A case Study of the Geological Risk Management using Suitable Investigation in Mountain Tunneling

In mountain tunneling, it is important to avoid or reduce the geological risks such as a weakness of the bedrock and a large volume of ground water beyond assumption from a safety and economic efficiency point of view. The author has experienced a mountain tunneling project in which uncommon geological risk management was conducted. The objectives of this study are, thus, to introduce this risk management, discuss its benefits, and describe future issues in geological risk management. In this project, a research was conducted on the risk management technique for occurrence prevention of the above-mentioned latent geological risks on a mountain tunnel in Japan. It was presumed that a ground had many geotechnical problems based on results of restricted investigation in a prior stage. In order to cope with the geological risks and to advance construction more smoothly, the tripartite council which consisted of owner, constructor and geological consultant were established before construction, and various technical advices were given on the construction and auxiliary method. Since geotechnical information was always shared in the tripartite council, quick investigation was conducted by the geological consultant when collapse of the crown and water inflow occurred in tunnel. The geological consultant played important role about the revaluation for unexcavated section and the proposal of a suitable construction method from geotechnical viewpoint. The geological risk management was continued until end of the construction using detailed geomorphic analysis and observational construction control system in addition to various kinds of investigations. Eventually, designed case was compared with actual case adopting modification index (i). A geotechnical base line of ground classification was drawn up on the basis of the information acquired by this construction and it is applied also in other tunnel. Moreover, the degree of the geological uncertainty in a ground at the planning phase will become more clear by application of this geotechnical base line. That serves as an important factor to examine the construction method, cost and planning of construction. The geological risk management research of ground evaluation utilizing this experience is continued in other mountain tunnel with the same kind of geological features. It is an important future subject to quantify the geological risks as the degree of deviation and extent from assumed geological condition, and to reflect them in the design and construction