ABSTRACT:

Determination of the initial state of stress in rock masses remains one of the most important and complex problems in Rock Mechanics. The paper presents a brief review of the Stress Tensor Tube (STT) and the Small Flat Jack (SFJ) methods, developed at the LNEC, pointing out the most recent advances concerning apparatus, testing and result interpretation. A synthesis of the data collected by the LNEC about the initial stress fields in rock masses is presented, as well as some significant applications of rock stress measurements in the design of structures.

RÉSUMÉ:

La détermination de l''état de contrainte initiale dans les massifs rocheux est encore un des problèmes les plus importants et complexes de la Mécanique des Roches. On décrit deux méthodes - la cellule tridimensionelle (STT) et Ie verin plat (SFJ) - devélopées par LNEC pour la détermination de l''état de contrainte, en présentant les derniers perfectionnements sur les appareils, les éssais in situ et leur interpretation. Une synthèse des donnés obtenues par Ie LNEC, pendant des années, sur l''état de contrainte est présentée, ainsi que quelques applications significatives de ces mesures à des ouvrages.

ZUSMMENNFASSUNG:

Die Bestimmung des ursprünglichen Spannungszustandes in Gebirgen ist immer noch eines der wichtigsten und Komplexesten Probleme der Felsmechanik. Das Referat gibt einen kurzen Überblick über die im LNEC entwickelten STT -(Stress Tensor Tube - Spannungs tensorröhre) und SFJ - (Small Flat Jack - Kleines Druckkissen) - Verfahren, wobei die neuesten Fortschritte bezüglich der Apparate, der Versuchsausführung und der Interpretation der Ergebnisse herausgestellt werden. Eine Synthese der durch das LNEC zusammengetragenen Daten über die ursprünglichen Spannungsfelder in Gebirgen wird vorgestellt, sowie auch einige bedeutsame Anwendungen von Felsspannungsmessungen.

I - INTRODUCTION

It is well known that the initial state of stress in rock masses is a factor of the utmost importance for the behaviour of structures in rock, such as dams and underground structures. However stress determinations in rock masses undoubtly remain one of the major problems in Rock Mechanics. Since the virgin stresses depend on factors such as the weight of the overly ing formations and forces associated with the geological history of the rock mass, such as genetic, orogenic and erosive actions, and they are conditioned by lithological, structural and topographic patterns, accurate forecasts of the stress state require in situ measurements. The severe local meaning of the tests and the scattering of their results, connected both with the available techniques and the complex morphology of rock masses, have, however, in most cases, so far prevented a satisfactory knowledge of the initial state of stress for design and construction purposes. LNEC has developed two methods for in situ stress determinations, both of them based on the stress relief concept: the Stress Tensor Tube (STT), based on an overcoring process, which requires a know ledge of the deformability characteristics of the rock mass, and the Small Flat Jack Method (SFJ), using slot openings, which allows the direct measurement of the stresses in the rock mass. The most recent developments concerning apparatus, testing methodologies and interpretation of results are referred to for both techniques, and a summary of LNEC experience on stress field measurements in rock masses, collected for several years, is reported, as well as four significant case histories dealing with dams, mines, tunnels and caverns.

2 - TECHNIQUES FOR MEASURING THE STATE OF STRESS
2.1 - SFJ method

Execution of at least three tests of this type, with the slots forming a rosette, makes it possible to determine the state of stress at the point of the plane in question. Normally, three slots in rosette at 45° are used, and by means of an additional test a confirmation of the results is obtained. By carrying cut this_ study on two further planes with different orientations it is possible to determine the complete state of stress at the point concerned (Fig. 1).

3 - SOME ROCK STRESS MEASUREMENTS
3.1 - State of stress in a coal mine in Spain

This mine is located along sedimentary formations, which consist of alternate sandstone, claystone and coal layers at the test site. Strata have a 200 dip towards the entry of the tunnel where tests with STT are performed. Given the depth at which the tunnel lies (about 325 m), joints are closed. The four states of stress measured in the rock mass show that the direction of one of the principal stresses is roughly normal to the strata, whereas stresses parallel to the strata are not very different in any direction. The stresses measured show that act - ins stresses are not only due to the dead weight, presenting concentrations between 2 and 10 in the plane of strata, whereas following the normal to the strata there is a relief that corresponds to about 0.65 and 1.75 of the expected stresses.

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