Conical strain gauge probes for stress measurement

We have been engaged in the stress measurement in rock massif by the hydrofracturing method. This method enables measurement of stress in plane normal to the axis of borehole, when one of the principal stresses parallel to the axis is assumed. But, in complex geomechanical conditions connected with the geology and mining action, the parallelism of the principal stress to the axis of a borehole cannot be assumed with certainty. That is why we decided to develop a device making the determination of the total state of stress tensor possible. In design we started from experiences of K.Sugawara and Y.Obara (Nakamura et al., 1999) that have been using the compact conical borehole overcoring system CCBO. The conical shape of the strain gauge probe CCBO makes the fixation of the probe in borehole easy and makes the measurement of stress in independent directions possible. The principle of the measuring method, design of the probe for continual measurement of stress in course of overcoring and first results of laboratory testing of the development sample are described. Simplified conical probe was designed for long term monitoring of the stress changes in rock massif.