A series of measurements of induced stress by the Compact Conical-ended Borehole Overcoring (CCBO) technique was performed within rock mass around a tunnel excavated in a region that an underground powerhouse is constructed under high initial rock stress. In the measurements, a phenomenon of core disking was observed during overcoring. The selecting method of strain data uninfluenced by the core disking was proposed, using X-ray CT scanner. Then, the state of induced stress determined from the selected strain data was discussed, comparing with the state of initial stress.

Eine Serie von Messungen von induzierter Belastung mit kompaktem konisch-beendetem Überbohrverfahren (CCBO) wurde in Felsmassen um einen Tunnel ausgefuehrt, der in einer Region gebohrt wurde wo ein unterirdisches Kraftwert unter hoher anfanglicher Felsbelastung gebaut wird. Bei den Messungen wurde beim Überbohrverfahren ein Phanomen des Core Disking beobachtet. Die gewahlte Methode der Belastungsdaten, unbeeinflusst durch Core Disking wurde vorgeschlagen, unter Verwendung von Röntgen-CT-Scanner. Dann wurde der Zustand der induzierten Belastung aus den gewahlten Belastungsdaten diskutiert, unter Vergleich mit der urspruenglichen Belastung.

Une serie de mesures de contrainte induite effectuee avec la technique de forage de sur-carottage compacte à embout conique a ete executee dans la masse rocheuse autour d'un tunnel excave dans une region de construction d'une centrale electrique souterraine sous haute contrainte de roche initiale. Au cours des mesures, un phenomène de discage de carotte a ete observe pendant le sur-carottage. La methode de selection des donnees de contrainte non influencee par le discage de carotte a ete proposee, en utilisant un scanneur de tomographie calculee aux rayons X. Il s'ensuit que l'etat de contrainte induite provoquee et determine à partir des donnees de la contrainte selectionnees a ete traitee, en effectuant une comparaison avec l'etat de contrainte initiale.

Introduction

The initial rock stress measurement for the design of an underground powerhouse was firstly performed by Compact Conical-ended Borehole Overcoring (CCBO) technique 1, 2, which is one of the stress relief methods. The state of initial stress was that the maximum principal direction in the horizontal plane is East-West, and that its magnitude was larger comparing the overburden pressure. Therefore, the longitudinal axis of the powerhouse was changed and determined as the maximum horizontal stress is acted almost parallel to it, considering the initial rock stress and the direction of predominant joint set 3, 4, 5.

Secondly, a series of measurements of induced stress by CCBO was performed within rock mass around a tunnel excavated in a region that the underground powerhouse is constructed under high initial rock stress. Since the initial stress in this field was high and the measurement stations were located near the tunnel wall, the very high stress concentration was prospected. In fact, a phenomenon of core disking was observed during overcoring in the measurement.

In stress measurements by the stress relief method under high rock stress, a phenomenon of core disking is sometimes observed during overcoring. In the case that the core disking was observed.

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