A recently developed 2-D stressmeter, namely the "borehole slotter", satisfies important operational requirements. It is reuseable, does not rely on overcoring and allows for a large number of measurements at very reasonable costs. Extensive in-situ tests at the Burdekin Falls damsite (Queensland/Australia) have been carried out in parallel with overcoring and hydraulic fracturing tests. Whilst providing results on the in-situ stress state with at least comparable accuracy, the borehole slotting technique was found to have major advantages over the established methods with regard to speed of operation, reliability and economy. Borehole slotting allows an internal check of the consistency of the stress measuring results with the degree of redundancy of the readings being adjustable to the particular situation. This proved particularly useful when interpreting the stress measurements.
Le "Borehole Slotter", sonde-compteur récemment développée pour mesurer l'' état de contrainte biaxial, répond pleinement à d''importantes exigences opérationnelles. II est réutilisable, ne nécessite pas de surcarottage et permet d''enregistrer un grand nOmbre de mesures à des frais peu élevés. On a effectué de nombreux tests sur Ie site de construction du barrage de Burdekin Falls (au Queensland, Australie), ainsi que des tests de surcarottage et de fracture hydraulique. A l''aide des tests effectués avec le "Borehole Slotter" on a mesuré l''état de contrainte avec tout au moins autant de précision qu''avec les autres méthodes. Mais quant à la vitesse, la sûreté de fonctionnement et Ie coût, Ie "Borehole Slotter" l''emporte. Le "Borehole Slotter" permet un contrôle interne dans l''obtention de résultats consistants. Le niveau de redondance des mesures peut être adapté à chaque situation; ce qui a été très utile, surtout quant a l''interprétation des mesures d''état de contrainte.
Eine vor kurzem entwickelte 2-D Spannungsmessonde ("Bohrloch- Schlitzsonde") hat gegenüber bestehenden Sonden wesentliche versuchstechnische vorzüge. Sie ist wiedergewinnbar, nicht auf ein Überbohren angewiesen und erlaubt die Durchführung einer groβen Anzahl von Messungen bei vertretbaren Kosten. Umfangreiche Bohrlochschlitzversuche wurden an der Burdekin Falls Dammbaustelle (Queensland/Australien) parallel mit Spannungsmessungen nach dem Überbohrverfahren und der Hydraulic Fracturing Methode durchgeführt. Sie ergaben, daβ das Bohrlochschlitzverfahren den in-situ spannungszustand in Fels mit mindestens vergleichbarer Genauigkeit zu bestimmen gestattet wie bestehende Verfahren. Das neue Verfahren erwies sich den bestehenden gegenüber als eindeutig überlegen hinsichtlich Schnelligkeit, Zuverlässigkeit und Kosten. Es gestattet darüberhinaus eine interne Überprüfung der Meβdaten mit einem der jeweiligen Situation angepaβten Grad an Redundanz der Meβdaten. Dies erwies sich als besonders hilfreich bei der Auswertung der Spannungsmessungen.
The borehole slotter is a recently developed 2-D stressmeter which satisfies important operational and scientific requirements. It is a stressmeter probe Details of the borehole slotting technique and its instrumentation were first published in 1983 and 1984 by Bock and Foruria. In these publications a stage of the borehole slotter development was documented at which both laboratory and field tests suggested a great potential for quick and reliable 2-D stress measurements. At that time the number of field tests, however, was rather limited. A definite conclusion with regard to the suitability of the borehole slotting method was deferred until the results of additional in-situ tests, including comparisons with established stress measuring methods, became available. This is now the case. Extensive in-situ borehole slotting tests have been carried out in 1984 and 1985 at a major dam construction site in parallel with overcoring and hydraulic fracturing techniques. An account of these investigations and the conclusions drawn with regard to the borehole slotting method are presented in this paper.
The borehole slotting stress measuring method is based on the principle of local stress relief. A half-moon shaped radial slot is cut into the borehole wall by means of a small diamond-impregnated saw (Figs. 2 and 3). In the prototype borehole slotter (Fig. 1), which is designed for HQ Boreholes of 96 mm diameter, the slot is 0.8 mm wide and maximally 32 mm deep. The saw is pneumatically driven and is part of the stressmeter. Before, during and after slotting, the change of tangential strain is measured at the borehole surface in the immediate vicinity of the slot where practically full stress release occurs (Bock and Foruria, 1984; their Fig. 4). The change is picked up by an innovative strain sensor which is recoverable and also is part of the stressmeter probe. Compared with the sensor documented in Bock and Foruria (1984; their Fig. 9), the currently-used type of sensor has been improved with regard to sensitivity and stability (Azzam, 1985). At a particular test location, at least three slotting tests with cuts in independent directions are made for a single 2-D measurement. The theory of linear elasticity is employed to transfer the strain readings into stresses. This means that the Young''s modulus E of the tested rock must be known for the stress measurement.
