MONITORING AND MODELLING GROUND DEFORMATIONS DURING TUNNELLING Michael J. Kavvadas Civil Engineering Faculty, National Technical University of Athens, 9 Iroon Polytechniou St., GR-15780 Zografou, Greece Abstract Monitoring of ground deformations in tunnelling is a principal means for selecting the appropriate excavation and support methods among those foreseen in the design, for ensuring safety during tunnel construction (including personnel safety inside the tunnel and safety of structures located at ground surface) and, finally, for ensuring construction quality management according to ISO 9000. This paper briefly describes the types of ground deformation measurements often used in tunnelling, the difficulties in obtaining ground measurements and their subsequent evaluation, and the application of these measurements (a) in modeling tunnel excavation and support and (b) in establishing early warning systems against incipient ground collapses or damage to structures at ground surface. Examples of ground deformation monitoring and their application in tunnel design and construction are illustrated via cases from the Jubilee Line Extension of the London Underground, from Lines 2 and 3 of the Athens Metro and from a nine-kilometer long mountain tunnel in Greece. In the first two examples, ground deformation monitoring aimed to ensure that structures at ground surface would not be harmed by the tunnelling operations. In the third case, the objective was the optimization of the temporary support requirements as well as early warning against potential collapses. 1. Introduction The need to upgrade and further develop transportation infrastructure (high-speed railway, highway and urban transit lines) has lead to the on-going construction of large-diameter, long tunnels under difficult conditions. Such conditions usually arise from a combination of adverse ground and groundwater regimes, very high overburden pressures or, in the case of urban tunnels, the existence of sensitive structures within the zone of influence of the tunnel. Typical examples of such tunnels are the high-speed railway Alpine base tunnels in Switzerland, at present in the early stages of construction (the 57 km Gotthard and the 43 km Lotschberg) as well as several large Metro tunnelling projects recently completed (e.g. in London and Athens). Difficult tunnels also include several highway and railway mountain tunnels at present under construction in Greece, due to the very difficult ground conditions caused by the intense tectonisation and heterogeneity of the geology in Greece. Until quite recently, tunnel excavation and temporary support was based solely on the experience of the Contractor and support methods were based on analogies from the mining industry (steel sets and timber lagging). The engineer was mainly involved with the design of the final lining (concrete arch) using assumed loads, often based on the classical recommendations by Terzaghi (1946), as shown in Fig. 1. Proceedings, 11 th FIG Symposium on Deformation Measurements, Santorini, Greece, 2003.