The use of a tunnel boring machine (TBM) has become the first choice when planning the excavation of a circular tunnel. The basic advantages of TBMs are high safety and rapid excavation speed, with low overbreak (extra-excavation) and low manpower. TBM tunneling also has only a very small effect on the surrounding rock mass and constructions. However, the machine is comparatively expensive, is not as effective in the excavation of extremely hard rock, and lacks flexibility toward rapidly changing ground conditions. Furthermore, setup and dismantle times are significant, and the range of available tunnel cross-section shapes is limited.
The history of TBMs extends back to the beginning of the 19th century, and improvements have been made gradually but continually until today. However, the advantages and disadvantages mentioned above still remain to a greater or lesser degree. These features of TBM give rise to very good excavation rates under ideal conditions, yet poor performance under adverse conditions. In the conventional tunneling method (drill and blasting), high excavation rates cannot be achieved even under ideal conditions, regardless of the number of workers employed, because a consistently high work rate cannot be realized. In comparison, using a TBM, 100 meters per day is readily achievable under ideal conditions with continuous operation, however under adverse conditions, the penetration rate could fall radically. It is therefore essential to clarify in advance whether a TBM is suitable for a particular project, and in some cases a feasibility study may be necessary.
A considerable amount of research regarding TBM both in the field and in fundamental tests has been published, and the applicability of TBM has been discussed to some extent. However, there still exists considerable differences in opinion from author to author. In the first part of this paper, we review previous studies and outline the present situation, focusing on TBMs with disc cutters. In the second part of the study we discuss how to apply this knowledge in practice and propose an expert system for the applicability of TBM tunneling. Okubo (1990) previously published basic research on the expert system, and this paper represents further development and improvement on that work.