Visualizing Stress

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Pore Fluid Pressure

Pore fluids are fluids that occupy pore spaces in a soil or rock. This fluid reduces the normal stress thus reducing the principal stresses. Pore fluid pressure affects the Mohr circle by shifting it to the left along the normal stress axis towards the shear stress axis. Subtracting pore fluid pressure from a stable stress system can cause it to become unstable resulting in fracturing or faulting.

To illustrate the effects of pore fluid pressure on a stress system open PorePressureExample.txt, and then plot the failure envelope and Mohr circles for this data set without correcting for pore pressure. If the resulting Mohr circle lies completely under the failure envelope, then the overall state of stress does not exceed the conditions that would cause the material to fail. In the Plot Controls dialog box remove the check mark from the box labeled Mohr Circle in the No Pore Pressure group and place a check mark in the box labeled Mohr Circle in the Pore Pressure Corrected group and then select Plot. Note that for the given data, after being corrected for pore pressure, the Mohr circle is shifted to the left resulting in a portion of the circle lying above the fracture envelope (Figure 23). Thus, we conclude that failure is likely to occur under these conditions.

Figure 23. Mohr circles in both a stable stress regime with no pore pressure and an unstable stress regime with pore pressure.

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