Estimation of initial stress by Deformation Rate Analysis (DRA)

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Deformation Rate Analysis (DRA) is one of the methods to estimate the initial-stress of rock mass. In DRA, more than two cyclic compressive load are added to a cored-rock specimen. The graph, stress-difference of axial strain curve in cyclic loading test, have a bend point. And, this stress expresses in-situ initial-stress of rock mass. DRA can be used in a laboratory, so this method estimates initial-stress more easily than in-situ method such as over-coring method or hydraulic fracturing method. But in any case, especially long elapsed time, initial-stress can't be accurately estimated because of less clear bend point of this curve. Some parameters have, therefore, been investigated to influence on the evaluated value of initial stress by DRA. In this paper, the effects of lateral stress, elapsed time after pre-stress loading, and loading rate are investigated. In our method, not only axial pre-stress but also lateral one could be estimated for a single uniaxial cyclic loading test. Just after previous loading, the DRA curve has a clear bend, since delay time is short. With the increase in delay time, the bend of DRA curve became less clear. Even in that case, however, the clear bend of the DRA curve can be recognized at the pre-stress point on and after the second loading. Therefore, it was possible to estimate the pre-stress, even when the bend point of the DRA curve was not clearly observed in the first loading. Loading rate didn't produce any significant influence in evaluated values. So, it was confirmed that DRA is useful technique to estimate initial stress of cored-rock which have lateral stress even in case where elapsed time after coring is long.