Figure F12. A. Differential fluid pressure (the fluid pressure difference between the ends of the sample) vs. time during undrained shear of friable claystone (Sample 170-1040C-13R-5, 88-105 cm). Note the fluid pressure increases initially (indicative of contractive deformation) followed by larger drop-offs in fluid pressure, associated with fluid drawn into a dilating shear zone. B. Porosity vs. permeability before and after shear of the same sample. Note how after deformation, smaller changes in porosity yield larger changes in permeability (seen as a flattening of the relative curves). C. Permeability vs. effective stress both before and after shear. First permeability test after deformation was conducted after reduction of the confining pressure to 100 kPa effective stress (marked on the graph). At the lowest values of effective stress after deformation, permeability is more than three orders of magnitude greater than before deformation.