A THEORETICAL AND EXPERIMENTAL STUDY FOR ESTIMATING THREE-DIMENSIONAL IN-SITU STRESSES FROM CORE DISCING

采用有限元法分析了77种地应力条件下长HQ岩心桩内及桩下拉主应力的方向和大小。确定了可能发生岩心剥离的26种应力条件。然后，通过评估在整个岩心横截面上形成轮廓面所必需的最大拉伸主应力，提出了岩心切割的标准。对主应力方向分布的详细分析表明，圆盘端面的中心部分可能形成近乎平面的表面，最大主应力与中间主应力之间的较大差异可能产生所谓的鞍形外围。总结分析结果，提出了一种基于端面对称、端面中心法线方向和端面凹凸轴方向，从圆盘几何形状确定主地应力方向的方法。为了验证上述通过岩心剖切估算三维地应力的方法，对白川焊接凝灰岩和木町砂岩进行了岩心剖切实验，通过三轴加载系统加载了一个包含长岩心的区块。本文提出的岩心剥离准则近似地估计了白川焊接凝灰岩岩心剥离的起始应力。然而，该准则预测的应力与Kimachi砂岩的实验结果并不一致，Kimachi砂岩的岩心切割伴随着井底压缩破坏。即使两个水平应力不同，在这些岩石中也没有观察到马鞍状的切割。

The direction and the magnitude of tensile principal stresses within and below a long HQ core stub were analyzed with a finite element method for 77 in-situ stress conditions. 26 stress conditions were identified to be those under which core discing is likely to occur. A criterion for core discing was then proposed by evaluating the maximum tensile principal stress which is necessary for the contour plane to be formed throughout a cross-section of the core. A detail analysis on the distributions of the direction of tensile principal stresses showed that almost planar surfaces might be formed in the central part of the end surfaces of a dics and that large difference between the maximum and intermediate principal stresses might produce a so-called saddle shaped periphery. Summarizing the results obtained by the analysis, a procedure for determining principal in-situ stress directions from the geometry of a disc was proposed, based upon the symmetry of the end surfaces, the normal direction in the central part of the end surfaces and the direction of the concave or the convex axes in the end surfaces.To verify the methods for estimating three-dimensional in-situ stresses from core discing, described above, experiments on core discing were carried out for Shirakawa welded tuff and Kimachi sandstone by loading a block containing a long core with a triaxial loading system. The criterion for core discing proposed in this study approximately estimated the stresses at which core discing initiated for Shirakawa welded tuff. However, the stresses predicted by the criterion did not coincide with the experimental results for Kimachi sandstone, for which the core discing was accompanied by compressive failure at the borehole bottom. A saddle shaped dicing was not observed for these rocks even if two horizontal stresses were different.