Databank

Databank

Posiva publishes Working Reports and Posiva Reports. From the year 2006 nearly all the reports have been published on our webpage and they can be found in the databank. In the databank you can also find our Annual Reviews and some other publications as well. You can also find print-quality pictures and useful links in the databank.

Recent publications


Workreport 2018-18

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Name:

ONKALO POSE Experiment - FRACOD2D Back-Analyses

Writer:

Siren, T., Suikkanen, J.

Language:

English

Page count:

102

Summary:

The POSE experiment was conducted in ONKALO® underground research laboratory as an in situ rock mass strength test that revealed the rock mass failure in Olkiluoto being governed by fracture growth at lithological borders. In this study, the experiment outcomes were back-analyzed with FRACOD2D code which is based on principles of fracture mechanics, capable of modelling explicit fracture initiation and growth in continuous medium.

The FRACOD2D simulations were aimed to act as an extension to the conducted 3DEC back-analyses of POSE, by providing more realistic and detailed information of the observed fracturing process in specifically selected 2D cross-sections. Consequently, the methodology in the FRACOD2D approach involved elastic simulations to achieve consistent stress levels and temperatures, equivalent to the monitored or back-analysed values during the experiment, after which fracture initiation and growth was implemented within the simulations. The overall nature and extent of the simulated damage was evaluated against the POSE observations. The strength properties for the rock matrix and developing fresh fractures were adjusted, within the limit of associated uncertainties in the available data, until a sufficient match against in situ observations was achieved. 

The FRACOD2D back-analysis results indicate a damage extent in the order of centimetres to decimetres. The simulated damage extent can be perceived to be in the same order of magnitude as observed during the execution of POSE. Damage depth estimation from scaling of the experimental holes indicated a damage extent of less than 100 mm, while hydraulic flow measurements indicated an average value of 118 mm.

No available means or literature estimations exist of the strength and deformation properties for freshly developing new fractures as a consequence of stress exceeding the rock strength. During the FRACOD2D back-analyses, it was discovered that the best fit between simulated results and in situ observations was obtained with conceptualizing the freshly developing fractures having 90 % strength of the intact rock properties, with the logic of fresh fractures having to be weaker than the intact rock material while stronger than existing old fractures. In addition, it was discovered that the simulation results were highly sensitive on the Mode II fracture toughness (KIIc), which in turn is dependent on the applied level of confining stress. FRACOD2D does not take into account the confinement dependency in applied fracture toughness values and it was necessary to determine appropriate confinement value for determining the KIIc as an input. 

Although FRACOD2D suffers from the two-dimensional simplification of the back-analysis problem at hand, the methodology can capture the observed structural failure and the variability of the results depending for example on the foliation angle, such as observed in situ during the execution of POSE. Uncertainty exists in all of the parameters as majority of the parameters given did not allow taking account the changing confining pressures during the test. Also the boundary conditions brought from 3D modelling were complex and were simplified in the 2D model, rendering the comparison between methods often impractical.

Keywords:

POSE, FRACOD2D, fracture mechanics, modelling, back-analysis, in situ

File(s):

WR 2018-18_web (pdf) (9.3 MB)


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