Rock Stress Measurement by means of the Compact Overcoring Method.
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概要
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The compact overcoring method is proposed for the cost effective stress relieving, of which the diameter equals to that of the pilot boring for in situ stress measurements. This is successfully combined with the conical-ended borehole technique to determine the stress tensor with a high accuracy. Subsequently the axialloading experiment in a conical -ended borehole is presented to evaluat e the elastic moduli of the rock, which are essential to determine the magnitude of the stress tensor.<BR>It is firstly shown how the strains on the bottom surface can be measured by means of the compact overcoring method. The measurement system tobe used in practice has been presented and the excellent accuracy is discussed, analyzing case examples. Conical diamond bits to convert the bottom into a required shape, a borehole bottom scope to survey the crack distribution, a conical strain-cell and a spring-type bonding device to bond the necessary strain gaugesdirectly to the rock surface have been developed successfully as well as a thin-well core-bit for a cost effective stress relieving. From their successful application and subsequent monitoring of the changes in strain with the compact overcoring by the continuous strain measurement system, it is concluded that the present combined method can be used effectively in an isotropic rock mass.<BR>Secondly it is shown how the elastic moduli of the rock can be determined prior to the stress relieving by means of the axial uniform loading on an appointed ring ina conical-ended borehole. The pressure-strain relationships have been analyzed by FEM and the procedure to determine the elastic moduli has been presented as well as the necessary strain coefficients. From the experiments in situ and the subsequent comparision with the conventional laboratory tests, it is concluded that the axial loading procedure is available for the Young's modulus determination.
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