Fault Zone Spatial Stress Variations in a Granitic Rock Mass: Revealed by Breakouts Within an Array of Boreholes

Abstract The in situ stress state within fault zones is technically challenging to characterize. At the Bedretto Underground Laboratory Swiss Alps, breakouts observed an array of eight inclined boreholes penetrating a zone offer unique opportunity characterize fault‐associated spatial variations. Synthesizing multiple geophysical logs, natural geologic structures intersecting these are identified, revealing hierarchy major along with secondary structures. Within boreholes, breakout rotations occur over scales, spanning individual fractures and entire zone. We first estimate rule out effect fracture‐induced anisotropy on rotations, which attributed mainly Based field around circular borehole Mohr‐Coulomb failure criterion, azimuths used invert information. Results show that outside features ratio (quantifying relative magnitude) about 0.9, overburden (inclination: 12°∼18°), maximum horizontal principal ( S Hmax ) oriented N100∼120°E. zone, substantial reduction complicated constrained, likely induced by drop local fractures. As result, less critical inside expected. Our work provides semi‐quantitative estimation in‐situ variations absence direct measurements, beneficial number scientific engineering applications.