Three borehole instruments have been investigated for their suitability for monitoring rock stress changes; the yoke gauge, a purpose designed, non-reusable, three component borehole deformeter, and the conventional and thin-wall versions of the CSIRO triaxial hollow inclusion stress measurement cell. Laboratory experiments and field trials were carried out to investigate the effect on instrument stability and sensitivity of such factors as moisture absorption, temperature variation, polymer shrinkage and time since installation.
On a étudié l''aptitude pour le contrôle des variations des tensions des roches de trois appareils de carottage: l''indicateur à étrier, un déformomètre à trois éléments concu dans ce but et incapable d''être remployé, et les modèles conventionnel et à paroi mince de la cellule de mesure triaxiale développée par la CSIRO. On a fait des essais au laboratoire ainsi que sur place pour étudier l''influence sur. la stabilité et la sensibilité des instruments exercée par des facteurs telles que l''absorption d''humidité, la variation de la température, la contraction du polymère et le délai suivant l''installation.
Drei Bohrlochgeräte sind nach ihrer Eignung zur Kontrolle von Änderungen der Gesteinsbeanspruchung untersucht worden: das Rahmenmessgerät, ein zielskonzipiertes, nicht wiederverwendbares Drei-Komponenten-Bohrlochverformungsmessgerät, sowie die heiden herkömmlichen und dünnwändigen Typen der von CSIRO entwickelten Spannungsmesszelle. Versuche wurden im Labor sowie im Feld durchgeführt, um die Auswirkung auf die Apparatsbeständigkeit und -Empfindlichkeit von Faktoren wie Feuchtigkeitsaufnahme, Temperaturschwankungen, Polymerschwindung und Zeitdauer nach dem Einsetzen zu untersuchen.
The requirement to measure changes in the stress field in rock is generally tWofold. Rock stresses surrounding excavations are monitored to assess the onset of unstable behaviour; this is particularly true for excavations for underground mining. Monitoring is also carried out to assess the performance of a particular design. The current frequent use of design tools such as finite and boundary element modeling techniques has greatly increased the importance of monitoring stress change in rock. The primary output from these design aids are estimates of rock deformation and stress, and the effectiveness of an adopted design can only be confirmed by monitoring either, or both of these variables in-situ. Currently, the most viable method used to determine the change in rock stress is to employ an instrument fixed into a borehole drilled into the rock mass. These instruments fall into two non-rigorous categories; rigid and soft inclusions. Both types have certain advantages and disadvantages in use. A soft inclusion instrument or strainmeter has a low modulus of elasticity relative to that of the host rock. In order to calculate the change of stress in the host rock it is necessary to know or assume the constitutive law between stress and strain for the rock. In practice, a linear elastic law is usually assumed. A soft inclusion may only be employed to measure compressive stress magnitudes up to the point when the rock yields at some position around the borehole. However, a soft inclusion does not usually suffer from instrument-to-rock contact problems (as may occur with rigid inclusion instruments), and is thus well suited to be configured in biaxial or triaxial form, and for measuring stress reductions. A triaxial instrument is required to measure any rotation of the principal stresses and to monitor stress changes in a situation where a borehole cannot be drilled perpendicular to the stress direction or plane of interest. This paper discusses the performance of three soft inclusion borehole instruments for monitoring stress change in rock. Two are capable of triaxial measurement; one of biaxial measurement. The investigation is part of an ongoing program of research being conducted by the CSIRO in collaboration with the Australian Minerals Industry Research Association (AMIRA) and its sponsor companies.
This instrument is similar to the USBM three component borehole deformation gauge (Hooker and Bickel 1974), but is designed specifically for stress change monitoring purposes. Three changes in diameter of a borehole are measured which enables the change in the secondary principal stresses in the plane perpendicular to the axis of the borehole to be calculated. Three such cantilevers, which have a length that exceeds the diameter of the borehole by appoximately Imm, are encased in the annulus formed between two, thin-walled PVC tubes and are located so that the cantilever tips protrude from the outer tube at intervals of 60° around the circumference (Fig. 1). For each cantilever, the electrical resistance strain gauges are wired into a full, active, Wheatstone bridge configuration and are powered by an individual voltage regulator. All electrical components are encapsulated in epoxy resin and the space surrounding the cantilevers filled with silicone moisture proofing compound to prevent moisture ingress. When installed into a borehole, the cantilevers are compressed and pre-mixed epoxy cement is extruded into the annular space between the outer PVC tube and the wall of the borehole.
