To simulate fracture paths in a realistic way, a dense grid of triangular elements is constructed. A contact between two elements becomes a crack ("activated") if its associated failure criterion is exceeded. The authors applied this approach successfully in the past to model fracture growth around a single discontinuity during tensile testing of rock material [1].

Strength parameters and elastic parameters in layered rock are commonly directional dependent and should be treated as such [2]. The distinction is therefore made between contacts that represent the schistosity (i.e. layering) and the contacts in other directions. The schistosity contacts are further subdivided in a weaker and a stronger subtype, in correspondence with laboratory observations.

Additionally, a plastic failure criterion is introduced for the discrete elements, which cannot break internally. Hence, two sets of modelling parameters are required: i.e. the element strength parameters that define the plastic limits on the one hand and the contact strength parameters that define fracture growth on the other hand. The mutual proportions of the different strength parameters on element scale are based on the macroscopic parameters derived in laboratory tests for layered slate rock.

In the simulations here, the layers have an inclination angle equal to their friction angle (20°) and fractures develop both in layer direction and in other directions. This agrees well with experimental observations.

Next, it is found that an increased amount of schistosity layers of the weaker subtype result in an increased degree of fracturation. It has little influence on the strength of the sample under this configuration, because the governing mechanism for loss of strength is the occurrence of fractures at a small angle with the loading direction.

Similarly, it is found that increasing the contact strength anisotropy results in more fractures in the schistosity direction, although the subvertical fractures still develop. On the contrary, for a decrease in the strength anisotropy, fractures in the schistosity direction are nearly absent.

The simulations have established the interdependence of the different modelling parameters and have identified the mechanisms at work during failure of anisotropic rock for a uniaxial loading configuration. This knowledge can help to predict rock failure under more complex in situ stress conditions.

1

B. Debecker, A. Vervoort, J.A.L. Napier, "Fracturing in and around a Natural Discontinuity in Rock: A Comparison between Boundary Discontinuity and Discrete Element Models" in "Proceedings of the Fifth International Conference on Engineering Computational Technology", B.H.V. Topping (Editor), Civil-Comp Press, Stirlingshire, United Kingdom, paper 168, 2006. doi:10.4203/ccp.84.168

2

B. Amadei, "Importance of anisotropy when estimating and measuring in-situ stresses in rock", International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts, 33(3), 293-325, 1996. doi:10.1016/0148-9062(95)00062-3

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