This paper describes a method for simulating the goaf of mechanized longwall panels at shallow depth in various coal mines in India for prediction of subsidence, abutment pressure and stresses over goaf and barrier pillars. Proper modelling of goaf is necessary for the accurate prediction of stress distribution around the face and subsidence at the surface. In this paper, the broken rock debris has been modeled as a non-linear stress strain characteristic. Two characteristic properties have been identified to describe the stress-strain behaviour. This study is undertaken to find out the goaf properties for seven previously worked longwall panels. A numerical model is developed in FLAC3D, a finite difference modelling tool and the modelling procedure has been described. The results obtained from modelling were compared with field observations in case of subsidence and theoretical values in case of stress distribution over the stabilized goaf and average load on barrier pillars. This paper provides the characteristic values for the goaf properties for the three different roof-rock type which may be used for simulating the goaf.
As the longwall face advances, the immediate roof overlying the coal seam, consisting of weak beds, readily cave behind the supports. The main roof lying above the immediate roof consists of strong rock beds and is selfsupporting within a certain roof span. This remains intact and exerts pressure on the barrier pillars and coalface. With the progress of mining, it eventually fails and caves in as main fall.
As the size of the worked out area i.e. the goaf increases, it gets filled up with fragmented rock material that provides resistance to the lowering of the relatively intact strata. The amount of resistance offered by the goaf depends on its compaction and caving behaviour of the overlying roof rocks. Proper modelling of goaf is necessary for the accurate prediction of stress distribution around the face and the load exerted on the face support system for designing the parameters such as support density and resistance.