ABSTRACT:

Although it has long been recognised that fractures in rock perturb the local stress field, it has been difficult to both simulate and visualise the stress heterogeneity developed in highly fractured systems. Here, we use the open source 3D FEM-DEM code Y3D, part of the Virtual Geoscience Simulation Tools (VGeST) suite, to model a discontinuous rock mass and hence rigorously determine the complete state of stress within it. The model replicates a 200 m cube dissected by two discontinuity sets, the properties of which are purely frictional. The results show that a significant degree of stress heterogeneity is induced. This heterogeneity is seen in both the orientation and magnitude of the principal stress components, and is largely independent of the frictional properties of the discontinuities. Although this is only a preliminary report of work in progress, it nevertheless suggests that it is important to understand the geological structure at a stress measurement location, and that identifying the in situ stress state in a discontinuous rock mass from a few stress measurements may be very difficult.

1. VARIABILITY OF IN SITU STRESS

It is widely recognized that measurements of in situ stress will often display significant variability. As stress is a second order tensor this variability is awkward to display visually, but Fig.1 shows it in terms of oriented and scaled principal stresses. Importantly for the context of this paper, Fig. 1 shows that the variability is, in fact, heterogeneity. Furthermore, in this case the stress heterogeneity is strongly linked to the geological structure of the host rock — the 'average' stress state in the diorite of Fig. 1 is very different from that in the granodiorite, with the boundary between the two domains being defined by a geological fault. If we assume that specific measurements of in situ stress represent values of a uniform far-field stress state that have been perturbed by the rock mass structure, then meaningful interpretation of stress measurements requires us to know how stress is perturbed by the existence of geological structures such as faults, fractures and other mechanical discontinuities. Given the mechanical complexity of such discontinua, numerical modeling will be of great benefit in this regard. Thus, in this paper we report on simple, preliminary experiments using a FEM-DEM code to investigate the nature of stress heterogeneity in a discontinuum.

2. FEM-DEM ANALYSIS OF A FRACTURED ROCK MASS

We have used the 3D FEM-DEM code Y3D from the open-source suite VGeST in our analyses [2]. The tools within the suite allowed a 3D model of a fractured rock mass to be defined directly using discontinuity orientation and location, and analyzed under polyaxial stress conditions. The individual blocks of intact material were assumed to be isotropic and linearly elastic, with elastic modulus 26GPa and Poisson's ratio 0.205. The discontinuities were given both normal and shear stiffness, and a cohesionless Coulomb frictional strength, and in common with other DEM models all blocks were free to displace relative to one another.

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