A RE-EVALUATION OF THE FILTER PAPER METHOD OF MEASURING SOIL SUCTION by RIFAT BULUT, B.S.C.E. A THESIS IN CIVIL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN CIVIL ENGINEERING Approved August, 1996 Eos I ^^^ ACKNOWLEDGMENTS UhO 1^ would like to express my sincere and deep appreciation to Dr. Warren Kent Wra> for his guidance, endless encouragement, and assistance throughout the course of this study. I also wish to thank Dr. Priyantha W. Jayawickrama for his kindK consentmg to serve on my thesis committee. 1 would sincerely like to thank Mr. Hsiu-Chung Lee for his wholehearted cooperation during performing of the experiments and guidance. My sincere appreciation to Mr. Brad Thomhill, Mario Torres, and Drex Little for their help in providing equipment for the experiments. Finally, 1 wish to thank my family and friends for their support and encouragement throughout the whole study period. n TABLE OF CONTENTS ACKNOWLEDGMENTS ii ABSTRACT v LIST OF TABLES vi LIST OF FIGURES vii CHAPTER L INTRODUCTION 1 1.1 Problem Statement 1 1.2 Statement of Objectives 4 1.3 Research Approach 6 11. SEARCH OF THE TECHNICAL LITERATURE 7 2.1 Soil Suction Concept 7 2.2 The Filter Paper Method 14 2.2.1 Historical Background ofthe Filter Paper Calibration 15 2.2.2 Working Principle ofthe Filter Paper Method 22 2.2.2.1 Principle of Total Suction Measurements 23 2.2.2.2 Principle of Matric Suction Measurements 25 2.2.3 Calibration Technique of Filter Paper 26 2.2.3.1 Total Suction Calibration 26 2.2.3.2 Matric Suction Calibration 29 2.2.4 Performance of Filter Paper Method 33 m. LABORATORY STUDIES 35 3.1 Apparatus Used in the Filter Paper Method 35 3.2 The Experimental Procedure 36 3.3 Total Suction Calibration Curve 39 in y '.'yy-'y ^^ IV. EVALUATION OF THE PROBLEM 44 4.1 The Problem 44 4.2 Sensitivity of Total Suction at High Water Content 44 4.2.1 Total Suction and Relative Humidity Relationship 44 4.2.2 Osmotic Suction versus Total Suction 45 4.2.3 Total Suction versus Filter Paper Water Content 46 4.3 Total Suction and Sah Solutions 46 4.4 Equilibration Time 50 V. CONCLUSIONS AND RECOMMENDATIONS 53 5.1 Conclusions 53 5.2 Recommendations 54 REFERENCES 56 APPENDICES A. CONVERSION UNITS 60 B. ASTM D 5298 - 92 STANDARD TEST METHOD FOR MEASUREMENT OF SOIL POTENTL\L (SUCTION) USING FILTER PAPER 63 C. RELATIVE HUMIDITIES AND OSMOTIC POTENTL\LS OF SOME SALT SOLUTIONS 69 D. "SEPTEMBER" DATA VALUES USED IN CALIBRATION OF TOTAL SUCTION CURVE 73 IV .....UUMM /• ABSTRACT The filter paper method is a soil suction measurement technique. Soil suction is one ofthe most important parameters describing the moisture condition of unsaturated soils. The measurement of soil suction is crucial for applying the theory' behind the unsaturated soils. The filter paper method is a laboratory test method, but it is inexpensive and relatively simple. It is also the only known method that covers the full range of suction. With the filter paper method, both total and matric suction can be measured. If the filter paper is allowed to absorb water through vapor flow (non-contact method), then only total suction is measured. However, if the filter paper is allowed to absorb water through fluid flow (contact method), then only matric suction is measured. In this research, the filter paper method was investigated, especially the lower (wet) part ofthe total suction calibration curve. A total suction calibration curve was constructed using sodium chloride, NaCl, salt solutions and S & S No. 589 White Ribbon filter papers. Salt solutions and filter papers were brought to equilibrium through vapor flow (total suction measurement) at isothermal conditions. Equilibrium time and temperature were two weeks and 25°C , respectively. It was found out that total suction is very sensitive below 2.5 pF and the reason for this sudden change in suction arise from double effect ofthe logarithms (i.e., natural logarithm from Kelvin's equation and base ten logarithm from pF scale) and sensitivity of filter papers to water at low suctions. Filter paper is a poorly graded material, so it has more tendency to absorb water at low suctions. Another conclusion from these experiments and literature review was that the calibration curve is highly salt solution-type dependent because every salt solution has a different chemical activity thus different osmotic potential. The filter paper method needs further investigation in regard to equilibrium time and temperature fluctuations. "^'(•Cv LIST OF TABLES 1.1 Instruments formeasuring soil suction 2 2.1 Water potentials of NaCl solutions at 25°C temperature 11 A.l Soil suction conversion factors 61 C.l Relative humidities of some saturated salt solutions 70 C.2 Water potentials of NaCl solutions in Bars 71 C.3 Water potentials of KCl solutions in Bars 72 VI '^••^^. LIST OF FIGURES 1.1 Selected calibration curves, constructed usine different types of filter papers and devices 5 2.1 Total suction versus relative humidit\, constructed using Kelvin's equation at a reference temperature of 20T 9 2.2 Total suction and relative humidity relationship using Kelvins equation at a reference temperature of20"C 12 2.3 Filter paper calibration curve for Whatman No. 42 filter papers using a combination of pressure membrane, pressure plate, and vacuum desiccator methods 17 2.4 Summry of calibration data using S and S White Ribbon filter papers 18 2.5 Filter paper calibration relationship using different types of filter papers 20 2.6 Filter paper test cofigurations 24 2.7 Total suction calibration test configuration 28 2.8 Total suction calibration curve for saturated salt solutions of MgNO?. NH4CI, and CaS04 and Fisher quantitative coarse filter papers 30 2.9 Matric suction calibration test configuration 31 2.10 Matric suction calibration curve for Fisher quantitative coarse filter papers 32 3.1 Data sheet for filter paper water content measurements 40 3.2 Total suction calibration data for NaCl salt solutions and J.J TotaS & lS suctio No. 58n 9calibratio White Ribbon curvn efilter for Spaper & S sNo . 589 White Ribbon 42 filter papers and NaCl sah solution 43 4.1 Total suction versus NaCl solutions in molality. constructed using Lang's Table 47 vii •SWHBTs 4.2 Osmotic suction versus NaCl solutions m molality, osmotic coefficients and water potentials of NaCl calculted from Lang's Table 48 4.3 Total suction calibration curve in kPa units 49 4.4 Comparison of calibration curves 52 A.l Graphical presentation of conversion units 62 Vlll •"*~"^''"™raHi CHAPTER I INTRODUCTION 1 • 1 Problem Statement Soil suction can simply be defined as the unit attractive force ofthe soil for water (McKeen, 1977). The measurement of soil suction in engineering practice is very important for the application ofthe theory and practice behind unsaturated soil mechanics. Soil suction is one ofthe most important stress variables describing the behavior ofthe unsaturated soils. In many cases, the soils are mostly unsaturated and behave quite differently from that predicted by saturated soil mechanics theory. Soil suction and positive pore water pressure are two similar important parameters in regard to describing the behavior of unsaturated and saturated soils, respectively (Houston et al., 1994). With a reliable soil suction measurement technique, the initial and final soil suction profiles can be obtained at convenient depth intervals. The change in suction with seasonal moisture movement is valuable information for many engineering applications. There are many soil suction measurement techniques in the fields of soil science and engineering. Some ofthe instruments used in soil suction measurement are shown in Table 1.1. Most of these instruments have limitations with regard to range of measurement, equilibration times, and cost. Therefore, there is a need for a method which can cover the full range of measurement, be adopted for a routine basis, and is inexpensive. The filter paper method, which was evolved in Europe in the 1920s and came to the United States in 1937 with Gardner (1937), has been accepted among soil scientists and engineers. The filter paper is a specially designed circular porous paper. .IJIUM Table 1.1 Instruments for measuring soil suction, taken from class notes for PSS 5335 Soil Physics, Department of Soil Science, Texas Tech University, 1995. Name of Measured Range Comments Instrument Suction (MPa) Psychrometer Total 0.2-5 •Constant temperature environment is required. •Resolution and accuracy are good. Temperature can be measured also. •Calibration may be required approximately every six months. Tensiometer Matric 0-0.08 •Problems with air diffusion through ceramic cup. •Limited moisture range. •Good precision and accuracy. Pressure Plates Matric 0-1.5 •Range of measurement is a function of the air entry value of the ceramic disc. The application of the method is versatile. •Expensive method. Resistance Blocks Matric 0.1-1 •Must be calibrated for individual soils. •Very low precision in wet range. •Plaster and gypsum blocks deteriorate rather rapidly. •Inexpensive method. Heat Dissipation Matric 0.01-1 •Can be interfaced with a data logger. Blocks •Easy to operate. •Hysteresis may be a problem y The filter paper method can cover the full range of suction measurement. It is a laboratory test method, but it is inexpensive and relatively simple. The working principle behind the filter paper method is that the filter paper will come to equilibrium with the soil either through vapor flow or liquid flow, and at equilibrium suction value ofthe filter paper and the soil will be the same. If the filter paper is allowed to absorb water through vapor flow (no contact between the filter paper and soil), then only total suction is measured.