MECHANICAL PROPERTIES OF ST. PETER SANDSTONE A COMPARISON OF FIELD AND LABORATORY RESULTS A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY MICHAEL EUGENE DITTES IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE DECEMBER, 2015 This thesis contains previously published material (Appendix E) in the “Journal of Geotechnical and Geoenvironmental Engineering” © 2002 ASCE. All other material © 2015 Michael E. Dittes All Rights Reserved. ACKNOWLEDGMENTS I would like to extend my thanks to those, who without their help I would not have been able to bring this project to completion. First and foremost, I thank Professor Joseph Labuz who took a chance on me, by offering me a TA-ship. Through my time in school, Joe helped me stay focused when I started to move off on tangents, offered timely advice, and over the years has become much more than my advisor. I am proud to think of him as a friend. Professors Andrew Drescher and Peter Huddleston also deserve special thanks for providing critical review of my work, and agreeing to sit on my thesis committee. To Ms. Tiffany Ralston who had an uncanny way of knowing what I needed to do, and how, before I knew myself. To Charles Nelson who helped me streamline my thesis project and helped me gain access to the Minnesota Library Archives construction site. To my fellow graduate students with whom I debated, joked and laughed, you helped me see things that I was missing. Your input was invaluable and you have my deep appreciation. To my parents who always pushed myself, and my sisters, to never stop learning and achieving. Thank you. Finally, to my wife Chantale a very special thank you. She has supported me while seeing the best and the worst of me, without flinching, and her talents in everything Word was indispensable in completing this work. You are my friend and my love. i DEDICATION This work is dedicated to my late father, Richard E. Dittes, who constantly pushed me to “finish what I start.” Through the twists and turns in life, I did not forget that advice, and though it has taken some years, this project is finally complete. This one is for you, Dad. ii ABSTRACT The St. Peter sandstone is an arenaceous, ortho-quartzitic, sublittoral cratonic sheet sand of middle Ordovician age. It is remarkable in geographic extent, mineralogical composition, thickness and engineering properties. A vast majority of its extent is buried; only in the Upper Mississippi River valley is it readily exposed. The St. Peter sandstone behaves as a locked sand, a nearly cohesionless geo- material characterized by brittle behavior, a lack of interstitial cement, and high dilation rates at failure. When confined it is capable of supporting large loads with small deformations, even under saturated conditions, yet when confinement is removed it disintegrates and readily flows. Dry intact samples have a uniaxial strength of approximately 1 MPa and a Young’s Modulus of about 1 GPa. Triaxial tests conducted at confining pressures of less than 150 kPa yield an angle of internal friction of approximately 60°. Because the St. Peter sandstone disintegrates so readily when unconfined, sampling the material is quite difficult. The St. Peter sandstone has been excavated as foundation material, and for tunnels and sewers, by the cities of Minneapolis and St. Paul for decades. Earliest testing was for those endeavors. As the interest in the design of underground spaces has grown, testing of the St. Peter sandstone has changed to meet that need but testing has been done predominantly in the laboratory. The purpose of this project was to evaluate the mechanical properties of St. Peter sandstone by comparing in situ tests with laboratory test results. Direct shear tests were conducted to evaluate strength-dilatancy behavior. Transmitted light and scanning electron microscopy were used to help explain the high friction angle of the material. At iii low confining pressures the St. Peter sandstone exhibits a friction angle of around 60° but with small cohesion, less than 100 kPa. The high angle of internal friction at failure may be due to locked sand particles or to post-depositional quartz overgrowths. Tests on pulverized, densely packed samples (with void ratios similar to intact samples) and loosely packed samples were conducted in the same fashion as the intact samples and yielded friction angles of approximately 45° and 35° respectively. Pressuremeter tests were performed in situ and the results were interpreted using elasto-plastic analysis. By properly considering system stiffness, a Young’s modulus of approximately 0.5 GPa was determined and a friction angle between 60° and 40° was estimated, depending on the assumed dilation angle. iv Table of Contents INTRODUCTION ........................................................................................................................... 1 GEOLOGY ...................................................................................................................................... 6 Introduction .................................................................................................................................. 6 Geographic Distribution ............................................................................................................... 7 Stratigraphic Position ................................................................................................................... 8 Lithology .................................................................................................................................... 10 Structure ..................................................................................................................................... 13 Depositional Environment ......................................................................................................... 17 Petrography ................................................................................................................................ 20 Economic Value ......................................................................................................................... 23 INDEX PROPERTIES................................................................................................................... 24 Specific Gravity, Gs ................................................................................................................... 24 Unit Weight, Porosity, and Voids Ratio .................................................................................... 25 Grain-Size Distribution .............................................................................................................. 26 Shear Strength ............................................................................................................................ 31 Direct Shear ............................................................................................................................... 33 Results ........................................................................................................................................ 36 Triaxial Compression ................................................................................................................. 46 Uniaxial Compression ................................................................................................................ 50 v Plate Load Tests ......................................................................................................................... 61 Permeability ............................................................................................................................... 61 MICROSCOPIC ANALYSIS ........................................................................................................ 62 Thin Section Analysis ................................................................................................................ 62 Locked Sand Grains ................................................................................................................... 64 Quartz Overgrowths ................................................................................................................... 67 FIELD RESULTS .......................................................................................................................... 73 Pressuremeter Testing ................................................................................................................ 73 Introduction ............................................................................................................................ 73 System Saturation .................................................................................................................. 73 Bladder Stiffness Calibration ................................................................................................. 74 System Stiffness Calibration .................................................................................................. 75 Pressuremeter Tests ............................................................................................................... 86 Borehole Preparation ............................................................................................................. 87 Data Correction ...................................................................................................................... 88 Determination of Young’s modulus E ......................................................................................