Geology 3120 - Failure Models
Powerpoint notes are available online at: Outline Virtual rock deformation experiment
Influence of pore fluid pressure Andersonian faulting Byerlees law Other failure models Virtual Rock Deformation Experiment
Run s1 (MPa) s3 (Mpa) Failure (Q) 1 250 150 none 2 50 +37 3 490 190 4 690 310 s1 s3 +Q Run 1: s1= 250 MPa; s3=150 MPa; no fracture Run 1: s1= 250 MPa; s3=150 MPa; no fracture Run 2: s1= 250 MPa; s3= 50 MPa; 37 fracture Run 2: s1= 250 MPa; s3=150 MPa; no fracture
74 Run 3: s1= 490 MPa; s3=190 MPa; 37 fracture Run 3: s1= 490 MPa; s3=190 MPa; 37 fracture
74 Run 4: s1= 690 MPa; s3=310 MPa; 37 fracture Run 4: s1= 690 MPa; s3=310 MPa; 37 fracture
74 Determining the Failure Envelope
sc = 0.29sn + 60 MPa f f = 16 tan f = 0.29 s0 = 60 MPa sc = 0.29sn + 60 MPa Predicting Failure Run 5: s3= 250 MPa; at what s1 fracture occur? Run 5: s3= 200 MPa; at what s1 fracture occur?
Predicting Failure 74 Run 5: s3= 200 MPa; at what s1 fracture occur? 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. Principal Stresses s2 - intermediate principal stress
s1 - greatest principal stress s2 - intermediate principal stress s3 - minimum principal stress Principal stress directions are mutually perpendicular to each other 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 Assumptions for Andersonian Faulting
sn sc Y = mX + b ( Coulomb brittle failure - no pre-existing faults f = Q Most rocks have f = 30 so Q = 30 Assumptions for Andersonian Faulting
Normal stress (s1 , s2, s3) Zero shear stress No shear stress exists at the Earths surface One principal stress must act normal to the surface s1 , s2, or s3 must be perpendicular to the surface Rules of Thumb for Stresses
s1 bisects the acute angle s2 is parallel to the intersection of conjugate faults s3 bisects the obtuse angle Normal Fault Strike-slip Fault Thrust Fault Normal faulting South North
Find the conjugate faults and determine the orientations of principal stresses. Normal faulting SouthNorth Normal faulting SouthNorth s1 s3 s2 s1 Determining Sense of Slip Determining Sense of Slip Determining Sense of Slip Determining Sense of Slip Determining Sense of Slip Determining Sense of Slip Byerlees Law of Rock Friction
ss mf = sn mf = coefficient of sliding friction 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 Failure Models 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.