Analysis and Evaluation of Tunnel Contour Quality Index

Abstract

Abstract Overbreak, which is inevitably caused by geological and operational conditions in a drill & blast tunnel and is one of elements consisting of tunnel contour quality, has been a major cause to increase construction time and costs. Thus, many attempts have been made to reduce it worldwide. Against this backdrop, this paper suggests Tunnel Contour Quality Index (TCI) established based upon laser profiling technique for the effective management of tunnel contour quality, and analyzes TCI as well as the factors on TCI through field investigations using laser profilers in Norway and Korea. TCI is defined as a fraction form consisting of three elements (overbreak depth, contour roughness, longitudinal overbreak variation) that are calculated from surveyed tunnel profiles, three weights for the elements and one constant for range adjustment. The result shows a large difference in TCI, i.e., TCI for the Korean tunnels is average 63.4 by overbreak depth of 21.4 cm, contour roughness of 1.066 (meaning a 6.6% increase than the planned contour area by blasting) and longitudinal overbreak variation of 5.7 cm, while TCI for the Norwegian tunnels is average 45.9 by 64.2 cm, 1.120 and 11.1 cm respectively. The main causes for the large difference are the different philosophy in tunnel design between the two countries, as well as the hard rock having distinct joints of Norway that makes it easier for blasting pressure to break off rock masses along existing joints than to develop new cracks in the hard rock. In addition, it is found that the major element affecting TCI is overbreak depth because large overbreak depth increases the possibility of high contour roughness. The results show that most overbreak primarily results from large look-out and poor rock masses, and major causes resulting in high contour roughness are heterogeneous geological conditions in the tunnel face and big deviation in drilling position of contour holes. Finally, it is thought that TCI established based upon laser profiling technique can be used for the more effective management of tunnel contour quality.