The effective stress principle (Terzaghi's law) states that the stress governing rock/soil mechanical behavior (including brittle failure via the Mohr-Coulomb criterion) is the effective stress, σ' = σ − Pf, where σ is total normal stress and Pf is pore fluid pressure. Increasing fluid pressure directly reduces effective normal stress without changing the total (differential) stress, moving the Mohr circle leftward toward the failure envelope.
In Mohr-Coulomb failure theory, brittle failure occurs when the shear stress on a potential failure plane exceeds the failure envelope defined by cohesion and effective...
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