| Round length in conventional tunneling can pose economical effect. In Japan, however, the round length is generally determined by ground class and support pattern according to the guidelines provided by owners. Round lengths which are against the guidelines are not easily adopted mainly due to lack of experiments. In this study, numerical analysis is conducted to grasp a rough outline of influence of round length on displacement and stress of support during excavation. Based on the results, field tests are carried out to examine the actual behavior of tunnel when the round length is changed to 1.2 - 2.0 m from 1.0 - 1.5 which are originally provided by the guideline. Major conclusions include: According to the numerical analysis of continuous ground, influence of changing round length is small in terms of displacement and stress of support. The trend is the same in the field tests in this study. However, frequency of fall of loosen rocks increased in the case of discontinuity-dominant rock mass. Based on the results, an index for determination of round length is proposed in this paper. |
