We have been engaged in the stress measurement in rock massif by the hydro fracturing 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.
Detection of stress condition in rock massif is one of basic conditions for the successful solution of numerous projects within the domain of an underground construction both exploratory and technological service nature. Rough estimates of field of stress based solely on assumption of an elastic behavior of rock massif and derived only from the corresponding geostatic load can be very unfavorably affected in view of accuracy especially in tectonically faulted areas, or in areas disturbed by mining activity, or eventually in areas influenced by additional forces due to geological actions.
In the Institute of Geonics of ASCR we have dealt for a number of years with matter of detection of the Cress Condition of in situ rock massif, above all in the Czech region of Upper Silesian hard coal basin. Predominantly measuring equipment based on hydraulic fracturing of borehole walls has been applied. This equipment comprises many advantages and many limitations, as well. One of its principal limitations is the practical impossibility of measuring absolute stress tensor in a given location. We decided, therefore, to develop some different equipment, which enables such kind of measurement.
The newly developed equipment is based on well- known Principle of stress measurement based on evaluating of deformation changes of drill hole core by means of an overcoring method. We apply the CCBO modification of the method (compact conical-ended borehole overcoring) designed and tested in Japan by Sugawara and Obara (in Nakamura et al., 1999).
The choice of conical shape of surface to be measured is actually a compromise between other types of this kind of stress measuring method. By this approach the advantages have been merged both of the possibility of measuring of absolute stress tensor in a single place and of a simple installation which is additionally featured