Measuring the displacement of a tunnel is prerequisite for the evaluation of stability and also makes it possible to understand the geological condition ahead of the tunnel face. In fact, the displacement of a tunnel occurs before, during and after the excavation. The measurement, in general, is undertaken only behind the face allowing a certain amount of displacement, so-called ‘premeasurement- displacement’. Measured displacement is only a part of the total radial displacement. Therefore, pre-measurementdisplacement should be determined in order to characterize a complete radial displacement of the tunnel. A horizontal inclinometer was used for the measurement of the settlement ahead of the tunnel face in the study site. It was relatively easy to be installed from the other portal of the tunnel. The results from the measurement of the settlement ahead of the face enabled us to construct a complete radial displacement curve by a nonlinear regression analysis. It was found that the crown displacement of the tunnel of the study site started to occur at a distance equivalent to a three-tunnel diameter ahead of the tunnel face. Taking into consideration the complete displacement characteristics, the settlement could be about 40% of the total displacement of the tunnel. Based on the complete radial displacement, it was possible to determine the appropriate support characteristics and the time of installation.
The New Austrian Tunneling Method (NATM) is based on a concept whereby the ground (rock or soil) surrounding an underground opening becomes a load bearing structural component through activation of a ring like body of supporting ground and is also defined a method of producing underground space by using all available means to develop the maximum selfsupporting capacity of the rock or soil itself to provide the stability of the underground opening [1, 2]. For the maximum self-supporting capacity of the rock or soil, it is very essential to determine the time of installation and the stiffness of the supports. The design schemes have been determined according to various empirical support patterns, which depend on geological condition. To understand the geological condition ahead of the tunnel face, measurement of the displacement is prerequisite for the stability evaluation of the tunnel. In general, the measurement is undertaken only behind the face allowing a certain amount of displacement, socalled ‘pre-measurement-displacement’, which is only a part of the total radial displacement. The value of pre-deformation like the settlement ahead of a tunnel face is very important in the ‘convergence–confinement method’, in which the interaction between the rock and support is studied according to the ground reaction curve and support characteristics [3, 4, 5, 6]. In this study, a horizontal inclinometer was used to identify complete radial displacement. The inclinometer is relatively easy to be installed from the other portal of the tunnel. The settlement ahead of the face is evaluated with the analysis of the total radial displacement.