Co-operative research for developing reliable core-based methods for measuring in-situ stresses

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Fundamental research as well as actual in-situ stress measurements have been carried out by eight researchers to enhance reliability of core-based methods for measuring in-situ stresses at great depth. In the stress measurements common rock cores were distributed to each investigator who has performed stress measurements according to each way. Two series of rock core, a granodiorite and a limestone, were extracted from 550m and 250m level from Kamaishi mine.The methods of stress measurement were Deformation Rate Analysis (DRA) for Dr.Fujii and Prof.Yamamoto, the combined method of Acoustic Emission (AE) and the DRA for Prof.Furuzumi and Dr.Seto and Differential Strain (Curve) Analysis (DSCA) for Mr.Oikawa and Prof.Sano. The members of Tohoku Univ., Prof.Matsuki and Dr.Kojima, performed all the core-based methods. Mr.Oikawa performed a midified DRA under hydrostatic pressure in addition to the conventional DSCA, and Dr.Sano measured the anisotropy of elastic wave velocity to apply it to stress measurement. The comparison among the results given by the researchers has not been completed, but some agreements as well as some disagreements have been found.As basic research work, Prof.Yamamoto has proposed the mechanism which describes how inelastic deformation occurs when uniaxial compressive stress reaches the normal in-situ stress by being based on the concept of stress homoginization in rock in the earth crust. Prof.Matsuki and Dr.Kojima also have proposed another mechanism which may explain the behavior of rock, on which all of the core-based methods are based by model experiments of a glass plate which contained a closed crack with two main wing cracks at the tips.