This limestone bed (see above) from Lilstock Beach on the southern margin of the Bristol Channel, England, is broken by numerous fractures, providing visual evidence of inelastic behavior (Rawnsley et al., 1998; Engelder and Peacock, 2001). In this chapter we contemplate this singular phenomenon and describe the modern concepts, laboratory data, and fracture mechanics required to understand it. We infer from the fractures that these rocks have deformed in a brittle state and seek to understand the consequences of brittle behavior for the state of stress and strain, the nature of the physical mechanisms that operate during such deformation, and the reasons why inelastic deformation may be localized into thin tabular zones. Brittle deformation is manifest in rock structures including microcracks, joints, veins, dikes, deformation bands, compaction bands, and faults.
Concepts from Chapter 9
These exercises explore concepts from Chapter 9 including the strength of rock in triaxial laboratory tests as a function of confining pressure, pore pressure, and effective stress, Coulomb's criterion for shearing failure, Griffith's criterion for brittle failure, linear elastic fracture mechanics and the propagation of opening and shearing fractures, and the development of a damage zone and secondary structures during the growth of dikes and faults.
Dike and joint propagation at Willow Wash
This exercise explores the structural relationships between dikes and joints using field examples from the Colorado Plateau where Tertiary dikes intrude nearly horizontal sedimentary rocks. A common hypothesis about this relationship is found to be inconsistent with field observations and a new hypothesis is developed and examined using a mechanical model based on linear elastic fracture mechanics.
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