Final Report

Project Title: Soil Transport Characteristics

SCERP Project Number: H-5

Principal Investigator: Sandra Houston
Arizona State University

CHAPTER ONE

Laboratory Filter Paper Suction Measurements Abstract

Results of filter paper measurements for both matric and total suction values are reported for a sand, silt, and clay. Calibration curves for the matric and total suction are given for the filter paper used in this study, and comparisons with the results of several other researchers are made. For low suction values, the matric suction is more easily measured than the total suction using the filter paper method. This difference is due primarily to the insensitivity of the water content of the noncontact filter paper to changes in suction. A pressure membrane, saturated salt solutions, and tenisometers were used in developing the filter paper calibration curves. Potential pitfalls in making filter paper measurements and limitations of the method are discussed. A procedure for obtaining filter paper suction measurements is given.
 

CHAPTER TWO

Accurate measurement and prediction of the hydraulic properties of unsaturated soils remain a major challenge, especially in the case of undisturbed field soils. These soils are often layered and very heterogeneous. This complicates the analysis and often results in an inaccurate prediction of the hydraulic conductivity and the suction as a function of the water content. One of the most commonly used methods for obtaining unsaturated hydraulic conductivity is the instantaneous profile method of Rose et al., 1965. This method is very time-consuming since it requires water content and pressure head measurements at regular intervals of time and space. The method has been used both in the laboratory (11) and in the field (14). It has also been applied successfully to both a wetting cycle (11) and a drying cycle (4). In the drying cycle, it is limited to pressure heads higher than 200 cm of water because the redistribution rate of water becomes very small at low pressures. The results of the instantaneous method are often less accurate for a heterogeneous material, and it is therefore often difficult to justify the money spent on this type of testing.

In order to estimate the hydraulic conductivity as a function of the water content using the instantaneous profile method, the variation of the pressure head with the water content must be predetermined. The methods used for obtaining suction measurements include (13): tensiometer, thermal conductivity sensor (matric), psychrometer, filter paper, and pore fluid squeezer (osmotic). The tensiometer is commonly used and works well in the field, but cannot be used for suctions greater than 1 atm. The psychrometer can handle pressures as low as -80 bar, but is very sensitive to changes in temperature. It is therefore very difficult to use the psychrometer in the field for many circumstances. However, at depths of more than one meter, the temperature change is normally small enough to allow the use of the psychrometer (80). A method commonly used in determining the suction of partially wet soils is the filter paper. There is no theoretical limit on the pressure range for this method, but its use is normally limited to laboratory testing. Use of the filter paper method was discussed in detail in chapter 1.

Many researchers have tried to find a simplified solution to the problem of estimating the hydraulic conductivity of unsaturated soils. Most of these new methods are based on empiricallydetermined analytical expressions describing the relationship between hydraulic conductivity as a function of the water content, based on only a few soil parameters such as K(sat), and the moisture characteristic curve of the soil. Most of the empirical equations are intended to produce results that are very close to those obtained from tests using the instantaneous profile method. Some of the empirical equation-based methods and their assumptions are discussed below.

One of the most commonly used analytical expressions is that proposed by van Genuchten (14) describing the soil moisture retention characteristics. The computer program RETC.F77 uses this equation together with Mualem's predictive model. the required inputs for this program are the dry density and K(sat) for the sample plus the water content, , versus action, , relationship. The output is K versus volumetric water content for the entire range of the degree of saturation.

Ahuja et al. (5) assumed the unsaturated K to be a piecewise continuous power function of soil suction and the water content to be alogarithmic function of the soil suction. Wosten and van Genuchten (15) fit analytical expressions for the hydraulic conductivity to experimental data. This enabled them to study the effects of different model paramenters. As a result, they could relate the variation in unsaturated K to more easilymeasured properties. Warrick and Zhang (6) assumed that the unsaturated conductivity varies exponentially with the pressure head and that the material is isotropic. With the pressure head variation known, they investigated two- and three-dimensional flow from a saturated soil into an unsaturated region. Several other researchers have proposed other empirical correlations between unsaturated hydraulic conductivity and water content ((1),(7),(10),(12)).

One final note must be made. According to Hills et al. (4), the hysterisis between the wetting and drying curves is significant. It is therefore important to simulate the expected field conditions for any analysis or testing program used to determine the unsaturated hydraulic conductivity.
 

CHAPTER THREE

A Study of the Gradient Components and the Hydraulic Conductivity for Partially Saturated Soils Using One- Dimensional Infiltration Tests Introduction

The movement of water through dry soils, such as those found in arid regions, is governed by the sum of the gravity (elevation) and the suction gradients. The suction for a typical silty soil, with an in situ water content of about 3 percent, may be as high as 2000m head of water. The effect of gravity is therefore often assumed to be negligible, and the flow assumed to be governed by gradients in soil suction alone for dry, unsaturated flow.

A testing program using one-dimensional infiltration tests was carried out to examine the importance of each of the components of total potential on infiltration into initially dry soils. For the infiltration of ponded water, the infiltration tests were performed using undisturbed samples in transparent plastic tubes. Since the tube used was transparent, it was possible to monitor the movement of the wetted front as the sample was given free access to water. Some specimens were ponded at the top, and the others at the bottom. At the end of each infiltration test, the sample was extruded and sliced up into segments of 2 to 3 cm. These slices were oven dried for 24 hours to determine the final water content profile throughout the sample. Based on these data, the variation of the hydraulic conductivity with the degree of saturation was back-calculated. The results from these tests, to be discussed subsequently, show that the effect of the gravity gradient is significant for the case of ponding, and cannot be neglected. The method for estimating the hydraulic conductivity yielded reasonable results.

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Last updated 7/1/99