Numerical modelling of the goaf: methodology and application

The mechanical behavior of the goaf is a critical issue that may affect the efficiency of longwall mining. Goaf numerical modelling as a continuous material is a challenge, especially because its large-scale mechanical properties are not precisely known. Many different values of the elastic modulus may be found in the literature to be used for representing the mechanical behavior of the goaf area. In the present study, the elastic numerical modelling is shown to be a useful tool for simulating the stress redistribution and displacement due to longwall mining, while taking into account the goaf geometry and its equivalent mechanical properties. The analysis is applied on the Provence coalmine, in the south of France, which had been in operation for more than 50 years, using the longwall mining method was used. A finite difference numerical model of the mine is constructed and two approaches are carried out in order to simulate the goaf area above the excavated panels where the panels have various length to width ratios. In the first approach, the caved zone and the fractured zone have different but homogeneous elastic modulus, both zones have elastic modulus lower than the unaffected host rock. In the second one, their elastic modulus varies linearly with the vertical distance above the panel, up to the elastic modulus of the host rock. In both cases with and without goaf, the subsidence at the ground surface is calculated and compared with in-situ measured values. Results show that attributing to the goaf area a low elastic modulus increases the vertical stress within the rib pillars as well as the subsidence at the surface. The elastic modulus for the direct roof above the panel after excavation has found to be 225 <= Eimmediate-roof (MPa) <= 180 in order to satisfy the total convergence between the roof and the floor. Representing the goaf area as a material with linearly varying elastic modulus gives rational results in terms of convergence and ground surface subsidence.