SUMMARY:

In this paper a technique monitoring the stability of underground caverns during a construction period is described. The technique is based on displacement measurements such as Convergence, borehole inclinometer and extensometer measurements. The stability of the caverns is convinced directly from strains evaluated by the measured displacements, without analyzing stress distributions around a cavern. The practical examples are shown to demonstrate an applicability of this technique. The failure criterion of rock and soil masses is also discussed in connection with the proposed monitoring technique.

ZUSAMMENFASSUNG:

In diesem Beitrag wird eine Technik vorgestellt, mit der die Standsicherheit von unterirdischen Kavernen während der Bauperiode überwacht werden kann. Die Methode beruht auf Verformungsmessungen, wie Konvergenzen, Bohrlochinklinometer und Extensometermessungen. Die Standsicherheit von Kavernen wird hauptsächlich von den Dehnungen hergeleitet, die aus den gemessenen Verformungen errechnet werden, ohne dass es notwendig ist Spannungen im Bereich der Kaverne zu berechnen. Die angeführten praktischen Beispiele sollen die Anwendbarkeit dieser Methode zeigen. Das Bruchkriterium von Fels und Boden wird ebenfalls in Verbindung mit der vorgeschlagenen Oberwachungsmethode diskutiert.

RESUME:

Cet exposé décrit une technique de contrôle de la stabilité des cavernes durant leur période de construction. Cette technique est basée sur des mesures de déplacement telles que la convergence, l''inclinaison des trous de forage et la dilatation. La stabilité des cavernes est démontrée directement à partir des variations de longueur évaluées en fonction des déplacements mesurés, sans analyser les répartitions des contraintes autour de la caverne. Des exemples pratiques sont présentés pour prouver une applicabilité de cette technique. Le critére de rupture des masses de sol et de roche est aussi présenté en rapport avec la technique de contrôle propose.

INTRODUCTION

Field measurements have been recognized as a potential tool for monitoring the stability of underground caverns during a construction period. Many different types of field instrumentations are now available for measuring stresses and displacements in the underground media. The most reliable and recommended monitoring system, however, must be the one which is based on the displacement measurements such as convergence and borehole extensometer measurements.

The displacement measurements are desirable because of simplicity of instrumentation and reliability of the data obtained, while the direct stress measurements are occasionally questionable and less reliable. Nevertheless, in order to give a quantitative interpretation to the results of displacement measurements, the stress distribution around caverns must be back-analyzed from the measured displacements, because the failure criterion of rocks is in general given in terms of stress. Therefore the stability of caverns can be assessed by comparing the computed stress with the failure criterion of the material.

This back-analysis of stress, however, definitely depends upon the stress-strain relationship of rocks which is not easily evaluated for in-situ rock masses.

In order to overcome such a shortcoming involved in the stress analyses, the author has proposed a monitoring technique which is based on strain rather than stress, so that the stress analyses are unnecessary. Since strain is kinematically related to displacement, the proposed technique can achieve monitoring the cavern stabilities directly from the measured displacements. Accordingly it is named "Direct Strain Evaluation Technique (DSET) 1).. which can be used in any complex geological formations because stress-strain relationship is unnecessary, and it is applicable to any three-dimentionally complex shape of caverns.

The DSET is based on convergence, borehole inclinometer and extensometer measurements perfoming during a construction period. The procedure of the technique is as follows. That is, when the displacements are measured at several measuring points around a cavern, a continuous displacement function is assumed so as to interpolate the measured values at the measuring points. The displacement function yields a strain distribution around the cavern by differentiating the function. If the derived strains tend to be greater than critical failure strains, then the additional supports such as shotcrete, rock bolts and steel ribs must be installed.

It is noted in the proposed technique that the most significant problem is how to evaluate the allowable strains of in-situ rock masses properly.

(Figure in full paper)

In order to determine failure criteria in terms of strain, the author has analyzed the results of laboratory and ill-situ tests, and h~ found out the fact that the critical failure strains obtained by laboratory tests are the same order of magnitude as those observed in in-situ tests. This shows that a failure criterion in terms of strain, which is utilized in monitoring the stability of caverns, may be determined by only the laboratory tests. Since the DSET is simple in operation, the stability of caverns can be assessed immediately after taking the displacement measurements. In order to achieve a quick interpretation of the data, the author has developed a monitoring system by introducing a microcomputer that makes the DEST applicable to any construction sites where a big computer system is not available.

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