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ABSTRACT: New monitoring techniques, particularly the determination of absolute displacements instead of convergence measurement have greatly enhanced the potential for interpretation of monitoring results. Traditional analysis procedures, like time histories of displacements provide a very local information of tunnel behaviour only. For tunnels a vector plot of displacements in the cross-section has brought a considerable improvement, while the developement of so called lines of influence has actually opened completely new fields. The evaluation of influence-lines allows a 3-dimensional assessment of the tunnel behaviour. Weakness zones can be clearly localized and the required countermeasures designed. Certain specific evaluations allow the identification of typical tunnel failure modes. A case history is given.
KURZFASSUNG: Neue Meβtechniken, besonders die Bestimmung von absoluten Verschiebungen anstatt von Konvergenzen. haben die Möglichkeiten der Interpretation von Meβdaten wesentlich erweitert. Die traditionellen Darstellungen, wie Zeitgeschichten von Verschiebungen, geben nur eine sehr lokale Information ueber das Tunnelverhalten. Die Darstellung von Verschiebungsvektoren im Querschnitt hat schon eine erhebliche Verbesserung gebracht, ein Durchbruch gelang durch die Entwicklung der sogenannten Einfluβlinien. Deren Auswertung erlaubt eine raumliche Beurteilung der Deformationen. Schwachezonen im Baugrund können klar erkannt und entsprechende Maβ- nahmen ergriffen werden. Bestimmte Methoden ermöglichen das Erkennen von typischen Versagensmechanismen. Ein Fallbeispiel wird erlautert.
ABSTRAIT: Les techniques de mesurage nouveaux, particulièrement de deplacements absoluts au lieu de convergence, ont ameliore beaucoup les possibilites d'interpreter les donnees. Les presentations traditionels, comme les diagrammes de deplacement vers temps, rendrent Wle information très locale seulement. La valeur d'information a beaucoup ameliore avec les graphiques donnant les vecteurs dans las section. Finalement les Iignes d'influence permettent une interpretation 3-dimensionel des deplacements du tunnel. Des mechaniques de defaillance peuvent ètre determine par quelques evaluations specifiques. Un cas d'un tunnel en Autriche est donne.
INTRODUCTION Tunnelling in difficult ground conditions, particularly near the surface, relies greatly on daily measurement of displacements to control the stability of the structure and to continuously verify or adjust the excavation and support strategies. An optional performance, analysis and interpretation of tunnel monitoring to guide an economic construction - while safety aspects are fully warranted - is a difficult task. A task that has sometimes not been achieved where tunnel collapses occurred. Frequently the monitoring results were blamed not to show the unfavourable developements in time. The chance to identify such developements, however, depends on the kind of monitoring systems, their professional and experienced performance and interpretation. A number of tunnel collapses show that the traditional monitoring technique of diagrams showing measurement results against time is an insufficient tool for the responsible engineer at site to assess the situation with confidence. Physical models, theoretical considerations and particularly the analysis of tunnel failures show clearly that critical situation in tunnels are mostly caused by the developement of shear zones in the ground or uncontrolled relaxation of natural arches. Mostly, such developements cause irregularities in the 3- dimensional developement of deformations in the tunnel and could be identified at small magnitudes. To perform reliable interpretation the monitored data need to be visualised in a way that enables a direct correlation of excavation activities with developements of displacements in both time and space.
MONITORING TECHNIQUES The classical method to determine tunnel deformations has been the convergence measurement by means of tape extensometers. Usually, it has been supplemented by levelling of the roof Special sections also included stress measurements in the lining, borehole extensometers, measuring rock bolts, and others. Most of these classical methods have shown significant insufficiencies. As an example, convergence readings only provide information on relative displacements between two points.
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