To assess the stability of underground openings by using the numerical modeling tool, the stress state (i.e., initial and induced stresses) should be generated accurately. The objective of the present paper is to propose a methodology to numerically initialize the pre-mining stress field in complex case study, where the measured vertical stresses are in disagreement with the overburden weight and the exploited coal seam dips with 10° in which numerous in-situ stress measurements were carried out. The measured principal stresses in the current case study are highly heterogeneous and anisotropic. In addition, many faults pass nearby the mining zone. To reproduce such complex pre-mining stress state in a numerical model (Finite difference numerical model FLAC3D), stress gradients are estimated based on the existing stress measurements. The existing discontinuities in the mining area are introduced numerically by using Discrete Fracture Network (DFN) elements and Ubiquitous mechanical model. Two approaches are adopted to compare the initial stresses, the first approach is based on using the 3D stress gradient in a continuum rockmass without implementing the effect of discontinuities. The second approach is based on implementing the discontinuities (DFNs) in a discontinuum numerical model, in addition to use the 3D stress gradient, to test their influence in the pre-mining stress initialization step. The results reveal that the 3D stress gradients can reproduce such complex and anisotropic initial stress state and the presence of the discontinuities in the numerical model influenced the pre-mining stress state in the mine.
Numerical modeling tool plays an important role to describe the rockmass behavior before and during mining activities compared with traditional empirical formulas. It is well known that to obtain a relied stress state, after mining activities were took place, a coherent initial stress state and well matched with the in-situ measurements, if available, must be obtained. Many researchers used the traditional approach to estimate the vertical stress based on the overburden weight (Brown and Hoek, 1978), and the horizontal stresses is a function of the vertical stress (Terzaghi and Richart, 1952). The traditional approach for stress initialization in numerical modeling is efficiently used for homogeneous rockmass subjected to isotropic stress field. But, in case of heterogenous rockmass with anisotropic stress field, a much complex stress initialization method should be adopted.
