Measurement and Prediction of Ground Movements Around Three NATM Tunnels.

This research is concerned with the measurement and interpretation of ground displacements around three NATM tunnels forming part of the Heathrow Express Terminal 4 underground station and the prediction of these displacements using the finite element method. Ground displacements, using precise levelling and multiple-rod extensometers, were measured as part of the construction process. In addition the displacement of the shotcrete tunnel lining was monitored using three dimensional displacement monitoring. This database of information was supplemented with ground displacements measured ahead of the advancing concourse tunnel using chain deflectometers and inclinometers. All the monitoring results were interpreted and typical ground displacement patterns around the tunnel heading were established. An unique set of ground displacement vectors in a longitudinal plane on the tunnel centreline was produced. Such a set of field measurements is extremely valuable for the benchmarking of 3D finite element analyses. Based on the results obtained from the monitoring it was possible to evaluate the performance of the various instrumentation systems used. It was the first time that chain deflectometers had been used to measure ground displacements ahead of an advancing tunnel and their performance was very encouraging. Chain deflectometers provide an alternative for measuring vertical displacements and in addition provide horizontal displacements, which is unusual for horizontally installed instrumentation. The performance of the inclinometer system was also judged to be highly satisfactory and the use of sacrificial inclinometers ahead of the tunnel may usefully be considered for future tunnel instrumentation. The performance of the precise levelling and multiple-rod extensometers was adequate, while the performance of the in-tunnel lining deformation monitoring proved to be disappointing. Both 2D and 3D finite element analyses were conducted to investigate the effect of the different categories of assumptions and simplifications inherent to the method. The finite element analyses showed that the initial stress conditions, material model used and the modelling of construction details all have a significant effect on the predictions. Although the prediction of displacements in the far-field was disappointing for all analyses, reasonable predictions of the ground displacements around the tunnel heading were obtained with a 3D analysis using a non-linear elastic perfectly plastic material model