TECHNICAL REPORT S-72-12 IN SITU TESTS FOR THE DETERMINATION OF ROCK MASS SHEAR STRENGTH by T. W. Zeigler November I972 TA 7 Sponsored by Office, Chief of Engineers, U. S. Army .W 34t S-7 2-1 2 Conducted by U. S. Army Engineer Waterways Experiment Station 1972 Soils and Pavements Laboratory Vicksburg, Mississippi APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED Ä * « %M • ¿ i s I LIBRARY FE B 2 3 1973 Bureau of Reclamation Denver, Colorado Destroy this report when no longer needed. Do not return it to the originator. The findings in this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. k. BUREAU OF RECLAMATION DEI 92021814 IN SITU TESTS FOR THE DETERMINATION OF ROCK MASS SHEAR STRENGTH by T. W. Zeigler JOI ioiu u u lOlIffiDD]IDI November 1972 Sponsored by Office, Chief of Engineers, U. S. Army Engineering Study 553 Conducted by U. S. Army Engineer Waterways Experiment Station Soils and Pavements Laboratory Vicksburg, Mississippi ARMY-MRC VICKSBURG, MISS. APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED / FOREWORD This study was conducted by the U. S. Army Engineer Waterways Experi­ ment Station (WES). The work was sponsored by the Office, Chief of Engi­ neers (OCE), under Engineering Study (ES) 553. The study was conducted intermittently during the period from July 1969 to July 1971. Mr. T. W. Zeigler conducted the research under the general supervision of Messrs. W. C. Sherman, Chief, Soil and Rock Mechanics Branch, and D. C. Banks, Chief, Rock Mechanics Section. Messrs. J. P. Sale and R. G. Ahlvin were Chief and Assistant Chief, respectively, of the Soils and Pavements Laboratory during the period of investigation. This report was prepared by Mr. Zeigler. COL Levi A. Brown, CE, and COL Ernest D. Peixotto, CE, were Directors of WES during preparation of this report. Mr. F. R. Brown was Technical Director. iii CONTENTS Page FOREWORD ............. .................. , ...........................iii CONVERSION FACTORS, BRITISH TO METRIC UNITS OF MEASUREMENT........ vii SUMMARY ..................... .. ........... ...................... ix PART I: IN TRODUCTION.............. 1 Purpose of R e p o r t......................................... 1 S c o p e ............ 1 PART II: METHODS OF DETERMINING IN SITU SHEAR S T R E N G T H .......... 3 Direct Shear T e s t ............................... 3 Triaxial T e s t .................................................. 7 Torsion Shear T e s t ......................... 9 Pull-Out Shear Test ............................. 13 PART III: DIRECT SHEAR T E S T ............ 15 Shear of Discontinuities ..... ........................... 15 Shear of Intact Rock Including Hard Clays and Weak Shales . 54 Shear of Concrete Blocks on Rock Surfaces ........... 68 PART IV: INTERPRETATION OF IN SITU DIRECT SHEAR TEST RESULTS . 79 Introduction............................................. 79 Failure Criteria .............................................. 79 Failure Envelopes............ 84 Data from Displacement Observations ................ ..... 89 Reliability of Test Results............ 92 Application of Test Results................................... 94 Summary of Shear S t r e n g t h s ........................... 101 PART V: CONCLUSIONS AND RECOMMENDATIONS..............................107 LITERATURE C I T E D .......................................................Ill SELECTED BIBLIOGRAPHY...................... 117 TABLES 1-4 APPENDIX A: CASE HISTORIES INVOLVING TESTS OF CLEAN DISCONTINUITIES APPENDIX B: CASE HISTORIES INVOLVING TESTS OF FILLED DISCONTINUITIES APPENDIX C: CASE HISTORIES INVOLVING TESTS OF INTACT MATERIAL APPENDIX D: CASE HISTORIES INVOLVING TESTS OF CONCRETE BLOCKS CAST ON ROCK SURFACES APPENDIX E: CASE HISTORIES INVOLVING TESTS OF VARIOUS CONDITIONS v CONVERSION FACTORS, BRITISH TO METRIC UNITS OF MEASUREMENT British units of measurement used in this report can be converted to metric units as follows: Multiple By To Obtain inches 2.54 centimeters feet 0.3048 meters square inches 6.4516 square centimeters square feet 0.092903 square meters pounds 0.45359237 kilograms tons (2000 lb) 907.185 kilograms pounds per square inch 0.070307 kilograms per square centimeter pounds per square foot 4.88243 kilograms per square meter pounds per cubic foot 16.0185 kilograms per cubic imeter vii SUMMARY Rock shear strength is often determined from in situ tests. Many cases have been reported in the literature describing in situ shear test procedures and test results. The purpose of this report is to summarize this information for guidance in planning and evaluating such tests. The types of tests discussed are: (1) direct shear test, (2) triaxial or multiaxial test, (3) torsion shear test, and (4) pull-out test. The direct shear test is most widely used, and some 48 case histories of such tests are summarized in Appendixes A through E. The main advantage of the direct shear test is the ability to measure the shear resistance in any desired direction along potentially critical discontinuities. The test is also popular due to its adaptability to field conditions; tests can be conducted in trenches, adits, tunnels, and even calyx drill holes. Direct shear tests have been carried out on clean and filled discon­ tinuities, intact materials, and concrete-on-rock contacts. Specimen size, types of encasements (when used), loading procedures, and test moisture conditions are varied. Instrumentation is generally provided for measuring horizontal and vertical displacements. Illustrations of various direct shear test setups that have been employed are presented. Failure envelopes determined from in situ direct shear test results reported in the literature were generally straight lines defined by Mohr- Coulomb shear strength parameters 0 and c . Average peak strength para­ meters are given below: ______Test Zone__________ Average 0, deg Average c, psi Clean discontinuities 39 24 Filled discontinuities 30 23 Intact rock 50 70 Concrete-on-rock contacts 38 40 As expected, intact specimens exhibited the greatest average shear strength, and filled discontinuities the least. ix IN SITU TESTS FOR THE DETERMINATION OF ROCK MASS SHEAR STRENGTH PART I: INTRODUCTION Purpose of Report 1. A reliable assessment of the shear strength of rock masses is often required to ensure a safe and economical design of many civil engi­ neering projects. In most cases the rock mass contains joints, bedding planes, shear zones, faults, and foliation surfaces. Since the shear strength of a rock mass is usually governed by the number, location, orientation, and physical properties of these geological discontinuities, it is often quite difficult to determine the shear strength accurately. 2. A preferred method of determining the shear strength is by testing relatively large samples in the field. Many cases have been reported in the literature describing in situ shear test procedures and test results. The purpose of this report is to summarize this information for guidance in planning and evaluating such tests. Scope 3. The report discussed methods of conducting various in situ field tests, such as the direct shear tests, the triaxial or multiaxial test, and the torsion shear test. Most of the discussion pertains to the direct shear test since it was found to be the one most often used for field testing. The shear mechanism, test procedures, and data interpretation are discussed for direct shear tests on clean and filled discontinuities, intact material, and concrete-on-rock contacts. Strengths determined from in situ direct shear tests reported in the literature are summarized. The information contained in this report came from a review of case histories on field direct shear testing (see Appendixes A to E) and discussions of rock mass shear strength in the literature. 2 PART II: METHODS OF DETERMINING IN SITU SHEAR STRENGTH Direct Shear Test Description 4. By far the most common in situ test used for the determination of shear strength is the direct shear test. Clean and filled discontinuities, intact material, and concrete-on-rock contacts are often tested in direct shear. The selection of critical zones for testing should be preceded by a detailed engineering geology investigation of the project site. The purpose and methods of engineering geology investigations related to shear strength are discussed by Clar et al. (1964), Duncan (1965), Deere et al. (1967), Mizukoshi et al. (1967), Muller and John (1963), Rocha (1964a), and Serafim and Guerriero (1966). In situ shear tests in rock conducted from 1934 through 1965 are discussed by Link (1969). Recommended field direct shear test procedures were presented to the American Society for Testing and Materials (ASTM) as part of an effort to establish an ASTM standard for measuring the in situ shear strength of rock (Dodds, 1970). 5. In the simplest form of the direct shear test, the specimen to be sheared is subjected to a load perpendicular to and another load parallel to a predetermined failure plane or failure zone; these loads are termed the normal load N and shear load S , respectively, as shown in figs, la, lb, and lc. The normal and shear stresses are assumed to be distributed uniformly over the test surface and are computed by dividing the respective loads by the test surface area. The normal force is applied first and is generally held constant until the specimen fails under an increasing shear force. The normal and shear loads are usually applied by hydraulic jacks. The only exceptions are cases where the normal load is produced solely by the weight of the test block. Consolidation time, rate of loading, and moisture conditions can be varied. The essential features of the direct shear test are presented in Part III. 3 NORMAL LOAD N a. TEST OF DISCONTINUITY NORMAL LOAD N b. TEST OF INTACT ROCK NORMAL LOAD N c. TEST OF CONCRETE BLOCK CAST ON ROCK Fig. 1. In situ direct shear test 4 Advantages and disadvantages 6. The popularity of the direct shear test is mainly due to its adaptability to field conditions. The test can be conducted in a trench, an underground adit, or a tunnel, and in at least one case at the bottom of a calyx drill hole as illustrated in fig. 2 (Case History B-4)*.