3D Numerical Simulation of the Goaf Due to Large-Scale Longwall Mining

Abstract

Due to longwall excavations, the upper strata disturb, the roof and the floor of the opening become in contact. This disturbed area is commonly known as the “goaf area”. The challenge of simulating numerically the goaf area is to identify its geometry and its equivalent mechanical properties. The main objective of this study is to improve the 3D numerical simulation of longwall mining and its accompanying goaf area, which will permit us to observe the stress changes due to longwall excavations. The Provence coal mine in the South of France has been chosen to be the case study of the current research, where the mined coal seam has 2.5 m thickness and the average depth of the mine is 1000 m. The longwall panels have a regular width of 200 m and various lengths from 400 up to 1400 m. A large-scale finite difference numerical model of the mine was constructed by using FLAC^3D. The numerical modeling of the goaf area was performed in two steps. The first step was to identify the geometry of the goaf area above longwall panels. The second step was to calibrate its equivalent mechanical properties with the total convergence between the roof and the floor. The goaf geometry and the mechanical properties were then calibrated with the in-situ surface subsidence. The results show that applying a linearly varying elastic modulus within the goaf area is a very effective method to express its heterogeneity, which also gave rational surface subsidence values for panels width less than 1000 m. In addition, in terms of stress redistribution, the induced vertical stress increases progressively with panel width, and it becomes close to the initial values at the center of the goaf for panel width larger than 1000 m.