Geology - In Situ Stress Interaction Modelling in Olkiluoto, Finland

摘要

The geological conditions at Posiva’s planned spent nuclear fuel disposal site on the western coastrnof Finland have proven to be challenging when attempting to measure and validate the in situ stressrnfield. The dominant rock type at the site is migmatite and may be a reason for the encounteredrnuncertainties. As a part of this work stress modelling of the Olkiluoto stress state began in 2009 withrnthe aim of determining the interaction of in situ stress and geological features at Olkiluoto with thernONKALO area under more specific focus. The current interpretations of the brittle deformation zonesrn(BFZ) were used in the model.rnClarification of the effect of discontinuity parameters on stress magnitude and orientation andrnwhich of the major brittle deformation zones detected in the ONKALO region could have potentialrneffects on local in situ stress states were the primary modelling goals. The default boundary conditionrnwas compression from the northwest - southeast, i.e. middle Atlantic ridge push, although numerousrnother thrust boundary conditions and an ice-age were also simulated.rnOnly minor interactions developed at shallow depths when using shear strength parameters derivedrnfrom Q parameters obtained from drillhole core-logging and ONKALO tunnel mapping. Factors thatrnaffect the shear strength of shallow BFZ’s as well as their specific shear strengths are, however, notrnvery well known. BFZ cohesion and joint friction angle, i.e. shear strength, were determined to have arncritical effect on stress geology interaction in the first 350 m of bedrock, essentially in this order ofrnimportance. With very conservative BFZ strength parameters i.e. a friction angle of 20 degrees and arncohesion of 0.3 MPa, stress rotations and an increase in stress magnitude occurred related torndeformation zones with shallow dips and oriented roughly in the direction of applied compression.rnGlacial simulations caused heavy stress rotations and a release of horizontal stresses in the first 300 mrnof bedrock due to BFZ slip driven by excess horizontal stresses caused by viscous bending of the crustrnat post glacial phase. Below this depth level stress magnitudes and trends were similar to those seenrnprior to loading.rnBased on the results of this study a cohesion of 0.7 MPa and a friction angle of 35° limits thernstress-geology interaction of shallow dipping BFZs to the first 100 m of bedrock, but values of 0.3rnMPa and 25° enables it to extend to a depth of 300 - 400 m. If interaction is possible the magnitudernand plunge of the maximum principal stress can vary considerably although the bearing is more stable.rnThis result demonstrates the value of accurately characterising and identifying the parameters ofrnbrittle deformation zones.