ABSTRACT:

Sedimentary rocks exhibit a geological structure known as "mechanical layering" where vertical to sub-vertical joints are bounded by bedding plane boundaries, and a ratio between bed thickness and joint spacing is typically defined. This paper demonstrates the use of geologically based discrete fracture models (geoDFN) which incorporate the "mechanical layering" concept and its integration with the powerful block cutting algorithm (DC) of the numerical discontinuous deformation analysis (DDA) method. As such, we propose a new hybrid geoDFN-DDA approach which can simulate natural and complex geological structures. Once the geoDFN-DDA block mesh is obtained, a more realistic forward modeling of discontinuous deformation can be performed. The geoDFN-DDA approach begins by generating a three dimensional, mechanically layered fracture pattern. In this study we use FracMan® which allows us to assign different statistical distributions to the different structural parameters and to impose realistic geological constraints of mechanical layering on the generated fracture network. For the purposes of the 2D-DDA analysis, a 2-D trace plane is then cut through the three dimensional discrete fracture network to provide a 2D trace model which can be simulated using DDA. All trace lines generated by DFN on the trace plane of interest are then imported into the DDA line generation and block cutting programs for the computation of the block mesh. In the resulting block mesh all block areas, centroids, and edge coordinates are stored in a format accessible for geomechanical analyses. This approach is demonstrated for two case studies of the computation of rock mass deformations around underground opening. These case studies provide a comparison of DDA analyses using the previous statistically based DFN approach, and the new geologically based geoDFN approach. For the case studies presented, the stability of the immediate roof above the opening is not dependent on accurate modeling of mechanical layering. However, surface settlements above the underground opening are sensitive to the accurate modeling of mechanical layering geometries.

1. INTRODUCTION

Sedimentary rock masses exhibit a geological structure known as "mechanical layering" [1] where vertical to sub-vertical joints are bounded by bedding plane boundaries (Figure 1A), and a ratio between bed thickness and joint spacing is typically defined [e.g.1, 2, 3]. This paper demonstrates the use of discrete fracture models which incorporate the "mechanical layering" concept to improve stability analysis for underground opening. This potentially represents a significant advance over earlier rock engineering approaches which relied on simplified, statistically based, fracture patterns. These simplified models have typically been parameterized in terms of for example joint persistence [4] joint trace length [5] and bridge [6, 7] (Figure 1B). This paper presents an approach which combines the "mechanical layering" fracture spatial model [8] for sedimentary rock (referred to below as a geologic discrete fracture network or geoDFN) with the discrete element DDA method [9]. The DDA approach is applicable for rock masses in which the significant fractures effecting stability must be modeled explicitly. This includes rock masses with more fractures than can be analyzed using the clamped beam models [10] and the Voussoir beam analogue [11-14], and rock masses where the number of fractures is insufficient to particle flow code [15]or plastic continuum approximations [e.g. 16].

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