Lab 3 - the mohr circle, pore pressure, and rock strength name: . plot the specified o, (confining pressure) on the a,-axis. we kmowthat the rock failed by sliding, so by trial a and eror, use a compass to find the mohr circle that passes through the 150 mpa mark and is tangent to the failure envelope. then, w to the ith a compass, draw a circle centered at this point and tangent tres sdingnvelope. use your mohr circle to determine the mean stress (o) and the differential (aa) at the time of failure. consdngthe mean stress that you determined, and byerlee's results, was it reasonable to use equation 2 rather than equation i to specify the failure envelope? /0.5 5. draw a line from the center of the mohr circle to the point of tangency between the circle and the aiu evelope and measure the angle 20. given this angle, we can determine the orientation of the fracture that probably slid first 4a what is it? note that this plane is not the plane on which shear stress was greatest /0.5 4.b. why did it slide first? why do you expect the other fractures to be stable under these stress conditions? /0.5) consider a hypothetical state of stress in the upper crust of the earth. the vertical stress ơais due to the weight of overlying rock. the magnitude of o is pgh. in the absence of tectonic stress, horizontal stress in the upper elastic crust is due to lateral expansion of the rock in response to the vertical load. assume the value of horizontal stress to ben« σ-ys (gh) first determinethe depth (h) in the crust at which σ2 150 mpa (use rhoξ 2.7 gcm , and g 9.8 ms, then determine the mean stress and differential stress at such depth. 6. assuming that the predictions made above are correct, how do these stresses compare with the stresses at the time of failure in the experiment? do you expect that at a depth of 10 km in the earth that one or both fracture sets will slide in dry rock? if not, by how much can pore fluid pressure increase before slip on a fracture set will occur 7. 12/12