Triaxial Shear of Soil with Stress Path Control by Performance Feedback Charles Hadley Cammack Iowa State University

Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1978 Triaxial shear of soil with stress path control by performance feedback Charles Hadley Cammack Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Civil Engineering Commons Recommended Citation Cammack, Charles Hadley, "Triaxial shear of soil with stress path control by performance feedback " (1978). Retrospective Theses and Dissertations. 6377. https://lib.dr.iastate.edu/rtd/6377 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. 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University Microfilms international 300 N, ZEEB ROAD, ANN ARBOR, Ml 48106 18 BEDFORD ROW, LONDON WC1R 4EJ, ENGLAND 7 907240 CAMMACK, CHARLES HADLEY TSIAXIAL SHEAR OF S3IL WITH STRESS PATH CONTROL BY PERFORMANCE FEEDBACK. IOWA STATE UNIVERSIFY, PH.D., 1978 UniversiW MicrOTlms International soon, zeeb road, ann arbor, mi «siœ © 1978 CHARLES HADLEY CAMMACK All Rights Reserved Triaxial shear of soil with stress path control by performance feedback by Charles Hadley Cammack A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Department; Civil Engineering Major: Geotechnical Engineering Approved: Signature was redacted for privacy. or Work Signature was redacted for privacy. For the Major*Department Signature was redacted for privacy. For the Graduate College Iowa State University Ames, Iowa 1978 Copyright (g) Charles Hadley Cammack, 1978. All rights reserved. ii TABLE OF CONTENTS Page INTRODUCTION 1 LITERATURE REVIEW 4 THEORETICAL DEVELOPMENT 15 Basic Test Procedure 15 'Philosophy 15 Theory 2 7 A consideration of energy principles 45 Summary of theoretical development 46 Pore Pressure Parameter 50 REQUIRED MEASUREMENTS AND CORRECTIONS 56 Radial Strain 56 Volumetric Strain 58 Effectiveness of Measurements and Corrections 66 SOIL PROPERTIES AND APPLICATIONS 69 Methods of Estimating Settlement 69 Stress-path method 69 Case study I 77 Soil moduli/elastic solution method 78 Case study II 81 Summary 90 Soil Strength Parameters - Cohesion and Angle of Internal Friction 95 Soi] Moduli - Application to the Finite Element Method 107 DETAILS OF THE INCREMENTAL TRIAXIAL SHEAR TEST PROCEDURE 122 SUMMARY AND CONCLUSIONS 12 5 iii Page RECOMMENDATIONS FOR FURTHER RESEARCH 129 BIBLIOGRAPHY 132 ACKNOWLEDGEMENTS 134 APPENDIX A: PRECISION OF THE DETERMINATION OF MEASURED RADIAL STRAIN 135 APPENDIX B: COMPUTER PROGRAMS 136 Computer Program For Drained Conditions 137 Computer Program For Undrained Conditions 142 List of Variables Used in the Computer Programs 147 iv LIST OF FIGURES Page Figure la. Mohr-circle representation of stress states. 5 Figure lb. Stress-path representation of stress states. 5 Figure Ic. Relationship between stress-path notation for the failure state and the Mohr-Coulomb envelope. Figure 2. Several proposed relationships between K and (j)' 10 Figure 3a. The Iowa Continuous K-Test Apparatus. 13 Figure 3b. Typical stress path from the Iowa Continuous K-Test. 13 Figure 4. Stress path for Lambe's stress-path method combined with that of the conventional triaxial shear test. 16 Figure 5. Stress paths and line from an idealized triaxial shear test series. 18 Figure 6. Relationship between angle of internal fric tion and earth pressure coefficient K and K^. ° 22 Figure 7a. Stress path, Kf line, and Mohr-Coulomb fail ure envelope for a cohesionless soil having an angle of internal friction of 30°. 23 Figure 7b. Relationship of confining pressure to axial stress from the same test. 23 Figure 8. Relationship between incremental stress ratio and both Poisson's and incremental strain ratio. 34 Figure 9. Various relationships between incremental strain ratio and Poisson's ratio. 36 Figure 10. Two definitions of a cycle. 39 V Page Figure 11. Relationship between strain ratio (y) and the coefficient f(y). 4 2 Figure 12a. Conventional and apparent incremental triaxial shear stress paths. 51 Figure 12b. An enlarged view of the origin of Figure 12a. 51 Figure 13. Diagram of the air-over-water triaxial shear test equipment. 59 Figure 14. Relationship between confi ing pressure and change in volume change re ding. 61 Figure 15. Relationship of confining pressure to volume change error attributed to lack of fit be tween the membrane and sample. 63 Figure 16a. Drawing of ruled, stainless steel straps. 68 Figure 16b. Typical relationship between the measured radial strain ("maximum), and the calculated radial strain (average), 68 Figure 17. Stress - strain relationships determined from oedometer and incremental triaxial shear tests of a saturated soil. 73 Figure 18. Stress-strain relationships determined from oedometer and incremental triaxial shear tests of an unsaturated soil. 76 Figure 19. Plan and profile of grain elevator founda tion. 83 Figure 20. Generalized subsurface profile of site. 84 Figure 21. Stress-strain relationship from test of soil from fourth layer. 86 Figure 22a. Stress path from an incremental triaxial shear test. 96 Figure 22b. Relationship of axial stress to axial, radial and volumetric strain from the same incremental triaxial shear test. 98 vi Page Figure 23. Stress path from an incremental triaxial shear test. 98 Figure 24. Two incremental triaxial shear test stress paths and three Mohr circles from conven tional triaxial tests of remolded soil. 105 Figure 25. Geometric relationship between coordinate systems used in describing the hyperbolic stress function. 114 Figure 26. Relationship of Poisson's ratio to axial stress in an incremental triaxial shear test. 119 Figure 27. Relationship of Young's modulus to axial stress in an incremental triaxial shear test. 120 y vii LIST OF TABLES Page Table 1. Settlement analysis of rigid mat using stresses at the corner and incremental triaxial shear test data. 88 Table 2. Settlement analysis of rigid mat using stresses at the center and incremental triaxial shear test data 89 Table 3. Settlement analysis of rigid mat using stresses at the center and oedometer test data. 91 Table 4. Settlement analysis of rigid mat using stresses at the corner and oedometer test data. 92 1 INTRODUCTION The conventional triaxial shear test is primarily a means of determining two soil strength parameters --cohesion and angle of internal friction. The test has as a theoretical basis Mohr-Coulomb Failure Theory, which states that failure will occur along a plane when the angle between the normal stress and the resultant of the normal and shearing stresses is at a maximum. This state of stress exists when the Mohr circle intersects the Coulomb failure envelope. These most severe combinations of stresses that a material can sustain can be represented by a straight line (failure envelope) on a Mohr diagram. The slope of the line is indicative of the angle of internal friction, while the intercept represents co hesion. Triaxial shear tests are widely used today to determine soil strength and material properties. These tests are considered to be among the most reliable available. This is true despite the fact that conventional test procedures uti lizing constant confining pressures fail to correctly model field stress conditions. This shortcoming has been described as being unimportant since it has been shown that variations in test procedures, such as the use of decreasing confining

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