Rock stress measurement is cast in the more general context of the determination of boundary conditions for rock engineering. A study on the effect of elastic anisotropy on underground excavations reveals that this effect is actually controlled by the imposed boundary conditions. Applications to tunneling and to the Underground Research Laboratory (URL), Canada, exemplify a procedure for boundary condition estimation formerly developed by the authors. The procedure displayed fast convergence despite the complex geometry, and the high degree of non-linearity of the models. In the URL application, the 3-D model was able to completely reproduce the complex measured stress pattern, and the 90° rotation of the principal in situ stresses with depth. In order to reliably estimate the boundary conditions reproducing the current in situ state of stress, response measurements of the rock mass to current disturbances are necessary as input data.

La mesure des contraintes in situ tombe dans le domaine general de la determination des conditions aux limites pour les problèmes de mecanique des roches. Une etude sur le rôle de l'anisotropie elastique sur les excavations souterraines montre que ce rôle est contrôle par les conditions aux limites qui sont imposees. L'application aux tunnels et au Laboratoire Souterrain de Recherche (URL) au Canada, nous permet de presenter une methode de determination des conditions aux limites. La methode converge rapidement bien que le geometrie soit complexe et que le problème soit non-lineaire. Pour le URL, le modèle 3D est capable de reproduire la distribution des contraintes qui ont ete mesurees ainsi que la rotation de 90 degres des contraintes in situ avec la profondeur. Afin de determiner avec precision les conditions aux limites responsables pour la distribution actuelle des contraintes, la reponse du massif rocheux doit être connue et mesuree.

In der Felsentechnik sind die Spannungsmessungen in Gestein an die Bestimmung der Randbedingungen gebunden. Eine Studie legt dar, dass der Effekt elastischer Anisotropie in unterirdischen Aushoehlungen von den Randbedingungen kontrolliert ist. Eine von den Authoren entwickelte Methode zur Bestimmung der Randbedingungen wird an Beispielen im Tunnelbau und im Underground Research Laboratory (URL) in Kanada illustriert. Auch bei komplexer Geometrie und einem hohen Grad an Nicht-Linearitaet zeigt diese Methode ein schnelles Konvergenzverhalten. Ein 3D-Model und dessen 90° Drehung der lokalen Hauptspannungen mit der Tiefe, kann bei einer URL Anwendung vollstaendig reproduziert werden.

Um mit dieser Methode die Randbedingungen zuverlaessig abschaetzen zu koennen, werden der gegenwaertige lokale Spannungszustand sowie die Antwort des Gesteins und die resultierenden Stoerungen als Eingabedaten benoetigt.

Introduction

Any rock mechanics study starts with the determination of the force fields in which the rock mass under study is embedded. Body forces and boundary conditions determine these force fields. The latter fields may be very complex, due to local variations in rock mass structure, or to major structural features, such as faults at the regional scale. In the majority of the cases, the boundary conditions are assumed (no lateral strain, uniform "far field" state of stress etc.). At best, they are derived by averaging the results of some (local) stress measurements over the volume of rock of interest, which, in the case of civil and mining projects, can be of the order of 103 - 109 m3.

Because the behavior of a rock mass is determined by its boundary conditions, it is proposed to shift one's attention from rock stresses to boundary conditions. Hence, the central question is how to assign boundary conditions to a (numerical) model of a rock mass, which, in the following, will be called M, for brevity. M is the tool that allows one to forecast the rock mass response to any disturbance (e.g. excavation, slope, foundation etc.).

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