Seventeenth Canadian Geotechnical Colloquium: The effect of cohesion loss and stress path on brittle rock strength

Stress-strain curves for brittle rocks show three characteristic stress levels: crack initiation, long-term strength, and peak strength. Damage-controlled testing at low confining stresses has shown that the long-term and peak strengths are sensitive to the amount of induced damage, i.e., the greater the amount of damage, the lower the long-term and peak strengths. These tests also showed that the brittle-failure process is characterized by a loss of cohesion as friction is mobilized. Excavation of a circular test tunnel in massive brittle rock resulted in failure around the tunnel. The back-calculated strength for the failed rock around the tunnel is approximately one-half of that measured in laboratory tests. Crack-induced damage of Lac du Bonnet granite, both in the laboratory and in situ, begins when the load exceeds approximately one-third of the unconfined compressive strength. However, the stress level associated with failure is a function of loading path. In the laboratory, where the loading path monotonically increases, the ultimate strength of an unconfined sample is 225 MPa. Numerical studies suggest that in situ the loading path around the tunnel is more complex, involving stress increase and decrease and stress rotation. For this loading path, failure initiates at a stress between 100 and 120 MPa. Conventional frictional failure criteria did not adequately predict the extent of brittle failure measured around the circular tunnel. The results from the damage-controlled laboratory tests and the microseismic monitoring carried out during tunnel construction indicate a constant-deviatoric-stress criterion is a reliable indicator for predicting the onset of damage. This criterion was also found to give a reasonable prediction for the maximum depth of failure around the test tunnel. The fundamental assumption in the constant-deviatoric-stress criterion is that at low confining stresses, such as those which occur around underground openings, the brittle-failure process is dominated by cohesion loss.

[1]  E. T. Brown,et al.  Underground excavations in rock , 1980 .

[2]  James D. Byerlee,et al.  Chapter 1 Observations of Quasistatic Fault Growth from Acoustic Emissions , 1992 .

[3]  Paul Tapponnier,et al.  Development of stress-induced microcracks in Westerly Granite , 1976 .

[4]  J. B. Martino,et al.  Comparison of borehole breakouts from laboratory and field tests , 1994 .

[5]  John M Kemeny,et al.  Micromechanics of Deformation in Rocks , 1991 .

[6]  E. T. Brown,et al.  Rock characterization testing and monitoring , 1981 .

[7]  Evert Hoek,et al.  The Hoek-Brown failure criterion-a 1988 update , 1988 .

[8]  Z. T. Bieniawski,et al.  The Geomechanics Classification In Rock Engineering Applications , 1979 .

[9]  Paul G. Richards,et al.  Quantitative Seismology: Theory and Methods , 1980 .

[10]  A. Jones.  10 – Rock Mechanics and Rock Engineering , 1989 .

[11]  D. R. McCreath,et al.  Rockmass Damage Initiation Around the Sudbury Neutrino Observatory Cavern , 1996 .

[12]  C. Derek Martin,et al.  The effect of sample disturbance on laboratory properties of Lac du Bonnet granite , 1994 .

[13]  B. J. Pestman,et al.  An acoustic emission study of damage development and stress-memory effects in sandstone , 1996 .

[14]  C. Scholz,et al.  Dilatancy in the fracture of crystalline rocks , 1966 .

[15]  R. P. Young,et al.  Microseismic monitoring in highly stressed granite: relation between shaft-wall cracking and in Situ stress , 1992 .

[16]  P Cundall,et al.  A MICROCOMPUTER PROGRAM FOR MODELLING LARGE-STRAIN PLASTICITY PROBLEMS. PROCEEDINGS OF THE SIXTH INTERNATIONAL CONFERENCE ON NUMERICAL METHODS IN GEOMECHANICS, 11-15 APRIL 1988, INNSBRUCK, AUSTRIA. VOLUMES 1 - 3 , 1988 .

[17]  N. A. Chandler,et al.  The progressive fracture of Lac du Bonnet granite , 1994 .

[18]  A. McGarr,et al.  Some comparisons between mining-induced and laboratory earthquakes , 1994 .

[19]  N. A. Chandler,et al.  Stress heterogeneity and geological structures , 1993 .

[20]  P. K. Kaiser,et al.  An interpretation of ground movements recorded during construction of the Donkin-Morien tunnel , 1991 .

[21]  John M Kemeny,et al.  Effective moduli, non-linear deformation and strength of a cracked elastic solid , 1986 .

[22]  C. Martin,et al.  Characterizing in situ stress domains at the AECL Underground Research Laboratory , 1990 .

[23]  Bezalel C. Haimson,et al.  Laboratory study of borehole breakouts in Lac du Bonnet granite: a case of extensile failure mechanism , 1993 .

[24]  Z. Bieniawski.  Mechanism of brittle fracture of rock: Part III—fracture in tension and under long-term loading , 1967 .

[25]  J. Houska.  Fundamentals of Rock Mechanics , 1977 .

[26]  Monitoring Progressive Failure Around a Tunnel In Massive Granite , 1995 .

[27]  R. Read,et al.  Interpreting excavation-induced displacements around a tunnel in highly stressed granite , 1994 .

[28]  A. A. Griffith.  The Phenomena of Rupture and Flow in Solids , 1921 .

[29]  C. Derek Martin,et al.  Asymmetric borehole breakouts at the URL , 1995 .

[30]  C. Martin,et al.  The strength of massive Lac du Bonnet granite around underground openings , 1993 .

[31]  B. J. Carter.  Size and stress gradient effects on fracture around cavities , 1992 .

[32]  E. Z. Lajtai.  Effect of tensile stress gradient on brittle fracture initiation , 1972 .

[33]  R. Ewy,et al.  Deformation and fracture around cylindrical openings in rock—II. Initiation, growth and interaction of fractures , 1990 .

[34]  W. R. Wawersik,et al.  Post-failure behavior of a granite and diabase , 1971 .

[35]  Evert Hoek,et al.  Rock fracture under static stress conditions , 1965 .

[36]  N. Cook,et al.  Analysis of borehole breakouts , 1989 .

[37]  E. Z. Lajtai,et al.  The long-term strength of lac du bonnet granite , 1985 .

[38]  Christopher H. Scholz,et al.  Microfracturing and the inelastic deformation of rock in compression , 1968 .

[39]  A. T. Starr.  Slip in a crystal and rupture in a solid due to shear , 1928, Mathematical Proceedings of the Cambridge Philosophical Society.

[40]  J. Gramberg,et al.  A non-conventional view on rock mechanics and fracture mechanics , 1988 .

[41]  John A. Hudson,et al.  Shape of the Complete Stress-Strain Curve for Rock , 1972 .

[42]  John H. Schmertmann,et al.  An Experimental Study of the Development of Cohesion and Friction with Axial Strain in Saturated Cohesive Soils , 1960 .

[43]  Estimation of In-situ Compressive Strength of Rocks From In-situ Rock Stress Measurements In Highly Stressed Rock Structures , 1991 .

[44]  D. J. Holcomb,et al.  Determining peak stress history using acoustic emissions , 1985 .

[45]  B. J. Carter,et al.  Mapping the state of fracture around cavities , 1991 .

[46]  T. R. Stacey.  A simple extension strain criterion for fracture of brittle rock , 1981 .

[47]  P. K. Kaiser,et al.  Support of underground excavations in hard rock , 1995 .

[48]  R. T. Ewy,et al.  DEFORMATION AND FRACTURE AROUND CYLINDRICAL OPENINGS IN ROCK - I. OBSERVATIONS AND ANALYSIS OF DEFORMATIONS , 1990 .

[49]  E. Orowan,et al.  Fracture and strength of solids , 1949 .

[50]  J. B. Martino,et al.  Observations of brittle failure around a circular test tunnel , 1997 .