Workreport 2018-16



ONKALO POSE Experiment – Literature Study of Damage Based Approaches on Heterogeneous and Anisotropic Rock Material


Siren, T.



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In 2009 Posiva started to design the three phases of the in situ Posiva Olkiluoto Spalling Experiment (POSE) in ONKALO underground rock characterization facility. The outcome of the POSE in situ experiment was that the experimental holes exhibited damage mainly as a result of structurally controlled factors. The results initiated a back-calculation of the results to understand and predict failure of Olkiluoto rock. As part of the back-calculation a literature study of damage based approaches and heterogeneous and anisotropic material was initiated, including typical and often observed strength anisotropy, a review of the laboratory behaviour of heterogeneous and anisotropic material and the available computer modelling tools.

The main problem with Olkiluoto rock is understanding and hence being able to predict the type of rock failure that can occur, when such failure is significantly influenced by the inhomogeneity and anisotropy inherent in the rock. The weakest planes are associated with foliation planes that developed after mineral segregation occurred at high pressures and temperature gradients. To describe this inherent anisotropy the Pariseau continuum model seems to be the most promising weakness model since it is also the most flexible model. However, it seems there is no applicable existing rock mass failure model for anisotropic rock masses, such as the rock in Olkiluoto. 

The literature study concluded that no clear damage based approach exists for predicting the heterogeneous and anisotropic failure observed in ONKALO. Good weakness models describing the rock anisotropy exist (Pariseau’s continuum model) and lately the hybrid continuum-discontinuum (e.g. ELFEN and IRAZU) and discontinuum models (e.g. PFC and YADE) exist for modelling complex heterogeneous material. However, there is a gap between weakness models and modelling tools, which demands a vast set of laboratory tests and model calibrations.


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