considering that the bottom of the layer of Fresno sand in the layered column corresponds to a height of 49.5 cm in a homogeneous 

column, the moisture content and degree of saturation of the Fresno 

sand (fig. 29) correspond closely, especially in the upper part of the 

layer, to that obtained in the drainage of columns filled only with 

Fresno sand (fig. 14).

MOISTURE DISTRIBUTION AND DISCHARGE IN RELATION" TO TIME

OP DRAINAGE

The tension data, except those for one-half hour, were converted 

to moisture content by using the moisture-tension curve for Del Monte 

sand (fig. 23) and for Fresno medium sand (fig. 15). These moisture 

content data for various periods of drainage are shown in figure 30. 

The quantity of discharge of water in relation to time of drainage is 

shown in figure 31.

Because the bottom layer of Del Monte sand was functioning somewhat as a hanging water column during the early period of drainage, 

the tensions in this sand and for some distance in the overlying Fresno 

sand would not be an accurate reflection of moisture content. Consequently, the tensions for one-half hour of drainage were not converted 

to moisture contents. For the Del Monte sand, the moisture distribution for various periods of drainage in a layered column (fig. 30) was 

quite similar to that obtained from the drainage of columns filled only 

with Del Monte sand (figs. 24 and 25). As discussed in the section 

on tension, the vertical scale in figure 30 should be adjusted so that it 

reads 30.5 cm at the bottom of the layer of Fresno sand. The moisture 

contents in this layer then are similar to those obtained in the drainage 

of homogeneous columns of Fresno medium sand.
B48 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

180

160

140

CC

UJ

| 120

Z

UJ

O

Z

1 100

^J

o 

o

LL.

O

UJ

< 80

CD 

UJ

0

CD

^

hg 60

UJ

I

40

20

0

-

_

_

~

-

-

_

-

«

<

4

41

4

4

4

A

1

*

£

,

1

^

*

'

i

\

.

*

j

-a

c 

as

V)

4-J

c

o

"5

Q

'

E

3

b

<D

E 

o

c

!/)

0)

II

^

y

c

roin

IV

?

o

Q

\

\

^

U)

IV

E

0

~~*

^

"D

C

ro

U)

^

!/) 

IV

E

§

'

0

_

*

0

1 

<

»

'

«

> 

>

'

^

*

4p

0

0

,

»

>

*

»

-J

-

~

-

-

_

-

-

 70

 60

-50 w

UJ

I

O

Z

z

z"

-40 § 

O

LL.

o

UJ

^1

CD

UJ

-30 g

hg

UJ I

 20

 10

0

1.4 1.6 1.8 30 40

DRY UNIT WEIGHT, IN POROSITY, IN

GRAMS PER CUBIC PERCENT

CENTIMETER

FIGORE 28. Graph showing distribution of dry unit weight and 

porosity after draining a layered column of 20-mesh Del Monte 

sand and Fresno medium sand.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B49

180

160 -

140

120

100

70

50

100

PERCENT

FIGURE 29. Graph showing moisture retained after drainage of a layered column 

of 20-mesh Del Monte sand and Fresno medium sand.
B50 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

ELAPSED TIME, IN HOURS

180

10 15 20 25 30 

MOISTURE CONTENT, IN PERCENT OF VOLUME

40

FIGURE 30. Graph showing moisture distribution during drainage of a layered column 

of 20-mesh Del Monte sand and Fresno medium sand.

The outflow from the layered column (fig. 31) was greater, for 

equivalent time of drainage, than the outflow from a homogeneous 

column of Fresno sand (fig. 18) . The outflow from the layered column 

was 72 ml after 20 minutes and 99 ml after 30 minutes of drainage. 

The outflow for column 1 of Fresno sand was 54 ml after 20 minutes 

and 70 ml after 30 minutes of drainage. This was probably due to the 

bottom layer of Del Monte sand acting as a hanging water column 

while at the same time the top layer of Del Monte sand was contributing a greater quantity of drainage, than would be contributed by a 

similar layer of Fresno sand.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B51

240

100

60

40

20

20 30 40 50 60 70 

ELAPSED TIME, IN HOURS

90 100

FIGURE 31. Graph showing quantity of water drained from a layered column of 

 20-mesh Del Monte sand and Fresno medium sand.

SUMMARY

Three sand-size materials, 0.120-mm glass beads (very fine sand size), 

a sand collected near Fresno, Calif, (medium-sand size) and 20-mesh 

Del Monte sand (coarse-sand size) were packed in columns consisting 

of 2- and 4-cm lucite segments having an inside diameter of 2.8 cm. A 

column consisting of layers of 20-mesh Del Monte sand and Fresno 

medium sand was also packed. The thickness of the media in the 

columns ranged between 141 and 170 cm. The media was wetted from 

below and drained under controlled conditions of temperature and 

humidity while outflow measurements were recorded. Tensiometers 

were installed at selected locations and tension readings were recorded 

during drainage. After drainage the media contained in each lucite 

segment was sampled for determinations of dry unit weight, porosity, 

moisture content, and degree of saturation, and these values were compared. The tension values recorded during drainage of the media were 

converted to moisture content by use of a curve that related moisture 

content to tension.

Test results from the drainage of 20-mesh Del Monte sand indicated 

that this sand had a high permeability at low tensions 0 to approximately 12 cm of water and that the permeability decreased very 

rapidly with increasing tension up to tensions of about 25 cm, at which 

time the flow rate was only a small fraction of the initial flow rate 

and all but a small fraction of the potential drainage had occurred.

Drainage data for the 0.120-mm glass beads and Fresno medium 

sand indicated a lower permeability at the lower tensions than did the
B52 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

coarser textured 20-mesh Del Monte sand, but the permeability did not 

decrease as rapidly with increasing tension.

The data indicate that moisture equilibrium in the Del Monte sand 

would require tensions of as much as 150 cm and periods of drainage 

much in excess of the period of this study. Furthermore, experimental 

techniques capable of developing this tension for such long periods, 

as much as 1 year, would require precision humidity and temperature 

controls beyond the capabilities of present installations in the Hydrologic Laboratory.

Tne hydraulic gradient in the lower sections of the columns remained close to zero throughout the drainage test. The thickness of 

this zone of low hydraulic gradient increased with time of drainage.

In the upper sections of the columns of all three materials the hydraulic gradient was close to one during the early periods of drainage. 

It remained close to one throughout the drainage test for 20-mesh Del 

Monte sand but decreased with time of drainage for the 0.120-mm glass 

beads and Fresno medium sand, reaching a hydraulic gradient close to 

seven-tenths for Fresno medium sand and for the 0.120-mm glass beads 

at the end of the drainage tests.

The zone of capillary saturation was approximately 12 cm for the 

20-mesh Del Monte sand, 53 cm for the 0.120-mm glass beads, and 12 

cm for the Fresno medium sand. Measurements of the degree of saturation of these zones after drainage indicated that they were more than 

90 percent saturated.

The moisture-distribution curve for the homogeneous 0.120-mm glass 

beads showed a much thicker zone of capillary saturation and a thinner 

and more sharply defined funicular zone than did the moisture-distribution curve for the heterogeneous Fresno medium sand.

A plot of moisture content and tension for the three materials 

indicated a much narrower range of scatter for data on the drainage 

of 0.120-mm glass beads than on the Fresno medium and 20-mesh Del 

Monte sand. This is probably caused by the more homogeneous 

nature of the glass beads.

Adjacent values of porosity and dry unit weight were comparable 

throughout the columns for the 0.120-mm glass beads and the 20-mesh 

Del Monte sand. For columns of Fresno medium sand, however, 

adjacent values of the porosity and dry unit weight were similar in the 

middle and upper sections by dissimilar in the lower section of the 

column.

The moisture-distribution data and the outflow data indicated the 

time when drainage occurred, as well as the location within the 

column where this discharge occurred. Most of the outflow from 

these sand-size materials occurred in the earlier hours of drainage.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B53

After 1 hour of drainage, approximately 90 percent of the total 

measured discharge had occurred in the 20-mesh Del Monte sand, 

60 percent in the 0.120-mm glass beads, and 50 percent in the Fresno 

sand. After 5 hours 90 percent of the total measured discharge had 

occurred in the 0.120-mm glass beads, and after 25 hours 90 percent 

had occurred in the Fresno sand.

In drainage of a layered column consisting of a bottom layer of 

20-mesh Del Monte sand, a middle layer of Fresno medium sand, and 

a top layer of 20-mesh Del Monte sand, the bottom layer functioned 

as a hanging water column, accelerating the drainage of the overlying media until the visual zone of saturation had passed through the 

layer of Fresno sand. After the bottom layer started to drain, the 

pattern of tension and moisture distribution in the bottom and top 

layers showed similar trends to that observed in homogeneous columns 

of Del Monte sand.

The pattern of tension and moisture distribution in the middle 

layer of Fresno medium sand after drainage was similar to that in a 

homogeneous column of this sand.

In the laboratory this study demonstrates the phenomena of slow 

drainage of porous materials even where those materials may be as 

coarse as sands. Meinzer (1923) and Wenzel (1942) both cautioned 

that the maximum specific yield is reached only after a drainage period 

of considerable length. The drainage period to reach maximum specific yield may range from a few hours for coarse-textured materials 

to months for the fine-textured materials.

The effect of slow drainage on the specific yield of porous materials 

is emphasized by the experimental data in this report. These data 

indicate that 90 percent of the maximum specific yield was reached 

in 1 hour for 20-mesh Del Monte sand (uniform, coarse sand), in 5 

hours for 0.120-mm glass beads (very uniform, very fine to fine sand), 

and in 25 hours for Fresno sand (less uniform, medium sand).

Although most of the drainage, or the largest percentage of the 

specific yield, was reached in this study during the early hours of 

drainage, a very long time would have been required to reach 

drainage equilibrium. Even for the sand-size materials used in this 

study, the authors estimate that from 2 months to more than one year 

would be required to reach drainage equilibrium, and thus give the 

maximum specific yield. Equipment for control of humidity and 

temperature was not sufficiently refined to permit drainage of samples 

for longer periods without the effects of evaporation being too great. 

However, sufficient drainage probably had taken place for all practical 

purposes.
B54 HYDROLOGIC PROPERTIES OF EARTH MATERIALS

Such slow drainage, especially if even finer textured materials were 

present, naturally would have a great effect upon the specific yield 

obtained by short-term pumping tests in the field. Under most field 

conditions, the maximum specific yield may not be obtained until after 

many days of pumping.

REFERENCES

Buckingham, Edgar, 1907, Studies on the movement of soil moisture: U.S.

Dept. Agriculture Bull. 38, p. 29-61. 

Coleman, E. A., 1946, A laboratory study of lysimeter drainage under controlled

soil-moisture tension: Soil Sci., v. 62, p. 365. 

Day, P. R., and Luthin, J. N., 1956, A numerical solution of the differential

equation of flow for a vertical drainage problem: Soil Sci. Soc. America

Proc., v. 20, p. 443-447. 

Eckis, Rollin, and Gross, P. L. K., 1934, South Coastal Basin investigation 

geology and ground-water storage capacity of valley fill: California Div.

Water Resources Bull. 45,273 p. 

Gatewood, J. S., and others, 1950, Use of water by bottom-land vegetation in

lower Safford Valley, Arizona: U.S. Geol. Survey Water-Supply Paper

1103, p. 81-92. 

Hazen, Alien, 1892, Experiments upon the purification of sewage and water at

the Lawrence Experimental Station, No. 1, 1889 to Dec. 31, 1891: Massachusetts State Board Health Pub., no. 34, p. 425-633. 

Johnson, A. I., Morris, D. A., and Prill, R. C., 1961, Specific yield and related

properties an annotated bibliography: Denver, Colo., Hydrologic Laboratory Bibliography No. 2, U.S. Geol. Survey open-file rept., 245 p. 

Johnson, A. I., Prill, R. C., and Morris, D. A., 1963, Specific yield column

drainage and centrifuge moisture content: U.S. Geol. Survey Water-Supply

Paper 1662-A, 60 p. 

King, F. H., 1899, Principles and conditions of the movement of ground water:

U.S. Geol. Survey 19th Ann. Rept., pt. 2, p. 85-91. 

Lambe, T. W., 1951, Capillary phenomena in cohesionless soils: Am. Soc. Civil

Engineers Trans., v. 116, p. 401-432. 

Lebedeff, A. F., 1927, The movement of ground and soil waters: Internal Gong.

Soil Sci. Proc., v. 1, p. 459-494. 

Leverett, M. G., 1941, Capillary behavior in porous solids: Am. Inst. Mining

Metall. Engineers Trans., v. 1942, p. 152-168. 

Luthin, J. N., and Miller, R. D., 1953, Pressure distribution in soil columns

draining into the atmosphere: Soil Sci., Soc. America Proc., v. 17, p. 329-333. 

Marx, J. W., 1956, Determining gravity-drainage characteristics on the centrifuge:

Jour. Petroleum Technology, v. 8, p. 88-91. 

Meinzer, O. E., 1923, Outline of ground-water hydrology: U.S. Geol. Survey

Water-Supply Paper 494, 71 p. 

Morris, D. A., and Kulp, W. K., 1961, Mechanical uniform packing of porous

media : U.S. Geol. Survey Prof. Paper 424-D, p. D31-D32. 

Piper, A. M., and others, 1939, Geology and ground-water hydrology of the

Mokelumne area, California: U.S. Geol. Survey Water-Supply 780, p. 101-

122. 

Prill, R. C., 1961, Comparison of drainage data obtained by the centrifuge and

column drainage methods, in Geological Survey Research 1961: U.S. Geol.

Survey Prof. Paper 424-D, Art. 434, p. D399-D401.
SPECIFIC YIELD EFFECT OF TIME ON COLUMN DRAINAGE B55

Prill, B. C., and Johnson, A. I., 1959, Effect of temperature on moisture contents 

as determined by centrifuge and tension techniques: Am. Soc. Testing Materials, Spec. Tech. Pub. 254, p. 340-349.

Bichards, L. A., and Moore, D. C., 1952, Influence of capillary conductivity and 

depth of wetting on moisture retention in soil: Am. Geophys. Union Trans., 

v. 33, no. 4, p. 531-540.

Smith, W. O., 1933, The final distribution of retained liquid in an ideal uniform 

soil: Physics, v. 4, p. 425-438.

 1961, Mechanism of gravity drainage and its relation to specific yield 

of uniform sands: U.S. Geol. Survey Prof. Paper 402-A.

Stahl, B. F., Martin, W. A., and Huntington, B. L., 1943, Gravitational drainage 

from unconsolidated Wilcox sand: Am. Inst. Mining Metall. Engineers 

Trans., v. 151, p. 138-146.

Stearns, H. T., Bobinson, T. W., and Taylor, G. H., 1930, Geology and water 

resources of the Mokelumne area, California: U.S. Geol. Survey WaterSupply Paper 619, p. 151-172.

Terwillinger, P. L., and others, 1951, An experimental and theoretical investigation of gravity drainage performance: Am. Inst. Mining Metall. Engineers Trans., v. 192, p. 285-296.

Versluys, Jan, 1917, Die Kapillaritat der Boden: Inst. Mitt. J. Bodenk, v. 7, 

p. 117-140.

 1931, Can absence of edge-water encroachment in certain oil fields be

ascribed to capillarity?: Am. Assoc. Petroleum geologists Bull., v. 15, pt. 1,

p. 189-200. 

Wenzel, L. K., 1942, Methods for determining permeability of water-bearing

materials, with special reference to discharging-well methods: U.S. Geol.

Survey Water-Supply Paper 887. 

White, W. N., 1932, A method of estimating ground-water supplies based on

discharge by plants and evaporation from soil: U.S. Geol. Survey WaterSupply Paper 659-A, 106 p.

O