Geology 3120 - Failure Models

1. Geology 3120 - Failure Models Powerpoint notes are available online at:http://www.colorado.edu/geolsci/courses/GEOL3120

2. Outline • Virtual rock deformation experiment • Influence of pore fluid pressure • Andersonian faulting • Byerlee’s law • Other failure models

3. Virtual Rock Deformation Experiment s1 s3 +Q s1

4. Run 1: s1= 250 MPa; s3=150 MPa; no fracture

5. Run 1: s1= 250 MPa; s3=150 MPa; no fracture

6. Run 2: s1= 250 MPa; s3= 50 MPa; 37° fracture

7. Run 2: s1= 250 MPa; s3=150 MPa; no fracture 74°

8. Run 3: s1= 490 MPa; s3=190 MPa; 37° fracture

9. Run 3: s1= 490 MPa; s3=190 MPa; 37° fracture 74°

10. Run 4: s1= 690 MPa; s3=310 MPa; 37° fracture

11. Run 4: s1= 690 MPa; s3=310 MPa; 37° fracture 74°

12. Determining the Failure Envelope sc = 0.29sn + 60 MPa f f = 16 tan f = 0.29 s0 = 60 MPa sc = 0.29sn + 60 MPa

13. Predicting Failure Run 5: s3= 250 MPa; at what s1 fracture occur?

14. Predicting Failure 74° Run 5: s3= 200 MPa; at what s1 fracture occur?

15. Influence of Pore Fluid Pressure Effective Stress Applied Stress pf Pore fluid pressure decreases normal stresses by the fluid pressure amount. Rock can then fail under the Mohr-Coulomb Law.

16. Principal Stresses • s1 - greatest principal stress • s2 - intermediate principal stress • s3 - minimum principal stress • Principal stress directions are mutually perpendicular to each other

17. Conjugate Faults Most simply - two fault planes that intersect to form a straight line Perhaps more typical - two fault surfaces that intersect to form a line Acute angle - < 90° angle Obtuse angle - > 90° angle Acute Obtuse

18. Assumptions for Andersonian Faulting ( ( Y = mX + b sc sn sn • Coulomb brittle failure - no pre-existing faults • f = 90 - 2Q • Most rocks have f = 30° so Q = ±30°

19. Assumptions for Andersonian Faulting Normal stress (s1 , s2,s3) Zero shear stress • No shear stress exists at the Earth’s surface • One principal stress must act normal to the surface • s1 , s2,ors3 must be perpendicular to the surface

20. Rules of Thumb for Stresses • s1 bisects the acute angle • s2 is parallel to the intersection of conjugate faults • s3 bisects the obtuse angle

21. Normal Fault

22. Strike-slip Fault

23. Thrust Fault

24. South North Normal faulting Find the conjugate faults and determine the orientations of principal stresses.

25. South North Normal faulting

26. South North Normal faulting s1 s3 s2 s1

27. Determining Sense of Slip s2 s3 s1

28. Determining Sense of Slip s1

29. Determining Sense of Slip s1

30. Determining Sense of Slip s1

31. Determining Sense of Slip s1

32. Determining Sense of Slip s2 s3 s1

33. Byerlee’s Law of Rock Friction ss mf = sn mf = coefficient of sliding friction

34. Byerlee verses Mohr-Coulomb Failure For a given differential stress, brittle failure will occur by frictional sliding on pre-existing fractures (if they exist) prior to Coulomb failure

35. Failure Models

36. References Slides 21, 22, 24, 39, 40 Davis. G. H. and S. J. Reynolds, Structural Geology of Rocks and Regions, 2nd ed., John Wiley & Sons, New York, 776 p., 1996. Slide 41 Twiss, R. J. and E. M. Moores, Structural Geology, W. H. Freeman & Co., New York, 532 p., 1992.