Q4) The Mohr circles representing the states of stress at two points, A and B, and in the same rock type at different levels in the crust are shown in the figure below with the Mohr Failure envelope for the rock type. The principal stresses at each point are vertical and horizontal as shown. The rock at B is at the point of failure. Answer the following questions. (Choose 8 only) (10 Marks) 50 350 100 MPa 0 би 1. Which point is deeper in the crust, A or B? 2. What is the mean stress at pointA? 3. What is the differential stress at point B? 4. What is the value of 03 at point A? 5. Is σ3 at point A tensile or compressive? Why? 6. What is the orientation, relative to the principal axes, of the fractures that form at point B? 7. Are these fractures at B shear or tensile fractures? Why? 8. If The pore fluid pressure at A is increased until the rock hydraulically fractures, show on the figure at top right the orientation of these fractures with the sense of movement that they would undergo under the given stress. 9. Calculate the critical shear stress to induce onset of fractures at point B. 10. Calculate the normal stress at point A then compare it to the shear stress at point B. |Page 2