The authors carried out laboratory experiments using cubic specimens of mortar in order to seek the possibility of direct measurement of the maximum principal stress from the shut-in pressure data by making a (pair of) artificial longitudinal slot(s) along the minimum principal stress direction. After the authors got a feel of applicability under some extent of limitation related to stress state, slot depth and permeability of rock, they carried out in-situ measurement by using the borehole slotter developed by them.
Les auteurs ont effectue des experiences de laboratoire utilisant des specimen cubiques de mortier pour etudier la possibilite de mesurer directement la contrainte principale maxima à partir des donnees de la pression de confinement, en pratiquant une (paire de) perforation(s) longitudinale(s) artificielle(s) le long de la direction de contrainte principale minima. A l"issue de ces experiences, les auteurs ont eu le sentiment que la methode etait applicable sous reserve de quelques restrictions liees à l"etat de contrainte, à la profondeur de perforation et à la permueabilite de la roche, et ils ont realise des mesures in-situ en employant Ie perforateur qu"ils ont developpe.
Die Autoren haben Laborversuche unter Verwendung von kubischen Mörtelproben durchgefuehrt, um nach einer Möglichkeit fuer direkte Messung der maximalen Hauptspannung aus den Daten fuer den eingeschlossenen Druck zu suchen, indem sie (ein Paar) kuenstiliche Langsschlitze entlang der minimalen Hauptspannungsrichtung anbrachten. Nachdem die Autoren ein Gefuehl fuer die Anwendbarkeit unter gewissen Begrenzungen in Bezug auf den Spannungszustand, die Schlitztiefe und die Permeabilitat des Gesteins gewonnen hatten, fuehrten sie mit einem von ihnen entwickelten Bohrlochschlitzgerat Messungen an Ort und Stelle durch.
The use of hydraulic fracturing as a technique for stress determination has grown considerably in the past decades and today, it is employed not only at great depths from the surface of sphere but also at shallow depths from a point of access such as underground tunnel or drift in order to contribute to rational design of underground construction or mining. The reason is because stress determination by small scale hydraulic fracturing can compete with over coring method in price and more recently, also in accuracy for three dimensional stress determination. In practice, it is very important to know three dimensional stress state for rational design of underground excavation. However, parallel with the development of in-situ stress measurement techniques, such as over-coring and hydraulic fracturing, an awareness of limitations associated with use of the methods has grown. A significant limitation in hydraulic fracturing is that the level of confidence, in the measured maximum principal stress is lower than in the minimum principal stress. In many instances, the maximum principal stress, generally the more important, is not directly measured, therefore, the authors have proposed a procedure for direct determination of the maximum principal stress by using the borehole slotter developed by them. After the authors checked confidence of the procedure through numerical interpretation of laboratory experimental data, they carried out in-situ experiments where the borehole slotter was actually used. In coupled borehole slotting and hydraulic fracturing test, the direction of the maximum principal stress is determined as it is perpendicular to the primary fracture produced by means of double fracture method which was developed by the authors in cooperation with Serata Geomechanics Inc..
As shown in Fig. 1, 190mm cubes of mortar were modeled as the specimens. In order to make an opening of borehole with slot, an alminum pipe with slot(s) and a (pair of) stripped plate with a projection is fixed at the center of the mold where the projection is inserted into the slot from the inside of the pipe.