Structural Evolution of the Overlying Strata of the Retreating Roadway and Roof Stability Under Monorail Crane Loading

The retreating roadway is a critical passage for the safe and efficient retreat of equipment from a fully mechanized longwall face. Its roof stability directly affects the transportation safety of large equipment such as hydraulic supports and the shearer. However, the structural evolution of the overlying strata of the retreating roadway and the roof stability under monorail crane loading have not been systematically investigated. Therefore, taking the retreating roadway of the 1093 fully mechanized longwall face in a coal mine in Anhui Province as the engineering background, this study combined physical similarity simulation, digital image correlation (DIC), and theoretical analysis to investigate the evolution of overlying strata fracture, caving, displacement, and stress fields during face extraction and retreating roadway formation. An analytical model was established to calculate bed separation between the immediate roof and the main roof under an equivalent static concentrated monorail crane load. The results indicate that the vertical displacement field of the overlying strata exhibits an overall trapezoidal distribution and continuously extends toward the higher overlying strata as the longwall face advances. In the physical model, no further propagation of fractures or bed separation toward the retreating roadway was observed after either roof-cutting operation. The withdrawal of hydraulic supports caused no significant changes in the stress or displacement of the overlying strata of the retreating roadway, indicating that the integrity of the overlying strata structure was well maintained. The model predicted a maximum bed separation of 6.39 mm between the immediate roof and the main roof at the gob-side end. The model can assist in identifying critical roof locations susceptible to bed separation and in prioritizing roof monitoring and support optimization during monorail-assisted equipment withdrawal.