Accuracy of Rock Stress Measurement by means of Conical-ended Borehole Technique.

Abstract

The conical-ended borehole technique is presented to measure the threedimensional state of stress in a rock mass with a high accuracy and low costs by the stress relief method, and the optimum shape of the bottom surface is discussed as well as the gauge arrangement. It is shown how the stress tensor can be determined from the strains on the conical bottom surface of a single borehole, and how the error of stress determined can be evaluated theoretically. The sensitivity and the practicality of the present method are discussed, comparing the conventional methods of in situ stress measurement. The axisymmetrical arrangement of cross gauges on a measuring circle with equal spaces of rotating angle, shown in Fig. 5, is suggested and the sensitivity of the present method is clarified to depend on the vertical angel: of the conical bottom surface, theradius: 0 0f the measuring circle, the number: of the strain gauges, and the Poisson's ratio:of rock. The smaller the vertical angle, the higher the accuracy in the stress tensor determination. Simultaneously, the accuracy improves with increase of 0 and n. From such an analysis of the sensitivity and the subsequent discussion on the measuring procedure in practice, the conical bottom of co=60 degrees and the measuring circle of 0 equals to a half of the borehole radius are concluded to be convenient for the over-coring operation having a diameter equals to that of the conical-ended borehole. The observation equation to be used in practice is presented, and from the time and the costs involved it is concluded that the present method is a promising method.