A soft hollow inclusion strain measuring device that employs the overcoring technique of stress relief is described. The instrument has an inflatable rubber membrane with electrical resistance strain gauges mounted on the outer surface. These strain gauges are bonded with epoxy cement directly to the periphery of a 38 mm diameter pilot hole by inflating the rubber membrane. A pressuremeter test can be conducted in situ, prior to the overcoring operation, to verify correct strain gauge operation and to determine the modulus of the rock. The instrument has been used to measure in situ stresses in both a sub-bituminous coal and an anisotropic schist, and also to monitor stress changes in coal.
Un appareil de mesure de contraintes basé sur une inclusion souple et creuse est décrit. Cet instrument emploie la technique de relachement du stress par "overcoring". L''instrument est composé d''une membrane en caoutchouc gonflable sur la surface extérieure de laquelle sont installées des résistances électriques mesurant les stress. Ces jauges de mesures des contraintes sont fixées par résine epoxy directement sur la périphérie d''un trou pilote EX en gonflant la membrane. Un test de me sure de pression peut être effectué sur place avant l''opération d''''''overcoring''''pour vérifier que la jauge fonctionne correctement et pour déterminer le module de la roche. Cet instrument a été utilisé à la fois dans un charbon subbituminex et dans un schiste anisotropique pour mesurer les contraintes en place, et dans un charbon pour suivre les changements de tension.
Ein weiches, hohles Einschliessungs-Instrument zur Ausdehnungsmessung, das die Kernbohrungs-Technik der Spannungsbefreiung ausnutzt, ist beschrieben. Das Instrument besitzt eine aufblasbare Gummimembrane auf deren Oberflaeche eletrische Dehnmessstreifen exponiert sind. Durch Aufblasen der Gummimembrane werden diese Dehnmessstreifen mit Epoxyharz direkt auf die Peripherie eines Pilotloches von 38 mm Durchmesser geklebt. Ein Dilatometer Test kann vor der Kernbohrung vor Ort durchgefuehrt werden, um die korrekte Funktion der Dehnmessstreifen zu sichern und den Modul des Gesteines zu bestimmen. Das Instrument wurde sowohl in unter-bituminoeser Kohle als auch in anisotropischem Schiefer zur Messung von Spannungen vor Ort benuetzt sowie zur Beobachtunq von Spannungsaenderungen in kohle.
This paper describes an instrument called the Auckland New Zealand Soft Inclusion (ANZSI) cell that was developed to measure in situ stress in coal. The instruments mode of operation is described together with the advantages that it offers for in situ stress measurements and stress change monitoring in coal as well as other rock types. Results of in situ stress measurements using the instrument are described elsewhere in these proceedings (Mills et al 1986). Stress relief by overcoring of an installed instrument is a well established technique for measuring in situ stress in rock masses. Many instruments have been developed which use electrical resistance strain gauges either cemented directly to the rock (Leeman & Hayes 1966), or cast within an epoxy inclusion which is then cemented to the rock (Rocha & Sil veira 1969), (Worotnicki & Walton 1976). These instruments have been used in a wide variety of rock types with best results achieved in hard competent rock. Practical advantages of waterproofing and general robustness ?f the instrument result from hav1ng stra1n gauges included within an epoxy annulus or cylinder which has a low elast1c modulus relative to the rock. For soft rocks, such as coal, the modulus of an epoxy inclusion is relatively high compared to the modulus of the rock. Consequently, large tensile stresses are induced in a radial direction at the instrument-rock interface during stress relief (Duncan-Fama & Pender 1980). Tensile stresses of the order of 800 kPa were generated when the CSIRO Hollow Inclusion cell was used in coal (Depledge et al 1980). These stresses were sufficient to cause tensile failure of the rock itself adjacent to the interface. Stress measurement instruments of the type described above rely on a biaxial test to verify correct gauge function and to determine rock properties. In a competent rock, the chances of recovery of intact core are relatively high, so rock properties can be determined in a biaxial compression test. In a closely jointed rock, however, the chances of recovery of intact core are much less. Even if overcoring has been successful, as indicated by the stress relief curves, it may not be possible to determine the rock properties or verify correct gauge operation because the recovered core cannot be tested in biaxial compression. Although the ANZSI cell was develored mainly to overcome the high tens le stresses generated during overcoring in coal with conventional instruments, its utilisation of the pressuremeter principle offers a number of other advantages. Among these is the ability to conduct a pressure test in situ and thereby to provide, before overcoring, information from which the rock properties can be determined and the correct operation of all strain gauges can be checked.
