Influences of hydro-mechanical conditions to fracturing behavior of sandstone analogs: approach and implications

The compressive torsion (C-T) fractures are widely distributed in water-saturated, clay-poor, and cementation-weak Cretaceous sandstones in Western China due to asymmetric tectonic convergence. The formation of these fractures significantly depends on the characteristics of sandstones, particularly the dynamics of sandstone physics induced by water entry and its timing. This study prepared synthetic sandstones possessing main hydro-mechanical characteristics of natural sandstones by establishing dual intergranular cementation while retaining free grains, overcoming the challenges of natural core sampling and single-variable test implementing. Furthermore, a mode with passive control of principal stresses was proposed to simulate the load boundaries for fracture formation. Through a series of tests under various water conditions, C-T paths and pressure levels, combined with real-time X-ray CT observations, it was found that a selection of low pressures and C-T paths capable of achieving low peak normalized generalized shear stress facilitates the reproduction of penetrated C-T fractures within synthetic specimens. The C-T path influences strength characteristics before fracture penetration by governing microcrack evolution derived from principal stress axis rotation, where the maximum rotation angle determines the inclination angle of the penetrated fracture. Whereas timing of pore water pressure enhances the complexity of the C-T fracture system by altering the specimen’s pore structure. These findings provide insights for clarifying the physical mechanisms of C-T fracture formation.