Vision metrology using digital cameras has been used for accurately measuring the configuration of large industrial products such as ships, aircraft and automobiles. This paper refers to the application of the technique to the measurement for engineering structures, mainly tunnels. Vision metrology has been nearly established, but its use in civil engineering requires solution of numerous problems. A test was therefore conducted in an actual tunnel to verify measurement accuracy for tunnel configurations for assessing the applicability to convergence measurement in tunnels. As a result, the measurements taken were as accurate as those obtained by Total Station instruments. This paper provides an outline of the test and reports how the technique is applied to various objects on a trial basis.
Rock convergence measurement in the tunnel is essential to the New Austrian Tunneling Method (NATM). Convergence measurement devices or Total Station instruments have been generally used. The former equipment requires installing pins on the wall surface and stressing the measuring device. Thus, it is an impediment to work in the tunnel and poses safety problems because it takes much time and involves high place work. The practical measurement accuracy is estimated at 0.5 mm. The device measures expansion between pins but cannot identify three-dimensional displacements. The latter type can basically take measurements accurately if a satisfactory measurement environment or sufficient time is available. It can, however, make three-dimensional position measurement only at several points per cross section intervals of 10 to 20 m because of restrictions of measurement time and labor. The measurement accuracy is a few millimeters at the best. Neither can identify area wide configuration in a tunnel. Construction conditions for mountain tunnels have recently been undergoing drastic changes. Construction projects in urban areas and those involving large cross sections have been increasing. It has been increasingly important to rapidly obtain more reliable measurements of rock convergence in tunneling and other phenomena, and reflect the data in subsequent construction. In the meantime, availability of cheaper digital cameras of higher resolutions and cheaper and faster personal computers with larger capacities for processing image data has been making vision metrology using digital cameras more applicable. Vision metrology has evolved from the photography using dry plate stereo cameras, or close range photogrammetry. It has recently been used mainly for industrial measurement including precision configuration measurement of such objects as ships, aircraft and large parabolic antennas. There is also high possibility of real-time field measurement. Thus, the technique offers numerous benefits. The authors are now studying the application of the technique for measuring deformations of civil engineering structures including rock convergence in tunneling. This paper presents the principle and procedure of vision metrology, and outlines an measurement accuracy verification test in an actual tunnel. It also reports the studies made so far of displacement measurement for an urban tunnel excavated by NATM, for steel segmental rings in a tunnel excavated by TBM, and of measurement while burying a railway system in service underground surface.
