Pore Pressure in Montmorillonite During Frictional Sliding

Abstract

AbstractEarth's shallow crust is pervasively breached by fractures and faults that are often filled with saturated clays such as montmorillonite. Pore pressure in montmorillonite is difficult to measure due to its extremely low permeability, resulting in large uncertainties in its friction, which, in turn, may impact our understanding on the mechanical behaviors of the shallow crust. This difficulty motivates us to investigate pore pressure in montmorillonite during frictional sliding. Here we provide a first order understanding on the evolution of pore pressure in montmorillonite during frictional sliding by combining experimental data with an analytical consolidation model. Our result shows large variations in pore pressure in montmorillonite during frictional sliding, which need correction for the evaluation of friction of montmorillonite. We re‐visit this problem with the measured stresses at the end of our slow loading experiments where the modeled pore pressure approaches zero and obtain a new relation that shows a significant cohesive strength. The new relation may be united with our modeled pore pressure with a Biot‐Willis effective stress coefficient of α ∼ 0.5. X‐ray powder diffraction analysis of our samples shows evidence that intense deformation occurred during the frictional sliding with strain‐hardening, consistent with the occurrence of shear localization in the clay matrix following extended frictional sliding (Tembe et al., 2010, https://doi.org/10.1029/2009JB006383). These results suggest that our new relation may represent a constitutive relation for an intensely sheared, saturated montmorillonite in frictional sliding. Our result also suggests that substantial cohesion may appear on some natural clay‐rich faults.