Abstract: In order to grasp the large deformation mechanism of roadway driving along goaf under unstable overlying rock strata, the digital panoramic borehole camera system was adopted to observe the roof rock strata of the roadway driving along goaf. Count Fracture Numbers in a Basic Measuring Scale Method was proposed to undertake borehole fracture statistical analysis. According to the method, the fractures in the roof of the roadway driving along goaf were statistically analyzed; fractures distribution as well as the evolution law was also investigated. Observation results indicate that longitudinal fractures and transverse fractures are two major fracture forms in roof rock strata. As for the distribution, the longitudinal fractures are mainly found on the top of the roof strata in the roadway, while the transverse fractures distribute in the bottom of the roof strata along goaf. As the stabilization time of the overlying strata of the goaf went by, the longitudinal fractures gradually evolved into fracture zone, and the transverse fractures evolved into roof separation zone and radial dislocation. When the two forms of fracture cross together, a wide range of abnormal fracture zone would come into being very easily. The field observation confirms that the primary causes of the overlying strata deformation of the roadway driving along goaf are the fracture of the main roof, the rotating of the key block and its sinking and slippage. Therefore, in order to control the rotating and slippage of the key block after the fracture of the main roof will be a fundamental method to control the deformation of the roadway driving along goaf.

Key words: roadway driving along goaf, unstable overlying rock strata, large deformation mechanism, borehole observation