Polyaxial Strength Criteria and the Assessment of the In Situ Stress Magnitudes in the KTB Ultra Deep Borehole

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9418738 Haimson The KTB project, located in southeastern Germany, is the world's deepest scientific drilling project outside the former Soviet Union (TD 9.1km). This is a joint project between the University of Wisconsin and Stanford University and represents a continuation of a cooperative US/German research project to determine stress magnitude and orientation as a function of depth in the KTB borehole through implementation of an integrated stress measurement strategy, relying heavily on evaluation of compressive and tensile failures observed in the borehole. To date, this work has been quite successful but an important weakness in this effort has been the lack of sufficient data on rock strength. The purpose of the this project is to fill this critical gap and thus greatly improve our ability to constrain stress magnitudes at depth. Specifically, the principal objective of the research is to comprehensively investigate the strength of core samples from the KTB ultra-deep borehole under a variety of stress conditions: tensile, uniaxial, compressive, triaxial compressive, and in particular true polyaxial. Comprehensive polyaxial compressive strength tests will be conducted in a newly designed cell that minimizes friction at the faces of prismatic rock specimens. The results of these studies will be used to improve the reliability of estimates of the maximum horizontal principal in situ stress in the KTB hole. ***