True　triaxial　experiment　with　a　synchronous　acoustic　emission　(AE)　monitoring　system　is　a　popular　measure　for　the　assessment　of　strength　and　cracking　behaviors　of　brittle　flawed　rocks　under　anisotropic　stress　circumstances.　This　measure　is　employed　to　invest　the　dependency　of　the　strength　and　fracturing　characteristics　of　flawed　sandstone　under　true　triaxial　stress　on　the　flaw　orientation.　Observations　first　quantitatively　reveal　the　three　types　of　failure　mechanisms　including　structure-control,　stress-structure　collaborative　control　and　stress-control,　dependent　on　the　flaw　orientation　with　respect　to　the　3D　(three-dimensional)　stress.　Second,　the　elastoplastic　strain　increment　ratio　(EPSIR)　method　is　creatively　introduced　to　identify　the　stress　thresholds　in　flawed　sandstone　under　true　triaxial　stress.　Third,　a　significant　increase　in　the　true　triaxial　strength　of　flawed　sandstone　among　the　tested　flaw　configurations　is　correlated　with　the　intensification　of　the　shear　mechanism.　The　3D　stress-driven　progressive　failure　of　flawed　sandstone　is　also　investigated　experimentally,　as　well　as　the　AE　(acoustic　emission)　characteristics　and　the　strength　prediction.　Current　experiments　provide　insights　into　the　brittle　failure　mechanisms　of　flawed　rocks　and　help　interpret　the　excavation　instability　in　deep　underground　engineering.　©　2025