Cd. Martin, 17TH-CANADIAN-GEOTECHNICAL-COLLOQUIUM - THE EFFECT OF COHESION LOSS AND STRESS PATH ON BRITTLE ROCK STRENGTH, Canadian geotechnical journal, 34(5), 1997, pp. 698-725
Stress-strain curves for brittle rocks show three characteristic stres
s levels: crack initiation, long-term strength, and peak strength. Dam
age-controlled testing at low confining stresses has shown that the lo
ng-term and peak strengths are sensitive to the amount of induced dama
ge, i.e., the greater the amount of damage, the lower the long-term an
d peak strengths. These tests also showed that the brittle-failure pro
cess 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 fai
led rock around the tunnel is approximately one-half of that measured
in laboratory tests. Crack-induced damage of Lac du Bonnet granite, bo
th in the laboratory and in situ, begins when the load exceeds approxi
mately 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 u
ltimate strength of an unconfined sample is 225 MPa. Numerical studies
suggest that in situ the loading path around the tunnel is more compl
ex, involving stress increase and decrease and stress rotation. For th
is 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 microseis
mic monitoring carried out during tunnel construction indicate a const
ant-deviatoric-stress criterion is a reliable indicator for predicting
the onset of damage. This criterion was also found to give a reasonab
le prediction for the maximum depth of failure around the test tunnel.
The fundamental assumption in the constant-deviatoric-stress criterio
n is that at low confining stresses, such as those which occur around
underground openings, the brittle-failure process is dominated by cohe
sion loss.