Carbonaceous　shale　is　the　common　carrier　of　underground　engineering,　and　its　strength　assessment　and　failure　behavior　prediction　are　essential　for　the　analysis　of　engineering　stability.　Carbonaceous　shale　naturally　contains　defects　in　different　forms　such　as　bedding　plane　and　fissures,　acting　as　the　potential　sources　for　the　crack　development　and　failure.　To　investigate　the　shale　behaviors　in　underground　engineering,　true　triaxial　compression　experiments　are　performed　on　transversely　isotropic　shale　samples　containing　twin　fissures,　and　the　mechanical　properties　and　failure　characteristics　are　the　main　concern.　Results　show　that　the　strength　difference　of　shales　with　the　same　bedding　angle　under　different　σ　3　directions,　respectively,　reaches　33.34 MPa　and　9.63 MPa,　and　the　strength　difference　with　the　same　σ　3　direction　under　different　bedding　angles　attains　43.8 MPa　and　20.09 MPa.　A　new　mechanism　of　the　＂effect　of　σ　1-defect-coupling-induced　fracture＂　is　proposed　to　interpret　the　σ　1-dominated　crack　damage,　the　fissure-dominated　crack　damage,　and　bedding　plane　effect.　For　the　shale,　the　shear　mechanism　is　dominated　during　the　initial　loading　stage,　while　the　tensile　mechanism　is　progressively　enhanced　as　the　loading　proceeds　toward　the　failure.　Moreover,　nine　crack　types　and　five　crack　coalescence　modes　are　found　in　current　true　triaxial　experiments.　This　study　provides　the　experimental　basis　for　the　evaluation　of　the　shale　behaviors　in　response　to　a　truly　three-dimensional　stress　circumstance.　©　2023,　The　Author(s),　under　exclusive　licence　to　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.