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

What is it? (quiz)

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

Experimental studies are fundamental in the study of rock failure

Common types of deformation experiments

Compressive strength tests: The Goal

Compressive strength tests: The Approach

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)

Tensile + Compressive strength tests

Result: Failure envelope is parabolic0 < < 30

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

Preexisting fractures of suitable orientation may fail before a new fracture is formed

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