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Abstract The onset of brittle failure in rocks includes dilatancy and strain localization. To better understand this nucleation process, we analyze the evolution of the local three‐dimensional strain tensor using X‐ray tomograms acquired during triaxial compression experiments on granite and sandstone. The onset of the localization of the compaction, dilation, and shear strain occurs when ∼65% of the rock volume experiences dilation. Tracking the locations of the high strains throughout loading suggests that the deformation that occurs early in loading influences the location of the system‐sized fracture network that produces macroscopic failure. This influence is larger in the sandstone experiments than the granite experiments, likely due to the microstructure of the sandstone. These results have important implications for detecting precursors to catastrophic failure.
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The competition between fracture nucleation, propagation, and coalescence in dry and water-saturated crystalline rock
Abstract. The continuum of behavior that emerges during fracturenetwork development in crystalline rock may be categorized into threeend-member modes: fracture nucleation, isolated fracture propagation, andfracture coalescence. These different modes of fracture growth producefracture networks with distinctive geometric attributes, such as clusteringand connectivity, that exert important controls on permeability and theextent of fluid–rock interactions. To track how these modes of fracturedevelopment vary in dominance throughout loading toward failure and thushow the geometric attributes of fracture networks may vary under theseconditions, we perform in situ X-ray tomography triaxial compressionexperiments on low-porosity crystalline rock (monzonite) under upper-crustalstress conditions. To examine the influence of pore fluid on the varyingdominance of the three modes of growth, we perform two experiments undernominally dry conditions and one under water-saturated conditions with 5 MPa ofpore fluid pressure. We impose a confining pressure of 20–35 MPa and thenincrease the differential stress in steps until the rock failsmacroscopically. After each stress step of 1–5 MPa we acquire athree-dimensional (3D) X-ray adsorption coefficient field from which weextract the 3D fracture network. We develop a novel method of trackingindividual fractures between subsequent tomographic scans that identifieswhether fractures grow from the coalescence and linkage of several fracturesor from the propagation of a single fracture. Throughout loading in all ofthe experiments, the volume of preexisting fractures is larger than that ofnucleating fractures, indicating that the growth of preexisting fracturesdominates the nucleation of new fractures. Throughout loading until close tomacroscopic failure in all of the experiments, the volume of coalescingfractures is smaller than the volume of propagating fractures, indicatingthat fracture propagation dominates coalescence. Immediately precedingfailure, however, the volume of coalescing fractures is at least double thevolume of propagating fractures in the experiments performed at nominallydry conditions. In the water-saturated sample, in contrast, although thevolume of coalescing fractures increases during the stage preceding failure,the volume of propagating fractures remains dominant. The influence ofstress corrosion cracking associated with hydration reactions at fracturetips and/or dilatant hardening may explain the observed difference infracture development under dry and water-saturated conditions.
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- Award ID(s):
- 1761912
- PAR ID:
- 10227366
- Date Published:
- Journal Name:
- Solid Earth
- Volume:
- 12
- Issue:
- 2
- ISSN:
- 1869-9529
- Page Range / eLocation ID:
- 375 to 387
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/se-12-375-2021
