Research on fracture characteristics and support mechanism of shallow buried double-soft composite roof

• The instability process of an anchored double-soft composite roof consists of four stages: elastic deformation, crack propagation, alternating fracture, and failure collapse. • Increasing pre-tightening force suppresses interfacial sliding between upper/lower rock layers and delays crack initiation and propagation, thereby enhancing roof stability. • A higher upper/lower rock strength ratio (I/II) shifts the failure mode from tensile failure of the steel strip to shear failure of the anchor rod, indicating a need for tailored support design. • The steel strip improves the continued bearing capacity of the anchored roof during crack propagation and alternating fracture stages, providing critical guidance for roof support in coal mines.