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This dissertation has bean microfilmed exactly as received 68*8863 NOR M A N , Carl Edgar, 1931- MICRO FRACTURES IN BRITTLE ROCKS: THEIR RELATIONSHIP TO LARGER SCALE STRUCTURAL FEATURES AND EXISTING GROUND STRESSES. The Ohio State University, Ph.D., 1967 Geology University Microfilms, Inc., Ann Arbor, Michigan MICROFRACTURES IN BRITTLE ROCKS: THEIR RELATIONSHIP TO LARGER SCALE STRUCTURAL FEATURES AND EXISTING GROUND STRESSES DISSERTATION Presented in Partial Fulfillment of the Requirements for Degree Doctor of Philosophy In the Graduate School of The Ohio State University By Carl Edgar Norman,, B.A.* M.Sc. ****** The Ohio State University 1967 Approved by Advi«ers Department of Geology ACKNOWLEDGMENTS The writer wishes to express sincere appreciation to Dr. H. J. Pincus who introduced him to the study of rock mechanics and provided guidance and stimulation of thought throughout the study. The Applied Physics Laboratory of the U. S. Bureau of Mines financed most of the field work by providing employment and field expenses during the summers of 1 9 6 3 1964; it also paid for the cost of the rock thin sections. Special thanks are due Dr. Leonard Obert of the U. S. Bureau of Mines who guided the in situ stress determination work and provided laboratory data on the physical properties of the rocks. The writer also wishes to acknowledge a National Science Foundation Graduate Fellowship which sponsored part of the research and field work. The writer expresses his indebtedness to Dr. Coiin Bull who served as adviser in the late stages of manu­ script preparation, to Mrs. Pat Price for her expert typing of the manuscript, and to his wife, Judith, for her patience, encouragement, and nontechnical advice. ii VITA February 1, 1931 B o m - Cokato, Minnesota 1957 ......... B.A., University of Minnesota, Minneapolis, Minnesota 195^1959 - • • Research and Teaching Assistant, Department of Geology, The Ohio State University, Columbus, Ohio 1959 ......... M.Sc., The Ohio State University, Columbus, Ohio 1959-1962 . Exploration Geologist, Humble Oil and Refining Company, Grand Rapids, Michigan; Durango, Colorado; and Mattoon, Illinois 1 9 6 2 -1 9 6 3 . Teaching Assistant, Department of Geology, The Ohio State University, Columbus, Jphio 1963-1964 . Geophysicist, Applied Physics (Summers) Research Lab, U. S. Bureau of Mines, College Park, Maryland 1963-1965 • • • National Science Foundation Graduate Fellow, Department of Geology, The Ohio State University, Columbus, Ohio 1965-1967 • • • Instructor, Department of Geology, University of Houston, Houston, Texas PUBLICATIONS "Classification of the Limestones of the Type Cincinna- tian." (Abstract with M. P. Weiss): Geol. Soc. America Bull., v. 71, p. 2 0 2 8, i9 6 0 iii "The American Upper Ordovician Standard; II. Develop­ ment of Stratigraphic Classification of Ordovician Rocks in the Cincinnati Region." (with M. P. Weiss): Ohio Geol. Survey Info. Circ. No. 26, 14 p., i9 6 0 "The American Upper Ordovician Standard; IV. Classifi­ cation of the Limestones of the Type Cincinnatian." (with M. P. Weiss): Jour. Sed. Petrol., v. 30, p. 2 8 3- 2 9 6, i9 6 0 "The American Upper Ordovician Standard; VII. Stratig­ raphy and Petrology of the Cynthiana and Eden Forma­ tions of the Ohio Valley." (with M. P. Weiss, W. R. Edwards, and E. R. Sharp): Geol. Soc. America Special Paper 8l, 7 6 p., 1 9 6 5 FIELDS OF STUDY Major Field: Geology Studies in Geophysics: Professor Colin B. Bull Studies in Engineering Mechanics: Professors Edgar C. Clark and Samuel B. Folk Studies in Structural Geology and Engineering Geology: Professor Howard J. Plncus Studies in Stratigraphy: Professors Edmund M. Speiker, Malcolm P. Weiss, and Robert L. Bates Studies in Hydraulics: Professor George P. Hanna iv TABLE OF CONTENTS Page ACKNOWLEDGMENTS . ii VITA ............. ill LIST OF TABLES . vii LIST OF ILLUSTRATIONS viii INTRODUCTION .... 1 Chapter I. RATIONALE OF THE INVESTIGATION .... 7 Griffith Theory Influence of grain anisotropy on fracture orientation Microfractures and stress magnitudes Persistence of microfractures in time II. FIELD M E T H O D S .......................17 Borehole deformation method Core deformation method Field procedure Photoelastic gages III. LABORATORY METHODS ................. 27 Compositional analysis Recording and error analysis of microfracture data Recording and error analysis of quartz c-axis data PetrofabrTc diagrams XV. MICROFRACTURES ..................... 40 Definition Description Frequency of Occurrence Origin of microfractures In ex­ tension or shear Origin of microfractures during preparation of thin section Origin of microfractures during coring of rock Microfracture frequency according to grain size and composition in polymineralic rocks v V. MICROFRACTURES, STRUCTURE, AND IN SITU STRESSES AT NORAD COC, C3I3TRADO SPRINGS, C O L O R A D O ......... 6 3 Geological setting Petrography In situ stress determinations Microfracture orientation data Open microfractures Filled and healed microfractures Summary of investigations at NORAD COC, Colorado VI. MICROFRACTURES, STRUCTURE, AND IN SITU STRESSES IN THE ATLANTA, SEMGIA, REGION ................... 102 Rock Chapel Mountain Petrography Structure Stress Determinations Microfractures Pine Mountain Petrography Structure Stress Determinations Microfracture s Arabia Mountain Petrography Structure Stress Determinations Microfractures Stone Mountain Petrography Structure Stress Determinations Microfractures Douglasville Petrography Structure Stress Determinations Microfractures Summary of investigations at At­ lanta, Georgia VII. CONCLUSION ....................... 144 BIBLIOGRAPHY 149 LIST OP TABLES 1 Frequency of microfractures in four rock t y p e s ...................................... 48 2 Comparison of microfracture frequency with grain mineralogy .......................... 6 0 3 Photoelastic strain gage data* NORAD COC, 1 C o l o r a d o .................................. 78 4 Poisson probabilities of randomness in orientations of microfractures at NORAD COC, Colorado.............................. 8 8 5 Comparison of orientations of fluid in­ clusion planes, open microfractures, and common joints, NORAD COC, Colorado ........ 9 6 6 Calculated secondary principal stresses at Rock Chapel, Georgia....................113 vii LIST OP ILLUSTRATIONS Fluid inclusion planes transect two ad­ jacent grains without changing direction . Borehole deformation method of in situ stress determination ........ ........ Core deformation method of in situ stress determination ................... Stress-relief drilling equipment ........ Photoelastic gage mounted on smoothed back of a 3-inch diameter hole .......... Borehole polariscope ................... Influence of thin section orientation on number of microfractures visible in thin section ....................... Composite petrofabric diagram of data from two perpendicular sections, show­ ing areas to be contoured separately . Partially healed microfracture in a quartz grain ........................... Microfracture filled with a material having high birefringence ............. Quartz grain with fluid inclusion plane Plagioclase grain showing open micro­ fractures transecting planes of fluid Inclusions........... ................ Open microfracture transecting four quartz grains differing in crystal- lographic orientation ............... viii 14 Set of three parallel* open micro­ fractures in a quartz g r a i n .............. 46 15 Frequency of microfractures vs. princi­ pal strain magnitudes and differences . 5 5 16 Microfracture frequency vs. grain size . 57 17 Microfracture frequency per unit grain area vs. grain diameter .................. 5 8 18 Percent of total microfractures vs. percent total grain area in two rock s a m p l e s .................................. 6 l 19 Plan and cross-sectional views of stress-relief holes at NORAD COC* Colorado.................................. 64 20 Cyclographic diagram of major dike and joint systems at NORAD COC* Colorado . 6 8 21 Stereonet plot of poles to joints* Chamber A* NORAD COC* Colorado............ 6 9 22 Equal area diagram of quartz c-axes* samples N2-4 and N5-4 72 23 Secondary principal stress directions and magnitudes at NORAD COC* Colorado . 79 24 Microfracture filled sequentially with two secondary minerals .................. 84 25 Equal area diagrams of poles to filled* healed* and open microfractures* NORAD COC* C o l o r a d o ............................ 93 2 6 Geologic map of the Atlanta* Georgia* r e g i o n ................................... 103 27 Aplite vein concordant with foliation cut by later discordant v e i n ............. 1 0 7 28 Structure of the Lithonia gneiss* Consolidated Quarries* Rock Chapel* G e o r g i a ................................. 108 ix 29 Equal area diagram of poles to foliae and concordant veins, Rock Chapel, Georgia ............................. 30 Small valley developed along a natural fracture in Lithonia gneiss, Rock Chapel, Georgia .................. 31 Sheeting in Lithonia gneiss exposed in quarry at Rock Chapel, Georgia ........ Ill 32 Vertical, nearly planar fracture sur­ face induced in Lithonia gneiss by quarry operator ...................... 33 Upwarped slab of rock on pavement sur­ face at Rock Chapel, Georgia .......... 117 34 En echelon array of vertical fractures Induced by detonation of explosives in vertical drill holes .................. 119 35 Orientations of open microfractures at Rock Chapel, Georgia .................. 36 Structure of the Lithonia gneiss, Pine Mountain, Georgia ................ 37

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