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

采用有限元方法，分析了77种地应力条件下，HQ长芯桩内部和下方的主拉应力方向和大小。26种应力条件被确定为可能发生岩心盘状现象的条件。然后，通过计算在整个岩心横截面上形成轮廓面所需的最大主拉应力，提出了岩心圆盘破裂的判据。对拉伸主应力方向分布的详细分析表明，在芯片端面的中心部位可能形成几乎平坦的表面，最大主应力与中间主应力相差较大可能形成所谓的马鞍形周边。在总结分析结果的基础上，提出了一种基于端面对称性、端面中心法向和端面凹凸轴方向由圆盘几何形状确定主要地应力方向的方法。为了验证上述由岩心圆盘计算三维地应力的方法，对白川焊接凝灰岩和木池砂岩进行了三轴加载系统加载含有长芯的块体的圆心圆盘实验。本研究提出的芯盘破裂判据大致估算了白川焊接凝灰岩芯盘萌生的应力。然而，对于岩心圆盘伴随着井底压缩破坏的木池砂岩，该准则预测的应力与实验结果并不一致。即使两个水平应力不同，这些岩石也没有观察到马鞍形的切屑。

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.