Assessment of the roof behavior in longwall gob and estimation of the occurrence of caving and fracturing zones above mined panels are the main factors used in evaluating abutment stresses, ground subsidence, face support and adjacent structures design. The combined height of caving and fracturing zones is taken as equivalent to the height of destressed zone (HDZ) in this study. The long-term estimation of this height plays a key role in accurate determination of maximum ground surface subsidence and the amount of transferred loads towards the neighboring solid sections. In this paper, the roof behavior in longwall gob has been studied in long-term condition. For this purpose, a time-dependent model based on the energy balance in longwall mining combined with a rheological model of caved materials with time-varying parameters was used to calculate HDZ. Also, parametric study was conducted to evaluate the effects of geometrical and geomechanical parameters on the roof caving and fracturing. Moreover, the calculated HDZ from the current study is compared with the results of the existing numerical, analytical and empirical models and the in-situ measurements reported in literatures. Comparative study confirms a good agreement that exists between the results of the present study with those of the in-situ measurements and empirical models. It can be concluded that the results of this research can be successfully used to evaluate HDZ in longwall gob in long-term condition to determine the induced stress and surface subsidence in longwall mining.
Key words: Longwall mining, height of destressed zone, time-dependent model, strain energy, rheological model.