Round length in conventional tunneling can pose an economical effect. However, in Japan, the round length is generally determined through a ground class and support pattern according to the guidelines provided by owners. Round lengths, which are against the guidelines, are not easily adopted mainly owing to the lack of experiments. In this study, numerical analysis was conducted to grasp a rough outline of the influence of round length on the displacement and stress of support during excavation. Based on the results, field tests were conducted to examine the actual behavior of a tunnel when the round length was changed to 1.2–2.0 m from 1.0–1.5 m, which are originally provided by the guideline. Accordingly, the following major conclusions were drawn. According to the numerical analysis of continuous ground, the influence of the changing round length is small in terms of displacement and stress of support. The same trend was observed in the field tests of this study. However, the frequency of the fall of loose 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.
The construction of tunnels in mountains requires low cost and a short construction period. This includes the extension of the round length as a method of rationalization. The installation interval of excavation support has expanded compared to that of the standard round length, and the reduction of support members and increase in the construction speed are considered possible. However, the area of unsupported ground has increased, raising some concerns about the lowering of safety at the time of construction and instability of support due to deformation or increase in stress on the support. To solve such problems, we performed a three-dimensional (3D) numerical analysis or field test by using the extended round length to determine the effect of the extended round length on ground and support.
The standard round length of a mountain tunnel is decided according to the ground classification. This length is calculated between 1 and 2 m and is related to the thickness of the spray concrete, pitch and size of the steel-arched support, and pitch and size of the rock bolt. These combinations are termed as the support patterns and are classified into five categories, as listed in Table 1.