SYNOPSIS:

This paper describes the results of short- and long-term borehole di1atometer tests performed in confined blocks of rock salt under simulated in-situ conditions. It is shown that, by using an appropriate interpretation method, such tests make it possible to determine certain basic rheological parameters of rock salt, needed in the design of underground openings.

RESUME:

On presente les resultats des essais dilatometriques à court et a long terme, effectues dans des blocs confines du sel gemme, en conditions quasi-naturelles. En utilisant une methode d'interpretation appropriee, on peut tirer à partir de tels essais certains paramètres rheologiques fondamentaux du sel gemme, necessaires pour la conception de souterrains.

ZUSAMMENFASSUNG:

In dieser Arbeit werden die Ergebnisse des kurz- und langzeitigen Bohrlochdilatometerversuche, welche in biaxialbelasteten Steinsalzquadern unter simulierten Feldbedingungen durchgefuehrt wurden, beschrieben. Es wird gezeigt, daß man mit Hilfe einer dazu geeigneten Bewertungsmethode aus solchen Versuchen einige grundsatzliche rheologische Kennwerte von Steinsalz, die beim Entwurf der unterirdischen Kavernen brauchbar sind, bestimmen kann.

1. INTRODUCTION

The mechanical behaviour of rock salt has for many years been studied essentially in connection with the design and operation of salt mines. In recent years, however, the interest and activity in this field has grown very rapidly because of energy-related projects, such as underground storage of oil, natural gas and radioactive waste. On the basis of numerous past laboratory studies of the response of rock salt and potash to changes in stress and temperature, a rather complex picture of the behaviour of such materials has been obtained. In addition, it was found that laboratory test results obtained on rock cores may not necessarily well represent the behaviour of the rock mass in situ, because of the effects of scale, destressing and changes in humidity and temperature. For that reason there is presently an increasing interest in developing reliable in-situ testing methods that would be able to furnish a set of most important parameters needed in the design of underground openings. The paper describes one such method which has recently been developed by the authors. The method uses a high capacity borehole dilatometer, such as the Colorado School of Mines Cell, for performing" short-term and longterm borehole loading tests in-situ. A proper evaluation method, proposed by the authors, enables to determine from such a test: the first loading and cyclic loading rigidity moduli, the short term tensile strength, a portion of the stress-strain curve up to the peak strength and the basic creep parameters needed for writing the constitutive creep equation. The paper describes the method and presents an analysis of borehole dilatometer test results obtained in the rock salt from a salt mine in Eastern Canada.

2. BOREHOLE DILATOMETER TESTING IN ROCK MECHANICS

The borehole dilatometer test has been in a frequent use in rock mechanics since at least two decades, especially after valuable instrumentation developments made Rocha et al. (1966) and by Hustrulid and Hustrulid (1975). These authors have shown that a borehole dilatometer test can furnish reliable in-situ values of short-term deformation properties of the rock mass around a borehole. On the other hand, there has been until recently very little interest in performing long-term hole expansion tests with this instrument for determining the creep properties of rock materials, so that practically no such information can be found in the present rock mechanics literature. The situation was quite different in other fields, such as in ice and permafrost mechanics, where the long-term borehole dilatometer testing has been in current use since the beginning of the 1970-ies (Ladanyi and Johnston, 1973). In the recent years, there has been a growing interest in testing short- and long-term mechanical properties of rock salt mainly in connection with the design of underground storage cavities in salt deposits. Although from a large amount of valuable laboratory test results, and from in-situ closure observations in salt mines accumulated through the years, a fairly clear picture of the behaviour of rock salt under changes of stress and temperature has been obtained, it is nevertheless recognized that it would be useful to be able to test the rock salt directly in-situ by using a relatively simple method, such as the borehole dilatometer test. The former showed the results of two short-term dilatometer tests to failure, carried out in unconfined thick cylinders of rock salt, using an improvised dilatometer device. Albrecht et al. in turn, used a high capacity dilatometer of their own design to determine deformation properties of rock salt in situ. However, although a short creep stage was included in their test, there is no mention in the paper how this creep information was used. In this connection it should also be mentioned that some attempts have been made to test creep properties of rock salt in situ by pressurizing unlined underground cavities (Passaris, 1979) and boreholes (Nelson and Kocherhans, 1981).

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