# Geology 3120 Powerpoint notes available online at:

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Geology 3120 Powerpoint notes available online at: http://www.colorado/edu/geolsci/courses/GEOL3120

Geology 3120 - The Mohr Stress Diagram nnnn  Stress Space Stress Space 0 ssss

Outline Setting up the Problem The Mohr Stress Diagram Mohr-Coulomb Law of Failure Exercises

Setting up the Problem  is defined as the angle between the plane and the force vector. Clockwise is positive (+).

Decomposing Stresses After several trigonometric and algebraic simplifications, the two equations left are ……

 n = (  1 +  3 ) - (  1 -  3 )cos 2 22  s = (  1 -  3 )sin 2 2 Fundamental Stress Equations Normal Stress Normal Stress Shear Stress Shear Stress

Physical and Stress Space 1111 1111 3333 3333 nnnn ssss   Physical Space Physical Space Stress Space Stress Space 0

Conventions - Physical Space 3333  is defined as the angle between the plane and the  1 stress direction. A clockwise direction is positive (+).  is defined as the angle between the plane and the  1 stress direction. A clockwise direction is positive (+). 1111 1111 3333 

Conventions - Stress Space nnnn ssss  0 3333 1111  n = compression  n = tension 

Conventions - Stress Space nnnn ssss  0 3333 1111  n (p),  s (p)  n (p) = (  1 +  3 ) - (  1 -  3 )cos 2 22  s  p   = (  1 -  3 )sin 2 2  s (p)  n (p)

Conventions - Stress Space nnnn ssss  0 3333 1111 (  1 -  3 )sin 2 2  n (p),  s (p)

Conventions - Stress Space nnnn ssss  0 3333 1111 (  1 +  3 )2  n (p),  s (p) Mean Stress - center of circle

Conventions - Stress Space nnnn ssss  0 3333 1111 (  1 -  3 )2  n (p),  s (p) Deviatoric Stress - radius of circle

Conventions - Stress Space nnnn ssss  0 3333 1111 (  1 -  3 )  n (p),  s (p) Differential Stress - diameter of circle

nnnn ssss  0 3333 1111 (  1 -  3 )cos 2 2  n (p),  s (p) Difference between mean stress and normal stress on plane Conventions - Stress Space

Laboratory Experiments in Rock Deformation Deformed marble rock cylinders

Repeated Failure Experiments

Stress Requirements for Rock Failure

Mohr-Coulomb Law of Failure nn cc  = angle of internal friction tan  = coefficient of internal friction [slope; m]  n = normal stress [X]  c = critical shear stress required for faulting [Y]  0 = cohesive strength [y-intercept; b] nn Y = mX + b ( (

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

Problems 1 & 2 1. 1. For the maximum and minimum principal stresses of 600 and 200 MPa oriented as a vertical vector and a horizontal, E-W striking vector, respectively, determine the normal and shear stress on a plane oriented N0°E, 45°E. 2. For the stress state in the problem above determine the deviatoric stress and mean stress.

Problems 1 & 2 1. 1. For the maximum and minimum principal stresses of 600 and 200 MPa oriented as a vertical vector and a horizontal, E-W striking vector, respectively, determine the normal and shear stress on a plane oriented N0°E, 45°E.  n = 400 Mpa,  s =200 MPa 2. For the stress state in the problem above determine the deviatoric stress and mean stress. Deviatoric Stress = 200 MPa, Mean Stress = 400 MPa WE 1111 3333 

Problem 3 3. Given two planes P1 and P2 oriented where  equals 90° (P1) and 45° (P2), P1 has a normal stress of 500 MPa and P2 has a normal stress of 300 MPa and a shear stress of 200 MPa, determine the magnitudes of the principal stresses, the deviatoric stress and the mean stress. Is this stress state more or less likely to produce failure as that in Problem 1?

Problem 3 3. 3. Given two planes P1 and P2 oriented where  equals 90 (P1) and 45 (P2) degrees, P1 has a normal stress of 500 MPa and P2 has a normal stress of 300 MPa and a shear stress of 200 MPa, determine the magnitudes of the principal stresses, the deviatoric stress and the mean stress.  1 = 500 MPa and  3 = 100 MPa Deviatoric Stress = 200 MPa Mean Stress = 300 MPa

Problem 3 3. 3. Is this stress state more or less likely to produce failure as that in Problem 1? The stress state of Problem 3 is more likely to produce failure than in Problem 1 since the Mohr circle is closer to the failure envelope.

References Slide 15 Twiss, R. J. and E. M. Moores, Structural Geology, W. H. Freeman & Co., New York, 532 p., 1992. Slides 16-19 Davis. G. H. and S. J. Reynolds, Structural Geology of Rocks and Regions, 2nd ed., John Wiley & Sons, New York, 776 p., 1996.