Semi-integration of overcoring stress data and review of rock stress data at the Olkiluoto site

This project involves a semi-integration of collected rock stress data at Olkiluoto. The data included in the study involves overcoring and hydraulic fracturing data reported in the 2006 Site Report, supplemented with the additional data gathered since then, measurements for the power plant construction, and measurements in ONKALO. Despite the extensive data, and although the data display a fair correspondence with respect to stress magnitudes between methods, a few discrepancies between methods exist. The discrepancies primarily concern the orientation of the horizontal stresses and the magnitude of maximum horizontal stress. The objective of the study is help answer the remaining discrepancies in the collected data using a semi-integration approach on the overcoring data, i.e. a simplified application of the Integrated Stress Determination Method. The semi-integration involves two steps: (i) a brief re-evaluation of data; and (ii) forced overcoring stress calculations based on constraints derived from the other stress measurement techniques. The overall intension was to force overcoring data, which is the method displaying the largest scatter in results at the Olkiluoto site, to be consistent with a number of constraints that were derived from other stress measurement techniques. This exercise would outline the most probable constraints and in the extension, help identifying a stress model for the site. Regrettably, the results of the semi-integration were not completely satisfactory. The most important factor of this outcome is believed to be the failure to reduce the initial scatter through a re-evaluation. The remaining scatter after re-evaluation is judged too large to represent solely in situ stress variation; hence, measurement related errors are likely still present in the re-evaluated data set. A more in-depth analysis may to some extent decrease the scatter in data, but perhaps more effectively, develop a tool that simultaneously can derive the stress field from all stress measurement techniques. The variability in results is visualized by the low consistency of overcoring strains with the various constraints, although they clearly may represent also improperly defined constraints but may also reflect a reduced data quality or limitations of the acceptance criteria and algorithm. What can be concluded is that one principal stress is indeed vertical (or near vertical) and closely resembles the theoretical weight of the overburden. The results of constraining also horizontal stress orientations and magnitudes are though much less clear. Most likely, σH is directed NW-SW, i.e. a quite imprecise result. The report also involved analysis of observed core discing. The results of this study show that dominating disc type at the Olkiluoto site is the incomplete disc (52 % of all observations) and the dominating host rock pegmatitic granite (PGR; 49 % of all observations). Discing events with other disc shapes and in other rock types are significantly less frequent. The discing events could not be correlated with existing and larger deformation zones, although discing events may possibly be correlated to zone OL-BFZ002. Note that the analysis has only been made with respect to the major deformation zones, which does not preclude that discing events may be correlated to smaller scale zones