Abstract: The direct roof of weak stratum in roadway has three kinds of failure modes: deflection failure, integral caving and arch caving. The change of roof thickness is one of the main reasons affecting the failure modes. Therefore, based on beam theory and block theory, a mechanical model of roof rock beam-block was constructed to reveal the changing law of the direct roof failure form in weak strata of roadway. On this basis, the safety factors of flexural failure and shear failure were defined, and the calculation models of critical thickness of the roof with different failure modes were established to determine the corresponding failure modes of the roof with different roof thicknesses. The case study shows that with the increase of roof thickness, the failure mode of the roof gradually changes from flexural failure to integral caving and then to arch caving. At the same time, when the thickness of the roof exceeds a certain value, the height of the caving arch no longer increases in a substantial manner. Based on the same engineering conditions and mechanical parameters, a comparative analysis between numerical simulation and a case calculation was carried out. The critical thickness of the roof under the case calculation conditions was 0.14 m from flexural failure to overall collapse, and 0.8 m from overall collapse to arch collapse. The numerical simulation and case calculation results were consistent. It shows that the calculation model of the critical thickness of the roof can effectively determine the failure form of the roof. The research results can provide theoretical reference for direct roof control and support scheme design of mine roadway in weak stratum.

Key words: soft layer direct roof, roof thickness, rock beam- block theory, failure form, critical value