The progressive fracture of Lac du Bonnet granite

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.

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

Some observations concerning the microscopic and mechanical behaviour of quartzite specimens in stiff, triaxial compression tests

A grain-based Universal Distinct Element Code model was developed to generate a deformable polygonal grain-like structure to simulate the microstructure of brittle rock. It takes into account grain-scale heterogeneity including microgeometric heterogeneity, grain-scale elastic heterogeneity, and microcontact heterogeneity. The microgeometric heterogeneity can be used to match the grain size distribution of the rock. The discrete element approach is able to simulate the microheterogeneity caused by elastic variation and contact stiffness anisotropy. The modeling approach was evaluated using Lac du Bonnet granite and A "Aspo" Diorite. The microheterogeneity played an important role in controlling both the micromechanical behavior and the macroscopic response when subjected to uniaxial compression loading. The crack-initiation stress was found to be controlled primarily by the microscale geometric heterogeneity, while the microcontact heterogeneity controlled the strength characteristics. The effect of heterogeneity on the distribution and evolution of tensile stresses and associated extension cracks was also examined.

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Effect of heterogeneity of brittle rock on micromechanical extensile behavior during compression loading

Fatigue properties of intact sandstone samples subjected to dynamic uniaxial cyclical loading

Soft, stiff and servo-controlled testing machines: a review with reference to rock failure

Shape of the Complete Stress-Strain Curve for Rock

Optimizing the Control of Rock Failure in Servo-Controlled Laboratory Tests
Recent studies of rock failure have been made with a closed loop servo-controlled testing machine. Feedback from the failing specimen is electronically compared with a pre-determined program and the hydraulic system is continuously and automatically adjusted to ensure that feedback and program are coincident. It is suggested that optimum control of rock failure in any laboratory test is achieved when the closed loop feedback represents the most sensitive indicator of failure. Any required experimental output can then be independently monitored. This method was applied to a variety of rock specimens loaded in uniaxial compression using lateral displacement as feedback. It was possible to control the failure of some rocks which had failed explosively in previous tests where the axial displacement had been programmed to increase monotonically.

Optimizing the Control of Rock Failure in Servo-Controlled Laboratory Tests

The failure of rock beams: Part I—Theoretical studies

Controlled Failure of Hollow Rock Cylinders in Uniaxial Compression
Detailed consideration is given to the behavior of hollow rock cylinders loaded in uniaxial compression Elastic stress distributions calculated by the finite element method show that radial stresses are considerably lower in hollow than in solid cylinders, and that the uniformity of the stresses can be improved considerably by using loading platens having the same cross-section as the specimen Servo-controlled uniaxial compression tests carried out on solid and thick-walled cylinders of white Tennessee marble show no essential differences in the behavior of the two specimen types with similar strengths and fracture phenomena being observed In tests carried out in servo-controlled, stiff, and conventional machines, the progressive formation of large numbers of short subaxial cracks is followed by the development of macrofractures such as slabbing and shearing well past the peak of the stress-strain curve

https://link.springer.com/content/pdf/10.1007/BF01239210.pdf

John A. Hudson

Papers

Soft, stiff and servo-controlled testing machines: a review with reference to rock failure

Shape of the Complete Stress-Strain Curve for Rock

Optimizing the Control of Rock Failure in Servo-Controlled Laboratory Tests

The failure of rock beams: Part I—Theoretical studies

Controlled failure of hollow rock cylinders in uniaxial compression