Part 5Part 5

CONCEPTS CONCEPTS AND AND

IMPACTS OF IMPACTS OF ROCK ROCK

STIFFNESSSTIFFNESSTunnel Gallery 3 failure Zion-Mt Carmel Tunnel Gallery 3 failure Zion-Mt Carmel

Tunnel in April 1958Tunnel in April 1958

Rock Stiffness Rock Stiffness Controls Controls Fracture Fracture SpacingSpacing

• Rock stiffness, more Rock stiffness, more than any other factor, than any other factor, appears to control appears to control spacings between spacings between secondary fractures, secondary fractures, such as these stress such as these stress relief joints around relief joints around adits in the Gerstley adits in the Gerstley Mine, Death ValleyMine, Death Valley

• The area beneath the stress vs strain curve The area beneath the stress vs strain curve is a measure of the stored elastic strain is a measure of the stored elastic strain energy of te rock specimen.energy of te rock specimen.

• The compressive strength exhibited by a specimen The compressive strength exhibited by a specimen of layered rock varies with the inclination of the rock of layered rock varies with the inclination of the rock fabric, as shown in these data. This variance is fabric, as shown in these data. This variance is termed “termed “anisotropyanisotropy.”.”

• Full-range stress strain plotsFull-range stress strain plots record behavior in the record behavior in the post-failure envelope. The testing machine must be post-failure envelope. The testing machine must be significantly stiffer than the specimen in order to carry significantly stiffer than the specimen in order to carry out such tests, because elastic strain energy is stored out such tests, because elastic strain energy is stored in the machine frame.in the machine frame.

• The shape of the post-failure envelope The shape of the post-failure envelope depends on depends on rock stiffnessrock stiffness, , testing machine vs testing machine vs rock specimen stiffness ratiorock specimen stiffness ratio, and , and scale of the scale of the specimenspecimen being tested. This is an extremely being tested. This is an extremely brittle material, the Nutall Quartzite.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 stiffnessTesting machine stiffness is a function of is a function of elastic distortion during loading. It can be elastic distortion during loading. It can be evaluated by advancing opposing rams without evaluated by advancing opposing rams without the rock specimen, as sketched here. the rock specimen, as sketched here.