To estimate three dimensional in-situ stress at great depth, Downward Compact Conical-ended Borehole Overcoring (DCCBO) technique, which is one of stress relief methods, was developed. DCCBO technique can be applied to a water-filled vertical/sub-vertical borehole. In in-situ tests, the bonding device and the cleaning system were verified to work well in a water-filled borehole with cuttings on the bottom.

Um dreidimensionale in-situ Beanspruchungen in großen Tiefen abzuschatzen, wurde das Downward Compact Conical-ended Borhole Overcoring (DCCBO) Verfahren entwickelt, wobei es sich um ein diese Beanspruchungen entlastendes Verfahren handelt. Die DCCBO Technik kann auf wassergefuellte vertikale und subvertikale Bohrlöcher angewandt werden. Bei den in-situ Tests wurde die Einsatzfahigkeit der Verbindungsvorrichtung und des Reinigungssystems fuer wassergefuellte Bohrlöcher mit Einschnitten am Boden bestatigt.

La technique de Downward Compact Conical-ended Borehole Overcoring (DCCBO) ou ‘Surcarottage par sondage à extremite conique compact dirige vers le bas’, qui est une methode de detente des tensions, a ete developpee pour calculer les tensions in-situ à trois dimensions à grande profondeur. Cette technique peut s'appliquer à des sondages verticaux/sub-verticaux remplis d'eau. Au cours des essais in-situ il a pu être verifie que le dispositif de liaison et le système de nettoyage fonctionnaient bien dans un trou de forage rempli d'eau avec saignees au fond.

1. Introduction

As underground space expands in scale and as the spaces being used becomes deeper, accurate determination of rock stress at great depth has become one of the most important subjects that must be faced. The principal methods used to measure rock stress at great depth include hydraulic fracturing [1] and core based methods (ASR [2], DSCA [3] and AE [4]), and many results by these method have been reported previously. However, the disadvantages of these methods are that the three-dimensional stress cannot always be measured and that the reliability of the results is not sufficient. Recently, great potential has been shown for such methods as core discing [5], borehole breakout [6] and DTF [7]. However, these methods are still under development; they are yet not matured enough to be used as in-situ measurement methods.

The stress relief method [8, 9] is now considered the most reliable method, but the present procedure is not suitable for rock stress measurements at great depth, since it is necessary to install strain sensors or displacement sensors, using adhesive or mortar, on the wall or at the bottom of a borehole before stress relief by overcoring. So, if the borehole points downward, it is difficult to install the sensors because the borehole is filled with water. In addition, overcoring with a greater diameter than that of the pilot borehole encounters difficulties at great depth. Recently, trials have been reported in which the door stopper method [10], which is one of the borehole bottom strain methods, has been applied to vertical boreholes [11, 12].

The compact conical-ended borehole overcoring (CCBO) technique[13, 14] is one of stress relief methods. It has been frequently used as a method which enables in-situ measurements to be made accurately and economically.

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