INTERPRETATION OF DEFORMATIONAL BEHAVIOR OBSERVED DURING EXCAVATION CONDUCTED TO ENLARGE CROSS SECTION OF AN OLD TUNNEL

A two lane road tunnels, which were constructed by a side drift method more than 35 years ago, in a very difficult ground condition had to be extended. Knowing that the initial construction had experienced large deformation and high pressure on concrete lining, a special care was taken for planning, selection of construction method, and the actual extension of the tunnel cross section was completed only with minor deformation with fair confidence in tunnel stability. This paper attempts to reveal a reason behind this fact and tries to associate the success of the operation with an existence of so-called “ground ring” which could have been formed around tunnel lining over time.