Announcements This week's lab: 1-3 PM with Andrew McCarthy. Please come prepared with specific...
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Transcript of Announcements This week's lab: 1-3 PM with Andrew McCarthy. Please come prepared with specific...
Announcements
This week's lab: 1-3 PM with Andrew McCarthy. Please come prepared with specific questions.
There will be no lecture this Wednesday!
Please use the time to:1) Study important terms/concepts listed at end of
Powerpoint files2) Study the practice problems
3) Fault project
Reminder: Midterm Oct. 14
1
13
3
instantaneous strain ellipse
Stress and Deformation: Part I(D&R, 122-126; 226-252)
The goal for today is to explore the stress conditions under which rocks fail (e.g., fracture), and the orientation of failure with respect to the principal stress directions.
1. Coulomb law of failure
2. Byerlee's law
Compressive strength tests: The resultsLinear envelope of failure. The fractures form at angles of 25 to 35 degrees from 1- very consistent!
c = critical shear stress required for failure0 = cohesive strengthtan = coefficient of internal friction () N = normal stress
Coulomb's Law of Failure
c = 0 + tan(n)
Tensile strength tests with no confining pressureApproach: Similar to compressive strength testsResults: (1) Rocks are much weaker in tension than in compression (2) Fracture oriented parallel to 1 (= 0)
Failure envelopes for different rocks: note that slope of envelope is similar for most rocks
c = 0 + tan(n)c = critical shear stress required for failure
0 = cohesive strength
tan = coefficient of internal friction
N = normal stress
Byerlee's Law
Question: How much shear stress is needed to cause movement along a preexisting fracture surface, subjected to a certain normal stress?
Answer: Similar to Coulomb law without cohesionFrictional sliding envelope: c = tan(N), where tan is the coefficient of sliding friction
Increasing pore fluid pressure favors failure!-Also may lead to tensile failure deep in crust
Effective stress = n – fluid pressure
What about pore fluid pressure?
What is it?
What is it?1 is parallel to the structure. What does this suggest about the magnitude of effective stress?What mechanism may help produce this structure within the deeper crust?
Tensile fracture filled with vein during dilation
very low
high fluid pressure to counteract lithostatic stress
What happens at higher confining pressures?
Von Mises failure envelope- Failure occurs at 45 degrees from 1
Next Lecture
Stress and Deformation II
...A closer look at fault mechanics and rock behavior during deformation
( D&R: pp. 304-319; 126-149)
Important terminology/concepts
Uniaxial vs. axial states of stress
Coulomb law of failure: known how it is determined and equation
values for compression
values for tension
Cohesive strength
Coefficient of internal friction
Byerlee's Law / frictional sliding envelope- know equation
Important role of pore fluid pressure