Effective Parameters on Crack Initiation Stress in Low Porosity Rocks

Laboratory testing of rocks is traditionally carried out to determine the peak strength using the ISRM Suggested Methods or other suitable standards. However, it is well known that in low-porosity crystalline rocks there are at least three distinct stages of compressive loading that can be readily identified if the stress–strain response is monitored during the loading process: (1) crack initiation, (2) unstable crack growth, i.e., crack coalescence and (3) peak strength. Crack initiation is noted as the first stage of stress-induced damage in low-porosity rocks. In addition, recent research suggests that crack initiation can be used as an estimate for the in situ spalling strength, commonly observed around underground excavations in massive to moderately jointed brittle rocks. Various methods have been proposed for identifying crack initiation in laboratory tests. These methods are evaluated using ten samples of Äspö Diorite and the results are compared with a simplified method, lateral strain response. Statistically, all methods give acceptable crack-initiation values. It is proposed that the ISRM Suggested Methods be revised to include procedures suitable for establishing the crack-initiation stress.;The stress-strain data from 376 laboratory tests carried out on samples of igneous, sedimentary and metamorphic rocks were analyzed to establish the onset of Crack Initiation (CI) stress. A statistical approach was used to find the geological parameters influencing crack initiation stress. Among various rock properties such as grain size and mineralogy, the proportion of the hardest constituent mineral were found to correlate with CI stress. Foliation-induced anisotropy was found to affect the peak strength but its effect on CI stress was less pronounced. The CI stress to peak stress ratio ranged from 0.42 to 0.47 regardless of the material properties in uniaxial compression whereas this ratio ranged from 0.50 to 0.54 when confined. The crack initiation parameters for the Hoek-Brown spalling criterion for igneous rocks can be expressed in terms of the CI stress ratio and the tensile strength. A comparison of tensile strength from Brazilian and direct tension tests showed that the direct tensile strength was approximately 0.78 of the Brazilian tensile strength.;Crack initiation in the uniaxial compressive loading in rocks occurs well before the peak strength is reached. The factors that may influence the onset of cracking and possible initiating mechanisms were explored using a Discrete Element numerical approach. The numerical approach was based on grain-based model that utilized voronoi tessellation scheme to represent low porosity crystalline rocks such as granite. This approach enabled complete tracking of the failure process along the mineral grain boundaries. The effect of grain-size distribution (sorting coefficient ranging from 1.5 to 1.03), grain size (ranging from 0.75 mm to 2.25 mm), and the heterogeneities of different mineral grains (quartz, K-feldspar, plagioclase) were examined. The modelling revealed that crack initiation is a tensile mechanism in the low porosity rocks simulated, and that shear cracking along grain boundaries is only a prominent mechanism near the peak strength. It was also shown that the grain size distribution had the most significant effect on peak strength and crack initiation stress. The peak strength ranges from 140 to 208 MPa as the grain size distribution varies from heterogeneous to uniform, respectively. However, the ratio of crack initiation to peak stress showed only minor variation, as the heterogeneity decreases. The other factors investigated had only minor effects on crack initiation and peak strength, and the crack initiation ratio.