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Part 5. CONCEPTS AND IMPACTS OF ROCK STIFFNESS. Tunnel Gallery 3 failure Zion-Mt Carmel Tunnel in April 1958. Rock Stiffness Controls Fracture Spacing.

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part 5
Part 5


Tunnel Gallery 3 failure Zion-Mt Carmel Tunnel in April 1958

rock stiffness controls fracture spacing
Rock Stiffness Controls Fracture Spacing
  • Rock stiffness, more than any other factor, appears to control spacings between secondary fractures, such as these stress relief joints around adits in the Gerstley Mine, Death Valley
The area beneath the stress vs strain curve is a measure of the stored elastic strain energy of te rock specimen.
The compressive strength exhibited by a specimen of layered rock varies with the inclination of the rock fabric, as shown in these data. This variance is termed “anisotropy.”
Full-range stress strain plots record behavior in the post-failure envelope. The testing machine must be significantly stiffer than the specimen in order to carry out such tests, because elastic strain energy is stored in the machine frame.
The shape of the post-failure envelope depends on rock stiffness, testing machine vs rock specimen stiffness ratio, and scale of the specimen being tested. This is an extremely brittle material, the Nutall Quartzite.
Class II brittle behavior is often exhibited by brittle rocks, or cores of varying size and length-to-wide ratios
Testing machine stiffness is a function of elastic distortion during loading. It can be evaluated by advancing opposing rams without the rock specimen, as sketched here.