A physical and numerical investigation of sudden massive roof collapse during longwall coal retreat mining

Abstract One of the most challenging safety problems in underground longwall coal mining is massive roof collapse in mined-out area during retreat mining. Understanding the mechanisms and geotechnical precursors of massive roof collapse is of great importance for risk assessment and ground control. In this study, a large-scale physical model was created based on a real case to simulate massive roof collapse during longwall coal retreating mining. A good agreement was achieved between field observation and physical result in terms of monitored working resistance, indicating that the physical model is able to capture the realistic response of the rock strata above mined-out area. A numerical model was created based on the model configuration of the physical model. The physical result and the numerical result were in good agreement with each other in terms of many features including roof failure mechanism, abutment pressure distribution and collapsed roof pattern. The physical and numerical result suggested that massive roof collapse is a typical snap-through failure where bedded roof crack through cross joint at midspan. Substantial increase in horizontal stress at the center of the potential failure zone is a reliable geotechnical precursor of massive roof collapse. It is suggested that horizontal stress increasing up to five times pre-mining can be considered as a threshold of the precursor of massive roof collapse.