A SIMPLIFIED METHOD FOR IDENTIFYING THE PREDOMINANT CLAY MINERAL IN SOIL by Eugene C. Mojekwu, B.S. in C.E. A THESIS IN CIVIL ENGINEERING Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirement for the Degree of MASTER OF SCIENCE IN CIVIL ENGINEERING Approved Accepted December, 1979 ACKNOWLEDGMENTS I am deeply indebted to Dr. Warren K. Wray for his guidance and counseling during this research. I am also grateful to Dr. C.V.G. Vallabhan for his advice and useful criticisms. Grateful acknowledgment is also made to the Civil Engineer- ing Department of Texas Tech University for providing me with financial support during this research. Sincere appreciation is also extended to the following soil sample contributors, viz, McClelland Engineering, Houston, Texas; Southwestern Laboratories, Beaumont and Arlington, Texas; Amarillo Testing Laboratory, Amarillo, Texas; East Texas Testing Laboratory, Inc, San Antonio, Texas; Spencer J. Buchanan and Associates, Bryan, Texas; Trinity Engineering and Testing Corporation, Austin, Texas; and Rone Engineering, Arlington, Texas. n TABLE OF CONTENTS Paae ACKNOWLEDGMENTS ii ABSTRACT...' V LIST OF TABLES vi LIST OF FIGURES vii I. INTRODUCTION 1 1.1 The Problem 1 1.2 Purpose and Scope of Thesis 1 1.3 Review of Literature 2 1.3.1 Expansive Soils 2 1.3.2 Available Clay Mineral Identification Methods 5 1.3.3 Atterberg Limits 6 1.4 Definition of Terms 10 II. MATERIALS AND TEST METHODS 13 2.1 Materials 13 2.2 Test Methods • 13 2.2.1 Cation Exchange Capacity 15 2.2.2 Atterberg Limits 15 2.2.3 Hydrometer Analysis 16 íll. RESULTS AND DISCUSSION 17 3.1 Test Results and Discussion on Cati on Exchange Capaci ty 17 3.2 Test Results and Discussion on Atterberg Limits 19 3.3 Correlation of Data 21 3.3.1 Analysis of Test Data 22 3.3.2 Discussion on Correlation of Data 24 3.3.3 Integrity of Correlation Equation 24 3.3.4 Summary 29 IV. CONCLUSIONS AND RECOMMENDATION 30 LIST OF REFERENCES 31 APPENDIX A: COUNTIES, GEOLOGY, AND CLIMATE OF THE CITIES AND THE DEPTHS FROM WHICH SOIL SAMPLES WERE OBTAINED 34 m Page APPENDIX B: SIMPLIFIED PROCEDURE FOR DETERMINING CATION EXCHANGE CAPACITY USING A SPECTROPHOTOMETER (AFTER PEECH (10)) 43 APPENDIX C DETERMINATION OF ATTERBERG LIMITS 46 APPENDIX D HYDROMETER ANALYSIS TEST PROCEDURE 52 APPENDIX E TEST DATA FOR CATION EXCHANGE CAPACITY (CEC), ATTERBERG LIMITS (PLASTIC LIMIT (PL) AND LIQUID LIMIT (LL)), AND PLASTICITY INDEX 56 APPENDIX F: SUBROUTINE "INPUT" 59 APPENDIX G: PROCEDURE FOR REMOVING ORGANIC MATTER FROM SOIL 61 IV ABSTRACT Increased construction activity in sites that contain wery active clay minerals has greatly expanded the necessity for engineering know- ledge related to the type and amount of clay minerals in a given soil. Presently, there are varying methods of predominant clay mineral iden- tification. These methods are, however, frequently time consuming and laborious and require expensive equipment that is not commonly found in the ordinary commercial soils testing laboratories. Pearring in 1968 and, later, Holt in 1970 developed a correlation chart to aid in the identification of the predominant clay mineral in a given soil. The two parameters involved are Cation Exchange Acti- vity and Activity Ratio. These two parameters require the plasticity index, the cation exchange capacity, and the percent of clay in the soil fraction passing the No. 200 sieve. Presently, cation exchange capacity determination is not devoid of expense and time problems. These problems prompted this research which is intended to relate the cation exchange capacity of a clay soil to the easily obtainable Atterberg limits (plastic limit and liquid limit) and plasticity in- dex. A detailed study was made of selected soils of varying geographic locations and geologic origins to establish data related to the chem- ical (cation exchange capacities) and engineering index properties of such materials. A study of the test results discloses that it is possible to predict the cation exchange capacity of a soil and, hence, the predominant clay mineral in the soil, using the plastic limit. The result of the correlation study shows a strong relation to exist between the cation exchange capacity and the plastic limit of all soils tested. This relation may be approximated by the expres- sion CEC = PL^*^^. LIST OF TABLES Page 1. Reproducibility of Cation Exchange as Determined by the Peech Method Using the "Spectronic - 20" 18 2. Comparison of Methods of Determining Cation Exchange Capacity 18 3. Results of Regression Analysis 23 4. Comparison Betv/een Logarithmic Equations With and Without Intercepts 26 5. Comparison of Clay Mineral Identification Procedures 28 VI LIST OF FIGURES Pa^e 1. Correlation of Cation Exchange Activity, Activity Ratio, and Clay Minerals of Montmorillonitic and Latertic Soils (After Pearring and Holt) 7 2. Relationship Between Plasticity Index and Clay Fraction (After Skempton (23)) 9 3. Location of Soil Samples Tested 14 4. Correlation Between Cation Exchange Capacity and Plastic Limit of Fifty-Five Soil Samples 25 vn CHAPTER 1. INTRODUCTION Many methods are presently available that can be used to assess the potential volume change characteristics of a soil. However, it is frequently desirable to know the predominant clay mineralogy of the soil in order to better assess its potential for shrink-swell activity. Simple classification tests can only imply the soil acti- vity whereas more sophisticated X-ray diffraction, infrared analysis, and other tests are expensive and can require lengthy testing periods This paper will present the results of a substantial testing program that correlates such soil properties as Atterberg limits, cation ex- change capacity, clay content, and activity into a simplified means of identifying the predominant clay minerals of montmorillonite, ill- ite, kaolinite, halloysite, and attapulgite without performing any tests more sophisticated than the standard Atterberg limits and hydro- meter analyses. 1.1 The Problem The geotechnical engineer has frequently been faced with the problem of identifying the predominant mineral in an active clay. The basic solution to this problem has been based on experience and, mostly, on the techniques available to him as well as the clay min- eralogist and the soil physicist. The techniques commonly used in- clude X-ray diffraction analysis, chemical analysis, electron micro- scope resolution, differential thermal analysis, gravimetrical analy- sis, and infrared analysis. These available methods are frequently laborious, expensive, lengthy and require expensive,intricate equip- ment that is, in general, too sophisticated and expensive to be found in the ordinary commercial soils testing laboratory. 1.2 Purpose and Scope of Thesis The purpose of this study is to find a simplified method of identifying the predominant clay mineral in a soil. The correla- tion chart developed by Pearring (9) and Holt (3) offered a simple 1 way of doing this except thatcation exchange capacity (CEC) is one of the required parameters. Since the CEC is not typically evalua- ted in normal practice, a simplified way to evaluate CEC was needed without using the expensive equipment normally required to do so. The Atterberg limits are easily obtainable and usually performed and, hence, the objective of this study was to investigate the possi- bility of a relation between CEC and Atterberg limits as a way to avoid using the conventional tedious means of cation exchange capa- city determination. When the final relationship between CEC and Atterberg limits was determined, a check of the overall method was made. The approach was to arbitrarily identify some samples using the above method in conjunction with Pearring-Molt correlation chart and then compare the results to clay mineral identifications, on the same samples accom- plished by X-ray diffraction analysis. 1.3 Review of Literature The review of literature pertaining to this study v/as made and is presented under the three headings: (1) Expansive Soils, (2) Atterberg Limits, and (3) Available Methods for Clay Mineral Identification. 1.3.1 Expansive Soils The expansive soil problem has long instigated a lot of inves- tigations which began as early as 1931 (3). Terzaghi (13) was the first to look into the problem of expansive soils. In his work, he concluded that swell is directly related to the electrical charge on the clay mineral and the surface tension of the water it contains. Since Terzaghi (13), other important contributions have been made in the area of swelling soils and researchers (3, 13, 14, 22, 51) generally tend to agree that soil expansion is related to the type and amount of clay mineral, the hydration rate of the adsorbed ions, the amount of exhangeable cations the negatively charged min- eral is capable of adsorbing (cation exchange capacity), and the a- nount and composition of the pore water. 3 1.3.1-A The Effect of Clay Minerals on Expansion: Clay minerals are the primary cause of soil volume changes (swelling and shrinking). It has been reported that clay minerals are commonly crystalline and contain chiefly silicon, aluminum, oxygen and water (12). According to the combinations in which these constituents occur, most of the minerals can be divided into three major groups: smec- tite (montmorillonites being the most abundant), illite, and kaoli- nite. Terzaghi and Peck (12) reported that these minerals have the same laminated crystalline structure but wery different surface ac- tivities. They also reported that kaolinites are the least active, followed by illites. Smectites, they reported, are the most active and have the capacity to swell by taking water molecules directly into their space lattice. This type of swelling is referred to as intramicellar swelling. The other type of swelling, intermicellar swelling, occurs between clay minerals and is exhibited by all clay minerals. 1.3.1-B The Effect of lons on Expansion: Terzaghi and Peck (12) reported that the surface of ewery soil particle carries a negative electric charge.